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1

Shear-wave velocity variation in jointed rock: an attempt to measure tide-induced variations  

SciTech Connect

The use of the perturbation of seismic wave velocities by solid earth tides as a possible method of exploration for fractured media is discussed. Velocity of compressional seismic waves in fractured homogeneous rock has been observed to vary through solid earth tide cycles by a significant 0.5-0.9%. This variation of seismic velocities may be attributed to the opening and closing of joints by tidal stresses. In an attempt to see if shear-wave velocities show a similar velocity variation, a pneumatic shear-wave generator was used for the source. The 5 receivers, 3-component, 2.0 Hz, moving-coil geophones, were connected to a GEOS digital recorder. The two receivers located 120 m and 110 m from the source showed large shear-to-compression amplitude ratio and a high signal-to-noise ratio. A glaciated valley was chosen for the experiment site, since topography is flat and the granodiorite is jointed by a set of nearly orthogonal vertical joints, with superimposed horizontal sheeting joints. A slight velocity variation was noted in the first 200 consecutive firings; after which, the amplitude of the shear-wave begun to increase. This increase has been attributed to the compacting of the soil beneath the shear-wave generator (SWG). In the future, the soil will be compacted prior to placing the SWG or the SWG will be coupled directly to the rock to alleviate the amplitude fluctuation problem. This research may have application in exploration for fracture permeability in the rock mass between existing wells, by measuring seismic velocities from well to well through the tidal cycle.

Beem, L.I.

1987-08-01

2

Velocities Variations of the Near Surface Seismic Waves in a Soil Liquefaction Site in Taiwan  

NASA Astrophysics Data System (ADS)

Soil liquefaction is a natural hazard that we can never ignore during a large earthquake. Soil liquefaction is normally occurred at somewhere near the surface, while the sandy soil is loose and having higher level of ground water. Soil liquefaction can potentially cause the surface constructions subsiding, inclining or collapsing, and damage the shallow buried light structures and lifelines. Soil spurts to the surface will affect crops in the farmland as well. This study utilized the P-wave refraction method and the multichannel analysis of surface waves (MASW) method to obtain the variation of the near surface P-wave and S-wave velocities in a particular site in southern Taiwan, where liquefaction occurred in 1946 and again in 2010. The purpose of this study is to investigate the correlation between the velocities variations of the seismic waves near surface and the liquefaction within and outside the liquefaction area. Totally, seven survey lines were deployed; results of the study show that velocities of the P-wave and S-wave are directly related to the area with soil liquefaction phenomena. For the areas not having liquefaction, velocities of the P-wave and S-wave are almost the same in the horizontal direction, but varied vertically from the ground surface down to the depth of 18 m. However, within the liquefaction area, the velocities of P-wave and S-wave in the horizontal directions are not varied significantly within 5 meters beneath the surface; major variances of the P-wave and S-wave velocities appear at between 5 and 18 meters below the ground surface. Within the soil liquefaction area, the velocity of S-wave is low; however, the P-wave velocity is higher in contrast. In addition, because of the saturated water content, variations of the P-wave velocity in the horizontal direction remain the same below the depth of 8 m. For the area not having liquefaction seen on the surface, velocity of the S-wave below the depth of 8 m is similar to that regions having liquefaction in the horizontal direction, which inferred that the region would have liquefaction under the surface within the entire area of all liquefied positions. Although evaluating the potential of occurring soil liquefaction in a large earthquake was commonly conducted by using an engineering method. In this paper, we show that the potential of soil liquefaction can be evaluated by using the velocities variations of seismic waves as well. In addition, the seismic method can be applied more efficiently to a broader region.

Chen, T.; Chen, C.; Shih, R.

2012-12-01

3

Intraseasonal vertical velocity variation caused by the equatorial wave in the central equatorial Indian Ocean  

NASA Astrophysics Data System (ADS)

Intraseasonal vertical velocity variation in the central equatorial Indian Ocean was investigated using observations from the field experiment "Mirai Indian Ocean Cruise for the Study of the MJO-convection Onset" and from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction, in October-November 2006. Using an array of four subsurface moored acoustic Doppler current profilers, we estimated vertical velocity by applying the continuity equation. Results indicated alternating downwelling and upwelling episodes at around thermocline depth, with maximum amplitudes larger than 8.0 × 10-5 m s-1, or about 7 m per day. The vertical velocity variation was mainly produced by the divergence/convergence of meridional currents, with a quasi-biweekly period of 11-16 days. The temporal changes in temperature around thermocline depth were consistent with variations in vertical velocity, whereas upwelling had less impact on the surface layer temperature. Intraseasonal variations in the ocean may be a part of biweekly fluctuations by remotely forced mixed Rossby-gravity waves, which have a meridional current maximum at the equator, accompanied by divergence/convergence in the surface layer a few degrees from the equator.

Horii, Takanori; Masumoto, Yukio; Ueki, Iwao; Kumar, S. Prasanna; Mizuno, Keisuke

2011-09-01

4

CHINESE JOURNAL OF GEOPHYSICS Vol.50, No.1, 2007, pp: 183191 FINE STRUCTURE OF P-WAVE VELOCITY VARIATIONS UNDERNEATH  

E-print Network

VARIATIONS UNDERNEATH THE CENTRAL PACIFIC FROM PKP WAVES RECORDED AT THE CHINA SEISMIC NETWORK (CSN) ZHENG Si of the lowermost mantle underneath the Central Pacific. Here we use differential travel times between the AB and DF branches of PKP waves to constrain the small-scale variation of P velocity beneath the Central Pacific

Song, Xiaodong

5

Variation of seismic-wave velocities in westerly granite under stress  

E-print Network

Variation of Seismic-Wave Velocities in Westerly Granite Under Stress. (December 1994) Abdulaziz Muhareb Al-Shaibani, B. Sc. , King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia Co-Chairs of Advisory Committee: Dr. Anthony Gangi Dr..., 89-94, 1957. Todd, T. , G. Simmons and W. Baldridge, Acoustic double refraction in low-porosity rocks, Bull. Seismol. Soc. Am. , 63, 2007-2020, 1973. VITA Abdulaziz Muhareb Al-Shaibani was born in Taif, Saudi Arabia in 1964. He graduated with B...

Al-Shaibani, Abdulaziz Muhareb

1994-01-01

6

Apparent changes in seismic wave velocity related to microseism noise source variations  

NASA Astrophysics Data System (ADS)

Currently there is a strong interest of using cross correlation of ambient noise for imaging of the subsurface or monitoring of various geological settings where we expect rapid changes (e.g. reservoirs or volcanoes). Through cross correlation retrieved Green's function is usually used to calculate seismic velocities of the subsurface. The assumption of this method is that the wavefields which are correlated must be diffuse. That means that the ambient noise sources are uniformly distributed around the receivers or the scattering in the medium is high enough to mitigate any source directivity. The location of the sources is usually unknown and it can change in time. These temporal and spatial variations of the microseism noise sources may lead to changes in the retrieved Green's functions. The changed Green's functions will then cause apparent changes in the calculated seismic velocity. We track the spatial and temporal distribution of the noise sources using seismic arrays, located in Ireland. It is a good location in which to study these effects, as it is tectonically very quiet and is relatively close to large microseism noise sources in the North Atlantic, allowing a quantification of noise source heterogeneity. Temporal variations in seismic wave velocity are calculated using data recorded in Ireland. The results are compared to the variations in microseism source locations. We also explore the minimum noise trace length required in Ireland for the Green's functions to converge. We quantify the degree to which apparent velocity variations using direct arrivals are caused by changes in the sources and assess if and at what frequencies the scattering of the medium in Ireland is high enough to homogenise the coda wavefield.

Friderike Volk, Meike; Bean, Christopher; Lokmer, Ivan; Craig, David

2014-05-01

7

A note on the regional variations of P n wave velocities in the Indian-subcontinent  

Microsoft Academic Search

P-wave interval velocities between different sections in the Indian sub-continent have been calculated from data published by the India Meteorological Department, New Delhi. The interval velocities indicate the characteristics typical of shield areas. Further, since theories of continental drift and global plate tectonics are generally accepted, the present note suggests that the three structurally different unique features, namely the southern

B. S. R. Rao; P. Sitapathi Rao; T. K. S. Prakasa Rao; C. Udaysankar

1974-01-01

8

Variational data assimilation to estimate the velocity in the wave equation  

NASA Astrophysics Data System (ADS)

The estimation of wave speed from observations inside the domain has potential applications in transient elastography, which is an emerging medical imaging modality. The scalar wave model is studied here, and the solution depends on the initial condition and on the spatial distribution of the wave speed. A cost function is defined that measures the difference between predicted and observed displacements. Variational data assimilation techniques are presented to minimize this cost function, and the direct and adjoint derivatives of the cost function are calculated. This method has the following two advantages: the measurements are not differentiated, and no spatial smoothness on the wave speed is required. Numerical tests in two dimensions are presented, where both the initial condition and the spatial distribution of the wave speed are retrieved.

Fehrenbach, Jérôme; Oudry, Jennifer; Sandrin, Laurent

2010-11-01

9

Regional variations in near surface shear wave velocity in the Greater Memphis area  

Microsoft Academic Search

Site-specific and regional analyses were conducted for an area that includes southwestern Tennessee, eastern Arkansas, and northwestern Mississippi to establish characteristic shear wave velocity (Vs) profiles based on measured profiles compiled from several researchers. Site-specific studies were conducted at two locations near Memphis, Tennessee to determine the uncertainty and randomness in measuring Vs due to test-related factors and local variability.

Salome Romero; Glenn J Rix

2001-01-01

10

Shear wave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion  

USGS Publications Warehouse

The Taupo Volcanic Zone (TVZ), New Zealand is a region characterized by very high magma eruption rates and extremely high heat flow, which is manifest in high-temperature geothermal waters. The shear wave velocity structure across the region is inferred using non-linear inversion of receiver functions, which were derived from teleseismic earthquake data. Results from the non-linear inversion, and from forward synthetic modelling, indicate low S velocities at ???6- 16 km depth near the Rotorua and Reporoa calderas. We infer these low-velocity layers to represent the presence of high-level bodies of partial melt associated with the volcanism. Receiver functions at other stations are complicated by reverberations associated with near-surface sedimentary layers. The receiver function data also indicate that the Moho lies between 25 and 30 km, deeper than the 15 ?? 2 km depth previously inferred for the crust-mantle boundary beneath the TVZ. ?? 2004 RAS.

Bannister, S.; Bryan, C.J.; Bibby, H.M.

2004-01-01

11

P-wave velocity variation in the upper mantle beneath Tibetan Plateau  

NASA Astrophysics Data System (ADS)

We produced a P-wave model of the upper mantle beneath Tibeatan Plateau from a combination of International Seismological Centre (ISC) P and pP data, processed by Engdahl et al. (1998), P data of the Annual Bulletin of Chinese Earthquake (ABCE) [IG-CSB, 1990-1998], and travel times of P phases recorded at temporary arrays around Tibetan Plateau, including arrays operated by (a) MIT and the Chengdu Institute of Geology and Mineral Resources (MIT-CIGMR) - 09/2003-09/2004; (b) Lehigh university and CIGMR - 07/2003-10/2004; (c) the INDEPTH (InterNational DEep Profiling of Tibet and the Himalaya) project - 05/1994-10/1994, 07/1997-06/1999; and (d) various French and Chinese groups - 07/1991-06/1992. Incorporating the travel times of phases recorded at arrays allow us to constrain upper mantle structure with much greater resolution and confidence than before, and the preliminary models reveal intriguing variations in P-wavespeed in the upper mantle beneath Tibetan Plateau. Our images suggest that the Indian lithosphere subducts from the foreland basin and underlies only the southwestern margin of the Tibetan Plateau, implying that much of the Tibetan Plateau is not underlain by Indian but by Asian lithosphere. Some of the seismically slow structures beneath the eastern part of the plateau may connect to structures, and presumably, processes in the upper mantle beneath the South China Sea.

Li, C.; van der Hilst, R.; Chen, Z.; Anne, M.

2005-12-01

12

Spatial variation in coda Q around the Nobi fault zone, central Japan: relation to S-wave velocity and seismicity  

NASA Astrophysics Data System (ADS)

We investigate the spatial variation in coda Q around the Nobi fault zone in a high strain rate zone to assess the relation between coda Q, shear wave velocity, and seismicity. Waveform data were obtained from dense seismic observations. Low coda Q that follows the Niigata-Kobe Tectonic Zone in the high strain rate zone is distinct at the lowest frequency band of 1 to 2 Hz. However, at higher frequencies, such a spatial pattern in coda Q is unclear. A good positive correlation was found between coda Q at the 1- to 2-Hz frequency band and the S-wave velocity perturbation at 25-km depth, which suggests that the coda Q reflects the ductile deformation below the brittle-ductile transition zone. Furthermore, coda Q at the 1- to 2-Hz frequency band correlates negatively with seismicity at 10- to 15-km depth, which implies that there is a high stressing rate in the low coda Q area. These facts, together with results of previous studies, imply that a high deformation rate below the brittle-ductile transition zone produces the high strain rate observed by the Global Positioning System (GPS) on the surface in this region.

Tsuji, Sugane; Hiramatsu, Yoshihiro

2014-12-01

13

Geological variation in S-wave velocity structures in Northern Taiwan and implications for seismic hazards based on ambient noise analysis  

NASA Astrophysics Data System (ADS)

Ambient noise analysis in Northern Taiwan revealed obvious lateral variations related to major geological units. The empirical Green's functions extracted from interstation ambient noise were regarded as Rayleigh waves, from which we analyzed the group velocities for period from 3 to 6 s. According to geological features, we divided Northern Taiwan into seven subregions, for which regionalized group velocities were derived by using the pure-path method. On average, the group velocities in mountain areas were higher than those in the plain areas. We subsequently inverted the S-wave velocity structure for each subregion down to 6 km in depth. Following the analysis, we proposed the first models of geology-dependent shallow S-wave structures in Northern Taiwan. Overall, the velocity increased substantially from west to east; specifically, the mountain areas, composed of metamorphic rocks, exhibited higher velocities than did the coastal plain and basin, which consist of soft sediment. At a shallow depth, the Western Coastal Plain, Taipei Basin, and Ilan Plain displayed a larger velocity gradient than did other regions. At the top 3 km of the model, the average velocity gradient was 0.39 km/s per km for the Western Coastal Plain and 0.15 km/s per km for the Central Range. These S-wave velocity models with large velocity gradients caused the seismic waves to become trapped easily in strata and, thus, the ground motion was amplified. The regionalized S-wave velocity models derived from ambient noises can provide useful information regarding seismic wave propagation and for assessing seismic hazards in Northern Taiwan.

Lai, Ya-Chuan; Huang, Bor-Shouh; Huang, Yu-Chih; Yao, Huajian; Hwang, Ruey-Der; Huang, Yi-Ling; Chang, Wen-Yen

2014-12-01

14

Rapid lateral variation of P-wave velocity at the base of the mantle near the edge of the Large-Low Shear Velocity Province beneath the western Pacific  

NASA Astrophysics Data System (ADS)

We examine P-wave velocity structure at the base of the mantle beneath the western Pacific, near the western edge of the Pacific Large-Low Shear Velocity Province (LLSVP), using high-quality seismograms provided by a large-scale mobile broad-band seismic observation in northeastern China (the NECESSArray project). Forward modelling using the reflectivity method is conducted to explain the variation of P-wave traveltimes as a function of epicentral distance near the core shadow zone. Additionally, PcP-P traveltimes are examined to enlarge the survey area. As a result, a rapid variation of P-wave velocity is detected at the base of the mantle. Regions of thin (20-50 km thick) and low velocity (-2 to -5 per cent) layers at the base of the mantle are intersected by an 80-km-thick region with a high velocity (+2 per cent). A slightly fast region exists at the northwest of the region with the thin low-velocity layer. These layers are typically separated by several hundred kilometres and would be difficult to explain by thermal effects alone. These observations suggest that very complicated thermochemical reactions occur near the edge of the Pacific LLSVP.

Tanaka, Satoru; Kawakatsu, Hitoshi; Obayashi, Masayuki; Chen, Y. John; Ning, Jieyuan; Grand, Stephen P.; Niu, Fenglin; Ni, James

2015-02-01

15

Velocity field after wave breaking  

Microsoft Academic Search

The velocity field in breaking water waves is considered in this paper. A numerical simulation describes in detail the transition from a primary overturning and consequent rebounding jets into a bore front, where the vorticity in the coherent large-scale eddy structures devolves into turbulence. Spatial changes in the frequency spectra of the kinetic energy and the enstrophy are associated with

Yasunori Watanabe; Hiroshi Saeki

2002-01-01

16

Continuous subsurface velocity measurement with coda wave interferometry  

NASA Astrophysics Data System (ADS)

A 1-month field experiment was conducted near Kunming in Yunnan Province, China, to continuously monitor subsurface velocity variations along different baselines. The experiment site is located 10 km west to the seismically very active Xiaojiang fault zone. An electric hammer was used as a source to generate highly repeatable seismic waves, which were recorded by 5 short-period seismometers deployed at ˜10 m to 1.2 km away from the source. Velocity variation was estimated by using coda wave interferometry technique. The technique measures changes in differential time between the coda and the first arrival, which is in principal insensitive to timing errors. We obtained a fractional velocity perturbation (?v/v) of 10-3 to 10-2 with a precision of 10-4. The measured velocity variation is consistent among different components and stations and appears to well correlate with deep water level. The velocity variation is featured by a long-term linear trend and well-developed daily cycles. The latter is interpreted as the velocity response to the barometric pressure. A multivariate linear regression analysis of the data indicates that the velocity change exhibits a negative correlation with barometric pressure, with a stress sensitivity of 10-6/Pa at the experimental site.

Wang, Baoshan; Zhu, Ping; Chen, Yong; Niu, Fenglin; Wang, Bin

2008-12-01

17

Local surface skimming longitudinal wave velocity and residual stress mapping.  

PubMed

Local variation in surface skimming longitudinal wave (SSLW) velocity has been measured using a scanning acoustic microscope. A very narrow width electrical impulse has been used to excite the transducer of the acoustic lens. This permits the separation of the SSLW signal from the direct reflected signal in the time domain. A simple method of measuring the time delay between the directly reflected signal and the SSLW signal at two defocuses has been utilized for the local measurement of SSLW velocity. The variation in the SSLW velocity measured over an area of the sample is scaled and presented as an image. The method has been implemented to image the variation of the SSLW velocity around a crack tip in a sample of Ti-6Al-4V. Since the SSLW velocity is known to change linearly with the stress, the SSLW velocity image is considered as a representation of the image of stress around the crack tip. Local stress variation in the same region of the crack tip is directly measured using x-ray diffraction. The SSLW velocity image is compared with the x-ray diffraction stress image. The contrast in the two images, spatial resolution, and the penetration depth into the sample of acoustic waves and x rays are discussed. PMID:14759007

Sathish, Shamachary; Martin, Richard W; Moran, Thomas J

2004-01-01

18

Shear wave velocities in the earth's mantle.  

NASA Technical Reports Server (NTRS)

Direct measurement of the travel time gradient for S waves together with travel time data are used to derive a shear velocity model for the earth's mantle. In order to satisfy the data it is necessary to discard the usual assumption of lateral homogeneity below shallow depths. A shear velocity differential is proposed for a region between western North America and areas of the Pacific Ocean. Distinctive features of the velocity model for the upper mantle beneath western North America are a low-velocity zone centered at 100 km depth and zones of high velocity gradient beginning at 400, 650, and 900 km.

Robinson, R.; Kovach, R. L.

1972-01-01

19

Continuous subsurface velocity measurement with coda wave interferometry  

Microsoft Academic Search

A 1-month field experiment was conducted near Kunming in Yunnan Province, China, to continuously monitor subsurface velocity variations along different baselines. The experiment site is located 10 km west to the seismically very active Xiaojiang fault zone. An electric hammer was used as a source to generate highly repeatable seismic waves, which were recorded by 5 short-period seismometers deployed at

Baoshan Wang; Ping Zhu; Yong Chen; Fenglin Niu; Bin Wang

2008-01-01

20

Global Models of Surface Wave Group Velocity  

Microsoft Academic Search

v--vMeasurements of group velocity are derived from phase-velocity dispersion curves and modeled with global laterally-varying isotropic structure. Maps for both Love and Rayleigh waves are created in the period range 35 s to 175 s. The data set of group-velocity measurements includes over 50,000 minor-arc observations and 5,000 major-arc observations. The errors in the measurements are estimated by an empirical

E. W. F. Larson; G. Ekström

2001-01-01

21

Measuring refraction and modeling velocities of surface waves  

Microsoft Academic Search

Recent high-resolution global models of lateral variations in surface-wave velocities contradict each other. While the models parameterize structures as small as 500 km, they are consistent only for wavelengths longer than 4000 km. This disagreement causes confusion and frustration on the part of Earth scientists who try to make use of the models. This thesis addresses this ambiguity in surface-wave

Erik William Feldmeth Larson

2000-01-01

22

Lateral variations in lower mantle seismic velocity  

NASA Technical Reports Server (NTRS)

To obtain a theoretical model which provides a rationale for the observed high values of velocity variations, the effect of a 0.1 to 0.2 percent partially molten volatile-rich material in various geometries which are heterogeneously dispersed in the lower mantle is examined. Data obtained indicate that, depending on aspect ratio and geometry, 0.1-0.2 percent partial melting in conjunction with about 100 K thermal anomalies can explain the seismic variations provided the compressibility of the melt differs by less than about 20 percent from the surrounding solid.

Duffy, Thomas S.; Ahrens, Thomas J.

1992-01-01

23

Orbital velocities induced by surface waves  

NASA Technical Reports Server (NTRS)

During the third intensive observational period of the Surface Wave Dynamics Experiment (SWADE), an aircraft-based experiment was conducted on 5 March 1991 by deploying slow-fall airborne expendable current profilers (AXCPs) and airborne expendable bathythermographs (AXBTs) during a scanning radar altimeter (SRA) flight on the NASA NP-3A research aircraft. As the Gulf Stream (GS) moved into the SWADE domain in late February, maximum upper-layer currents of 1.98 m/s were observed in the core of the baroclinic jet where the vertical current shears were O(10(exp -2)/s). The SRA concurrently measured the sea surface topography, which was transformed into two-dimensional directional wave spectra at 5-6-km intervals along the flight tracks. The wave spectra indicated a local wave field with wavelengths of 40-60 m propagating southward between 120 deg and 180 deg, and a northward-moving swell field from 300 deg to 70 deg associated with significant wave heights of 2-4 m. As the AXCP descended through the upper ocean, the profiler sensed orbital velocity amplitudes of 0.2-0.5 m/s due to low-frequency surface waves. These orbital velocities were isolated by fitting the observed current profiles to the three-layer model based on a monochromatic surface wave, including the steady and current shear terms within each layer. The depth-integrated differences between the observed and modeled velocity profiles were typically less than 3 cm/s. For 17 of the 21 AXCP drop sites, the rms orbital velocity amplitudes, estimated by integrating the wave spectra over direction and frequency, were correlated at a level of 0.61 with those derived from the current profiles. The direction of wave propagation inferred from the AXCP-derived orbital velocities was in the same direction observed by the SRA. These mean wave directions were highly correlated (0.87) and differed only by about 5 deg.

Shay, Lynn K.; Walsh, Edward J.; Zhang, Pen Chen

1994-01-01

24

MEASUREMENT OF OCEAN WAVES VELOCITY FIELDS FROM A SINGLE SPOT-5 DATASET USING CORRELATION BETWEEN  

E-print Network

MEASUREMENT OF OCEAN WAVES VELOCITY FIELDS FROM A SINGLE SPOT-5 DATASET USING CORRELATION BETWEEN Energies Alternatives, Siège-Saclay, 75000, Paris, France 1. ABSTRACT Ocean waves represent an important of the local ocean waves' velocity field and its temporal variation can provide invaluable information about

Paris-Sud XI, Université de

25

Rayleigh wave phase velocities in the Atlantic upper mantle  

NASA Astrophysics Data System (ADS)

velocity in the period range 30-130 s is measured for approximately 10,000 fundamental-mode Rayleigh waves traversing the Atlantic basin. In order to isolate the signal of the oceanic upper mantle, paths with >30% of their length through continental upper mantle are excluded. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. In both scenarios, phase velocity shows a strong dependence on seafloor age at all periods, with higher velocity associated with older seafloor. Removing age-dependent velocity from the 2-D phase-velocity maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. The age-dependent phase velocities for the Atlantic are not consistent with a half-space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400°C. In contrast, age-dependent phase velocities for the Pacific basin determined by Nishimura and Forsyth (1989) can be fit reasonably well by a half-space cooling model with mantle temperature approximately 50°C warmer than the Atlantic. Comparison of Rayleigh wave phase velocity and fractionation-corrected Na concentrations in mid-ocean ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period, as expected if along-ridge variations in mantle potential temperature are controlling both quantities.

James, Esther K.; Dalton, Colleen A.; Gaherty, James B.

2014-11-01

26

A regional study of mantle velocity variations beneath eastern Australia and the southwestern Pacific using short-period recordings of P, S, PcP, ScP and ScS waves produced by Tongan deep earthquakes  

NASA Astrophysics Data System (ADS)

Deep earthquakes located in the Tonga-Kermadec region produce exceptionally clear and sharp short-period P, S, PcP, ScP, and ScS phases which are recorded at many stations at distances of less than 60°. The data used in this study are produced by short-period stations located in oceanic-type regions (Fiji and New Caledonia), a mobile continental region (eastern Australia) and a shield region (central Australia). Differential travel-time residuals of the above phases at these stations are investigated to determine the contribution to the differential residuals from: (1) the upper part of the mantle (S-P residuals); (2) the core-to-station portion of the mantle (ScS-ScP residuals); and (3) the hypocenter-to core portion of the mantle (ScP-PcP residuals). The use of differential travel-time residuals considerably reduces near-station effects and effects due to inaccurate determination of the source parameters, and hence the results can be interpreted as due to variations along the propagation paths. The results show that (S-P) residuals from phases traveling along event-to-station paths are about 7 s smaller at the shield station than at the oceanic stations. This correlation with surface tectonic environments is equally strong for the (ScS-ScP) residuals, with the shield/oceanic station difference being about 4 s. Moreover, the data suggest that this correlation between differential residuals and surface tectonic environments is caused by variations in shear velocity within the upper part of the mantle. However, the data cannot uniquely resolve the required depth of these variations within the mantle. For example, if the shear velocity variations extend to a depth of 400 km beneath the recording stations, then the average shear velocity difference between shield- and oceanic-type environments is about 4%. However, if the variations extend only to a depth of 200 km, this difference is more than 8%. (ScP-PcP) and (ScS-PcS) residuals vary from about +1 to about +4 s at the different stations, apparently because of compressional velocity variations in the mantle along the Pc path. If the variation in compressional velocity within the mantle below a depth of about 600 km is about 10% and occurs near the source region, these results suggest that, in the vicinity of deep earthquake zones, variations in compressional velocity extend to a depth of about 1000 km. However, these results can equally be explained by a 1% variation in compressional velocity, evenly distributed along the entire Pc path. An estimate of Q determined from the observed predominant frequency of ScS waves, as recorded at the shield station, suggests that the average < Qs> of the mantle beneath about 600 km is about 1050 at frequencies of about 1 Hz.

Frohlich, Cliff; Barazangi, Muawia

1980-01-01

27

Elastic wave velocities in single-walled carbon nanotubes  

Microsoft Academic Search

This paper reports an atomistic simulation of single-walled carbon nanotubes subjected to harmonic waves, using the molecular structural mechanics method and normal mode superposition. The velocities of longitudinal, transverse, and torsional waves propagating in single walled carbon nanotubes are obtained. The results indicate that the longitudinal wave velocity is roughly twice as much as that of torsional wave. The velocity

Chunyu Li; Tsu-Wei Chou

2006-01-01

28

Variational Principles for Water Waves  

E-print Network

We describe the Hamiltonian structures, including the Poisson brackets and Hamiltonians, for free boundary problems for incompressible fluid flows with vorticity. The Hamiltonian structure is used to obtain variational principles for stationary gravity waves both for irrotational flows as well as flows with vorticity.

Boris Kolev; David H. Sattinger

2007-12-01

29

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity  

NASA Astrophysics Data System (ADS)

We constructed the S-wave velocity structure of the crust and uppermost mantle (10-100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10-60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North-South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60-80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30-50 km) beneath the Beijing-Tianjin-Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.

Chen, Hao-peng; Zhu, Liang-bao; Wang, Qing-dong; Zhang, Pan; Yang, Ying-hang

2014-07-01

30

Determination of Shear Wave Velocity and Attenuation From Waveforms in Low Velocity Formations  

E-print Network

In boreholes where formation shear velocity is lower than borehole fluid velocity neither refracted shear waves nor pseudo-Rayleigh waves can propagate. When frequency response of the sonde does not extend to low frequencies ...

Toksoz, M. Nafi

1984-01-01

31

S velocity variations beneath North America Heather Bedle1,2  

E-print Network

S velocity variations beneath North America Heather Bedle1,2 and Suzan van der Lee1 Received 23 S and Rayleigh wave trains generated by earthquakes around North America that occurred between the years 2000 variations beneath North America, J. Geophys. Res., 114, B07308, doi:10.1029/2008JB005949. 1. Introduction [2

van der Lee, Suzan

32

3D anisotropic surface wave and shear wave velocity structure beneath Eastern Tibet  

NASA Astrophysics Data System (ADS)

Recent studies have suggested that uplift of the northern Tibetan plateau may be related to removal of lithospheric mantle, resulting in emplacement of hotter, less dense asthenosphere. Other studies propose that plateau uplift and crustal thickening have occurred through a process of lateral mid-crustal flow or coherent deformation between crust and lithospheric mantle. Some authors attribute the geophysical properties of upper mantle beneath the plateau to either delamination of thickened lithosphere, or asthenospheric counterflow associated with subduction of continental Indian lithosphere beneath central Tibet. In order to study the evolution and dynamics of the Tibetan plateau, we deployed 74 broadband seismic stations throughout northeastern Tibet within the scope of ASCENT/INDEPTH-IV experiment. In conjunction with Namche Barwa data, we have calculated fundamental mode Rayleigh wave phase velocities utilizing two-plane wave approach, for periods between 20-143 seconds. We also obtained preliminary phase velocities using Love waves. To invert for shear wave velocities, we use partial derivatives from Saito (1988), assuming a constant Poisson's ratio. Our azimuthal anisotropy measurements agree well with SKS splitting results; both indicate significant (>2%) average azimuthal anisotropy throughout the upper mantle down to depths exceeding 250 km, with a dominantly EW fast directions. Although we observe variations of fast directions with depth, they are generally consistent (i.e., within 15 degrees) up to ~200 km, indicative of vertically coherent deformation. Furthermore at crustal depths, azimuthal fast directions tend be sub-parallel to the strikes of major strike slip faults, suggesting that shearing is the dominant deformation mechanism in eastern Tibet. Our tomographic models show an uppermost mantle low velocity anomaly north of Bangong-Nujiang Suture (BNS) in northeastern Tibet, and a high velocity anomaly extending ~200 km centered on the BNS. We suggest that the low velocity zone is due to warmer, thinner lithosphere in the northern Qiangtang and Songpan-Ganzi terranes. At depth, we observe high velocity bodies to the south both in our phase velocity and shear wave velocity maps, indicative of an underthrusting Indian lithosphere. Our phase maps and shear wave velocity-anomaly isosurfaces strongly indicate that underthrusting of Indian plate is sub-horizontal, has variable geometry in EW direction, and does not extend north of the BNS. We propose that underthrusting is accompanied by lateral tearing along old weak zones into at least two fragments, and subsequent break-off of the western-most portion. Our models reveal low velocity zones concentrated along major strike slip faults, which we attribute to strain heating. Furthermore, the Qiadam Basin is characterized by high velocities in the mantle, and moderate to slow velocities below the Kunlun Shan and northern Qiangtang argue against continental subduction.

Ceylan, S.; Ni, J. F.; Chen, Y. J.; Tilmann, F. J.; Sandvol, E. A.

2011-12-01

33

Variation in ejecta size with ejection velocity  

NASA Technical Reports Server (NTRS)

The sizes and ranges of over 25,000 secondary craters around twelve large primaries on three different planets were measured and used to infer the size-velocity distribution of that portion of the primary crater ejecta that produced the secondaries. The ballistic equation for spherical bodies was used to convert the ranges to velocities, and the velocities and crater sizes were used in the appropriate Schmidt-Holsapple scaling relation of estimate ejecta sizes, and the velocity exponent was determined. The latter are generally between -1 and -13, with an average value of about -1.9. Problems with data collection made it impossible to determine a simple, unique relation between size and velocity.

Vickery, Ann M.

1987-01-01

34

Diffraction correction for precision surface acoustic wave velocity measurements  

NASA Astrophysics Data System (ADS)

Surface wave dispersion measurements can be used to nondestructively characterize shot-peened, laser shock-peened, burnished, and otherwise surface-treated specimens. In recent years, there have been numerous efforts to separate the contribution of surface roughness from those of near-surface material variations, such as residual stress, texture, and increased dislocation density. As the accuracy of the dispersion measurements was gradually increased using state-of-the-art laser-ultrasonic scanning and sophisticated digital signal processing methods, it was recognized that a perceivable dispersive effect, similar to the one found on rough shot-peened specimens, is exhibited by untreated smooth surfaces as well. This dispersion effect is on the order of 0.1%, that is significantly higher than the experimental error associated with the measurements and comparable to the expected velocity change produced by near-surface compressive residual stresses in metals below their yield point. This paper demonstrates that the cause of this apparent dispersion is the diffraction of the surface acoustic wave (SAW) as it travels over the surface of the specimen. The results suggest that a diffraction correction may be introduced to increase the accuracy of surface wave dispersion measurements. A simple diffraction correction model was developed for surface waves and this correction was subsequently validated by laser-interferometric velocity measurements on aluminum specimens. copyright 2002 Acoustical Society of America.

Ruiz M., Alberto; Nagy, Peter B.

2002-09-01

35

Effects of horizontal velocity variations on ultrasonic velocity measurements in open channels  

USGS Publications Warehouse

Use of an ultrasonic velocity meter to determine discharge in open channels involves measuring the velocity in a line between transducers in the stream and relating that velocity to the average velocity in the stream. The standard method of calculating average velocity in the channel assumes that the velocity profile in the channel can be represented by the one-dimensional von Karman universal velocity profile. However, the velocity profile can be described by a two-dimensional equation that accounts for the horizontal velocity variations induced by the channel sides. An equation to calculate average velocity accounts for the two-dimensional variations in velocity within a stream. The use of this new equation to calculate average velocity was compared to the standard method in theoretical trapezoidal cross sections and in the L-31N and Snapper Creek Extension Canals near Miami, Florida. These comparisons indicate that the two-dimensional variations have the most significant effect in narrow, deep channels. Also, the two-dimensional effects may be significant in some field situations and need to be considered when determining average velocity and discharge with an ultrasonic velocity meter.

Swain, E.D.

1992-01-01

36

Measurements of mantle wave velocities and inversion for lateral heterogeneity and anisotropy. I - Analysis of great circle phase velocities  

NASA Technical Reports Server (NTRS)

The global lateral heterogeneity of the upper mantle is investigated using the classical Fourier-transform method of Sato(1958) and IDA/GDSN data from 25 1980 earthquakes. The great-circle phase velocities of 200 Love and 250 Rayleigh 100-330-sec-period fundamental-mode wave paths are determined and interpreted in terms of regional phase-velocity variation, using additional data on surface tectonics to extrapolate odd-harmonic information from the even-harmonic data. The results are presented in extensive tables, maps, and graphs. Regionalized inversion using the seven-region model of Okal (1977) is found to give maximum variance reductions of 65 percent for Love waves and 85 percent for Rayleigh waves, compared to 60 and 90 percent for l(max) = 2 inversion. Significant interregion differences are found in the regionalized Love-wave phase velocities.

Nakanishi, I.; Anderson, D. L.

1983-01-01

37

S wave velocity structure of the Arabian Shield upper mantle from Rayleigh wave tomography  

Microsoft Academic Search

The shear wave velocity structure of the shallow upper mantle beneath the Arabian Shield was modeled by inverting Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurements between 10 and 45 s. For measuring phase velocities, we applied a modified array method to data from several regional networks that minimizes

Andrew A. Nyblade; Arthur J. Rodgers; Abdullah Al-Amri

2008-01-01

38

A simple method of predicting S-wave velocity  

USGS Publications Warehouse

Prediction of shear-wave velocity plays an important role in seismic modeling, amplitude analysis with offset, and other exploration applications. This paper presents a method for predicting S-wave velocity from the P-wave velocity on the basis of the moduli of dry rock. Elastic velocities of water-saturated sediments at low frequencies can be predicted from the moduli of dry rock by using Gassmann's equation; hence, if the moduli of dry rock can be estimated from P-wave velocities, then S-wave velocities easily can be predicted from the moduli. Dry rock bulk modulus can be related to the shear modulus through a compaction constant. The numerical results indicate that the predicted S-wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately 5 MPa. An advantage of this method is that there are no adjustable parameters to be chosen, such as the pore-aspect ratios required in some other methods. The predicted S-wave velocity depends only on the measured P-wave velocity and porosity. ?? 2006 Society of Exploration Geophysicists.

Lee, M.W.

2006-01-01

39

Surface wave phase-velocity tomography based on multichannel cross-correlation  

NASA Astrophysics Data System (ADS)

We have developed a new method to retrieve seismic surface wave phase velocity using dense seismic arrays. The method measures phase variations between nearby stations based on waveform cross-correlation. The coherence in waveforms between adjacent stations results in highly precise relative phase estimates. Frequency-dependent phase variations are then inverted for spatial variations in apparent phase velocity via the Eikonal equation. Frequency-dependent surface wave amplitudes measured on individual stations are used to correct the apparent phase velocity to account for multipathing via the Helmholtz equation. By using coherence and other data selection criteria, we construct an automated system that retrieves structural phase-velocity maps directly from raw seismic waveforms for individual earthquakes without human intervention. The system is applied to broad-band seismic data from over 800 events recorded on EarthScope's USArray from 2006 to 2014, systematically building up Rayleigh-wave phase-velocity maps between the periods of 20 and 100 s for the entire continental United States. At the highest frequencies, the resulting maps are highly correlated with phase-velocity maps derived from ambient noise tomography. At all frequencies, we observe a significant contrast in Rayleigh-wave phase velocity between the tectonically active western US and the stable eastern US, with the phase velocity variations in the western US being 1-2 times greater. The Love wave phase-velocity maps are also calculated. We find that overtone contamination may produce systemic bias for the Love-wave phase-velocity measurements.

Jin, Ge; Gaherty, James B.

2015-06-01

40

P And S Wave Velocity Determination  

E-print Network

There are three general methods that can be used to determine formation velocities from full waveform logs. The first approach is to make use of the data from the entire waveform. This type of velocity analysis is performed ...

Willis, M. E.

1983-01-01

41

Continuous subsurface velocity measurement with coda wave interferometry  

E-print Network

. The velocity variation is featured by a long-term linear trend and well-developed daily cycles. The latter is interpreted as the velocity response to the barometric pressure. A multivariate linear regression analysis of the data indicates that the velocity change exhibits a negative correlation with barometric pressure

Niu, Fenglin

42

Diffraction correction for precision surface acoustic wave velocity measurements  

E-print Network

Diffraction correction for precision surface acoustic wave velocity measurements Alberto Ruiz M is the diffraction of the surface acoustic wave SAW as it travels over the surface of the specimen. The results suggest that a diffraction correction may be introduced to increase the accuracy of surface wave

Nagy, Peter B.

43

Rayleigh wave group velocity dispersion across Northern Africa, Southern Europe and the Middle East  

SciTech Connect

THis report presents preliminary results from a large scale study of surface wave group velocity dispersion throughout Northern Africa, the Mediterranean, Southern Europe and the Middle East. Our goal is to better define the 3D lithospheric shear-wave velocity structure within this region by improving the resolution of global surface wave tomographic studies. We hope to accomplish this goal by incorporating regional data at relatively short periods (less than 40 sec), into the regionalization of lateral velocity variation. Due to the sparse distributions of stations and earthquakes throughout the region (Figure 1) we have relied on data recorded at both teleseismic and regions; distances. Also, to date we have concentrated on Rayleigh wave group velocity measurements since valuable measurements can be made without knowledge of the source. In order to obtain Rayleigh wave group velocity throughout the region, vertical component teleseismic and regional seismograms were gathered from broadband, 3-component, digital MEDNET, GEOSCOPE and IRIS stations plus the portable PASSCAL deployment in Saudi Arabia. Figure 1 shows the distribution of earthquakes (black circles) and broadband digital seismic stations (white triangles) throughout southern Europe, the middle east and northern Africa used in this study. The most seismicly active regions of northern Africa are the Atlas mountains of Morocco and Algeria as well as the Red Sea region to the east. Significant seismicity also occurs in the Mediterranean, southern Europe and throughout the high mountains and plateaus of the middle-east. To date, over 1300 seismograms have been analyzed to determine the individual group velocities of 10-150 second Rayleigh waves. Travel times, for each period, are then inverted in a back projection tomographic method in order to determine the lateral group velocity variation throughout the region. These results are preliminary, however, Rayleigh wave group velocity maps for a range of periods (10-95 sec) are presented and initial interpretations are discussed. Significant lateral group velocity variation is apparent at all periods. In general, shorted periods (10-45 sec) are sensitive to crustal structure as seen by the relatively low velocities associated with large sedimentary features (eastern Arabian shield, Persian Gulf, Eastern Mediterranean, Caspian Sea). At longer periods (50-95 sec), Rayleigh waves are most sensitive to topography on the Moho and upper mantle shear-wave velocity structure. This is observed in the group velocity maps as low velocities associated with features such as the Zagros Mountains, Iranian Plateau and the Red Sea. Analysis will eventually be expanded to include Love wave group and phase velocity. Knowledge of the lateral variation of group velocity and phase velocity will allow us to invert for shear velocity at each grid point. A detailed regionalization of shear-wave velocity will potentially lower the threshold for Ms determinations and improve event location capabilities throughout the region. Better Ms estimates and locations will improve our ability to reliably monitor the Comprehensive Test Ban Treaty (CTBT).

McNamara, D.E.; Walter, W.R.

1997-07-15

44

Longitudinal Variation and Waves in Jupiter's South Equatorial Wind Jet  

NASA Technical Reports Server (NTRS)

A detailed study of the chevron-shaped dark spots on the strong southern equatorial wind jet near 7.5 S planetographic latitude shows variations in velocity with longitude and time. The presence of the large anticyclonic South Equatorial Disturbance (SED) has a profound effect on the chevron velocity, causing slower velocities to its east and accelerations over distance from the disturbance. The chevrons move with velocities near the maximum wind jet velocity of approx 140 m/s, as deduced by the history of velocities at this latitude and the magnitude of the symmetric wind jet near 7 N latitude. Their repetitive nature is consistent with a gravity-inertia wave (n = 75 to 100) with phase speed up to 25 m/s, relative to the local flow, but the identity of this wave mode is not well constrained. However, for the first time, high spatial resolution movies from Cassini images show that the chevrons oscillate in latitude with a 6.7 +/- 0.7-day period. This oscillating motion has a wavelength of approx 20 and a speed of 101 +/- 3 m/s, following a pattern similar to that seen in the Rossby wave plumes of the North Equatorial Zone, and possibly reinforced by it. All dates show chevron latitude variability, but it is unclear if this larger wave is present during other epochs, as there are no other suitable time series movies that fully delineate it. In the presence of mUltiple wave modes, the difference in dominant cloud appearance between 7 deg N and 7.5 deg S is likely due to the presence of the Great Red Spot, either through changes in stratification and stability or by acting as a wave boundary.

Simon-Miller, Amy A.; Choi, David; Rogers, John H.; Gierasch, Peter J.; Allison, Michael D.; Adamoli, Gianluigi; Mettig, Hans-Joerg

2012-01-01

45

Estimating propagation velocity through a surface acoustic wave sensor  

DOEpatents

Techniques are described for estimating the propagation velocity through a surface acoustic wave sensor. In particular, techniques which measure and exploit a proper segment of phase frequency response of the surface acoustic wave sensor are described for use as a basis of bacterial detection by the sensor. As described, use of velocity estimation based on a proper segment of phase frequency response has advantages over conventional techniques that use phase shift as the basis for detection.

Xu, Wenyuan (Oakdale, MN); Huizinga, John S. (Dellwood, MN)

2010-03-16

46

Common Genetic Variation in the 3-BCL11B Gene Desert Is Associated With Carotid-Femoral Pulse Wave Velocity and Excess Cardiovascular Disease Risk The AortaGen Consortium  

PubMed Central

Background Carotid-femoral pulse wave velocity (CFPWV) is a heritable measure of aortic stiffness that is strongly associated with increased risk for major cardiovascular disease events. Methods and Results We conducted a meta-analysis of genome-wide association data in 9 community-based European ancestry cohorts consisting of 20,634 participants. Results were replicated in 2 additional European ancestry cohorts involving 5,306 participants. Based on a preliminary analysis of 6 cohorts, we identified a locus on chromosome 14 in the 3?-BCL11B gene desert that is associated with CFPWV (rs7152623, minor allele frequency = 0.42, beta=?0.075±0.012 SD/allele, P = 2.8 x 10?10; replication beta=?0.086±0.020 SD/allele, P = 1.4 x 10?6). Combined results for rs7152623 from 11 cohorts gave beta=?0.076±0.010 SD/allele, P=3.1x10?15. The association persisted when adjusted for mean arterial pressure (beta=?0.060±0.009 SD/allele, P = 1.0 x 10?11). Results were consistent in younger (<55 years, 6 cohorts, N=13,914, beta=?0.081±0.014 SD/allele, P = 2.3 x 10?9) and older (9 cohorts, N=12,026, beta=?0.061±0.014 SD/allele, P=9.4x10?6) participants. In separate meta-analyses, the locus was associated with increased risk for coronary artery disease (hazard ratio [HR]=1.05, confidence interval [CI]=1.02 to 1.08, P=0.0013) and heart failure (HR=1.10, CI=1.03 to 1.16, P=0.004). Conclusions Common genetic variation in a locus in the BCL11B gene desert that is thought to harbor one or more gene enhancers is associated with higher CFPWV and increased risk for cardiovascular disease. Elucidation of the role this novel locus plays in aortic stiffness may facilitate development of therapeutic interventions that limit aortic stiffening and related cardiovascular disease events. PMID:22068335

Mitchell, Gary F.; Verwoert, Germaine C.; Tarasov, Kirill V.; Isaacs, Aaron; Smith, Albert V.; Yasmin; Rietzschel, Ernst R.; Tanaka, Toshiko; Liu, Yongmei; Parsa, Afshin; Najjar, Samer S.; O’Shaughnessy, Kevin M.; Sigurdsson, Sigurdur; De Buyzere, Marc L.; Larson, Martin G.; Sie, Mark P.S.; Andrews, Jeanette S.; Post, Wendy S.; Mattace-Raso, Francesco U.S.; McEniery, Carmel M.; Eiriksdottir, Gudny; Segers, Patrick; Vasan, Ramachandran S.; van Rijn, Marie Josee E.; Howard, Timothy D.; McArdle, Patrick F.; Dehghan, Abbas; Jewell, Elizabeth; Newhouse, Stephen J.; Bekaert, Sofie; Hamburg, Naomi M.; Newman, Anne B.; Hofman, Albert; Scuteri, Angelo; De Bacquer, Dirk; Ikram, Mohammad Arfan; Psaty, Bruce; Fuchsberger, Christian; Olden, Matthias; Wain, Louise V.; Elliott, Paul; Smith, Nicholas L.; Felix, Janine F.; Erdmann, Jeanette; Vita, Joseph A.; Sutton-Tyrrell, Kim; Sijbrands, Eric J.G.; Sanna, Serena; Launer, Lenore J.; De Meyer, Tim; Johnson, Andrew D.; Schut, Anna F.C.; Herrington, David M.; Rivadeneira, Fernando; Uda, Manuela; Wilkinson, Ian B.; Aspelund, Thor; Gillebert, Thierry C.; Van Bortel, Luc; Benjamin, Emelia J.; Oostra, Ben A.; Ding, Jingzhong; Gibson, Quince; Uitterlinden, André G.; Abecasis, Gonçalo R.; Cockcroft, John R.; Gudnason, Vilmundur; De Backer, Guy G.; Ferrucci, Luigi; Harris, Tamara B.; Shuldiner, Alan R.; van Duijn, Cornelia M.; Levy, Daniel; Lakatta, Edward G.; Witteman, Jacqueline C.M.

2012-01-01

47

Low Velocity Waves Inside and Outside of Plants  

NASA Astrophysics Data System (ADS)

I have been reporting organizing waves in plants for many years. In 1989 I reported wave travel between plants. The waves travel at near 25 m/s horizontally through air on earth. Recently I built my own transmitters and receivers and found that the waves will penetrate mountains. Monitoring plants suggest that there is constant communication between plants with the cacophony peaking during the summer months. The location of the sun has a direct influence. I hypothesize that the observed waves are waves in dark matter as well as the other media involved. Apparently dark matter not only interacts with gravity but has much to do with the organization of nature. The velocities of waves in plants peak vertically. For example in Ponderosa pine the ratio of the vertical to horizontal velocity is 3/1 making a tall spindly tree. In apple the ratio is 4/3 making a nearly round tree. The velocity anisotropy may suggest that dark matter interacts differently with respect to the gravity direction. The penetrating qualities of the waves may provide useful communication. There appears to be a rather large velocity distribution, however, when the waves travel far through dense matter.

Wagner, Orvin

2010-03-01

48

Anisotropic parameter estimation using velocity variation with offset analysis  

SciTech Connect

Seismic anisotropy is defined as velocity dependent upon angle or offset. Knowledge about anisotropy effect on seismic data is important in amplitude analysis, stacking process and time to depth conversion. Due to this anisotropic effect, reflector can not be flattened using single velocity based on hyperbolic moveout equation. Therefore, after normal moveout correction, there will still be residual moveout that relates to velocity information. This research aims to obtain anisotropic parameters, ? and ?, using two proposed methods. The first method is called velocity variation with offset (VVO) which is based on simplification of weak anisotropy equation. In VVO method, velocity at each offset is calculated and plotted to obtain vertical velocity and parameter ?. The second method is inversion method using linear approach where vertical velocity, ?, and ? is estimated simultaneously. Both methods are tested on synthetic models using ray-tracing forward modelling. Results show that ? value can be estimated appropriately using both methods. Meanwhile, inversion based method give better estimation for obtaining ? value. This study shows that estimation on anisotropic parameters rely on the accuracy of normal moveout velocity, residual moveout and offset to angle transformation.

Herawati, I.; Saladin, M.; Pranowo, W.; Winardhie, S.; Priyono, A. [Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Jalan Ganesa 10, Bandung, 40132 (Indonesia)] [Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Jalan Ganesa 10, Bandung, 40132 (Indonesia)

2013-09-09

49

Spatiotemporal variations in the surface velocities of Antarctic Peninsula glaciers  

NASA Astrophysics Data System (ADS)

Velocity is an important parameter for the estimation of glacier mass balance, which directly signals the response of glaciers to climate change. Antarctic ice sheet movement and the associated spatiotemporal velocity variations are of great significance to global sea level rise. In this study, we estimate Antarctic Peninsula glacier velocities using the co-registration of optically sensed images and correlation (hereafter referred to as COSI-Corr) based on moderate-resolution imaging spectroradiometer Level 1B data (hereafter referred to as MODIS L1B). The results show that the glaciers of Graham Land and the Larsen Ice Shelf have substantially different velocity features. The Graham Land glaciers primarily flow from the peninsula ridge towards the Weddell Sea and Bellingshausen Sea on the east and west sides, respectively. There are very large velocity variations among the different ice streams, with a minimum of < 20 m a-1 and a maximum of 1500 m a-1 (with an average of 100-150 m a-1). Over the period 2000-2012, the glaciers of Graham Land accelerated in the south but slowed down in the north. In contrast, the Larsen Ice Shelf flows in a relatively uniform direction, mainly towards the northeast into the Weddell Sea. Its average velocity is 750-800 m a-1 and the maximum is > 1500 m a-1. During the period 2000-2012, the Larsen Ice Shelf experienced significant acceleration. The use of COSI-Corr based on MODIS L1B data is suitable for glacier velocity monitoring on the Antarctic Peninsula over long time series and large spatial scales. This method is clearly advantageous for analysing macro-scale spatiotemporal variations in glacier movement.

Chen, J.; Ke, C. Q.; Shao, Z. D.

2014-11-01

50

Density waves in traffic flow model with relative velocity  

NASA Astrophysics Data System (ADS)

The car-following model of traffic flow is extended to take into account the relative velocity. The stability condition of this model is obtained by using linear stability theory. It is shown that the stability of uniform traffic flow is improved by considering the relative velocity. From nonlinear analysis, it is shown that three different density waves, that is, the triangular shock wave, soliton wave and kink-antikink wave, appear in the stable, metastable and unstable regions of traffic flow respectively. The three different density waves are described by the nonlinear wave equations: the Burgers equation, Korteweg-de Vries (KdV) equation and modified Korteweg-de Vries (mKdV) equation, respectively.

Yu, L.; Shi, Z.-K.

2007-05-01

51

Irregular wave induced velocities in shallow water  

E-print Network

Significant Wave Height -10. 5 m mid-depth, near bottom 50, 30, 20, 15 cm 1. 0, 1. 5, 2. 0, 2. 5, 3. 0 seconds 4. 0, 9. 0 cm entered by hand onto a spreadsheet. The spreadsheet was mainly used for calcu- lating nondimensional parameters and tabulating... Spectral Period Significant Wave Height -10. 5 m mid-depth, near bottom 50, 30, 20, 15 cm 1. 0, 1. 5, 2. 0, 2. 5, 3. 0 seconds 4. 0, 9. 0 cm entered by hand onto a. spreadsheet. The spreadsheet was mainly used for calcu- lating nondimensional...

Sultan, Nels John

1991-01-01

52

High-resolution Rayleigh wave phase velocity maps from ambient noise tomography in North China  

NASA Astrophysics Data System (ADS)

We presented high-resolution Rayleigh wave phase velocity maps at periods ranging from 5 s to 30 s in the northeast part of the North China Craton (NNCC). Continuous time-series of vertical component between October 2006 and December 2008, recorded by 187 broadband stations temporarily deployed in the NNCC region, have been cross-correlated to obtain estimated fundamental mode Rayleigh wave Green's functions. Using the frequency and time analysis technique based on continuous wavelet transformation, we measured 3 667 Rayleigh wave phase velocity dispersion curves. High-resolution phase velocity maps at periods of 5, 10, 20 and 30 s were reconstructed with grid size 0.25° ×0.25°, which reveal lateral heterogeneity of shear wave structure in the crust and upper mantle of NNCC. For periods shorter than 10 s, the phase velocity variations are well correlated with the principal geological units in the NNCC, with low-speed anomalies corresponding to the major sedimentary basins and high-speed anomalies coinciding with the main mountain ranges. Within the period range from 20 s to 30 s, high phase velocity observed in eastern NCC is coincident with the thin crust, whereas low phase velocities imaged in central NCC is correlated to the thick crust. However, the low-velocity anomaly in the Beijing-Tianjin-Tangshan region displayed in the 20 s and 30 s phase maps may be associated with fluids.

Pan, Jiatie

2012-06-01

53

Minimizers with discontinuous velocities for the electromagnetic variational method  

SciTech Connect

The electromagnetic two-body problem has neutral differential delay equations of motion that, for generic boundary data, can have solutions with discontinuous derivatives. If one wants to use these neutral differential delay equations with arbitrary boundary data, solutions with discontinuous derivatives must be expected and allowed. Surprisingly, Wheeler-Feynman electrodynamics has a boundary value variational method for which minimizer trajectories with discontinuous derivatives are also expected, as we show here. The variational method defines continuous trajectories with piecewise defined velocities and accelerations, and electromagnetic fields defined by the Euler-Lagrange equations on trajectory points. Here we use the piecewise defined minimizers with the Lienard-Wierchert formulas to define generalized electromagnetic fields almost everywhere (but on sets of points of zero measure where the advanced/retarded velocities and/or accelerations are discontinuous). Along with this generalization we formulate the generalized absorber hypothesis that the far fields vanish asymptotically almost everywhere and show that localized orbits with far fields vanishing almost everywhere must have discontinuous velocities on sewing chains of breaking points. We give the general solution for localized orbits with vanishing far fields by solving a (linear) neutral differential delay equation for these far fields. We discuss the physics of orbits with discontinuous derivatives stressing the differences to the variational methods of classical mechanics and the existence of a spinorial four-current associated with the generalized variational electrodynamics.

De Luca, Jayme [Departamento de Fisica Rodovia Washington Luis, Universidade Federal de Sao Carlos, km 235 Caixa Postal 676, Sao Carlos, Sao Paulo 13565-905, SP (Brazil)

2010-08-15

54

Distinct velocity variations around the base of the upper mantle beneath northeast Asia  

NASA Astrophysics Data System (ADS)

Both the global and regional P wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions. In particular, low-velocity anomalies have been observed beneath the descending high-velocity slabs in a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, make it difficult to substantiate this feature and evaluate the vertical extent of the low-velocity structure. From broadband waveform modeling of triplicated phases near the 660-km discontinuity for three deep events, we constrained both the P and SH wave velocity structures around the base of the upper mantle in northeast Asia. For the two events beneath the Southern Kurile, the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending Pacific slab. Our preferred models consistently suggest normal-to-lower P and significantly low SH velocities above and below the 660-km discontinuity extending to about 760-km depth compared with the global IASP91 model, corroborating previous observations for a slow structure underneath the slab. In contrast, both high P and SH velocity anomalies are shown in our preferred models for the Japan subduction zone region, likely reflecting the structural feature of a slab stagnant above the 660-km discontinuity. The velocity jumps across the 660-km discontinuity were found to be on average 4.5% and 7% for P and S waves under the south Kurile, and 3% and 6% under the Japan subduction zone. The respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under-slab areas. Based on mineral physics data, the depth-averaged ˜1.5% P and ˜2.5% SH velocity differences in the depth range of 560-760 km between the two regions could be primarily explained by a 350-450 K temperature variation, although the presence of about 0.5-1 wt.% water might also contribute to the subtle velocity variations near the base of the transition zone in the Southern Kurile. From our modeling results, we speculate that the slow structure in the Southern Kurile may be correlated to the low-velocity zone observed previously around the 410-km discontinuity under Northern Honshu. If this is the case, both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast Asia.

Wang, Tao; Chen, Ling

2009-02-01

55

Uppermost mantle P wave velocities beneath Turkey and Iran  

SciTech Connect

The uppermost mantle P wave velocities beneath Turkey and Iran were estimated by applying the conventional travel time-distance relation method to arrival times of well located earthquakes recorded at a few stations. The average uppermost mantle P wave velocity under Turkey is estimated from two stations of the World Wide Standardized Seismograph Network (WWSSN), Istanbul and Tabriz. The data are consistent with a crust of uniform, but poorly determined, thickness and an uppermost mantle P wave velocity of 7.73 +- 0.08 km/s. This velocity is very similar to that for the Aegean Sea and suggests that its structure could be closely related to that beneath Turkey. For Iran, the results calculated from travel times to three WWSSN stations, Meshed, Shiraz, and Tabriz, can be explained by a crust dipping toward the south-southeast at about 1/sup 0/ with an uppermost mantle P wave velocity of 8.0 +- 0.1 km/s. If the crustal thickness were 34 km in the north it would reach about 49 km in the south. Based on these uppermost mantle velocities, the temperature at Moho beneath Turkey is probably close to the melting temperature of peridotite but that beneath Iran is probably lower.

Chen, C.; Chen, W.; Molnar, P.

1980-01-01

56

Anisotropic changes in P-wave velocity and attenuation during deformation and fluid infiltration of granite  

USGS Publications Warehouse

Fluid infiltration and pore fluid pressure changes are known to have a significant effect on the occurrence of earthquakes. Yet, for most damaging earthquakes, with nucleation zones below a few kilometers depth, direct measurements of fluid pressure variations are not available. Instead, pore fluid pressures are inferred primarily from seismic-wave propagation characteristics such as Vp/Vs ratio, attenuation, and reflectivity contacts. We present laboratory measurements of changes in P-wave velocity and attenuation during the injection of water into a granite sample as it was loaded to failure. A cylindrical sample of Westerly granite was deformed at constant confining and pore pressures of 50 and 1 MPa, respectively. Axial load was increased in discrete steps by controlling axial displacement. Anisotropic P-wave velocity and attenuation fields were determined during the experiment using an array of 13 piezoelectric transducers. At the final loading steps (86% and 95% of peak stress), both spatial and temporal changes in P-wave velocity and peak-to-peak amplitudes of P and S waves were observed. P-wave velocity anisotropy reached a maximum of 26%. Transient increases in attenuation of up to 483 dB/m were also observed and were associated with diffusion of water into the sample. We show that velocity and attenuation of P waves are sensitive to the process of opening of microcracks and the subsequent resaturation of these cracks as water diffuses in from the surrounding region. Symmetry of the orientation of newly formed microcracks results in anisotropic velocity and attenuation fields that systematically evolve in response to changes in stress and influx of water. With proper scaling, these measurements provide constraints on the magnitude and duration of velocity and attenuation transients that can be expected to accompany the nucleation of earthquakes in the Earth's crust.

Stanchits, S.A.; Lockner, D.A.; Ponomarev, A.V.

2003-01-01

57

The Effect of Area Variation on Wave Rotor Elements  

NASA Technical Reports Server (NTRS)

The effect of varying the cross-sectional flow area of the passages of a wave rotor is examined by means of the method of characteristics. An idealized expansion wave, an idealized inlet port, and an idealized compression stage are considered. It is found that area variation does not have a very significant effect on the expansion wave, nor on the compression stage. For the expansion wave, increasing the passage area in the flow direction has the same effect as a diffuser, so that the flow emerges at a lower velocity than it would for the constant area case. This could be advantageous. The inlet is strongly affected by the area variation, as it changes the strength of the hammer shock wave, thereby changing the pressure behind it. In this case, reduction in the passage area in the flow direction leads to increased pressure. However, this result is dependent on the assumption that the inlet conditions remain constant with area variation. This may not be the case.

Wilson, Jack

1997-01-01

58

Inferences on Upper Mantle Seismic Velocity and Anisotropy in Western North America From Surface Wave Analyses  

NASA Astrophysics Data System (ADS)

In this study, we examine surface wave dispersion to determine seismic velocity variations in the crust and upper mantle beneath the Great Basin. This will improve our understanding of the relationship between lithospheric and asthenospheric processes in the western United States, and what caused the relatively recent extension episode in the Basin and Range. We employ a two-station method using the excellent lateral and good azimuthal coverage enabled by USArray Transportable Array (TA) broadband seismic stations. We use data from TA stations to generate Rayleigh wave dispersion curves between periods of 16s and 170s, which enable modeling of seismic shear wave velocities to ~300km depth. In order to reduce trade-offs between structure in the crust and upper mantle, we supplement our measurements with published results of surface wave phase velocity data between 8s and 40s determined from ambient seismic noise analysis. Constraints on Moho depth are incorporated using the routinely determined receiver function results (http://www.seis.sc.edu/EARS). To model upper mantle shear wave velocities from the measured dispersion curves, we apply a forward modeling technique which allows us to determine quantitative model uncertainties and parameter trade-offs. As of summer 2007, we have found 22 events with high quality Rayleigh wave dispersion curves for ~900 station pairs with inter-station distances between ~70 and 400km. We combine phase velocity dispersion measurements for several sub-regions within the Great Basin and calculate an average and standard deviation for each group. We find a clear increase per group in phase velocities from north to south for periods above ~35s. In the northern part of the Great Basin (N41°- N42.5° latitude), dispersion curves for measurements along E-W paths show a well-defined reduction in phase velocity with respect to a slightly modified Tectonic North America (mTNA) shear wave velocity model between periods of ~35s and 100s. Dispersion curves for southern regions (between N39° and N41° latitude) in the E-W direction do not differ from the predictions of mTNA, and phase velocities for periods longer than 35s are generally larger than those for the northern paths. This increase in phase velocity from N to S is much less clearly visible for paths that are not in the E-W direction, most of which exhibit phase velocities lower than predictions from model mTNA around 35s-100s. Given that the sensitivity to shear wave velocities of 50s Rayleigh waves peaks at about 70km depth, these results suggest a reduction in shear wave velocities compared to mTNA, located below a thin (<100 km) lithospheric lid across the region. Our finding of a likely isotropic phase velocity structure in the northern Great Basin is similar to what is observed in most of the High Lava Plains to the north. Our results also suggest azimuthal anisotropy in the southern Great Basin with a fast direction aligned approximately E-W, which is generally consistent with regional shear-wave splitting results, though new splitting results within the Great Basin show shear wave splitting complexity that is not clearly manifested in the surface wave data. These variations are likely the result of extension across the Great Basin combined with (or the result of) well-organized asthenospheric flow across the region.

Beghein, C.; Snoke, J. A.; Fouch, M. J.

2007-12-01

59

Dip-movement processing for depth-variable velocity. [Correction for variation of velocity with depth  

SciTech Connect

Dip-moveout correction (DMO) has become commonplace in the seismic processing flow. The goal of DMO processing is to transform the NMO-corrected data to zero-offset, so that the application of zero-offset (poststack) migration is equivalent to full prestack migration of the recorded data. Nearly all DMO implementations assume that the seismic velocity is constant. Usually, this is an acceptable tradeoff because of the tremendous cost savings of DMO and poststack migration versus prestack migration. Where the velocity changes rapidly with depth, however, this constant velocity theory can yield inadequate results. For many areas, such as the Gulf Coast, a velocity function that varies with depth is a reasonable approximation to the true velocity field. Using ray tracing, I find the raypaths from the source and receiver to the reflection point with the given recording time. The time along the corresponding zero-offset ray gives the DMO correction. The relationships between the three rays are expressed by a system of nonlinear equations. By simultaneously solving the equations via Newton-Raphson iteration, I determine the mapping that transforms nonzero-offset data to zero-offset. Unlike previous schemes that approximately handle vertical velocity variation, this method makes no assumptions about the offset, dip, or hyperbolic moveout.

Artley, C.T.

1992-12-01

60

Reconfigurable Wave Velocity Transmission Lines for Phased Arrays  

NASA Technical Reports Server (NTRS)

Phased array antennas showcase many advantages over mechanically steered systems. However, they are also more complex, heavy and most importantly costly. This presentation paper presents a concept which overcomes these detrimental attributes by eliminating all of the phase array backend (including phase shifters). Instead, a wave velocity reconfigurable transmission line is used in a series fed array arrangement to allow phase shifting with one small (100mil) mechanical motion. Different configurations of the reconfigurable wave velocity transmission line are discussed and simulated and experimental results are presented.

Host, Nick; Chen, Chi-Chih; Volakis, John L.; Miranda, Felix

2013-01-01

61

An inexpensive instrument for measuring wave exposure and water velocity  

USGS Publications Warehouse

Ocean waves drive a wide variety of nearshore physical processes, structuring entire ecosystems through their direct and indirect effects on the settlement, behavior, and survivorship of marine organisms. However, wave exposure remains difficult and expensive to measure. Here, we report on an inexpensive and easily constructed instrument for measuring wave-induced water velocities. The underwater relative swell kinetics instrument (URSKI) is a subsurface float tethered by a short (<1 m) line to the seafloor. Contained within the float is an accelerometer that records the tilt of the float in response to passing waves. During two field trials totaling 358 h, we confirmed the accuracy and precision of URSKI measurements through comparison to velocities measured by an in situ acoustic Doppler velocimeter and those predicted by a standard swell model, and we evaluated how the dimensions of the devices, its buoyancy, and sampling frequency can be modified for use in a variety of environments.

Figurski, J.D.; Malone, D.; Lacy, J.R.; Denny, M.

2011-01-01

62

On electromagnetic waves with a negative group velocity  

SciTech Connect

Recent publications devoted to the electrodynamics of media in which waves with a negative group velocity can exist are discussed. The properties of such waves have been studied from the beginning of the past century, and the most important results in this field were obtained by Soviet physicists in the 1940s-1950s. However, in most recent publications, this circumstance has not been taken into account.

Makarov, V. P.; Rukhadze, A. A.; Samokhin, A. A. [Russian Academy of Sciences, Prokhorov Institute of General Physics (Russian Federation)

2010-12-15

63

Love waves propagation in functionally graded piezoelectric materials with quadratic variation  

Microsoft Academic Search

The propagation behavior of Love waves in a semi-infinite functionally graded piezoelectric material (FGPM) with a quadratic variation is addressed. The coupled electromechanical field equations are solved, and the dispersion relations, displacement, electric potential, and stress fields are obtained analytically for both electrically open and short conditions. The effects of gradient coefficient on phase velocity, group velocity, and electromechanical coupling

M. Eskandari; H. M. Shodja

2008-01-01

64

Time-lapse wave-equation migration velocity analysis  

NASA Astrophysics Data System (ADS)

Time-lapse analysis of seismic data acquired at different stages of oil production or CO2 injection has been very successful at capturing detailed reservoir changes (e.g., pressure, saturation, fluid flow). Conventional 4D analysis is performed in the time domain assuming a constant baseline model; however, this procedure becomes very difficult when the subsurface is significantly altered by production/injection, and significant time anomalies and complex 4D coda are recorded. We argue that a more robust 4D analysis procedure in these situations requires iterative wave-equation depth imaging and a time-lapse velocity analysis. Wave-equation depth migration of 3D seismic data requires accurate knowledge of the velocity field usually obtained by one of two approaches. First, data-space methods are where recorded data are matched to those calculated through a background velocity model. Differences between the two datasets are used in a tomographic back-projection to image velocity model update. Alternatively, image-space methods are where one uses discrepancies in migrated images (non-flat gathers) to estimate velocity model updates. These types of approaches are termed wave-equation migration velocity analysis (WEMVA). This abstract focuses on the methodology of extending 3D WEMVA approaches to time-lapse velocity analysis. We discuss the differences in 4D WEMVA inversion goals, and how we leverage the locality of 4D perturbations to focus the inversion procedure to provide high-resolution velocity model updates. We demonstrate the utility of 4D WEMVA in a synthetic CO2 geosequestration example by successfully imaging a thin layer (< 20m) of injected gas in a typical North Sea reservoir.

Shragge, J. C.; Lumley, D. E.; CentrePetroleum Geoscience; CO2 Sequestration

2011-12-01

65

Laboratory measurements of wave height variations and currents along a steep-sided channel  

E-print Network

A small-scale physical model test was conducted in a three-dimensional shallow water wave basin to document wave height variation, velocity and current along a steep-sided channel. The purpose of the test was to provide a benchmark data set...

Way, Francis

2000-01-01

66

Lithospheric structure beneath the East China Sea revealed by Rayleigh-wave phase velocities  

NASA Astrophysics Data System (ADS)

We explore the variations of Rayleigh-wave phase-velocity beneath the East China Sea in a broad period range (5-200 s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 373 interstation paths using vertical-component broad-band waveforms at 32 seismic stations around the East China Sea from 6891 global earthquakes. The resulting maps of Rayleigh-wave phase velocity and azimuthal anisotropy provide a high resolution model of the lithospheric mantle beneath the East China Sea. The model exhibits four regions with different isotropic and anisotropic patterns: the Bohai Sea, belonging to the North China Craton, displays a continental signature with fast velocities at short periods; the Yellow Sea, very stable unit associated with low deformation, exhibits fast velocities and limited anisotropy; the southern part of the East China Sea, with high deformation and many fractures and faults, is related to slow velocities and high anisotropic signature; and the Ryukyu Trench shows high-velocity perturbations and slab parallel anisotropy.

Legendre, C. P.; Chen, Q.-F.; Zhao, L.

2014-12-01

67

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis  

NASA Astrophysics Data System (ADS)

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong-Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ?34-38 km thick crust beneath the Shillong Plateau which increases to ?37-38 km beneath the Brahmaputra valley and ?46-48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2-8 km along with a low velocity zone within ?8-13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ?10-18 km and low velocity zone within ?18-36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

Bora, Dipok K.; Hazarika, Devajit; Borah, Kajaljyoti; Rai, S. S.; Baruah, Saurabh

2014-08-01

68

Shear Wave Velocity Imaging Using Transient Electrode Perturbation: Phantom and ex vivo Validation  

PubMed Central

This paper presents a new shear wave velocity imaging technique to monitor radio-frequency and microwave ablation procedures, coined electrode vibration elastography. A piezoelectric actuator attached to an ablation needle is transiently vibrated to generate shear waves that are tracked at high frame rates. The time-to-peak algorithm is used to reconstruct the shear wave velocity and thereby the shear modulus variations. The feasibility of electrode vibration elastography is demonstrated using finite element models and ultrasound simulations, tissue-mimicking phantoms simulating fully (phantom 1) and partially ablated (phantom 2) regions, and an ex vivo bovine liver ablation experiment. In phantom experiments, good boundary delineation was observed. Shear wave velocity estimates were within 7% of mechanical measurements in phantom 1 and within 17% in phantom 2. Good boundary delineation was also demonstrated in the ex vivo experiment. The shear wave velocity estimates inside the ablated region were higher than mechanical testing estimates, but estimates in the untreated tissue were within 20% of mechanical measurements. A comparison of electrode vibration elastography and electrode displacement elastography showed the complementary information that they can provide. Electrode vibration elastography shows promise as an imaging modality that provides ablation boundary delineation and quantitative information during ablation procedures. PMID:21075719

Varghese, Tomy; Madsen, Ernest L.

2011-01-01

69

Quasi-periodic variations in Doppler velocities of H ? spicules  

NASA Astrophysics Data System (ADS)

New series of CCD spectral observations of spicules were obtained using 53-cm Lyot coronagraph of Abastumani Astrophysical Observatory (Georgia) at 5500 km height above the solar limb on October 17, 2012 in H ? spectral line. The line-of-sight Doppler velocities of 34 spicules were measured with the cadence of 4.5 s and standard error equal to ±0.3 km/s. Life times of almost all measured spicules were 5-6 min (and longer), therefore they resemble the type I spicules. No short lived structures (similar to type II spicules) were identified during the time series neither inside nor outside the observed spicules. The Doppler velocity time series were processed using Lomb Periodogram Algorithm revealing 4 types of dominating period intervals centered around: 254 s, 136 s, 94 s and 65 s having confidence levels over 95 %. The oscillations with periods around 254 s can be caused by quasi-periodic rebound shocks after the propagation of photospheric pulses. The oscillations with periods around 136 s can be caused by the oscillation of spicules axis at the kink cut-off frequency in gravitationally stratified magnetic tubes. In this case, seismological estimations give the density scale height as 380-540 km for the kink wave speed of 70-100 km/s in spicules. Shorter period oscillations are probably caused by propagating kink waves in spicules.

Khutsishvili, E.; Kulidzanishvili, V.; Kvernadze, T.; Zaqarashvili, T. V.; Kakhiani, V.; Khutsishvili, D.; Sikharulidze, M.

2014-12-01

70

Non-linear surface wave phase velocity inversion based on ray theory  

NASA Astrophysics Data System (ADS)

The development of temporary and permanent broad-band seismic arrays reinforces the need for advanced interpretation techniques in surface-wave analysis. We present a new method based on 2-D paraxial ray theory of inverting teleseismic surface-wave phase information and constructing phase velocity maps on a regional scale. Measurements of local phase velocities and propagation directions of Rayleigh waves taken from full waveform synthetic seismograms are used to validate the ray theory for smooth structures on a regional scale. Curved wavefronts created by heterogeneous structure outside the study area are taken into account through joint inversion for the phase velocity field and the shape of the incoming wavefronts. In the forward ray tracing procedure, the curved wavefronts are introduced through the boundary conditions by equating the slowness vector of the ray at the edge of the study region with the known gradient of the arrival time of the wave. To make the inverse problem non-singular we constrain the parameters in the inversion primarily by applying a smoothness criteria on the velocity field and on the incoming wave-field. Inversions of synthetic data sets computed by direct ray tracing and by full waveform modelling show that for 100 km spacing between stations the minimum size of structure that we can image is approximately 150 km. Heterogeneities with a size approximately equal to the wavelength are reconstructed by the ray-based inversion even though velocity variations are underestimated due to the wave-field smoothing of the structures. A minimum signal-to-noise ratio of 3.5 is necessary in order to correctly retrieve the phase velocity field. Inversion of a subset of the SVEKALAPKO data for 60 s period demonstrates the applicability of the method on real data.

Bruneton, Marianne; Farra, Véronique; Pedersen, Helle Anette

2002-11-01

71

SAR imaging of waves in water and ice - Evidence for velocity bunching  

NASA Technical Reports Server (NTRS)

Synthetic aperture radar (SAR) images collected over the Arctic marginal ice zone show gravity wave patterns in both the open water and the ice. Diffuse wave patterns are visible in the water at near range (small incidence angles), while most distinct wave patterns are visible in the ice across the entire swath. The wave patterns in the ice appear as bright lines rather than sinusoidal intensity variations. Additionally, the images show a periodic displacement of the ice/water boundary, apparently due to Doppler shift effects associated with the gravity wave orbital motions. These observations are interpreted as evidence for the velocity bunching effect and also illustrate the effects of random scatterer motions in the open water.

Lyzenga, D. R.; Shuchman, R. A.; Lyden, J. D.; Rufenach, C. L.

1985-01-01

72

Lifting the Seismic Lid Beneath Cameroon Volcanic Line Using 1D Shear Wave Velocities  

NASA Astrophysics Data System (ADS)

The composition of the lithosphere beneath Cameroon and the origin of the Cameroon Volcanic Line (CVL) is a matter of debate. Although many studies based on regional or global observations provide models for the setting of the CVL, none of them are strong enough to be considered as definitive. We used the joint inversion of Rayleigh wave group velocities and Rayleigh wave group velocities to derive shear wave velocity profiles of the lithosphere beneath Cameroon andshow that lithosphere is, on average, faster beneath the Congo Craton than beneath the Pan-African age crust. Using recently published dispersion curves, we extend the depth of investigation from 60 to 200 km. The calculated velocity-depth profiles do not show any sharp discontinuity that could be interpreted as the lithosphere-asthenosphere transition. Furthermore, there is no clear evidence of the existence of a low velocity zone beneath any geologic province within Cameroon. The smooth velocity variations observed on the velocity models are believed to be influenced by lateral mantle heterogeneities rather than vertical ones. The shear wave velocities for the uppermost mantle are in general greater than 4.3 km/s at all stations. This is higher than the values obtained in the Main Ethiopian Rift, and suggest that the perturbation of the by thermal anomalies does not extend as far as the CVL. This suggests that the source of volcanism along the CVL is from small scale convection in the asthenosphere and controlled by lithospheric fractures that are probably driven by the cold (and fast) edge of the Congo Craton.

Tokam Kamga, Alain; Durrheim, Ray; Tabod, Charles; Nyblade, Andrew; Nguiya, Severin

2014-05-01

73

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements  

USGS Publications Warehouse

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated Vs and borehole measured Vs in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived Vs values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis. ?? 2002 Elsevier Science Ltd. All rights reserved.

Xia, J.; Miller, R.D.; Park, C.B.; Hunter, J.A.; Harris, J.B.; Ivanov, J.

2002-01-01

74

On the possibility of natural formation of a transverse wave with a phase velocity lower than the velocity of light  

NASA Astrophysics Data System (ADS)

The formation of a transverse wave with a phase velocity lower than the velocity of light, which can exist in an equilibrium plasma without a slow-wave structure in zero magnetic field, is described. It involves the transformation of a transverse wave with trapped electrons, traveling along the magnetic field, into a slow transverse wave after the removal of the magnetic field. During the evolution of the wave with trapped electrons, the magnetic induction decreases very slowly in the direction of the wave propagation. As a result, the velocity at which electrons are in resonant interaction with the wave increases; therefore, the electrons fall to the bottom of potential wells. Under the influence of the trapped electrons, the phase velocity of the wave decreases and becomes lower than the velocity of light. It becomes equal to the velocity at which the electrons are in resonance interaction with the wave at the instant when the magnetic field vanishes. It is demonstrated that a transverse wave with a velocity lower than the velocity of light can exist in an equilibrium plasma even after the magnetic field vanishes; in this case, the flow of trapped electrons serves as a slow-wave structure.

Matveev, A. I.

2012-12-01

75

Anisotropic shear-wave velocity structure of the Earth's mantle: A global model  

NASA Astrophysics Data System (ADS)

We combine a new, large data set of surface wave phase anomalies, long-period waveforms, and body wave travel times to construct a three-dimensional model of the anisotropic shear wave velocity in the Earth's mantle. Our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model, simultaneous inversion for velocity and anisotropy, as well as discontinuity topographies, and implementation of nonlinear crustal corrections for waveforms. A comparison of our new three-dimensional model, S362ANI, with two other models derived from comparable data sets but using different techniques reveals persistent features: (1) strong, ˜200-km-thick, high-velocity anomalies beneath cratons, likely representing the continental lithosphere, underlain by weaker, fast anomalies extending below 250 km, which may represent continental roots, (2) weak velocity heterogeneity between 250 and 400 km depths, (3) fast anomalies extending horizontally up to 2000-3000 km in the mantle transition zone beneath subduction zones, (4) lack of strong long-wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle-lower mantle boundary, and (5) slow-velocity superplumes beneath the Pacific and Africa. The shear wave radial anisotropy is strongest at 120 km depth, in particular beneath the central Pacific. Lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths. Tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle.

Kustowski, B.; EkströM, G.; Dziewo?Ski, A. M.

2008-06-01

76

Observations of Rapid Velocity Variations in the Slow Solar Wind  

NASA Astrophysics Data System (ADS)

The technique of interplanetary scintillation (IPS) is the observation of rapid fluctuations of the radio signal from an astronomical compact source as the signal passes through the ever-changing density of the solar wind. Cross-correlation of simultaneous observations of IPS from a single radio source, received at multiple sites of the European Incoherent SCATter (EISCAT) radio antenna network, is used to determine the velocity of the solar wind material passing over the lines of sight of the antennas. Calculated velocities reveal the slow solar wind to contain rapid velocity variations when viewed on a time-scale of several minutes. Solar TErrestrial RElations Observatory (STEREO) Heliospheric Imager (HI) observations of white-light intensity have been compared with EISCAT observations of IPS to identify common density structures that may relate to the rapid velocity variations in the slow solar wind. We have surveyed a one-year period, starting in April 2007, of the EISCAT IPS observing campaigns beginning shortly after the commencement of full science operations of the STEREO mission in a bid to identify common density structures in both EISCAT and STEREO HI datasets. We provide a detailed investigation and presentation of joint IPS/HI observations from two specific intervals on 23 April 2007 and 19 May 2007 for which the IPS P-Point (point of closest approach of the line of sight to the Sun) was between 72 and 87 solar radii out from the Sun's centre. During the 23 April interval, a meso-scale (of the order of 105 km or larger) transient structure was observed by HI-1A to pass over the IPS ray path near the P-Point; the observations of IPS showed a micro-scale structure (of the order of 102 km) within the meso-scale transient. Observations of IPS from the second interval, on 19 May, revealed similar micro-scale velocity changes, however, no transient structures were detected by the HIs during that period. We also pose some fundamental thoughts on the slow solar wind structure itself.

Hardwick, S. A.; Bisi, M. M.; Davies, J. A.; Breen, A. R.; Fallows, R. A.; Harrison, R. A.; Davis, C. J.

2013-07-01

77

A P wave velocity model of Earth's core  

NASA Astrophysics Data System (ADS)

Present Earth core models derived from the retrieval of global Earth structure are based on absolute travel times, mostly from the International Seismological Centre (ISC), and/or free-oscillation eigenfrequencies. Many core phase data are left out of these constructions, e.g., PKP differential travel times, amplitude ratios, and waveforms. This study is an attempt to utilize this additional information to construct a model of core P wave velocity which is consistent with the different types of core phase data available. In conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements, which were the highest-quality observations chosen from a large population of Global Digital Seismograph Network (GDSN) records. As a result of fitting these various data sets, a one-dimensional P wave velocity model of the core, PREM2, is proposed. This model, modified from the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981), shows a better fit to the combined data set than any of the existing core models. Major features of the model include a sharp velocity discontinuity at the inner core boundary (ICB), with a large jump (0.78 km/s), and a low velocity gradient at the base of the fluid core. The velocity is nearly constant over the lower 100 km of the outer core. The model features a depth-dependent Q? structure in the inner core such that a constant t* for the inner core fits the amplitude ratios and waveforms of short-period waves moderately well. This means the top of the inner core is more attenuating than the deeper part of the inner core. In addition, the P velocity in the lowermost mantle is reduced from that of PREM as a baseline adjustment for the observed separations of the DF and AB branches of PKP at large distances.

Song, Xiaodong; Helmberger, Don V.

1995-06-01

78

Measurements of parallel electron velocity distributions using whistler wave absorption  

SciTech Connect

We describe a diagnostic to measure the parallel electron velocity distribution in a magnetized plasma that is overdense ({omega}{sub pe} > {omega}{sub ce}). This technique utilizes resonant absorption of whistler waves by electrons with velocities parallel to a background magnetic field. The whistler waves were launched and received by a pair of dipole antennas immersed in a cylindrical discharge plasma at two positions along an axial background magnetic field. The whistler wave frequency was swept from somewhat below and up to the electron cyclotron frequency {omega}{sub ce}. As the frequency was swept, the wave was resonantly absorbed by the part of the electron phase space density which was Doppler shifted into resonance according to the relation {omega}-k{sub ||v||} = {omega}{sub ce}. The measured absorption is directly related to the reduced parallel electron distribution function integrated along the wave trajectory. The background theory and initial results from this diagnostic are presented here. Though this diagnostic is best suited to detect tail populations of the parallel electron distribution function, these first results show that this diagnostic is also rather successful in measuring the bulk plasma density and temperature both during the plasma discharge and into the afterglow.

Thuecks, D. J.; Skiff, F.; Kletzing, C. A. [Department of Physics and Astronomy, University of Iowa, 203 Van Allen Hall, Iowa City, Iowa 52242 (United States)

2012-08-15

79

Eulerian-Lagrangian analysis for particle velocities and trajectories in a pure wave motion using particle image velocimetry.  

PubMed

This paper investigates the velocity and the trajectory of water particles under surface waves, which propagate at a constant water depth, using particle image velocimetry (PIV). The vector fields and vertical distributions of velocities are presented at several phases in one wave cycle. The third-order Stokes wave theory was employed to express the physical quantities. The PIV technique's ability to measure both temporal and spatial variations of the velocity was proved after a series of attempts. This technique was applied to the prediction of particle trajectory in an Eulerian scheme. Furthermore, the measured particle path was compared with the positions found theoretically by integrating the Eulerian velocity to the higher order of a Taylor series expansion. The profile of average travelling distance is also presented with a solution of zero net mass flux in a closed wave flume. PMID:22393117

Umeyama, Motohiko

2012-04-13

80

Coseismic velocity variation of Wenchuan aftershocks measured from active source monitoring  

NASA Astrophysics Data System (ADS)

M8.0 Wenchuan earthquake ruptured about 300km northeast ward with an aftershock series migrating in the same direction. To monitor the stress changes in the north edge of Wenchuan earthquake fault zone, we conducted a field experiment to measure the subsurface velocity variation. In the experiment, an electric hammer was used as a repeatable seismic source and the seismic signals were recorded by eight short period seismometers composed of Guralp 40T sensor and Reftek 130B digitizer. Seismometers were deployed with epicenter distances ranging from several meters to 200 meters, forming a cross-type array. The digitizer recorded the data with sampling rate of 500 samples per second. The experiment was repeated once an hour. In each round, electric hammer generated several (usually 12) seismic pulses. The experiment began from 12:00, 7, June 2008, and lasted for three weeks. Two moderate aftershocks with magnitude 4.5 and 4.4 occurred during the period of active source experiment. To estimate the velocity change of subsurface media in the experiment site, we estimated the absolute arrival time of the direct arrival and surface waves. The arrival time of the direct waves, recorded by the station closest to the source, were used as reference time to calculate the travel time of different phases. The absolute arrival time of certain phase at one station, was measured by cross-correlating the waveform with the reference waveform, and then the cross-correlation function was interpolated with a cosine function to obtain sub-sample precision. The digitizers were timing continuously with GPS, and clock errors for most of digitizers were found to be less than 5 microseconds, which can be corrected by using Reftek utilities. Thus our absolute travel time measurements have precision to 10-5 to 10-6. We average the travel time for different shot in each round, and the standard errors are estimated to be ~ 10-4 and 10-5 for direct arrival and surface wave, respectively. The travel time variation of direct arrival and surface waves were measured for different stations, and results from different stations show the similar tendency. Both direct arrival and surface wave velocities show a general dependency on barometric pressure. But surface wave velocity appears to be negatively correlated with barometric pressure, while dependency of direct wave on barometric appears to be positive. We found two prominent drops by ~ 1% in the surface wave velocity measurement. They occurred right after the two moderate aftershocks. Surface wave velocity recovered in several hours. But these coseismic changes were not obviously shown in the direct arrival time. We speculate the coseismic surface velocity drops may be caused by fluid migration in the environment layer in responding to the dynamic stress changes.

Wang, B.; Ge, H.; Yuan, S.; Yang, W.; Song, L.; Wang, W.; Li, Y.

2008-12-01

81

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations  

NASA Technical Reports Server (NTRS)

Observed properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T(perpindicular)/T(parallell) approximately > 2). This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch-angle scattering

Vinas, A.F.; Gurgiolo, C.; Nieves-Chinchilla, T.; Gary, S. P.; Goldstein, M. L.

2010-01-01

82

Creatinine clearance, pulse wave velocity, carotid compliance and essential hypertension  

Microsoft Academic Search

Creatinine clearance, pulse wave velocity, carotid compliance and essential hypertension.BackgroundThe vascular hallmark of subjects with end-stage renal disease is increased arterial stiffness independent of blood pressure, wall stress, and cardiovascular risk factors such as hypertension, plasma glucose and cholesterol, obesity, and tobacco consumption. Whether arterial stiffness and kidney function are statistically associated in subjects with plasma creatinine ?130 ?mol\\/L has

Jean-Jacques Mourad; Bruno Pannier; Jacques Blacher; Annie Rudnichi; Athanase Benetos; Gerard M. London; Michel E. Safar

2001-01-01

83

Accuracy of arterial pulse-wave velocity measurement using MR  

Microsoft Academic Search

The performance of a one-dimensional MR technique for the estimation of pulse-wave velocity in the aorta was evaluated. An expression for the error in this es- timate was formulated and veri5ed both by simulation and by experiment. On the basis of this formulation. guidelines for increasing the emciency of the acquisi- tion were established. The technique was further vali- dated

Bradley D. Bolster; Ergin Atalar; Christopher J. Hardy; Elliot R. McVeigh

1998-01-01

84

Negative phase velocity of electromagnetic waves and the cosmological constant  

E-print Network

Examining the propagation of electromagnetic plane waves with the wavevector directed in opposition to the time-averaged Poynting vector in cosmological spacetime with piecewise uniform metric, we show that such negative-phase-velocity (NPV) propagation is possible in certain de Sitter spacetimes but not in anti-de Sitter spacetimes. This difference suggests the possibility of an optical/electromagnetic experiment to discern the cosmological constant of a four-dimensional universe stemming from a five-dimensional brane universe.

Tom G. Mackay; Sandi Setiawan; Akhlesh Lakhtakia

2005-03-15

85

Photospheric Logarithmic Velocity Spirals as MHD Wave Generation Mechanisms  

E-print Network

High-resolution observations of the solar photosphere have identified a wide variety of spiralling motions in the plasma. These spirals vary in properties, but are observed to be abundant on the solar surface. In this work these spirals are studied for their potential as magnetohydrodynamic (MHD) wave generation mechanisms. The inter-granular lanes, where these spirals are commonly observed, are also regions where the magnetic field strength is higher than average. This combination of magnetic field and spiralling plasma is a recipe for the generation of Alfv\\'en waves and other MHD waves. This work employs numerical simulations of a self-similar magnetic flux tube embedded in a realistic, gravitationally stratified, solar atmosphere to study the effects of a single magnetic flux tube perturbed by a logarithmic velocity spiral driver. The expansion factor of the logarithmic spiral driver is varied, multiple simulations are run for a range of values of the expansion factor centred around observational data. Th...

Mumford, S J

2015-01-01

86

Mapping Phase Velocities and Azimuthal Anisotropy of Rayleigh Waves in Iceland  

NASA Astrophysics Data System (ADS)

Using Rayleigh wave data recorded at both the HOTSPOT and the ICEMELT experiments in Iceland, we have applied the two-plane wave inversion technique and obtained phase velocities and azimuthal anisotropy from period 20 s to 100 s. The most striking feature is that the slow anomalies are generally confined beneath the Icelandic rift zones but not correlate with the plume center on the surface. Azimuthal anisotropy appears to be frequency dependent and also shows strong lateral variations especially between the western Iceland, the rift zones, and the eastern Iceland, as suggested by shear-wave splitting measurements. It is well known that tradeoffs exist between isotropic and anisotropic heterogeneity. We conducted resolution tests to estimate how robust the observed features of phase velocities and anisotropy are. Synthetic phase and amplitude data of Rayleigh waves were calculated from a typical phase velocity model that has low velocities beneath the Icelandic rift zones. Azimuthal anisotropy that uniformly distributes in the area or varies laterally by tectonic province was also included in the input models. The pattern of isotropic phase velocities with fast anomalies in the western and eastern Iceland and the slow in the rift zones is well recovered in both isotropic and anisotropic inversions. The azimuthal anisotropy larger than 1% in the input models can be largely retrieved. However, the amount of anisotropy when varying by tectonic province is not negligible in anisotropic solutions even for isotropic input models. Therefore, we suggest inverting synthetic data from the observed isotropic phase velocity models in order to detect whether the observed anisotropy reflects the real structure or the tradeoff with isotropic heterogeneity.

Li, A.; Detrick, R. S.

2002-05-01

87

Effects of neutral interactions on velocity-shear-driven plasma waves  

SciTech Connect

In a laboratory experiment, we demonstrate the substantial effects that collisions between charged and neutral particles have on low-frequency (?{sub i}????????{sub e}) shear-driven electrostatic lower hybrid waves in a plasma. We establish a strong (up to 2.5?kV/m) highly localized electric field with a length scale shorter than the ion gyroradius, so that the ions in the plasma, unlike the electrons, do not develop the full E?×?B drift velocity. The resulting shear in the particle velocities initiates the electron-ion hybrid (EIH) instability, and we observe the formation of strong waves in the vicinity of the shear with variations in plasma densities of 10% or greater. Our experimental configuration allows us to vary the neutral background density by more than a factor of two while holding the charged particle density effectively constant. Not surprisingly, increasing the neutral density decreases the growth rate/saturation amplitude of the waves and increases the threshold electric field necessary for wave formation, but the presence of neutrals affects the dominant wave frequency as well. We show that a 50% increase in the neutral density decreases the wave frequency by 20% while also suppressing the electric field dependence of the frequency that is observed when fewer neutrals are present. The majority of these effects, as well as the values of the frequencies we observe, closely match the predictions of previously developed linear EIH instability theory, for which we present the results of a numerical solution.

Enloe, C. L. [Physics Department, US Air Force Academy, Colorado Springs, Colorado 80840 (United States); Tejero, E. M.; Amatucci, W. E.; Crabtree, C.; Ganguli, G. [Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375 (United States); Sotnikov, V. [Sensors Directorate, Air Force Research Laboratory, Dayton, Ohio 45433 (United States)

2014-06-15

88

Analyses of the cross-hole method for determining shear wave velocities and damping ratios  

Microsoft Academic Search

The cross-hole test is a common method used for determining shear wave velocities and, hence, to characterize the shear modulus. The determination of the shear wave velocities can be greatly improved by signal processing techniques. With further signal processing the damping ratios can also be determined. In this paper, the signal processing techniques to determine shear wave velocity and damping

L. Hall; A. Bodare

2000-01-01

89

Stress-wave velocity of wood-based panels: Effect of moisture,  

E-print Network

Stress-wave velocity of wood-based panels: Effect of moisture, product type, and material direction Guangping Han Qinglin Wu Xiping Wang Abstract The effect of moisture on longitudinal stress-wave veloc- ity, particleboard, and southern pine lumber was evaluated. It was shown that the stress-wave velocity decreased

90

Shear velocity structure in the Aegean area obtained by inversion of Rayleigh waves  

NASA Astrophysics Data System (ADS)

The purpose of this work is to derive a 3-D tomographic image of the shear wave velocity structure of the crust-uppermost mantle in the Aegean area using the group velocities of Rayleigh wave fundamental mode. The database consists of 185 regional earthquakes recorded at broad-band stations that were installed for a period of 6 month in the Aegean area within the framework of a large-scale experiment. In a previous work (Karagianni et al. 2002), an averaged group velocity has been determined using the method of frequency time analysis (FTAN) for each epicentre-station ray path and the data were used in order to determine the local group velocities for different periods over the area covered by the seismic ray paths. Taking into account additional resolution results obtained for the local group velocities, a grid of 0.5° was adopted for the Aegean area and a local dispersion curve was defined for each gridpoint. More than 80 local dispersion curves were finally inverted using a non-linear inversion approach, deriving the corresponding 1-D shear velocity models. The interpolation of these models resulted in a 3-D S-wave tomographic image of the crust and uppermost mantle in the broader Aegean area. As expected, as a result of the complex tectonic setting of the Aegean area, strong lateral variations of the S-wave velocities of the crust and uppermost mantle of the studied area are found. In the southern Aegean sea, as well as in a large part of the central Aegean sea a thin crust of approximately 20-22 km is observed, whereas the remaining Aegean sea area exhibits a crustal thickness less than 28-30 km. On the contrary, a crustal thickness of 40-46 km is observed in western Greece along the Hellenides mountain range, whereas in the eastern continental Greece the crust has a typical thickness of approximately 30-34 km. For shallow depths (<10 km) low S-wave velocities are observed under the sedimentary basins of the north Aegean sea, the Gulf of Thermaikos (Axios basin) and western Greece. At depths ranging from 10 to 20 km, low S-wave velocities are mainly found in western Greece under Peloponnesus as well as in Rhodes. This low-velocity zone seems to extend along the Hellenic arc and can be correlated to the Hellenides mountain range and the Alpine orogenesis, in agreement with previous P-wave tomographic results. In the southern Aegean sea very low S-wave velocities (3.6-4.0 km s-1) are observed at depths of approximately 30-40 km just below the Moho discontinuity, while in the rest of the inner Aegean sea and continental Greece the uppermost mantle is characterized by velocities around 4.3-4.4 km s-1. This low-velocity zone in the southern Aegean sea can be associated with the high temperatures and the presence of significant percentage partial melt in the mantle wedge of the southern Aegean subduction zone, in agreement with previous studies.

Karagianni, E. E.; Papazachos, C. B.; Panagiotopoulos, D. G.; Suhadolc, P.; Vuan, A.; Panza, G. F.

2005-01-01

91

An upper-mantle S-wave velocity model for East Asia from Rayleigh wave tomography  

NASA Astrophysics Data System (ADS)

We present a new shear velocity model of the upper mantle beneath the East Asia region derived by inverting Rayleigh wave group velocity measurements between 10 and 145 s combined with previously published Rayleigh wave phase velocity measurements between 150 and 250 s. Rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2D dispersion maps for 10-145 s periods. The group velocity maps benefit from the inclusion of new data recorded by the China National Seismic Network and surrounding global stations. The increase in available data has resulted in enhanced resolution compared with previously published group velocity maps; the horizontal resolution across the region is about 3° for the periods used in this study. The new shear-wave velocity models indicate varying velocity structure beneath eastern China, which yields estimates of a lithosphere-asthenosphere boundary depth from around 160 km beneath the Yangtze block to approximately 140 km beneath the western part of the North China Craton (NCC), up to depths of 70-100 km beneath the eastern NCC, Northeast China, and the Cathaysia block. The models reveal the subduction of two opposite-facing continental plates under the southern and northern margin of Tibet. An obvious low-velocity anomaly appears in the top 200 km of the upper mantle beneath northern Tibet, which is inconsistent with the presence of subducted Asian or Indian mantle lithosphere beneath northern Tibet. The Cenozoic volcanism fields in the Mongolian plateau are characterized by an obvious upper mantle negative anomaly, but no signature of deep-seated plume was observed.

Li, Yonghua; Wu, Qingju; Pan, Jiatie; Zhang, Fengxue; Yu, Daxin

2013-09-01

92

An upper-mantle S-wave velocity model for East Asia from Rayleigh wave tomography  

NASA Astrophysics Data System (ADS)

We present a new shear velocity model of the upper mantle beneath the East Asia region derived by inverting Rayleigh wave group velocity measurements between 10 and 145 s combined with previously published Rayleigh wave phase velocity measurements between 150 and 250 s. Rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2D dispersion maps for 10-145 s periods. The group velocity maps benefit from the inclusion of new data recorded by the China National Seismic Network and surrounding global stations. The increase in available data has resulted in enhanced resolution compared with previously published group velocity maps; the horizontal resolution across the region is about 3° for the periods used in this study. The new shear wave velocity models indicate varying velocity structure beneath eastern China, which yields estimates of a lithosphere-asthenosphere boundary depth from around 160 km beneath the Yangtze block to approximately 140 km beneath the western part of the North China Craton (NCC), up to depths of 70-100 km beneath the eastern NCC, Northeast China, and the Cathaysia block. The models reveal the subduction of two opposite-facing continental plates under the southern and northern margin of Tibet. An obvious low-velocity anomaly appears in the top 200 km of the upper mantle beneath northern Tibet, which is inconsistent with the presence of subducted Asian or Indian mantle lithosphere beneath northern Tibet. The Cenozoic volcanism fields in the Mongolian plateau are characterized by an obvious upper mantle negative anomaly, but no signature of deep-seated plume was observed. This study was supported by the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120) and NSFC (41074067).

Li, Y.; Wu, Q.; Pan, J.; Zhang, F.; Sun, L.

2013-12-01

93

Shear wave velocity structure of the Bushveld Complex, South Africa  

NASA Astrophysics Data System (ADS)

In this study, upper crustal structure of the Bushveld Complex (BC) is investigated. The BC is a prominent layered igneous mafic intrusion located within the northern part of the Kaapvaal Craton, South Africa. Past studies suggest that the mafic outcrops of the western and eastern limbs of the BC may be continuous and connected at depth. The principal objective of this study is to map out the continuity and connectivity of mafic units of the BC in areas without surface exposure using high frequency Rayleigh wave tomography and the joint inversion of high frequency receiver functions and short period Rayleigh wave group velocity. Event-station, group velocities have been measured for 201 local and regional earthquakes. These earthquakes were recorded by 42 broadband stations selected from the temporary SASE (Southern African Seismic Experiment) network and two broadband stations each belonging to the GSN (Global Seismic Network) and SANSN (South African National Seismograph Network). Most of the earthquakes occurred within deep gold mines to the south of the BC and within mines within the BC itself. Results of the lower crust of the BC confirm the presence of about 5 km of crustal thickening under the BC and suggest that the upper crust in the middle of the BC may have high velocities indicative of mafic rock layers near the surface.

Kgaswane, E. M.; Nyblade, A.; Dirks, P.; Durrheim, R. J.

2010-12-01

94

A continuous record of intereruption velocity change at Mount St. Helens from coda wave interferometry  

NASA Astrophysics Data System (ADS)

In September 2004, Mount St. Helens volcano erupted after nearly 18 years of quiescence. However, it is unclear from the limited geophysical observations when or if the magma chamber replenished following the 1980-1986 eruptions in the years before the 2004-2008 extrusive eruption. We use coda wave interferometry with repeating earthquakes to measure small changes in the velocity structure of Mount St. Helens volcano that might indicate magmatic intrusion. By combining observations of relative velocity changes from many closely located earthquake sources, we solve for a continuous function of velocity changes with time. We find that seasonal effects dominate the relative velocity changes. Seismicity rates and repeating earthquake occurrence also vary seasonally; therefore, velocity changes and seismicity are likely modulated by snow loading, fluid saturation, and/or changes in groundwater level. We estimate hydrologic effects impart stress changes on the order of tens of kilopascals within the upper 4 km, resulting in annual velocity variations of 0.5 to 1%. The largest nonseasonal change is a decrease in velocity at the time of the deep Mw = 6.8 Nisqually earthquake. We find no systematic velocity changes during the most likely times of intrusions, consistent with a lack of observable surface deformation. We conclude that if replenishing intrusions occurred, they did not alter seismic velocities where this technique is sensitive due to either their small size or the finite compressibility of the magma chamber. We interpret the observed velocity changes and shallow seasonal seismicity as a response to small stress changes in a shallow, pressurized system.

Hotovec-Ellis, A. J.; Gomberg, J.; Vidale, J. E.; Creager, K. C.

2014-03-01

95

Determination of wave intensity in flexible tubes using measured diameter and velocity  

Microsoft Academic Search

Wave intensity (WI) is a hemodynamics index, which is the product of changes in pressure and velocity across the wave-front. Wave Intensity Analysis, which is a time domain technique allows for the separation of running waves into their forward and backward directions and traditionally uses the measured pressure and velocity waveforms. However, due to the possible difficulty in obtaining reliable

J. Feng; A. W. Khir

2007-01-01

96

A bio-impedance measurement system for portable monitoring of heart rate and pulse wave velocity using small body area  

Microsoft Academic Search

Continuous monitoring of cardiovascular system can prevent death and disabilities due to heart failure. In this paper, a bio-impedance measurement system that operates on a small local area of the forearm is presented for portable and continuous monitoring of the heart rate and pulse wave velocity (PWV). The impedance variation caused by the heart pump is measured by analog circuitries

Min-Chang Cho; Jee-Yeon Kim; Seong-Hwan Cho

2009-01-01

97

Shear Wave Velocity Structure beneath the African-Anatolian Subduction Zone in Southwestern Turkey from Inversions of Rayleigh Waves  

NASA Astrophysics Data System (ADS)

The ongoing subduction of the African Plate under western Anatolia results in a highly complex tectonic structure especially beneath Isparta Angle (IA) and the surroundings where the Hellenic and Cyprian slabs with different subduction geometries intersect. The primary objective is to accurately image the lithospheric structure at this convergent plate boundary and further understand the reasons responsible for the active deformation. Data was gathered from a temporary seismic network consisting of 10 broadband stations that was installed in August 2006 with the support from University of Missouri and nine more stations deployed in March 2007 with the support from Bogazici Research Fund (project ID:07T203). In addition, 21 permanent stations of Kandilli Observatory and Earthquake Research Institute (KOERI) and two from Süleyman Demirel University (SDU) together with five stations from IRIS/Geofon Network were also included to extend the station coverage. We used earthquakes in a distance range of 30-120 degrees with body wave magnitudes larger than 5.5. Depending on the signal to noise ratio, azimuthal coverage of events, and coherence from station, 81 events provided high-quality data for our analysis. The distribution of events shows a good azimuthal coverage, which is important for resolving both lateral heterogeneity and azimuthal anisotropy. We adopted a two-plane-wave inversion technique of Forsyth and Li (2003) to simultaneously solve for the incoming wave field and phase velocity. This relatively simpler representation of a more complex wavefield provided quite stable patterns of amplitude variations in many cases. To begin with, an average phase velocity dispersion curve was obtained and used as an input for tomographic inversions. Two-dimensional tomographic maps of isotropic and azimuthally anisotropic phase velocity variations were generated. Phase velocities can only tell us integrated information about the upper mantle. Furthermore, we inverted phase velocities for shear wave velocities (Saito,1988) in order to obtain direct information at a depth range of 30-300 km that can be interpreted in terms of major tectonic processes such as extension, slab detachment/tearing, STEP faults, volcanism, temperature anomalies, the presence of melt or dissolved water, etc. Resulting tomograms along horizontal and vertical depth sections provided valuable insights on the crustal and upper mantle structure beneath Southwestern Turkey down to almost 300 km.

Teoman, U. M.; Sandvol, E. A.; Kahraman, M.; Sahin, S.; Turkelli, N.

2011-12-01

98

Blood pulse wave velocity measured by photoacoustic microscopy  

NASA Astrophysics Data System (ADS)

Blood pulse wave velocity (PWV) is an important indicator for vascular stiffness. In this letter, we present electrocardiogram-synchronized photoacoustic microscopy for in vivo noninvasive quantification of the PWV in the peripheral vessels of mice. Interestingly, strong correlation between blood flow speed and ECG were clearly observed in arteries but not in veins. PWV is measured by the pulse travel time and the distance between two spot of a chose vessel, where simultaneously recorded electrocardiograms served as references. Statistical analysis shows a linear correlation between the PWV and the vessel diameter, which agrees with known physiology. Keywords: photoacoustic microscopy, photoacoustic spectroscopy, bilirubin, scattering medium.

Yeh, Chenghung; Hu, Song; Maslov, Konstantin; Wang, Lihong V.

2013-03-01

99

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS and SS - S{sub d}S travel time residuals  

SciTech Connect

The arrival times of seismic P- and S-waves are used in constructing a mantle velocity model and a model of boundary topography of the 410-km and 660-km discontinuities. Travel times obtained from events listed in the catalog of the International Seismology Centre (ISC) are relocated relative to the velocity model of the International Association of Seismology and Physics of the Earth`s Interior 1991 (IASP91). The role of perturbations of the 410-km and 660-km discontinuities on the models of P- and S-wave velocity variations is discussed. Correlations are not evident when velocity anomalies are plotted as a function of transition zone thickness.

Vasco, D.W.; Johnson, L.R. [Lawrence Berkeley Laboratory, CA (United States)] [Lawrence Berkeley Laboratory, CA (United States); Pulliam, J. [Univ. of Texas, Austin, TX (United States)] [Univ. of Texas, Austin, TX (United States)

1995-12-10

100

Shear-wave velocity estimation using multiple logs and multicomponent seismic AVO interpretation: Gulf of Thailand  

NASA Astrophysics Data System (ADS)

Shear-wave velocity (Vs) is very important to seismic interpretation. Shear-wave logs, however, are often missing. Existing Vs estimation methods either require detailed rock volumetric and fluid prediction first or give large errors in the study area: a gas field in the Gulf of Thailand where sediments consist of sand-shale mixtures and coals. Two petrophysical characteristics of the sand-shale mixtures are worth noting. First, when the clay content increases, their compressional-wave velocity (Vp) and Vs increase when the clay content is low and decrease when the clay content is high. This velocity behavior separates the sand-shale mixtures into two groups: the porous sand with low clay content and the low porosity sand-shale with high clay content. Second, the porous sands usually have higher velocities but lower density than the low porosity sand-shale. Based on these and other petrophysical characteristics, two methods of Vs estimation are given for the three groups of sediments: the porous sand, the low porosity sand-shale, and the coal. P-wave AVO technique, as a powerful hydrocarbon prospecting too], is applied almost routinely in practice now. However, current AVO analyses assume a generalized Gardner's equation for the density-Vp relationship across the interface, which is not true in some areas including the study gas field. Therefore, I derived an AVO analysis technique without a predefined density-Vp relationship. Multicomponent seismic data are becoming popular. Information extracted from converted-wave AVO should significantly enhance that obtained from P-wave AVO. Unfortunately, there is not much literature available. Therefore, I analyzed the characteristics of converted-wave AVO under different shear-wave acoustic-impedance contrasts and Vp/ Vs ratios, and derived a method for converted-wave AVO crossplotting to detect hydrocarbon and lithologic variations. Finally, it has always been difficult to discriminate low and high gas saturations theoretically and practically prior to drilling. Consequently, drilling into non-commercial gas reservoirs has been called a "technical success." However, I found very good partial gas indicators (PGIs) using multicomponent seismic AVO data. Theoretical results show that the proposed PGIs can separate regions of high gas saturation from low saturation areas and distinguish water saturation changes from porosity or clay content changes.

Zhu, Fuping

2000-10-01

101

Surface-wave propagation and phase-velocity structure from observations on the USArray Transportable Array  

NASA Astrophysics Data System (ADS)

We address questions relating to the velocity structure of the Earth in three ways: mapping the phase-velocity structure of the western United States, examining deviations of wave paths due to lateral variations in velocity, and demonstrating that Love wave fundamental-mode phase measurements from array methods can be significantly contaminated by overtone interference, dependent on differences in fundamental-mode and first-overtone phase-velocity structure. All of the studies presented in this work use USArray Transportable Array data, which allow for dense, high-quality measurements at an unprecedented level. To image the uppermost mantle beneath the western US, we improve upon single-station phase measurements by differencing them to produce a baseline data set of phase measurements along inter-station paths, for both Love and Rayleigh waves from 25--100 s. Additional measurements of the arrival angle and local phase velocity are made using a mini-array method similar to beamforming. The arrival-angle measurements are used to correct the two-station baseline measurements and produce a corrected data set. Both the baseline and corrected data sets are separately inverted, producing phase-velocity maps on a 0.5°-by-0.5° grid. We select the corrected maps as the preferred models for Rayleigh waves, with better fits to the data and more consistent measurements. We find that arrival-angle measurements for Love waves may be biased by overtone interference, and hence select the baseline maps as the preferred models for Love waves. The final set of phase-velocity maps is consistent with expectations from known geologic features, and is useful for both calculation of phase for regional paths and studies of radial anisotropy within the region. We use the mini-array method to make observations of the deviations of waves from the great-circle path. Measured arrival angles vary from 0° to +/-15°. We compile results from earthquakes in small source regions, allowing the observation of bands of arrival-angle anomalies crossing the footprint of the USArray Transportable array in the propagation direction. These bands of deviations may result from heterogeneous velocity structure within the array, or on the larger source-to-array path. We use two global tomographic models to predict arrival-angle anomaly patterns, with both ray-theory-based prediction methods and measurements on synthetic waveforms calculated using SPECFEM3D Globe, a finite element package. We show that both models predict well the long-wavelength patterns of anomalies observed, but not the short-wavelength variations. Experiments with crustal structure indicate that greater heterogeneity is needed in the models. Predictions from the spectral-element-method synthetic waveforms contain the type of complexity seen in the observed patterns, and not obtained with the ray-theoretical methods, indicating that full synthetics are needed to compare model predictions to observed arrival-angle anomalies. We further examine possible overtone interference in the mini-array arrival-angle and local phase-velocity measurements for Love waves at long periods. Love wave fundamental-mode and higher-mode waves at the same period travel with similar group velocity, making them difficult to separate; the waves have different phase velocities, resulting in a beating interference pattern that oscillates with distance. We show this interference pattern for single-station, two-station, and mini-array phase-velocity measurements. Using measurements on synthetic waveforms calculated using both mode summation and SPECFEM3D Globe, we show that contamination of single-station measurements can largely be explained by interference between the fundamental and first-higher mode only. Interference causes small variations in the single-station phase velocity, up to 1%, and the oscillations about the expected values are asymmetric. The two array-based measurement techniques can be thought of as a spatial gradient over the single-station phase measurements, and consequently much larger variations are o

Foster, Anna E.

102

Determination of the specific area of liquid gas and the velocity of weak pressure waves in aqueous foams  

NASA Technical Reports Server (NTRS)

The specific area of an aqueous foam contained in a tube is obtained by determining the fraction of the quantity of light emitted by a source and diffused by the specific area of the column of the foam. The velocity of the waves of weak pressure (which propagate in the air with the velocity of sound) is measured by noticing the moment when the wave penetrates the column and the moment when, having reached the opposite side, it determines a variation of the light diffused by the area of the latter.

Saint-Cloud, Jean; Guerraud, Claude; Moreau, Michel; Manson, Numa

1988-01-01

103

Gigahertz acoustic wave velocity measurement in GaN single crystals considering acousto-electric effect.  

PubMed

The resistivity-frequency characteristics of longitudinal wave velocities propagating parallel to the c-axis in a GaN single crystal were theoretically estimated by considering the piezoelectric acousto-electric effect. The temperature and frequency dependences of longitudinal and shear wave velocities in conductive and semiconductive GaN single-crystal samples were experimentally investigated by Brillouin scattering. The temperature dependence of longitudinal and shear wave velocities had a linear tendency in the conductive sample, whereas in the semiconductive sample, those had a similar tendency to the predicted velocity changes resulting from the piezoelectric stiffening effect. However, the temperature dependence of shear wave velocity, which does not possess piezoelectric coupling, had a tendency similar to that of the longitudinal wave in the semiconductive sample, unexpectedly. The frequency dependence of longitudinal wave velocities in the semiconductive sample had a tendency similar to the predicted velocity changes resulting from the piezoelectric stiffening effect. PMID:25073138

Ichihashi, Hayato; Yanagitani, Takahiko; Takayanagi, Shinji; Kawabe, Masahiko; Matsukawa, Mami

2014-08-01

104

A seismic waves velocity model for Gran Canaria Island from ambient noise correlations  

NASA Astrophysics Data System (ADS)

We have analysed continuous ambient seismic noise recorded by a temporary array in Gran Canaria (Canary Islands, Spain) in order to find a velocity model for the top few kilometers. The SISTEVOTENCAN-IGN seismic array consisted of five broadband stations surrounding a sixth central one placed close to Pico de las Nieves, at the center of the island. The array had a radius of 12-14 km, with interstation distances ranging from 10 to 27 km. This network was operative from December 2009 to November 2011. The Green's functions between the 15 pairs of stations have been estimated in the time domain by stacking cross-correlations of 60-s time windows for the whole recording period (~2 years). The effects of several processing adjustments such as 1-bit normalization and spectral whitening are discussed. We observe significant differences (mainly in amplitude) between causal and acausal parts of the estimated Green's functions, which can be associated to an uneven distribution of the seismic noise sources. The application of a phase-matched filter based on an average dispersion curve allowed the effective reduction of some spurious early arrivals and the selection of fundamental-mode Rayleigh wave pulses, making possible an automatic extraction of their group velocities. Then, Rayleigh-wave dispersion curves were retrieved for the set of paths by using frequency-time analysis (FTAN) as well as by following the procedure described by Herrin and Goforth (1977, BSSA) based on the iterative fitting of a phase-matched filter which optimally undisperses the signal. Reliable curves were obtained from 1 s to 6-7 s with group velocities ranging between 1.5 and 2.2 km/s. Some lateral variations in velocity have been detected in spite of the limited spatial coverage and path density, which substantially restricted the resolution. A mean S-wave velocity model has been inverted for this area down to ~3 km.

García-Jerez, Antonio; Almendros, Javier; Martínez-Arévalo, Carmen; de Lis Mancilla, Flor; Luzón, Francisco; Carmona, Enrique; Martín, Rosa; Sánchez, Nieves

2014-05-01

105

Gas hydrate and P-Wave Velocity Distribution in the Yaquina Basin at the Peruvian margin  

NASA Astrophysics Data System (ADS)

The lower boundary of the methane hydrate stability zone in continental margin sediments is often marked by a strong, phase reversed reflection subparallel to the seafloor, called the bottom simulating reflector (BSR). High resolution multichannel seismic (MCS) data from the Yaquina Basin offshore Peru at 8 deg S show a BSR that is varying laterally in amplitude as well as in continuity. The amplitudes of the reflections above the BSR also vary with the appearance of the BSR. Where the BSR is strong, the reflections above it are weaker compared to areas where the BSR is weak. And although the strong part of the BSR is underlain immediately by strong reflections, reflections several hundred meters beneath the BSR appear weaker than those where the BSR is weak. This variation indicates significant heterogeneity in the distribution of gas and gas hydrate in this area. Chemoherms observed at the Yaquina Basin sea floor indicate the presence of free gas in the sediments up to the seafloor. The presence of gas and gas hydrate within the sediment sequence significantly influences the P-wave velocity in the affected layers. Therefore a detailed analysis of velocity variations enables to understand the apparently different conditions for the formation of gas hydrate along the BSR and the migration paths of the free gas. Ocean bottom seismometer (OBS) data from profiles coincident with the MCS data can provide such detailed velocity depth information. Velocity analysis from OBS data included 2D-ray tracing and 1D-interval-velocity analysis by means of DIX-inversion. In order to find a trade-off between vertical resolution and minimization of errors caused by the sensitivity of the DIX' formula to velocity variations in thin layers, the data have undergone a Kirchhoff wave-equation datuming and adjacent coherence filtering was applied to the data to eliminate the one sided travel path through the water column of the OBS-observations. The derived velocity structure confirms the interpretation of the reflection pattern in terms of gas and gas hydrate distribution.

Huebscher, C.; Gajewski, D.; Grobys, J.; Kukowski, N.; Netzeband, G.; Wagner, M.; Bialas, J.

2003-04-01

106

Seismic velocity structure and anisotropy of the Alaska subduction zone based on surface wave tomography  

NASA Astrophysics Data System (ADS)

Southcentral Alaska is a complex tectonic region that transitions from subduction of Pacific crust to flat slab subduction—and collision—of overthickened Yakutat crust. Because much of the Yakutat crust has been subducted, seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting. Here we use teleseismic Rayleigh waves to image large-scale variations in shear wave structure. Our imaging technique employs a two-plane wave representation with finite frequency sensitivity kernels. Our 3-D isotropic model reveals several features: the subducting Pacific/Yakutat slab, slow wave speeds characterizing the onshore Yakutat collision zone, slow wave speeds of the Wrangell subduction zone, and a deep tomographic contrast at the eastern edge of the Pacific/Yakutat slab. We produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy. These maps show the dominance of the Yakutat slab on the observed pattern of anisotropy. West of the Yakutat slab the fast directions are approximately aligned with the plate convergence direction. In the region of the Yakutat slab the pattern is more complicated. Along the margins of the slab the fast directions are roughly parallel to the margins. We identify notable differences and similarities with published SKS splitting measurements. Integrative modeling using 3-D anisotropy models and different seismic measurements will be needed in order to establish a detailed 3-D anisotropic velocity model for Alaska. This study provides a large-scale starting point for such an effort.

Wang, Yun; Tape, Carl

2014-12-01

107

Note on the velocity and related fields of steady irrotational two-dimensional surface gravity waves.  

PubMed

The velocity and other fields of steady two-dimensional surface gravity waves in irrotational motion are investigated numerically. Only symmetric waves with one crest per wavelength are considered, i.e. Stokes waves of finite amplitude, but not the highest waves, nor subharmonic and superharmonic bifurcations of Stokes waves. The numerical results are analysed, and several conjectures are made about the velocity and acceleration fields. PMID:22393109

Clamond, Didier

2012-04-13

108

Laser-based ultrasonic generation and detection of zero-group velocity Lamb waves in thin plates  

NASA Astrophysics Data System (ADS)

A novel laser-based ultrasonic technique for the inspection of thin plates and membranes is presented, in which a modulated continuous-wave laser source is used to excite narrow bandwidth Lamb waves. The dominant feature in the acoustic spectrum is a sharp resonance peak that occurs at the minimum frequency of the first-order symmetric Lamb mode, where the group velocity of the Lamb wave goes to zero while the phase velocity remains finite. Experimental results with the laser source and receiver on epicenter demonstrate that the zero-group velocity resonance generated with a low-power modulated excitation source can be detected using a Michelson interferometer coupled to a lock-in amplifier. This resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable, for example, for the measurement and mapping of nanoscale thickness variations.

Prada, C.; Balogun, O.; Murray, T. W.

2005-11-01

109

Imaging seismic velocities for hydrate-bearing sediments using converted waves near Yuan-An Ridge, off southwest Taiwan  

NASA Astrophysics Data System (ADS)

Data from P-waves and from S-waves generated by P-S conversion on reflection from airgun shots recorded along four lines of ocean bottom seismometers were used to construct 2-D velocity sections near the Yuan-An Ridge, off southwest Taiwan. The locations of the ocean bottom seismometers were determined to high accuracy by an inversion based on the shot traveltimes. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P-wave (Vp) and S-wave (Vs) velocities of hydrate-bearing sediment layers. The inversion indicates a relatively high P-wave velocity beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. S-wave velocities of the sediments over the entire section, down to 400 m below seafloor, range from 320 to 570 ms-1. We suggested the lateral variation in Vp/Vs profiles in the hydrate-affected zones may be related to the migration conduit of gas-rich fluid and a characteristic of hydrate content. We model Vp using equations based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ?200 m below the seafloor to 10-15% of pore space close to the bottom simulating reflector (BSR) in the survey area.

Cheng, W. B.; Shih, T. Y.; Lin, W. Y.; Wang, T. K.; Liu, C. S.; Wang, Y.

2014-10-01

110

Imaging the Anisotropic Shear-wave Velocity in the Earth's Mantle using Free Oscillations, Body Waves, Surface Waves and Long-period Waveforms  

NASA Astrophysics Data System (ADS)

We incorporate normal-mode splitting functions into a framework containing surface-wave phase anomalies, long-period waveforms, and body-wave travel times to investigate the three-dimensional structure of anisotropic shear-wave velocity in the Earth's mantle. In contrast with earlier studies, our modeling approach spans a wider spectrum (0.3-50 mHz) of seismological observables, jointly inverts for velocity and anisotropy apart from the discontinuity topographies, and incorporates new crustal corrections for the splitting functions that are consistent with the nonlinear corrections we employ for the waveforms. Our preferred anisotropic model, S362ANI+, an update to S362ANI, gives better fits to the recently measured splitting functions of spheroidal and toroidal modes that are modeled in this study. The splitting functions require additional isotropic variations in the transition zone and the mid mantle that are geographically distributed in the southern hemisphere. The level of agreement in the isotropic shear-velocity structure is higher between S362ANI+ and other recent studies than in the earlier generation of models. The anisotropic part of S362ANI+ is similar to S362ANI and is restricted to the upper 300 km in the mantle since only small improvements in fits are observed on adding anisotropy at depth. We also show that modeling the splitting functions reduces the tradeoffs between lateral variations in velocity and anisotropy in the lowermost mantle. Therefore, more data should be included to constrain any radial anisotropy in the transition zone and in the lower mantle.

Moulik, P.; Ekstrom, G.

2013-12-01

111

The correlations between the saturated and dry P-wave velocity of rocks.  

PubMed

Sometimes engineers need to estimate the wet-rock P-wave velocity from the dry-rock P-wave velocity. An estimation equation embracing all rock classes will be useful for the rock engineers. To investigate the predictability of wet-rock P-wave velocity from the dry-rock P-wave velocity, P-wave velocity measurements were performed on 41 different rock types, 11 of which were igneous, 15 of which were sedimentary and 15 of which was metamorphic. In addition to the dry- and wet-rock P-wave velocity measurements, the P-wave velocity changing as a function of saturation degree was studied. Moreover, dry-rock S-wave velocity measurements were conducted. The test results were modeled using Gassmann's and Wood's theory and it was seen that the measured data did not fit the theories. The unconformity is due to the fact that the theories are valid for high-porosity unconsolidated sediments at low frequencies. Gassmann's equation was modified for the rocks except high-porosity unconsolidated sediments. The dry- and wet-rock P-wave velocity values were evaluated using regression analysis. A strong linear correlation between the dry- and wet-rock P-wave velocities was found. Regression analyses were repeated for the rock classes and it was shown that correlation coefficients were increased. Concluding remark is that the derived equations can be used for the prediction of wet-rock P-wave velocity from the dry-rock P-wave velocity. PMID:17624388

Kahraman, S

2007-11-01

112

The influence of temperature variations on ultrasonic guided waves in anisotropic CFRP plates.  

PubMed

Carbon Fibre Reinforced Polymer (CFRP) materials are lightweight and corrosion-resistant and therefore are increasingly used in aerospace, automotive and construction industries. In Structural Health Monitoring (SHM) applications of CFRP materials, ultrasonic guided waves potentially offer large area inspection or inspection from a remote location. This paper addresses the effect of temperature variation on guided wave propagation in highly anisotropic CFRP materials. Temperature variations cause changes in guided wave velocity that can in turn compromise the baseline subtraction procedures employed by many SHM systems for damage detection. A simple model that describes the dependence of elastic properties of the CFRP plates on temperature is presented in this paper. The model can be used to predict anisotropic velocity changes and baseline subtraction performance under varying thermal conditions. The results produced by the model for unidirectional and 0/90 CFRP plates are compared with experimental measurements. PMID:25812468

Putkis, O; Dalton, R P; Croxford, A J

2015-07-01

113

Ultrasonic Velocity Variations with Soil Composition for Moisture Measurement  

NASA Technical Reports Server (NTRS)

Soil moisture content may be measured by many methods, but the presently available techniques all have drawbacks when used in ground truth measurements for remote sensing. Ultrasonic velocity varies with soil moisture content, and may be used as the basis of a new measurement technique. In order to characterize a sensor capable of field use, soil particle size distribution data are compared to ultrasonic velocity in a variety of soils over a wide moisture range.

Metzl, R.; Choi, J.; Aggarwal, M. D.; Manu, A.

1998-01-01

114

Illumination Profile & Dispersion Variation Effects on Radial Velocity Measurements  

NASA Astrophysics Data System (ADS)

The Multi-object APO Radial-Velocity Exoplanet Large-Area Survey (MARVELS) measures radial velocities using a fiber-fed dispersed fixed-delay interferometer (DFDI) with a moderate dispersion spectrograph. This setup allows a unique insight into the 2D illumination profile from the fiber on to the dispersion grating. Illumination profile investigations show large changes in the profile over time and fiber location. These profile changes are correlated with dispersion changes and long-term radial velocity offsets, a major problem within the MARVELS radial velocity data. Characterizing illumination profiles creates a method to both detect and correct radial velocity offsets, allowing for better planet detection. Here we report our early results from this study including improvement of radial velocity data points from detected giant planet candidates. We also report an illumination profile experiment conducted at the Kitt Peak National Observatory using the EXPERT instrument, which has a DFDI mode similar to MARVELS. Using profile controlling octagonal-shaped fibers, long term offsets over a 3 month time period were reduced from ~50 m/s to within the photon limit of ~4 m/s.

Grieves, Nolan; Ge, Jian; Thomas, Neil B.; Ma, Bo; Li, Rui; SDSS-III

2015-01-01

115

Crustal shear wave velocity and radial anisotropy beneath the Rio Grande rift from ambient noise tomography  

NASA Astrophysics Data System (ADS)

Shear wave velocity and radial anisotropy beneath New Mexico are obtained from ambient seismic noise tomography using data from the Transportable Array. Besides the distinct seismic structure imaged across the Rio Grande rift from the Colorado Plateau to the Great Plains, both velocity and anisotropy models also reveal significant variations along the rift. The rift at Albuquerque is characterized by remarkably low velocity in the shallow crust, high velocity and strong positive anisotropy in the middle and lower crust, and low velocity in the upper mantle. These observations can be interpreted as magma accumulation in the shallow crust and significant mafic underplating in the lower crust with abundant melt supply from the hot mantle. We propose that the Albuquerque region has recently been experiencing the most vigorous extensional deformation in the rift. Positive anisotropy with Vsh > Vsv appears in the central and southern rifts with a stronger anisotropy beneath younger volcanoes, reflecting layering of magma intrusion due to past and recent rifting activities. The low velocities in the uppermost mantle are observed under high-elevation places, the Jemez Lineament, northern rift, and east rift boundary, implying that the buoyancy of hot mantle largely compensates the local high topography. Low mantle velocities appear at the boundary of the southern rift, corresponding to the large lithosphere thickness change, instead of the rift center, consistent with the prediction from the small-scale, edge-driven mantle convection model. We conclude that the edge-driven upper mantle convection is probably the dominant mechanism for the recent and current rifting and uplift in the Rio Grande rift.

Fu, Yuanyuan V.; Li, Aibing

2015-02-01

116

Measurement of shear-wave velocity by ultrasound critical-angle reflectometry (UCR)  

NASA Technical Reports Server (NTRS)

There exists a growing body of research that relates the measurement of pressure-wave velocity in bone to different physiological conditions and treatment modalities. The shear-wave velocity has been less studied, although it is necessary for a more complete understanding of the mechanical properties of bone. Ultrasound critical-angle reflectometry (UCR) is a noninvasive and nondestructive technique previously used to measure pressure-wave velocities both in vitro and in vivo. This note describes its application to the measurement of shear-wave velocity in bone, whether directly accessible or covered by soft tissue.

Mehta, S.; Antich, P.; Blomqvist, C. G. (Principal Investigator)

1997-01-01

117

Stochastic simulation for the propagation of high-frequency acoustic waves through a random velocity field  

NASA Astrophysics Data System (ADS)

In-service inspection of Sodium-Cooled Fast Reactors (SFR) requires the development of non-destructive techniques adapted to the harsh environment conditions and the examination complexity. From past experiences, ultrasonic techniques are considered as suitable candidates. The ultrasonic telemetry is a technique used to constantly insure the safe functioning of reactor inner components by determining their exact position: it consists in measuring the time of flight of the ultrasonic response obtained after propagation of a pulse emitted by a transducer and its interaction with the targets. While in-service the sodium flow creates turbulences that lead to temperature inhomogeneities, which translates into ultrasonic velocity inhomogeneities. These velocity variations could directly impact the accuracy of the target locating by introducing time of flight variations. A stochastic simulation model has been developed to calculate the propagation of ultrasonic waves in such an inhomogeneous medium. Using this approach, the travel time is randomly generated by a stochastic process whose inputs are the statistical moments of travel times known analytically. The stochastic model predicts beam deviations due to velocity inhomogeneities, which are similar to those provided by a determinist method, such as the ray method.

Lu, B.; Darmon, M.; Leymarie, N.; Chatillon, S.; Potel, C.

2012-05-01

118

Stochastic simulation for the propagation of high-frequency acoustic waves through a random velocity field  

SciTech Connect

In-service inspection of Sodium-Cooled Fast Reactors (SFR) requires the development of non-destructive techniques adapted to the harsh environment conditions and the examination complexity. From past experiences, ultrasonic techniques are considered as suitable candidates. The ultrasonic telemetry is a technique used to constantly insure the safe functioning of reactor inner components by determining their exact position: it consists in measuring the time of flight of the ultrasonic response obtained after propagation of a pulse emitted by a transducer and its interaction with the targets. While in-service the sodium flow creates turbulences that lead to temperature inhomogeneities, which translates into ultrasonic velocity inhomogeneities. These velocity variations could directly impact the accuracy of the target locating by introducing time of flight variations. A stochastic simulation model has been developed to calculate the propagation of ultrasonic waves in such an inhomogeneous medium. Using this approach, the travel time is randomly generated by a stochastic process whose inputs are the statistical moments of travel times known analytically. The stochastic model predicts beam deviations due to velocity inhomogeneities, which are similar to those provided by a determinist method, such as the ray method.

Lu, B.; Darmon, M.; Leymarie, N.; Chatillon, S.; Potel, C. [CEA, LIST, F-91191 Gif-sur-Yvette (France); Laboratoire d'Acoustique de l'Universite du Maine (LAUM), UMR CNRS 6613, 72085 Le Mans Cedex 9 (France)

2012-05-17

119

Elastic wave velocities in anorthosite and anorthositic gabbros from Apollo 15 and 16 landing sites  

NASA Technical Reports Server (NTRS)

Laboratory measurements of ultrasonic velocities in lunar samples 15065, 15555, 15415, 60015, and 61016 as well as in synthetic materials corresponding to compositions of anorthositic gabbros are presented as a function of hydrostatic pressure to about 7 kb. The author examined the seismic velocity distributions in the moon with reference to the variations to be expected in a homogeneous medium. The lunar mantle begins about 60 km, and the velocity of P waves in this area is about 7.7 km/sec. Variation of the seismic parameter with depth in the upper crust (about 20 km thick) is much too rapid to be explained by compression of a uniform material and the departure from expectation is so great that no reasonable adjustment of the material parameters can bring agreement; therefore, this author concludes that this result in this region of the moon is not due to self-compression but to textural gradients. In the lower crust (about 40 km thick), the region is shown to be relatively homogeneous, consisting probably of anorthositic rocks.

Chung, D. H.

1973-01-01

120

Use Your Own Computer Timer: Velocity of Stress Waves in a Solid Rod.  

ERIC Educational Resources Information Center

Describes how to measure the penultimate laboratory speed, a stress wave velocity in a solid rod. Also includes background information on stress waves, apparatus, and procedures. Employs a homemade interface for the IBM platform. (DDR)

Russell, David

1997-01-01

121

Utility of automated brachial ankle pulse wave velocity measurements in hypertensive patients  

Microsoft Academic Search

BackgroundWe examined whether pulse wave velocity (PWV), determined by brachial ankle arterial pressure wave measurements, using a newly developed, fully automated device could be a surrogate measure for carotid femoral PWV.

Masanori Munakata; Nobuhiko Ito; Tohru Nunokawa; Kaoru Yoshinaga

2003-01-01

122

Local and noncontact measurements of bulk acoustic wave velocities in thin isotropic plates and shells using zero group velocity Lamb modes  

NASA Astrophysics Data System (ADS)

An original method for material characterization with acoustic waves is presented. The measurement of the longitudinal and shear wave velocities in thin isotropic plates or shells is performed locally on the same face without any mechanical contact. We exploit the resonance that occurs at the minimum frequency thickness product of the first order symmetric (S1) and of the second order antisymmetric (A2) Lamb modes. At these frequencies the group velocity vanishes, whereas the phase velocity remains finite. Then, the energy, which cannot propagate in the structure, is localized in a zone of diameter half the wavelength. The vibrations are excited in the thermoelastic regime by a laser pulse and detected at the same point by an optical interferometer. For these two Lamb modes we have computed the variations of the frequency thickness product versus Poisson's ratio. The resonance frequency ratio, which is independent of the plate or shell thickness, provides an absolute and local measurement of Poisson's ratio. Provided that the plate thickness is known, each resonance frequency allows us to determine in a single shot the bulk acoustic wave velocities VL and VT. Since it is based on frequency measurements, the method, tested on a large number of materials, is very accurate.

Clorennec, Dominique; Prada, Claire; Royer, Daniel

2007-02-01

123

Shear wave velocity models retrieved using Rg wave dispersion data in shallow crust in some regions of southern Ontario, Canada  

NASA Astrophysics Data System (ADS)

Many crucial tasks in seismology, such as locating seismic events and estimating focal mechanisms, need crustal velocity models. The velocity models of shallow structures are particularly important in the simulation of ground motions. In southern Ontario, Canada, many small shallow earthquakes occur, generating high-frequency Rayleigh ( Rg) waves that are sensitive to shallow structures. In this research, the dispersion of Rg waves was used to obtain shear-wave velocities in the top few kilometers of the crust in the Georgian Bay, Sudbury, and Thunder Bay areas of southern Ontario. Several shallow velocity models were obtained based on the dispersion of recorded Rg waves. The Rg waves generated by an m N 3.0 natural earthquake on the northern shore of Georgian Bay were used to obtain velocity models for the area of an earthquake swarm in 2007. The Rg waves generated by a mining induced event in the Sudbury area in 2005 were used to retrieve velocity models between Georgian Bay and the Ottawa River. The Rg waves generated by the largest event in a natural earthquake swarm near Thunder Bay in 2008 were used to obtain a velocity model in that swarm area. The basic feature of all the investigated models is that there is a top low-velocity layer with a thickness of about 0.5 km. The seismic velocities changed mainly within the top 2 km, where small earthquakes often occur.

Ma, Shutian; Motazedian, Dariush; Corchete, Victor

2013-04-01

124

Radial velocity variations in the young eruptive star EX Lupi  

NASA Astrophysics Data System (ADS)

Context. EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by optical and near-infrared outbursts attributed to highly enhanced accretion from the circumstellar disk onto the star. Aims: The trigger mechanism of EXor outbursts is still debated. One type of theory requires a close (sub)stellar companion that perturbs the inner part of the disk and triggers the onset of the enhanced accretion. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of the EXor class, and test whether they can be related to a close companion. Methods: We conducted a five-year RV survey, collecting 54 observations with HARPS and FEROS. We analyzed the activity of EX Lup by checking the bisector, the equivalent width of the Ca 8662 Å line, the asymmetry of the Ca II K line, the activity indicator SFEROS, the asymmetry of the cross-correlation function, the line depth ratio of the VI/FeI lines, and the TiO, CaH 2, CaH 3, CaOH, and H? indices. We complemented the RV measurements with a 14-day optical/infrared photometric monitoring to look for signatures of activity or varying accretion. Results: We found that the RV of EX Lup is periodic (P = 7.417 d), with stable period, semi-amplitude (2.2 km s-1), and phase over at least four years of observations. This period is not present in any of the above-mentioned activity indicators. However, the RVs of narrow metallic emission lines suggest the same period, but with an anti-correlating phase. The observed absorption line RVs can be fitted with a Keplerian solution around a 0.6 M? central star with msini = (14.7 ± 0.7) MJup and eccentricity of e = 0.24. Alternatively, we attempted to model the observations with a cold or hot stellar spot as well. We found that in our simple model, the spot parameters needed to reproduce the RV semi-amplitude are in contradiction with the photometric variability, making the spot scenario unlikely. Conclusions: We qualitatively discuss two possibilities to explain the RV data: a geometry with two accretion columns rotating with the star, and a single accretion flow synchronized with the orbital motion of the hypothetical companion; the second scenario is more consistent with the observed properties of EX Lup. In this scenario, the companion's mass would fall into the brown dwarf desert, which, together with the unusually small separation of 0.06 au would make EX Lup a unique binary system. The companion also has interesting implications on the physical mechanisms responsible for triggering the outburst. This work is based in part on observations made with ESO Telescopes at the La Silla Paranal Observatory under program IDs 079.A-9017, 081.A-9005, 081.A-9023, 081.C-0779, 082.C-0390, 082.C-0427, 083.A-9011, 083.A-9017, 084.A-9011, 085.A-9027, 086.A-9006, 086.A-9012, 087.A-9013, 087.A-9029, and 089.A-9007.Tables 2 and 3 are available in electronic form at http://www.aanda.org

Kóspál, Á.; Mohler-Fischer, M.; Sicilia-Aguilar, A.; Ábrahám, P.; Curé, M.; Henning, Th.; Kiss, Cs.; Launhardt, R.; Moór, A.; Müller, A.

2014-01-01

125

Estimation of spatiotemporal variation of acoustic velocity in ocean and its modeling for GPS/Acoustic seafloor positioning  

NASA Astrophysics Data System (ADS)

We have been developing an observation system with the GPS/Acoustic combination technique for monitoring of seafloor crustal deformation. We installed two sets of triangular array of acoustic transponders as geodetic reference sites on the Suruga trough, central Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate at a rate of 2 cm/yr. In our campaign observation, we measured ranges to acoustic transponders from an on-board acoustic transducer whose position was determined by kinematic GPS. Repeated our campaign observations can reveal directly seafloor crustal deformation in focal area of subduction zone. Present analysis method simultaneously estimates temporal variation of acoustic velocity and positions of acoustic transponders assuming the horizontally-layered structure of acoustic velocity. However, actual structure might have stable spatial variation due to oceanic current and internal wave. The stable spatial variation causes bias error on the positioning. For reduction of the bias error and shortening observation time, we should measure and/or estimate the spatial variation of acoustic velocity. In this presentation, for an investigation of horizontal scales of the spatial variation of acoustic velocity, we estimated spectrum of oceanic internal wave using by continuous measurements of temperature and pressure in ocean. In addition, we evaluate a new seafloor positioning method which simultaneously estimates spatiotemporal variation of acoustic velocity through numerical experiments. The continuous measurements in parallel with acoustic ranging were conducted by mooring temperature and pressure sensors attached rope with an interval of 50 m. The continuous measurements were made for five hours with a sampling interval of three seconds in each day in August and October 2008. For reduction of measurement noise, we carried out two-dimensional B-spline fitting of temperature and pressure with ABIC minimization. By the fitting, we could carry out spectral analysis for time-series of temperature (or pressure) at arbitrary depth (or temperature). As a result, several spectra were consistent with Garrett-Munk Spectrum. We could estimated the frequency of internal wave of 10^-4 Hz and phase velocity of isothermal surface was ~1.5 cm/s. Accordingly, the horizontal wavelength of the internal wave was about less than1 km at one of the sites in consideration of inertial and buoyancy frequency. In the new seafloor positioning method, spatial variation of acoustic velocity was regarded as two-dimensional lateral gradient, and this method estimate position of seafloor transponder, temporal variation and, two-dimensional lateral gradient of acoustic velocity. In numerical experiment, for example, we first assumed certain temporal variation and one-dimensional (NS component) gradient which was 0.75 m/s in distance of about 2 km and synthesized dataset of travel-time which was given certain Gaussian error. As a preliminary result, we reproduced given positions of seafloor transponder within 3 cm under that condition.

Sugimoto, S.; Tadokoro, K.; Ikuta, R.; Watanabe, T.; Okuda, T.; Sayanagi, K.; Miyata, K.; Nagao, T.

2009-12-01

126

Crustal structure and anisotropy of Greenland from surface-wave group-velocity tomography  

NASA Astrophysics Data System (ADS)

Studies of the geology of Greenland - largely restricted to the edge of the continent due to the presence of the inland ice sheet - show that a significant portion of the continent is underlain by Archean and Paleoproterozoic rocks. The Greenland craton is bounded by numerous younger fold belts, continental margins and basins, and the Iceland hotspot is thought to have had an impact on the lithosphere at the time of North Atlantic opening. The crustal structure of certain regions of Greenland has been studied through receiver function analysis at permanent and temporary broadband seismograph stations; the continental margins and surrounding ocean basins have also been sampled by controlled-source seismic profiles. However, the 3D structure of the crust and uppermost mantle across the continent remains poorly resolved. In order to shed further light on the structure and tectonic evolution of the Greenland continent, we analysed group velocities from earthquakes on the Mid-Atlantic Ridge and northern Canada, recorded at broadband stations of the GEOFON, GSN, CNSN, GLATIS and GLISN seismograph networks. The region covered by the study spans the entire Greenland continent as well as the surrounding oceanic lithosphere. The dense path coverage afforded by the data set allows for a tomographic inversion for maps of group velocity variation and azimuthal anisotropy across the region, sampling over a depth range from the mid crust to the uppermost lithospheric mantle. In addition to the expected first-order difference between oceanic and continental crust beneath the region, we note substantial lateral variations in group velocity across periods ranging from 10 to 60 seconds. Certain zones of relatively low group velocity correspond to sediment infill in large extensional basins at the edge of the Greenland continent; local variations for periods sampling the uppermost mantle are more enigmatic. Reconstruction of group velocities at each well-resolved grid point in the inversion allows the modelling of isotropic 1D shear-wave velocity structure using linearised inversion and Monte-Carlo sampling. The models are combined into a 3D model of the crust and uppermost mantle of the region. We use the 3D velocity model to investigate the structural relationships between the Greenland craton and its surroundings, and search for evidence of pervasive lithospheric modification by the Iceland hotspot.

Darbyshire, F. A.; Dahl-Jensen, T.; Larsen, T.; Voss, P.

2013-12-01

127

Real-time vector velocity assessment through multigate Doppler and plane waves.  

PubMed

Several ultrasound (US) methods have been recently proposed to produce 2-D velocity vector fields with high temporal and spatial resolution. However, the real-time implementation in US scanners is heavily hampered by the high calculation power required. In this work, we report a real-time vector Doppler imaging method which has been integrated in an open research system. The proposed approach exploits the plane waves transmitted from two sub-arrays of a linear probe to estimate the velocity vectors in 512 sample volumes aligned along the probe axis. The method has been tested for accuracy and reproducibility through simulations and in vitro experiments. Simulations over a 0° to 90° angle range of a 0.5 m/s peak parabolic flow have yielded 0.75° bias and 1.1° standard deviation for direction measurement, and 0.6 cm/s bias with 3.1% coefficient of variation for velocity assessment. In vitro tests have supported the simulation results. Preliminary measurements on the carotid artery of a volunteer have highlighted the real-time system capability of imaging complex flow configurations in an intuitive, easy, and quick way, as shown in a sample supplementary movie. These features have allowed reproducible peak velocity measurements to be obtained, as needed for quantitative investigations on patients. PMID:24474137

Ricci, Stefano; Bassi, Luca; Tortoli, Piero

2014-02-01

128

Velocity shear effect on the longitudinal wave in a strongly coupled dusty plasma  

NASA Astrophysics Data System (ADS)

The characteristics of longitudinal dust acoustic wave (DAW) in presence of velocity shear have been investigated in a strongly coupled dusty plasma using the generalized hydrodynamic (GH) model. In the hydrodynamic regime ( ?? m ?1), i.e. when characteristic time ? m is slower than inverse of wave frequency, the viscosity in the GH model plays the usual role of wave damping, whereas in the kinetic regime ( ?? m ?1), i.e. when characteristic time ? m is larger than inverse of wave frequency, viscosity shows energy storing property in the wave. In the kinetic regime, we have studied the longitudinal mode (where ? is the frequency, k is the wave number, c d is the dust acoustic velocity and c l is the longitudinal velocity that arises due to viscosity) in presence of velocity shear. It is shown that velocity shear can destabilize this mode. Both nonmodal and modal techniques are employed to demonstrate the growth rate of the instability.

Garai, S.; Banerjee, D.; Janaki, M. S.; Chakrabarti, N.

2014-02-01

129

Rayleigh wave phase-velocity heterogeneity and multilayered azimuthal anisotropy of the Superior Craton, Ontario  

NASA Astrophysics Data System (ADS)

We study the azimuthally anisotropic upper-mantle structure of the Superior Craton and Grenville Province in Ontario, Canada, using Rayleigh wave phase-velocity data in the period range 40-160s. 152 two-station dispersion measurements are combined in a tomographic inversion that solves simultaneously for isotropic and anisotropic terms using a least-squares technique. We perform a series of tests to derive optimal regularization (smoothing and damping) and to assess the resolution of, and trade-offs between, isotropic and anisotropic anomalies. The tomographic inversion is able to resolve isotropic phase-velocity anomalies on a scale of 200-300km and to distinguish between different anisotropic regimes on a 500-km scale across the study region. Isotropic phase-velocity anomalies in the tomographic model span a range of up to +/-2 per cent around a regional average which is similar to the Canadian Shield dispersion curve of Brune & Dorman (1963), with phase velocities up to 3 per cent above global reference models. The amplitude of azimuthal phase-velocity anisotropy reaches a maximum of ~1.2 per cent. A clear east-west division of the study area, based on both isotropic phase-velocity anomalies and azimuthal anisotropy, is apparent. In the western Superior, isotropic phase velocities are generally higher than the regional average. Anisotropy is observed at all periods, with ENE-WSW to NE-SW fast-propagation directions. At periods <=120s, the anisotropy likely results from frozen lithospheric fabric aligned with tectonic boundaries, whereas the anisotropy at longer periods is interpreted to arise from present-day sublithospheric flow. The fast directions from published SKS measurements are close to the fast Rayleigh wave propagation directions throughout the period range sampled, and the large SKS splitting times may be accounted for by this near-coincidence of fast-propagation directions. Across most of eastern Ontario, phase velocities are lower than the regional average. Fast-propagation directions rotate from ~NW-SE at 40-130s period to WNW-ESE at periods 140-160s. The results suggest a difference in fast-propagation directions between the anisotropic fabric frozen into the lithosphere and the fabric due to current and recent sublithospheric flow. The Superior Craton and Grenville Province are characterized by large-scale structural variations that reflect the complex tectonic history of the region. This study highlights differences between the characteristics of eastern and western Ontario and indicates the occurrence of multiple layers of anisotropy in the subcratonic upper mantle.

Darbyshire, Fiona A.; Lebedev, Sergei

2009-01-01

130

Crustal and mantle shear velocity structure of Costa Rica and Nicaragua from ambient noise and teleseismic Rayleigh wave tomography  

NASA Astrophysics Data System (ADS)

The Costa Rica-Nicaragua subduction zone shows systematic along strike variation in arc chemistry, geology, tectonics and seismic velocity and attenuation, presenting global extremes within a few hundred kilometres. In this study, we use teleseismic and ambient noise derived surface wave tomography to produce a 3-D shear velocity model of the region. We use the 48 stations of the TUCAN array, and up to 94 events for the teleseismic Rayleigh wave inversion, and 18 months of continuous data for cross correlation to estimate Green's functions from ambient noise. In the shallow crust (0-15 km) we observe low-shear velocities directly beneath the arc volcanoes (<3 km s-1) and higher velocities in the backarc of Nicaragua. The anomalies below the volcanoes are likely caused by heated crust, intruded by magma. We estimate crustal thickness by picking the depth to the 4 km s-1 velocity contour. We infer >40-km-thick crust beneath the Costa Rican arc and the Nicaraguan Highlands, thinned crust (˜20 km) beneath the Nicaraguan Depression, and increasing crustal thickness in the backarc region, consistent with receiver function studies. The region of thinned, seismically slow and likely weakened crust beneath the arc in Nicaragua is not localizing deformation associated with oblique subduction. At mantle depths (55-120 km depth) we observe lower shear velocities (up to 3 per cent) beneath the Nicaraguan arc and backarc than beneath Costa Rica. Our low-shear velocity anomaly beneath Nicaragua is in the same location as a low-shear velocity anomaly and displaced towards the backarc from the high VP/VS anomaly observed in body wave tomography. The lower shear velocity beneath Nicaragua may indicate higher melt content in the mantle perhaps due to higher volatile flux from the slab or higher temperature. Finally, we observe a linear high-velocity region at depths >120 km parallel to the trench, which is consistent with the subducting slab.

Harmon, Nicholas; Cruz, Mariela Salas De La; Rychert, Catherine Ann; Abers, Geoffrey; Fischer, Karen

2013-11-01

131

Coseismic and postseismic wave velocity changes caused by large crustal earthquakes in Japan  

NASA Astrophysics Data System (ADS)

Using Passive Image Interferometry (PII), we analyzed coseismic and postseismic changes of seismic wave velocities caused by the following earthquakes which occurred in Japan between 2004 and 2011: The 2005 Fukuoka (MW6.6), 2007 Noto Hant¯o (MW6.6) and 2008 Iwate-Miyagi Nairiku (MW6.9) earthquakes, three earthquakes in Niigata Prefecture (2004 Mid-Niigata, MW6.8; 2007 Ch¯u etsu Offshore, MW6.6; 2011 Nagano/Niigata, MW6.2), as well as the 2011 Tohoku earthquake (MW9.0) in the four regions of the other earthquakes. The time series of ambient noise used for the different earthquakes spanned from at least half a year before the respective earthquake until three months after the Tohoku earthquake. Cross-correlations and single-station cross-correlations of several years of ambient seismic noise, which was recorded mainly by Hi-net sensors in the surrounding areas of the respective earthquakes, are calculated in different frequency ranges between 0.125 and 4.0 Hz. Between 10 and 20 seismometers were used in the different areas. The cross-correlations are calculated for all possible station pairs. Using a simple tomography algorithm, the resulting velocity variations can be reprojected on the actual station locations. The cross-correlation and single-station cross-correlation techniques give compatible results, the former giving more reliable results for frequencies below 0.5 Hz, the latter for higher frequencies. Our analysis yields significant coseismic velocity drops for all analyzed earthquakes, which are strongest close to the fault zones and exceed 1 % for some stations. The coseismic velocity drops are larger at higher frequencies and recover on a time scale of several years, but the coseismic velocity drops do not completely recover during our observation time. Velocity drops are also visible in all areas at the time of the Tohoku earthquake. Furthermore, we measured seasonal velocity variations of the order of 0.1 % in all areas which are, at least for frequencies above 0.5 Hz and for the areas in the central to northern part of Japan, consistent for all sensors of an area and correspond to seismic velocity maxima in autumn.

Hobiger, Manuel; Wegler, Ulrich; Shiomi, Katsuhiko; Nakahara, Hisashi

2014-05-01

132

Advantages of using multichannel analysis of Love waves (MALW) in determining near-surface shear-wave velocity  

NASA Astrophysics Data System (ADS)

Surface-wave techniques have been given increasingly more attention by the near-surface community with applications to a variety of problems. Studies on high-frequency surface-wave techniques have been focused primarily on Rayleigh waves. There is much less attention on utilizing Love waves than Rayleigh waves in the near-surface community. Recent improvements in data-acquisition techniques and development of software make SH-wave data acquisition and processing easier. In addition, Love-wave analysis only results in SH-wave velocities, which suggests that we may benefit from analyzing SH-wave data using Love-wave inversion. Numerical results of SH waves and data from Kansas, Wyoming, Arizona, Washington, and Wuhan, China demonstrated three advantages of analyzing SH-wave data using multichannel analysis of Love waves (MALW). 1) Generally images of Love-wave energy are cleaner and sharper than those generated from Rayleigh waves. Owing to a long geophone spread commonly used in an SH-wave refraction survey, images of Love-wave energy are even much cleaner and sharper, which makes picking phase velocities of Love waves easier and more accurate. 2) Numerical results showed that because Love waves are independent of P-wave velocity, dispersion curves of Love waves are simpler than Rayleigh waves. "Mode kissing" (suggested by Robert Stewart) is an undesired and frequently occurred phenomenon in Rayleigh-wave analysis that causes mode misidentification. Fortunately, this phenomenon is less common in images of Love-wave energy than Rayleigh waves. 3) Real-world examples showed that inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves. This is because of being independent of P-wave velocity fewer unknowns in the MALW method not only make dispersion curves of Love waves simpler also reduces the degree of nonuniqueness, which leads to more stable inversion of Love-wave dispersion curves. Results also demonstrated that a more accurate model can be obtained by simultaneous inversion of the fundamental and the first-higher mode data. Our experimental study shows that the MALW method is a useful tool to analyzin high-frequency SH-wave data. It may become a common practice in the near-surface community and the oil industry because it provides SH-wave velocities that are critical for analysis of S-wave anisotropy of a near-surface velocity model. Furthermore, we realize two benefits of utilizing the MALW method in the near-surface applications. 1) Abundant SH-wave refraction data containing non-utilized strong Love-wave energy have been acquired for various environmental and engineering applications. Revisiting these data with the MALW method can enhance our knowledge of existing velocity models. 2) Acquiring Love-wave data by simply increasing a record length during future SH-wave refraction surveys and analyzing these data with the MALW method can provide accurate SH-wave velocities without additional cost in field.

Xia, J.; Xu, Y.; Luo, Y.; Miller, R. D.; Cakir, R.

2011-12-01

133

Characterizing elastic and inelastic deformation using ultrasound P-wave transmission during triaxial deformation experiments: velocities vs. relative amplitudes  

NASA Astrophysics Data System (ADS)

Direct information about in-situ rock properties is often spatially limited (e.g. boreholes) or even inaccessible. Therefore, variations in the properties of elastic wave propagation are amongst the most important sources of information to monitor natural or induced subsurface deformation processes, e.g. related to the seismic cycle or geothermal reservoir stimulation. Investigating velocities and amplitudes of elastic waves under controlled laboratory conditions can significantly contribute to the understanding of field data, since the physical state and properties of a rock sample are known and can be modified. For example, active ultrasound transmission during laboratory deformation experiments has become established as an important methodology in experimental rock physics to investigate the micromechanical processes associated with elastic and inelastic deformation. Ultrasound velocities are studied more extensively during deformation experiments than ultrasound amplitudes, although amplitudes of elastic waves are generally accepted to be more sensitive to microstructural variations than velocities, particularly in the presence of pore fluids. However, quantitative amplitude measurements are subject to a low comparability between single experiments, because amplitudes are strongly affected by the geometry of the experimental setup and coupling conditions. Yet, relative amplitude changes during deformation may yield valuable information on micromechanical processes not exposed by velocity measurements. Triaxial deformation characteristics of dry and saturated sandstone samples (Ruhr sandstone, Wilkeson sandstone, Fontainebleau sandstone) were investigated with simultaneously recorded ultrasound signals (1 MHz) of P-waves propagating parallel to the applied deviatoric stress. Deformation experiments were performed on dry samples at various confining pressures up to 150 MPa, as well as on dry and saturated samples at equivalent effective pressures. To avoid contributions from changes in effective pressure during deformation drainage of saturated samples was provided by applying sufficiently low strain rates. By combining mechanical data with properties of ultrasound wave propagation the information on micromechanical processes provided by relative amplitude changes and velocity measurements are investigated. During brittle deformation velocities and amplitudes initially increase, exhibit a maximum prior to failure and subsequently decrease, whilst the magnitude of variations decreases with increasing confining pressure and strongly differs between the three sandstone varieties. Amplitudes and velocities yield different information on the microstructural state of a rock sample. Relative changes of amplitudes are larger than relative changes in velocities and maxima of amplitudes occur at different axial strains than maxima in velocities. In particular, amplitudes seem to be much more sensitive to inelastic damage for saturated samples than for dry samples.

Duda, M.; Renner, J.

2012-12-01

134

Estimation of near-surface shear-wave velocities and quality factors using multichannel analysis of surface-wave methods  

NASA Astrophysics Data System (ADS)

This overview article gives a picture of multichannel analysis of high-frequency surface (Rayleigh and Love) waves developed mainly by research scientists at the Kansas Geological Survey, the University of Kansas and China University of Geosciences (Wuhan) during the last eighteen years by discussing dispersion imaging techniques, inversion systems, and real-world examples. Shear (S)-wave velocities of near-surface materials can be derived from inverting the dispersive phase velocities of high-frequency surface waves. Multichannel analysis of surface waves—MASW used phase information of high-frequency Rayleigh waves recorded on vertical component geophones to determine near-surface S-wave velocities. The differences between MASW results and direct borehole measurements are approximately 15% or less and random. Studies show that inversion with higher modes and the fundamental mode simultaneously can increase model resolution and an investigation depth. Multichannel analysis of Love waves—MALW used phase information of high-frequency Love waves recorded on horizontal (perpendicular to the direction of wave propagation) component geophones to determine S-wave velocities of shallow materials. Because of independence of compressional (P)-wave velocity, the MALW method has some attractive advantages, such as 1) Love-wave dispersion curves are simpler than Rayleigh wave's; 2) dispersion images of Love-wave energy have a higher signal to noise ratio and more focused than those generated from Rayleigh waves; and 3) inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves.

Xia, Jianghai

2014-04-01

135

Comparison of phase velocities from array measurements of Rayleigh waves associated with microtremor and results calculated from borehole shear-wave velocity profiles  

USGS Publications Warehouse

Shear-wave velocities (VS) are widely used for earthquake ground-motion site characterization. VS data are now largely obtained using borehole methods. Drilling holes, however, is expensive. Nonintrusive surface methods are inexpensive for obtaining VS information, but not many comparisons with direct borehole measurements have been published. Because different assumptions are used in data interpretation of each surface method and public safety is involved in site characterization for engineering structures, it is important to validate the surface methods by additional comparisons with borehole measurements. We compare results obtained from a particular surface method (array measurement of surface waves associated with microtremor) with results obtained from borehole methods. Using a 10-element nested-triangular array of 100-m aperture, we measured surface-wave phase velocities at two California sites, Garner Valley near Hemet and Hollister Municipal Airport. The Garner Valley site is located at an ancient lake bed where water-saturated sediment overlies decomposed granite on top of granite bedrock. Our array was deployed at a location where seismic velocities had been determined to a depth of 500 m by borehole methods. At Hollister, where the near-surface sediment consists of clay, sand, and gravel, we determined phase velocities using an array located close to a 60-m deep borehole where downhole velocity logs already exist. Because we want to assess the measurements uncomplicated by uncertainties introduced by the inversion process, we compare our phase-velocity results with the borehole VS depth profile by calculating fundamental-mode Rayleigh-wave phase velocities from an earth model constructed from the borehole data. For wavelengths less than ~2 times of the array aperture at Garner Valley, phase-velocity results from array measurements agree with the calculated Rayleigh-wave velocities to better than 11%. Measurement errors become larger for wavelengths 2 times greater than the array aperture. At Hollister, the measured phase velocity at 3.9 Hz (near the upper edge of the microtremor frequency band) is within 20% of the calculated Rayleigh-wave velocity. Because shear-wave velocity is the predominant factor controlling Rayleigh-wave phase velocities, the comparisons suggest that this nonintrusive method can provide VS information adequate for ground-motion estimation.

Liu, Hsi-Ping; Boore, David M.; Joyner, William B.; Oppenheimer, David H.; Warrick, Richard E.; Zhang, Wenbo; Hamilton, John C.; Brown, Leo T.

2000-01-01

136

Temporal changes of surface wave velocity associated with major Sumatra earthquakes from ambient noise correlation  

PubMed Central

Detecting temporal changes of the medium associated with major earthquakes has implications for understanding earthquake genesis. Here we report temporal changes of surface wave velocity over a large area associated with 3 major Sumatra earthquakes in 2004, 2005, and 2007. We use ambient noise correlation to retrieve empirical Green's function (EGF) of surface waves between stations. Because the process is completely repeatable, the technique is powerful in detecting possible temporal change of medium. We find that 1 excellent station pair (PSI in Indonesia and CHTO in Thailand) shows significant time shifts (up to 1.44 s) after the 2004 and 2005 events in the Rayleigh waves at 10–20 s but not in the Love waves, suggesting that the Rayleigh time shifts are not from clock error. The time shifts are frequency dependent with the largest shifts at the period band of 11–16 s. We also observe an unusual excursion ?1 month before the 2004 event. We obtain a total of 17 pairs for June, 2007 to June, 2008, which allow us to examine the temporal and spatial variation of the time shifts. We observed strong anomalies (up to 0.68 s) near the epicenter after the 2007 event, but not in the region further away from the source or before the event or 3 months after the event. The observations are interpreted as stress changes and subsequent relaxation in upper-mid crust in the immediate vicinity of the rupture and the broad area near the fault zone. PMID:19667205

Xu, Zhen J.; Song, Xiaodong

2009-01-01

137

Intima media thickness, pulse wave velocity, and flow mediated dilation  

PubMed Central

The identification of vascular alterations at the sub-clinical, asymptomatic stages are potentially useful for screening, prevention and improvement of cardiovascular risk stratification beyond classical risk factors. Increased intima-media thickness of the common carotid artery is a well-known marker of early atherosclerosis, which significantly correlates with the development of cardiovascular diseases. More recently, other vascular parameters evaluating both structural and functional arterial proprieties of peripheral arteries have been introduced, for cardiovascular risk stratification and as surrogate endpoints in clinical trials. Increased arterial stiffness, which can be detected by applanation tonometry as carotid-femoral pulse wave velocity, has been shown to predict future cardiovascular events and to significantly improve risk stratification. Finally, earlier vascular abnormalities such as endothelial dysfunction in the peripheral arteries, detected as reduced flow-mediated dilation of the brachial artery, are useful in the research setting and as surrogate endpoints in clinical trials and have also been suggested for their possible clinical use in the future. This manuscript will briefly review clinical evidence supporting the use of these different vascular markers for cardiovascular risk stratification, focusing on the correct methodology, which is a crucial issue to address in order to promote their use in future for routine clinical practice. PMID:25148901

2014-01-01

138

Pulse Wave Velocity and Cognitive Function in Older Adults  

PubMed Central

Arterial stiffness may be associated with cognitive function. In this study, pulse wave velocity (PWV) was measured from the carotid to femoral (CF-PWV) and from the carotid to radial (CR-PWV) with the Complior SP System (Alam Medical, Vincennes, France). Cognitive function was measured by six tests of executive function, psychomotor speed, memory, and language fluency. A total of 1433 participants were included (mean age 75 years, 43% men). Adjusting for age, sex, education, pulse rate, hemoglobin A1C, HDL cholesterol, hypertension, CVD history, smoking ,drinking, and depression symptoms, a CF-PWV > 12 m/s was associated with a lower Mini-Mental State Examination score (coefficient: ?0.31, se: 0.11, p=0.005), fewer words recalled on Auditory Verbal Learning Test (coefficient: ?1.10, se: 0.43, p=0.01), and lower score on the composite cognition score (coefficient: ?0.10, se: 0.05, p=0.04) and marginally significantly associated with longer time to complete Trail Making Test-B (coefficient: 6.30, se: 3.41, p=0.06), CF-PWV was not associated with Trail Making Test-A, Digit Symbol Substation Test, or Verbal Fluency Test. No associations were found between CR-PWV and cognitive performance measures. Higher large artery stiffness was associated with worse cognitive function, and longitudinal studies are needed to confirm these associations. PMID:23632267

Zhong, Wenjun; Cruickshanks, Karen J; Schubert, Carla R; Carlsson, Cynthia M; Chappell, Richard J; Klein, Barbara EK; Klein, Ronald; Acher, Charles W

2013-01-01

139

Pulse wave velocity and cognitive function in older adults.  

PubMed

Arterial stiffness may be associated with cognitive function. In this study, pulse wave velocity (PWV) was measured from the carotid to femoral (CF-PWV) and from the carotid to radial (CR-PWV) with the Complior SP System. Cognitive function was measured by 6 tests of executive function, psychomotor speed, memory, and language fluency. A total of 1433 participants were included (mean age 75 y, 43% men). Adjusting for age, sex, education, pulse rate, hemoglobin A1C, high-density lipoprotein cholesterol, hypertension, cardiovascular disease history, smoking, drinking, and depression symptoms, a CF-PWV>12 m/s was associated with a lower Mini-Mental State Examination score (coefficient: -0.31, SE: 0.11, P=0.005), fewer words recalled on Auditory Verbal Learning Test (coefficient: -1.10, SE: 0.43, P=0.01), and lower score on the composite cognition score (coefficient: -0.10, SE: 0.05, P=0.04) and marginally significantly associated with longer time to complete Trail Making Test-part B (coefficient: 6.30, SE: 3.41, P=0.06), CF-PWV was not associated with Trail Making Test-part A, Digit Symbol Substation Test, or Verbal Fluency Test. No associations were found between CR-PWV and cognitive performance measures. Higher large artery stiffness was associated with worse cognitive function, and longitudinal studies are needed to confirm these associations. PMID:23632267

Zhong, Wenjun; Cruickshanks, Karen J; Schubert, Carla R; Carlsson, Cynthia M; Chappell, Richard J; Klein, Barbara E K; Klein, Ronald; Acher, Charles W

2014-01-01

140

Magnetic plethysmograph transducers for local blood pulse wave velocity measurement.  

PubMed

We present the design of magnetic plethysmograph (MPG) transducers for detection of blood pulse waveform and evaluation of local pulse wave velocity (PWV), for potential use in cuffless blood pressure (BP) monitoring. The sensors utilize a Hall effect magnetic field sensor to capture the blood pulse waveform. A strap based design is performed to enable reliable capture of large number of cardiac cycles with relative ease. The ability of the transducer to consistently detect the blood pulse is verified by in-vivo trials on few volunteers. A duality of such transducers is utilized to capture the local PWV at the carotid artery. The pulse transit time (PTT) between the two detected pulse waveforms, measured along a small section of the carotid artery, was evaluated using automated algorithms to ensure consistency of measurements. The correlation between the measured values of local PWV and BP was also investigated. The developed transducers provide a reliable, easy modality for detecting pulse waveform on superficial arteries. Such transducers, used for measurement of local PWV, could potentially be utilized for cuffless, continuous evaluation of BP at various superficial arterial sites. PMID:25570363

Nabeel, P M; Joseph, Jayaraj; Sivaprakasam, Mohanasankar

2014-08-01

141

Variation of the radial velocity of Epsilon Cygni A  

NASA Technical Reports Server (NTRS)

The paper reports a series of 217 measurements of the radial velocity of Epsilon Cygni A made between May 16, 1987 and May 17, 1991 with an uncertainty per observation of +/- 12 m/s. The results indicate a sustained drift of -60 m/s yr. Lower limits on the companion's mass are presented as functions of period and primary mass. The companion is probably more massive than a planet.

Mcmillan, R. S.; Smith, P. H.; Moore, T. L.; Perry, M. L.

1992-01-01

142

Determination of ultrasonic wave velocities and phase velocity dispersion curves of an Inconel 600 plate using resonant ultrasound spectroscopy and leaky Lamb waves  

Microsoft Academic Search

A plate of Inconel 600 was interrogated using the resonant ultrasound spectroscopy (RUS) and the reflected leaky Lamb waves (LLW). It was found that the plate used in the present work has anisotropy in its material properties by the RUS. The longitudinal and the transverse wave velocities of the Inconel 600 plate were determined by the RUS, ultrasonic pulse–echo method

Young H Kim; Sung-Jin Song; Sung-Duk Kwon; Yong-Moo Cheong; Hyun-Kyu Jung

2004-01-01

143

Modeling the permeability evolution of microcracked rocks from elastic wave velocity inversion at elevated isostatic pressure  

Microsoft Academic Search

A key consequence of the presence of microcracks within rock is their significant influence upon elastic anisotropy and transport properties. Here two rock types (a basalt and a granite) with contrasting microstructures, dominated by microcracks, have been investigated using an advanced experimental arrangement capable of measuring porosity, P wave velocity, S wave velocity, and permeability contemporaneously at effective pressures up

Philip Benson; Alexandre Schubnel; Sergio Vinciguerra; Concetta Trovato; Philip Meredith; R. Paul Young

2006-01-01

144

S-Wave Velocities of the Lithosphere-Asthenosphere System in the Caribbean Region  

Microsoft Academic Search

An overview of the S-wave velocity (V s) structural model of the Caribbean with a resolution of 2° × 2° is presented. New tomographic maps of Rayleigh wave group velocity dispersion at periods ranging from 10 to 40 s were obtained as a result of the frequency time analysis of seismic signals of more than 400 ray-paths in the region.

O'leary F. González; José Leonardo Alvarez; Bladimir Moreno; Giuliano F. Panza

2011-01-01

145

Joint Inversion of Crustal Structure with the Rayleigh Wave Phase Velocity Dispersion and the ZH Ratio  

NASA Astrophysics Data System (ADS)

Surface waves are essential for resolving Earth's structure on both regional and global scales, and surface wave data are mostly exploited with velocity dispersion. However, dispersion is mostly sensitive to the integral feature of velocity structure, resulting in ambiguities of the model interpretation. Recently, it has been demonstrated that the ZH amplitude ratio of a Rayleigh wave is an effective approach for providing extra constraints to reduce ambiguity in surface wave inversion. In this paper, we studied the sensitivities of the Rayleigh wave phase velocity dispersion and the ZH ratio with layered crustal structure via forward modeling. The forward modeling experiments indicate that the Rayleigh wave ZH ratio shows different sensitivity as compared to Rayleigh wave phase velocity dispersion. The ZH ratio is more sensitive to the shallower structure compared to phase velocity dispersion of the same period, and the ZH ratio provides independent constraints on the structure. Thus, the combination of these two datasets should help to better constrain the velocity model. A joint inversion tool is developed to jointly invert for the Rayleigh wave phase velocity and the ZH ratio observations. The inversion is based on a Fast Simulated Annealing algorithm, which generates models randomly and can achieve a global minimum without requiring a sensitivity kernel. Joint inversions based on synthetic datasets confirmed that the ZH ratio together with a phase velocity dispersion curve can reduce the non-uniqueness in crustal structure inversion.

Chong, Jiajun; Ni, Sidao; Zhao, Li

2014-08-01

146

Assessment of local pulse wave velocity in arteries using 2D distension waveforms.  

PubMed

The reciprocal of the arterial pulse wave velocity contains crucial information about the mechanical characteristics of the arterial wall but is difficult to assess noninvasively in vivo. In this paper, a new method to assess local pulse wave velocity (PWV) is presented. To this end, multiple adjacent distension waveforms are determined simultaneously along a short arterial segment, using a single 2D-vessel wall tracking system with a high frame rate (651 Hz). Each B-mode image consists of 16 echo lines spanning a total width of 15.86 mm. Dedicated software has been developed to extract the end-diastolic diameter from the B-mode image and the distension waveforms from the underlying radiofrequency (rf) information for each echo-line. The PWV is obtained by determining the ratio of the temporal and spatial gradient of adjacent distension velocity waveforms. The proposed method is verified in a phantom and in the common carotid artery (CCA) of humans. Phantom experiments show a high concordance between the PWV obtained from 2D distension velocity waveforms (4.21 +/- 0.02 m/s) and the PWV determined using two pressure catheters (4.26 +/- 0.02 m/s). Assuming linear spatial gradients, the PWV can also be obtained in vivo for CCA and averages to 5.5 +/- 1.5 m/s (intersubject variation, n = 23), which compares well to values found in literature. Furthermore, intrasubject PWV compares well with those calculated using the Bramwell-Hill equation. It can be concluded that the PWV can be obtained from the spatial and temporal gradient if the spatial gradient is linear over the observed length of the artery, i.e. the artery should be homogenous in diameter and distension and the influence of reflections must be small. PMID:12051275

Meinders, J M; Kornet, L; Brands, P J; Hoeks, A P

2001-10-01

147

3-D Shear Velocity Structure of Costa Rica and Nicaragua from Teleseismic and Ambient Noise Rayleigh Wave Tomography  

NASA Astrophysics Data System (ADS)

The Costa Rica-Nicaragua subduction zone shows systematic along strike variation in arc chemistry, geology and seismic velocity and attenuation, presenting global extremes within a few hundred kilometres. In this study we use teleseismic and ambient noise derived surface wave tomography to produce a 3-D shear velocity model of the region. We use the 48 stations of the TUCAN array, and up to 96 events for the teleseismic Rayleigh wave inversion, and 20 months of continuous data for cross correlation to estimate Green's functions from ambient noise. In the shallow crust (0-15 km) we observe low shear velocities directly beneath the arc volcanos (< 3 km/s) with higher velocities in the back arc of Nicaragua. The anomalies are likely caused by heated crust, possibly intruded by magma. We observe > 40 km thick crust beneath the Costa Rican arc and the Nicaraguan Highlands, with thinned crust (~20 km) beneath the Nicaraguan Depression, with increasing crustal thickness in the back arc region. At mantle depths (55-120 km depth) we observe lower shear velocities (~2%) beneath the Nicaraguan arc and back arc relative to Costa Rica. This is well-correlated with a Vp/Vs anomaly beneath Nicaragua. The lower shear velocity beneath Nicaragua may indicate higher melt content in the mantle perhaps due to higher volatile flux from the slab. Finally, we observe a linear high velocity region at depths > 120 km parallel to the trench, which is consistent with the subducting slab.

Harmon, N.; Salas, M.; Rychert, C. A.; Fischer, K. M.; Abers, G. A.

2012-12-01

148

Colour and radial velocity variations in pulsating subluminous B stars  

E-print Network

, the associated variations in light are of low-amplitude. Consequently the non-radial pulsations that occur., 1997). These pulsations o#11;er tremendous prospects for studying the internal structure and evolution the identi#12;cation of oscillation frequencies. Other observables may also con- tribute to a greater

Jeffery, Simon

149

Radial Velocity and Line Profile Variations in the Dwarf Nova Em-Cygni  

Microsoft Academic Search

Spectroscopic observations of the dwarf nova EM Cygni are presented. From these spectra we measure the radial velocity variations of the accretion disk emission lines and the secondary star absorption lines. The emission line profiles show that the accretion disk was very unstable during the nights of observation, leading to uncertainties in the emission line radial velocities. In spite of

R. J. Stover; E. L. Robinson; R. E. Nather

1981-01-01

150

Low Velocity Waves Inside and Outside of Plants  

Microsoft Academic Search

I have been reporting organizing waves in plants for many years. In 1989 I reported wave travel between plants. The waves travel at near 25 m\\/s horizontally through air on earth. Recently I built my own transmitters and receivers and found that the waves will penetrate mountains. Monitoring plants suggest that there is constant communication between plants with the cacophony

Orvin Wagner

2010-01-01

151

The shear wave velocity of the upper mantle beneath the Bay of Bengal, Northeast Indian Ocean from interstation phase velocities of surface waves  

NASA Astrophysics Data System (ADS)

The Bay of Bengal evolved along the eastern margin of the Indian subcontinent about 130 Ma with the breakup of India from eastern Gondwanaland. Since then the Indian lithospheric Plate has moved northward, along with the Bay of Bengal, and eventually collided with the Eurasian Plate. The age of the lithosphere beneath the central Bay of Bengal is ˜110 Ma. We evaluate the shear wave velocity structure of the upper mantle beneath the central Bay through inversion of phase velocities of fundamental mode Rayleigh and Love waves along two wave paths: (i) between Port Blair (PBA) and VIS (Visakhapatnam) and (ii) between DGPR (Diglipur) and VIS. The seismological observatories PBA and DGPR are located on the Andaman Island and to the east of the Bay and the observatory at VIS in located on the eastern coast of India to the west of the Bay. Using broad-band records of earthquakes, which lie along the great circle arc joining each pair of observatories, we obtain phase velocities between 20 and 240 s periods for Rayleigh waves and between 23 and 170 s for Love waves. These phase velocities are inverted to find the S-wave velocity structure of the upper mantle down to 400 km. The crustal structure is based on previous studies of the Bay and kept fixed in the inversion. We obtain a radially anisotropic upper-mantle structure, where the SH-wave velocity (VSH) is greater than the SV-wave velocity (VSV) down to 400 km. The S-wave velocity decreases sharply by ˜4.5 per cent for VSV and ˜1.5 per cent for VSH at a depth 110 km, which is considered as the Lithosphere-Asthenosphere boundary (LAB), that is, the bottom of the mantle lid. Based on recent studies, such sharp fall of S-wave velocity below the mantle lid appears to indicate a partially molten thin layer (G-discontinuity) at this depth. The thickness of the mantle lid is intermediate between oceanic and continental regions. The lid is also characterized by low radial anisotropy, which decreases to near isotropy at the bottom of the lid. These two characteristics show a `continental-like' mantle lid beneath the Bay. Rapid northward motion of the Indian Plate before its collision with Eurasia might have caused the large radial anisotropy observed below the mantle lid.

Bhattacharya, S. N.; Mitra, Supriyo; Suresh, G.

2013-06-01

152

Variational Structure of Inverse Problems in Wave Propagation and Vibration  

E-print Network

Variational Structure of Inverse Problems in Wave Propagation and Vibration James G. Berryman in wave propagation (traveltime tomography) and two examples in vibration (the plucked string and free.'' For vibrating systems, the apparently very complex behavior of an excited string, drumhead, or the Earth can

153

Compressional and shear-wave velocity versus depth relations for common rock types in northern California  

USGS Publications Warehouse

This article presents new empirical compressional and shear-wave velocity (Vp and Vs) versus depth relationships for the most common rock types in northern California. Vp versus depth relations were developed from borehole, laboratory, seismic refraction and tomography, and density measurements, and were converted to Vs versus depth relations using new empirical relations between Vp and Vs. The relations proposed here account for increasing overburden pressure but not for variations in other factors that can influence velocity over short distance scales, such as lithology, consolidation, induration, porosity, and stratigraphic age. Standard deviations of the misfits predicted by these relations thus provide a measure of the importance of the variability in Vp and Vs caused by these other factors. Because gabbros, greenstones, basalts, and other mafic rocks have a different Vp and Vs relationship than sedimentary and granitic rocks, the differences in Vs between these rock types at depths below 6 or 7 km are generally small. The new relations were used to derive the 2005 U.S. Geological Survey seismic velocity model for northern California employed in the broadband strong motion simulations of the 1989 Loma Prieta and 1906 San Francisco earthquakes; initial tests of the model indicate that the Vp model generally compares favorably to regional seismic tomography models but that the Vp and Vs values proposed for the Franciscan Complex may be about 5% too high.

Brocher, T.M.

2008-01-01

154

Surface wave phase velocities from 2-D surface wave tomography studies in the Anatolian plate  

NASA Astrophysics Data System (ADS)

We study the Rayleigh and Love surface wave fundamental mode propagation beneath the Anatolian plate. To examine the inter-station phase velocities a two-station method is used along with the Multiple Filter Technique (MFT) in the Computer Programs in Seismology (Herrmann and Ammon, 2004). The near-station waveform is deconvolved from the far-station waveform removing the propagation effects between the source and the station. This method requires that the near and far stations are aligned with the epicentre on a great circle path. The azimuthal difference of the earthquake to the two-stations and the azimuthal difference between the earthquake and the station are restricted to be smaller than 5o. We selected 3378 teleseismic events (Mw >= 5.7) recorded by 394 broadband local stations with high signal-to-noise ratio within the years 1999-2013. Corrected for the instrument response suitable seismogram pairs are analyzed with the two-station method yielding a collection of phase velocity curves in various period ranges (mainly in the range 25-185 sec). Diffraction from lateral heterogeneities, multipathing, interference of Rayleigh and Love waves can alter the dispersion measurements. In order to obtain quality measurements, we select only smooth portions of the phase velocity curves, remove outliers and average over many measurements. We discard these average phase velocity curves suspected of suffering from phase wrapping errors by comparing them with a reference Earth model (IASP91 by Kennett and Engdahl, 1991). The outlined analysis procedure yields 3035 Rayleigh and 1637 Love individual phase velocity curves. To obtain Rayleigh and Love wave travel times for a given region we performed 2-D tomographic inversion for which the Fast Marching Surface Tomography (FMST) code developed by N. Rawlinson at the Australian National University was utilized. This software package is based on the multistage fast marching method by Rawlinson and Sambridge (2004a, 2004b). The azimuthal coverage of the respective two-station paths is proper to analyze the observed dispersion curves in terms of both azimuthal and radial anisotropy beneath the study region. This research is supported by Joint Research Project of the Scientific and Research Council of Turkey (TUB?TAK- Grant number 111Y190) and the Russian Federation for Basic Research (RFBR).

Arif Kutlu, Yusuf; Erduran, Murat; Çak?r, Özcan; Vinnik, Lev; Kosarev, Grigoriy; Oreshin, Sergey

2014-05-01

155

Quarter wave field line resonances: variation with latitude  

Microsoft Academic Search

When for a particular field line there is a strong asymmetry in conductivity at conjugate ionospheres, quarter wavelength mode eigenoscillations may be sustained instead of the more usual half-wave oscillations. We have studied the latitudinal distribution of such quarter-wave mode standing Alfvén waves. The diurnal variation of the local field line eigenfrequency was examined for L=1.7-5.1 using cross-phase analysis of

Frederick Menk; Yuki Obana; Colin Waters; Murray Sciffer; Akimasa Yoshikawa; Ichiro Yoshikawa; Mark Moldwin; Ian Mann; David Boteler

2010-01-01

156

Imaging subtle microstructural variations in ceramics with precision ultrasonic velocity and attenuation measurements  

NASA Technical Reports Server (NTRS)

Acoustic images of a silicon carbide ceramic disk were obtained using a precision scanning contact pulse echo technique. Phase and cross-correlation velocity, and attenuation maps were used to form color images of microstructural variations. These acoustic images reveal microstructural variations not observable with X-ray radiography.

Generazio, Edward R.; Roth, Don J.; Baaklini, George Y.

1987-01-01

157

Shear Wave Velocities in the Pampean Flat Slab Region from Rayleigh Wave Tomography: Implications for Crustal Composition and Upper Mantle Hydration  

NASA Astrophysics Data System (ADS)

The Pampean flat slab region, located in Chile and western Argentina between 29° and 34° S, is characterized by the subducting Nazca plate assuming a sub-horizontal geometry for ~300 km laterally before resuming a more "normal" angle of subduction. The onset of flat slab subduction is associated with the cessation of regional arc related volcanism and the migration of deformation inboard from the high Andes into the thin-skinned Precordillera and thick-skinned Sierras Pampeanas. Developing a better understanding of this region's geology is of particular importance, as it is an ideal area to study flat slab subduction and serves as a modern analogue to Laramide flat slab subduction in the western US. To study the crustal and mantle structure in the region, we combine ambient noise tomography and ballistic surface wave tomography to produce a regional 3D shear wave velocity model that encompasses flat slab subduction in the north and normal subduction geometry in the south, allowing for a comparison of the two. Results from this work show that shear velocities within the upper crust are largely determined by composition, with sedimentary basins and areas with active volcanism exhibiting slower velocities than basement cored uplifts and other bedrock exposures. Though surface waves are not particularly sensitive to the depth of sharp velocity contrasts, we observe an eastward increase in shear velocity at depth that correlates with an eastward decrease in crustal thickness. In both the slab and overlying mantle, we observe significant variations in shear wave velocity. North of 32° S, where flat slab subduction is occurring, the Nazca plate contains low-velocity zones (LVZs) beneath the high Andes and Precordillera that are not present in the east beneath the Sierras Pampeanas. An opposite transition is observed in the overlying mantle, which changes from fast in the west to slow in the east. Both of these observations are consistent with an initially hydrated slab dehydrating and releasing water into the overlying mantle. Within this region we also observe a LVZ immediately above the slab as the subduction angle steepens. This zone potentially represents asthenosphere or hydrated lithospheric mantle. South of 32° S, where subduction is occurring at a more normal angle, the slab is visible as a high-velocity body with a low-velocity mantle wedge present beneath the arc and back arc. The variations in slab and upper mantle shear velocities are consistent with a hydrated flat slab and the presence of a LVZ above the flat slab as it steepens suggests that water is being transported to a significant depth or that an asthenospheric wedge is present between the slab and cratonic lithosphere.

Porter, R. C.; Gilbert, H. J.; Zandt, G.; Beck, S. L.; Warren, L. M.; Calkins, J. A.; Alvarado, P. M.; Anderson, M. L.

2011-12-01

158

Validation of recent shear wave velocity models in the United States with full-wave simulation  

NASA Astrophysics Data System (ADS)

Interpretations of dynamic processes and the thermal and chemical structure of the Earth depend on the accuracy of Earth models. With the growing number of velocity models constructed with different tomographic methods and seismic data sets, there is an increasing need for a systematic way to validate model accuracy and resolution. This study selects five shear wave velocity models in the U.S. and simulates full-wave propagation within the 3-D structures. Surface-wave signals extracted from ambient seismic noise and regional earthquakes are compared with synthetic waveforms at multiple-frequency bands. Phase delays and cross-correlation coefficients between observed and synthetic waveforms allow us to compare and validate these models quantitatively. In general, measurements from regional earthquakes are consistent with ambient noise results, but appear more scattered, which may result from uncertainty of the earthquake source location, origin time, and moment tensor. Our results show the improvement of model prediction with the increase of seismic data sets and implement of advanced methods. There exists a positive linear trend between phase delay and interstation distance for three models, indicating that on average, these models are faster than the real Earth structure. The phase delays from the jointly inverted model of ambient noise and receiver function have negative means at all periods while without obvious dependence on the interstation distance. The full-wave ambient noise tomographic model predicts more accurate phase arrivals compared to other models. This study suggests a need for an integrated model constructed with multiple seismic waveforms and consideration of anisotropy and attenuation.

Gao, Haiying; Shen, Yang

2015-01-01

159

Prediction of long-term settlement on soft clay using shear wave velocity and damping characteristics  

Microsoft Academic Search

This paper presents a method of calculating long-term settlement of a loaded pad on soft clay at Klang, Selangor, Malaysia where the soil model is treated as an anelastic material of viscoelastic property. Initially, an elastic shear modulus (G) value from shear wave velocity profiles of the seismic tests from spectral analysis of surface wave (SASW) and continuous surface wave

Mohamad Nor Omar; Colin Peter Abbiss; Mohd. Raihan Taha; Khairul Anuar Mohd. Nayan

2011-01-01

160

The velocities and accelerations accompanying ocean waves can impose large hydrodynamic forces on marine organisms,  

E-print Network

The velocities and accelerations accompanying ocean waves can impose large hydrodynamic forces no guarantee, however, that these plants will not be broken. Indeed, wave forces imposed during storms tear (Koehl and Wainwright, 1977; Seymour et al. 1989). Despite the recognition of wave-induced hydrodynamic

Denny, Mark

161

Shear wave velocities in the Pampean flat-slab region from Rayleigh wave tomography: Implications for slab and upper mantle hydration  

NASA Astrophysics Data System (ADS)

The Pampean flat-slab region, located in central Argentina and Chile between 29° and 34°S, is considered a modern analog for Laramide flat-slab subduction within western North America. Regionally, flat-slab subduction is characterized by the Nazca slab descending to ˜100 km depth, flattening out for ˜300 km laterally before resuming a more "normal" angle of subduction. Flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of deformation and the cessation of volcanism within the region. To better understand flat-slab subduction we combine ambient-noise tomography and earthquake-generated surface wave measurements to calculate a regional 3D shear velocity model for the region. Shear wave velocity variations largely relate to changes in lithology within the crust, with basins and bedrock exposures clearly defined as low- and high-velocity regions, respectively. We argue that subduction-related hydration plays a significant role in controlling shear wave velocities within the upper mantle. In the southern part of the study area, where normal-angle subduction is occurring, the slab is visible as a high-velocity body with a low-velocity mantle wedge above it, extending eastward from the active arc. Where flat-slab subduction is occurring, slab velocities increase to the east while velocities in the overlying lithosphere decrease, consistent with the slab dewatering and gradually hydrating the overlying mantle. The hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere, helping to drive flat-slab subduction.

Porter, Ryan; Gilbert, Hersh; Zandt, George; Beck, Susan; Warren, Linda; Calkins, Josh; Alvarado, Patricia; Anderson, Megan

2012-11-01

162

LITHOSTRATIGRAPHY AND SHEAR-WAVE VELOCITY IN THE CRYSTALLIZED TOPOPAH SPRING TUFF, YUCCA MOUNTAIN, NEVADA  

SciTech Connect

Evaluation of the seismic response of the proposed spent nuclear fuel and high-level radioactive waste repository at Yucca Mountain, Nevada, is in part based on the seismic properties of the host rock, the 12.8-million-year-old Topopah Spring Tuff. Because of the processes that formed the tuff, the densely welded and crystallized part has three lithophysal and three nonlithophysal zones, and each zone has characteristic variations in lithostratigraphic features and structures of the rocks. Lithostratigraphic features include lithophysal cavities, rims on lithophysae and some fractures, spots (which are similar to rims but without an associated cavity or aperture), amounts of porosity resulting from welding, crystallization, and vapor-phase corrosion and mineralization, and fractures. Seismic properties, including shear-wave velocity (V{sub s}), have been measured on 38 pieces of core, and there is a good ''first order'' correlation with the lithostratigraphic zones; for example, samples from nonlithophysal zones have larger V{sub s} values compared to samples from lithophysal zones. Some samples have V{sub s} values that are beyond the typical range for the lithostratigraphic zone; however, these samples typically have one or more fractures, ''large'' lithophysal cavities, or ''missing pieces'' relative to the sample size. Shear-wave velocity data measured in the tunnels have similar relations to lithophysal and nonlithophysal rocks; however, tunnel-based values are typically smaller than those measured in core resulting from increased lithophysae and fracturing effects. Variations in seismic properties such as V{sub s} data from small-scale samples (typical and ''flawed'' core) to larger scale traverses in the tunnels provide a basis for merging our understanding of the distributions of lithostratigraphic features (and zones) with a method to scale seismic properties.

D. BUESCH; K.H. STOKOE; M. SCHUHEN

2006-03-20

163

New statistical analysis of the horizontal phase velocity distribution of gravity waves observed by airglow imaging  

NASA Astrophysics Data System (ADS)

We have developed a new analysis method for obtaining the power spectrum in the horizontal phase velocity domain from airglow intensity image data to study atmospheric gravity waves. This method can deal with extensive amounts of imaging data obtained on different years and at various observation sites without bias caused by different event extraction criteria for the person processing the data. The new method was applied to sodium airglow data obtained in 2011 at Syowa Station (69°S, 40°E), Antarctica. The results were compared with those obtained from a conventional event analysis in which the phase fronts were traced manually in order to estimate horizontal characteristics, such as wavelengths, phase velocities, and wave periods. The horizontal phase velocity of each wave event in the airglow images corresponded closely to a peak in the spectrum. The statistical results of spectral analysis showed an eastward offset of the horizontal phase velocity distribution. This could be interpreted as the existence of wave sources around the stratospheric eastward jet. Similar zonal anisotropy was also seen in the horizontal phase velocity distribution of the gravity waves by the event analysis. Both methods produce similar statistical results about directionality of atmospheric gravity waves. Galactic contamination of the spectrum was examined by calculating the apparent velocity of the stars and found to be limited for phase speeds lower than 30 m/s. In conclusion, our new method is suitable for deriving the horizontal phase velocity characteristics of atmospheric gravity waves from an extensive amount of imaging data.

Matsuda, Takashi S.; Nakamura, Takuji; Ejiri, Mitsumu K.; Tsutsumi, Masaki; Shiokawa, Kazuo

2014-08-01

164

Corrosion and erosion monitoring in plates and pipes using constant group velocity Lamb wave inspection.  

PubMed

Recent improvements in tomographic reconstruction techniques generated a renewed interest in short-range ultrasonic guided wave inspection for real-time monitoring of internal corrosion and erosion in pipes and other plate-like structures. Emerging evidence suggests that in most cases the fundamental asymmetric A0 mode holds a distinct advantage over the earlier market leader fundamental symmetric S0 mode. Most existing A0 mode inspections operate at relatively low inspection frequencies where the mode is highly dispersive therefore very sensitive to variations in wall thickness. This paper examines the potential advantages of increasing the inspection frequency to the so-called constant group velocity (CGV) point where the group velocity remains essentially constant over a wide range of wall thickness variation, but the phase velocity is still dispersive enough to allow accurate wall thickness assessment from phase angle measurements. This paper shows that in the CGV region the crucial issue of temperature correction becomes especially simple, which is particularly beneficial when higher-order helical modes are also exploited for tomography. One disadvantage of working at such relatively high inspection frequency is that, as the slower A0 mode becomes faster and less dispersive, the competing faster S0 mode becomes slower and more dispersive. At higher inspection frequencies these modes cannot be separated any longer based on their vibration polarization only, which is mostly tangential for the S0 mode while mostly normal for the A0 at low frequencies, as the two modes become more similar as the frequency increases. Therefore, we propose a novel method for suppressing the unwanted S0 mode based on the Poisson effect of the material by optimizing the angle of inclination of the equivalent transduction force of the Electromagnetic Acoustic Transducers (EMATs) used for generation and detection purposes. PMID:24582555

Nagy, Peter B; Simonetti, Francesco; Instanes, Geir

2014-09-01

165

The influence of wafer dimensions on the contact wave velocity in silicon wafer bonding  

NASA Astrophysics Data System (ADS)

The contact wave velocity in silicon wafer bonding is experimentally found to decrease with wafer thickness and to be only weakly dependent on wafer diameter. Wafers of different thicknesses ranging from 270 to 5000 ?m, were dipped in HF:H2O before bonding to give the surfaces hydrophobic properties. A model based on energy conservation can explain the main characteristics of the experimental results. The contact wave velocity is determined by the amount of energy available as kinetic energy for the entrapped gas in the gap between the wafers. By increasing wafer thickness, the elastic energy stored in the material is increased, and the contact wave velocity is decreased.

Bengtsson, Stefan; Ljungberg, Karin; Vedde, Jan

1996-11-01

166

Cherenkov-like shock waves associated with surpassing the light velocity barrier  

E-print Network

The effects arising from accelerated and decelerated motion of a point charge inside a medium are studied. The motion is manifestly relativistic and may be produced by a constant uniform electric field. It is shown that in addition to the bremsstrahlung and Cherenkov shock waves, the electromagnetic shock wave arises when the charge particle velocity coincides with the light velocity in the medium. For the accelerated motion this shock wave forming an indivisible entity with the Cherenkov shock wave arrives after the arrival of the bremsstrahlung shock wave. For the decelerated motion the above shock wave detaches from the charge at the moment when its velocity coincides with the light velocity in the medium. This wave existing even after termination of the charge motion of the charge propagates with the light velocity in the medium. It has the same singularity as the Cherenkov shock and is more singular than the bremsstrahlung shock wave. The space-time regions, where these shock waves exist, and conditions under which they can be observed are determined.

G. N. Afanasiev; V. G. Kartavenko

1999-10-06

167

Influence of small temperature variations on the ultrasonic velocity in concrete  

NASA Astrophysics Data System (ADS)

Ultrasonic methods are valuable tools for quality assessment and structural imaging of concrete. In such applications, accurate and reliable determination of ultrasonic wave velocities is essential, as they are affected by various experimental and environmental factors. In this research coda wave interferometry (CWI) was used to determine the influence of temperature (0 to 50 °C) on ultrasonic wave velocity in concrete samples. A resolution of better than 10-4 was achieved in the measurement of relative velocity changes. Reversibility and repeatability as well as the influence of moisture were taken into account. In addition the influence of sensor (transmitter and receiver) positioning errors on the results of the interferometric algorithm was evaluated as well as the benefit of newly developed, permanently embedded sensors. The presented results can be used to evaluate and refine data from ultrasonic monitoring systems.

Niederleithinger, E.; Wunderlich, C.

2013-01-01

168

Spatial velocity distributions in pulse-wave propagation based on fluid-structure interaction.  

PubMed

In this paper, spatial velocity distributions in pulse-wave propagation based on a fluid-structure interaction model are presented. The investigation is performed using the assumption of laminar flow and a linear-elastic wall. The fluid-structure interaction scheme is constructed using the finite element method. The results show that velocity distributions embody an obvious time delay in an elastic tube model. Further, the fully developed flow is delayed and the velocity values are increased in comparison with a rigid tube model. The increase in the wall thickness makes the time delay between the velocity peaks of different sites smaller while the time delay between the velocity minima is unchanged. Similarly, the time delay between the velocity bottoms is more easily found when decreasing the internal radius. The model gives valid results for spatial velocity distributions, which provide important information for wave propagation. PMID:24935118

He, Fan; Hua, Lu; Gao, Li-Jian

2014-09-01

169

Longitudinal Variation and Waves in Jupiter's South Equatorial Wind Jet  

NASA Technical Reports Server (NTRS)

We have conducted a detailed study of the cloud features in the strong southern equatorial wind jet near 7.5 S planetographic latitude. To understand the apparent variations in average zonal wind jet velocity at this latitude [e.g.. 1,2,3], we have searched for variations iIi both feature latitude and velocity with longitude and time. In particular, we focused on the repetitive chevron-shaped dark spots visible on most dates and the more transient large anticyclonic system known as the South Equatorial Disturbance (SED). These small dark spots are interpreted as cloud holes, and are often used as material tracers of the wind field.

Simon-Miller, A. A.; Rogers, John H.; Gierasch, Peter J.; Choi, David; Allison, Michael; Adamoli, Gianluigi; Mettig, Hans-Joerg

2012-01-01

170

Anomalous translational velocity of vortex ring with finite-amplitude Kelvin waves  

SciTech Connect

We consider finite-amplitude Kelvin waves on an inviscid vortex assuming that the vortex core has infinitesimal thickness. By numerically solving the governing Biot-Savart equation of motion, we study how the frequency of the Kelvin waves and the velocity of the perturbed ring depend on the Kelvin wave amplitude. In particular, we show that, if the amplitude of the Kelvin waves is sufficiently large, the perturbed vortex ring moves backwards.

Barenghi, C. F. [School of Mathematics, University of Newcastle, Newcastle NE1 7RU (United Kingdom); Haenninen, R.; Tsubota, M. [Department of Physics, Osaka City University, Sugimoto 3-3-138, 558-8585 Osaka (Japan)

2006-10-15

171

Spatial variations in Achilles tendon shear wave speed.  

PubMed

Supersonic shear imaging (SSI) is an ultrasound imaging modality that can provide insight into tissue mechanics by measuring shear wave propagation speed, a property that depends on tissue elasticity. SSI has previously been used to characterize the increase in Achilles tendon shear wave speed that occurs with loading, an effect attributable to the strain-stiffening behavior of the tissue. However, little is known about how shear wave speed varies spatially, which is important, given the anatomical variation that occurs between the calcaneus insertion and the gastrocnemius musculotendon junction. The purpose of this study was to investigate spatial variations in shear wave speed along medial and lateral paths of the Achilles tendon for three different ankle postures: resting ankle angle (R, i.e. neutral), plantarflexed (P; R - 15°), and dorsiflexed (D; R+15°). We observed significant spatial and posture variations in tendon shear wave speed in ten healthy young adults. Shear wave speeds in the Achilles free tendon averaged 12 ± 1.2m/s in a resting position, but decreased to 7.2 ± 1.8m/s with passive plantarflexion. Distal tendon shear wave speeds often reached the maximum tracking limit (16.3m/s) of the system when the ankle was in the passively dorsiflexed posture (+15° from R). At a fixed posture, shear wave speeds decreased significantly from the free tendon to the gastrocnemius musculotendon junction, with slightly higher speeds measured on the medial side than on the lateral side. Shear wave speeds were only weakly correlated with the thickness and depth of the tendon, suggesting that the distal-to-proximal variations may reflect greater compliance in the aponeurosis relative to the free tendon. The results highlight the importance of considering both limb posture and transducer positioning when using SSI for biomechanical and clinical assessments of the Achilles tendon. PMID:24933528

DeWall, Ryan J; Slane, Laura C; Lee, Kenneth S; Thelen, Darryl G

2014-08-22

172

Observation of wave packet distortion during a negative-group-velocity transmission.  

PubMed

In Physics, causality is a fundamental postulation arising from the second law of thermodynamics. It states that, the cause of an event precedes its effect. In the context of Electromagnetics, the relativistic causality limits the upper bound of the velocity of information, which is carried by electromagnetic wave packets, to the speed of light in free space (c). In anomalously dispersive media (ADM), it has been shown that, wave packets appear to propagate with a superluminal or even negative group velocity. However, Sommerfeld and Brillouin pointed out that the "front" of such wave packets, known as the initial point of the Sommerfeld precursor, always travels at c. In this work, we investigate the negative-group-velocity transmission of half-sine wave packets. We experimentally observe the wave front and the distortion of modulated wave packets propagating with a negative group velocity in a passive artificial ADM in microwave regime. Different from previous literature on the propagation of superluminal Gaussian packets, strongly distorted sinusoidal packets with non-superluminal wave fronts were observed. This result agrees with Brillouin's assertion, i.e., the severe distortion of seemingly superluminal wave packets makes the definition of group velocity physically meaningless in the anomalously dispersive region. PMID:25631746

Ye, Dexin; Salamin, Yannick; Huangfu, Jiangtao; Qiao, Shan; Zheng, Guoan; Ran, Lixin

2015-01-01

173

Observation of Wave Packet Distortion during a Negative-Group-Velocity Transmission  

NASA Astrophysics Data System (ADS)

In Physics, causality is a fundamental postulation arising from the second law of thermodynamics. It states that, the cause of an event precedes its effect. In the context of Electromagnetics, the relativistic causality limits the upper bound of the velocity of information, which is carried by electromagnetic wave packets, to the speed of light in free space (c). In anomalously dispersive media (ADM), it has been shown that, wave packets appear to propagate with a superluminal or even negative group velocity. However, Sommerfeld and Brillouin pointed out that the ``front'' of such wave packets, known as the initial point of the Sommerfeld precursor, always travels at c. In this work, we investigate the negative-group-velocity transmission of half-sine wave packets. We experimentally observe the wave front and the distortion of modulated wave packets propagating with a negative group velocity in a passive artificial ADM in microwave regime. Different from previous literature on the propagation of superluminal Gaussian packets, strongly distorted sinusoidal packets with non-superluminal wave fronts were observed. This result agrees with Brillouin's assertion, i.e., the severe distortion of seemingly superluminal wave packets makes the definition of group velocity physically meaningless in the anomalously dispersive region.

Ye, Dexin; Salamin, Yannick; Huangfu, Jiangtao; Qiao, Shan; Zheng, Guoan; Ran, Lixin

2015-01-01

174

Measurements of plasma temperature in indirect drive targets from the shock wave velocity in aluminum in the Iskra-5 facility  

SciTech Connect

Results are presented from the development of a method for measuring plasma temperature in indirect (X-ray) drive targets by recording the shock wave velocity in the Iskra-5 facility. The samples under investigation were irradiated by X-rays in a converter box, and the shock wave velocity was determined from the time at which the wave reached the back surface of the sample and the surface began to emit visible radiation. This emission, in turn, was detected by a streak camera. The results of experiments on the interaction of X radiation with a hot dense plasma, as well as the accompanying gas-dynamic processes in aluminum samples, are analyzed both theoretically and numerically. In experiments with Al and Pb samples, the shock wave velocity was measured to vary in the range U = 8-35 km/s, and the range of variation of the temperature of the box walls was measured to be T{sub e} = 140-170 eV.

Vatulin, V. V.; Zhidkov, N. V.; Kravchenko, A. G.; Kuznetsov, P. G.; Litvin, D. N.; Mis'ko, V. V.; Pinegin, A. V.; Pleteneva, N. P.; Senik, A. V.; Starodubtsev, K. V.; Tachaev, G. V. [Russian Federal Nuclear Center, All-Russia Research Institute of Experimental Physics (Russian Federation)

2010-05-15

175

A comprehensive dispersion model of surface wave phase and group velocity for the globe  

NASA Astrophysics Data System (ADS)

A new method is developed to measure Rayleigh- and Love-wave phase velocities globally using a cluster analysis technique. This method clusters similar waveforms recorded at different stations from a single event and allows users to make measurements on hundreds of waveforms, which are filtered at a series of frequency ranges, at the same time. It also requires minimal amount of user interaction and allows easy assessment of the data quality. This method produces a large amount of phase delay measurements in a manageable time frame. Because there is a strong trade-off between the isotropic part of the Rayleigh-wave phase velocity and azimuthal anisotropy, we include the effect of azimuthal anisotropy in our inversions in order to obtain reliable isotropic phase velocity. We use b-splines to combine these isotropic phase velocity maps with our previous group velocity maps to produce an internally consistent global surface wave dispersion model.

Ma, Zhitu; Masters, Guy; Laske, Gabi; Pasyanos, Michael

2014-10-01

176

The microscopic state of the solar wind: Links between composition, velocity distributions, waves and turbulence  

NASA Technical Reports Server (NTRS)

An overview is given of the microscopic state of the solar wind with emphasis on recent Ulysses high-latitude observations and previous Helios in-ecliptic observations. The possible links between composition, ionization state. velocity distribution functions of electrons, protons and heavy ions. kinetic plasma waves and MHD-scale turbulence are elaborated. Emphasis is placed on a connection of interplanetary kinetic-scale phenomena with their generating microscopic processes in the corona. The fast streams seem to consist of mesoscale pressure-balanced plasma filaments and magnetic flux tubes, reminiscent of the supergranular-size structures building the open corona, from which copious Alfven waves emanate. The wind from the magnetically structured and active corona shows developed compressive turbulence and considerable abundance and ionization state variations. Some modelling attempts to explain the observed element fractionation are briefly reviewed. The causes of the nonthermal particle features. such as proton-ion differential streaming, ion beams. temperature anisotropies, and skewed distributions associated with collisionless heat conduction, are ultimately to be searched in the fact, that the corona is never quiescent but fundamentally variable in space and time. Consequently, the radial evolution of the internal state of the wind resembles at all latitudes a complicated relaxation process, in the course of which the free (in comparison with LTE conditions) particle kinetic energy is converted into plasma waves and MHD turbulence on a wide range of scales. This leads to intermittent wave-particle interactions and unsteady anomalous transport, mixed with the weak effects of the rare Coulomb collisions. Spherical expansion and large-scale inhomogeneity forces the wind to attain microscopically a complex internal state of dynamic statistical equilibrium.

Marsch, E.

1995-01-01

177

Temporal changes of surface wave velocity associated with major Sumatra earthquakes from ambient noise correlation.  

PubMed

Detecting temporal changes of the medium associated with major earthquakes has implications for understanding earthquake genesis. Here we report temporal changes of surface wave velocity over a large area associated with 3 major Sumatra earthquakes in 2004, 2005, and 2007. We use ambient noise correlation to retrieve empirical Green's function (EGF) of surface waves between stations. Because the process is completely repeatable, the technique is powerful in detecting possible temporal change of medium. We find that 1 excellent station pair (PSI in Indonesia and CHTO in Thailand) shows significant time shifts (up to 1.44 s) after the 2004 and 2005 events in the Rayleigh waves at 10-20 s but not in the Love waves, suggesting that the Rayleigh time shifts are not from clock error. The time shifts are frequency dependent with the largest shifts at the period band of 11-16 s. We also observe an unusual excursion approximately 1 month before the 2004 event. We obtain a total of 17 pairs for June, 2007 to June, 2008, which allow us to examine the temporal and spatial variation of the time shifts. We observed strong anomalies (up to 0.68 s) near the epicenter after the 2007 event, but not in the region further away from the source or before the event or 3 months after the event. The observations are interpreted as stress changes and subsequent relaxation in upper-mid crust in the immediate vicinity of the rupture and the broad area near the fault zone. PMID:19667205

Xu, Zhen J; Song, Xiaodong

2009-08-25

178

Downdip velocity changes in subducted oceanic crust beneath Northern Japan—insights from guided waves  

NASA Astrophysics Data System (ADS)

Dispersed P-wave arrivals observed in the subduction zone forearc of Northern Japan suggest that low velocity subducted oceanic crustal waveguide persists to depths of at least 220 km. First arrivals from events at 150-220 km depth show that the velocity contrast of the waveguide reduces with depth. High frequency energy (>2 Hz) is retained and delayed by the low velocity crustal waveguide while the lower frequency energy (<0.5 Hz) travels at faster velocities of the surrounding mantle material. The guided wave energy then decouples from the low velocity crustal waveguide due to the bend of the slab and is seen at the surface 1-2 s after the low frequency arrival. Dispersive P-wave arrivals from WBZ earthquakes at 150-220 km depth are directly compared to synthetic waveforms produced by 2-D and 3-D full waveform finite difference simulations. By comparing both the spectrogram and the velocity spectra of the observed and synthetic waveforms we are able to fully constrain the dispersive waveform, and so directly compare the observed and synthetic waveforms. Using this full waveform modelling approach we are able to tightly constrain the velocity structures that cause the observed guided wave dispersion. Resolution tests using 2-D elastic waveform simulations show that the dispersion can be accounted for by a 6-8 km thick low velocity oceanic crust, with a velocity contrast that varies with depth. The velocities inferred for this variable low velocity oceanic crust can be explained by lawsonite bearing assemblages, and suggest that low velocity minerals may persist to greater depth than previously thought. 2-D simulations are benchmarked to 3-D full waveform simulations and show that the structures inferred by the 2-D approximation produce similar dispersion in 3-D. 2-D viscoelastic simulations show that including elevated attenuation in the mantle wedge can improve the fit of the dispersed waveform. Elevated attenuation in the low velocity layers can however be ruled out.

Garth, Tom; Rietbrock, Andreas

2014-09-01

179

Accurate seismic phase-velocities from interfering surface-waves using homomorphic deconvolution  

E-print Network

ACCURATE SEISMIC PHASE-VELOCITIES FROM INTERFERING SURFACE-WAVES USING HOMOMORPHIC DECONVOLUTION A Thesis by ROBERT BRUCE SZERBIAK Submitted to the Graduate College of Texas A6B University in partial fulfillment of the requirement... for the degree of MASTER OF SCIENCE December 1981 Major Subject: Geophysics ACCURATE SEISMIC PHASE-VELOCITIES FROM INTERFERING SURFACE-WAVES USING HOMOMORPHIC DECONVOLUTION A Thesis by ROBERT BRUCE SZERBIAK Approved as to style and content by: i...

Szerbiak, Robert Bruce

1981-01-01

180

S-Wave Velocities of the Lithosphere–Asthenosphere System in the Caribbean Region  

Microsoft Academic Search

An overview of the S-wave velocity (V\\u000a s) structural model of the Caribbean with a resolution of 2° × 2° is presented. New tomographic maps of Rayleigh wave group\\u000a velocity dispersion at periods ranging from 10 to 40 s were obtained as a result of the frequency time analysis of seismic\\u000a signals of more than 400 ray-paths in the region. For each cell

O’Leary F. González; José Leonardo Alvarez; Bladimir Moreno; Giuliano F. Panza

181

Aortic pulse wave velocity index and mortality in end-stage renal disease  

Microsoft Academic Search

Aortic pulse wave velocity index and mortality in end-stage renal disease.BackgroundAortic pulse wave velocity (PWV) is a strong independent predictor of overall and cardiovascular mortality in patients with end-stage renal disease (ESRD). Nevertheless, because age, blood pressure, heart rate, and gender are strong determinants of both arterial stiffness and mortality, the individual relevance of PWV measurements remains controversial.MethodsA cohort of

Jacques Blacher; Michel E Safar; Alain P Guerin; Bruno Pannier; Sylvain J Marchais; Gérard M London

2003-01-01

182

Seismic piezocone interpretation for shear wave velocity ( V S ) determination in the Persian Gulf  

Microsoft Academic Search

Shear wave velocity is one of the important factors representing the dynamic characteristics of soil layers. Hence, many researchers\\u000a have focused their studies on determining shear wave velocity by direct field measurements or expressions developed by other\\u000a soil parameters. The shear module and damping ratio of the soil layers also play a similar role in the majority of dynamic\\u000a soil

Aliakbar Hajimohammadi; Seyed Majdeddin Mir Mohammad Hosseini; Akbar Cheshomi

2010-01-01

183

Spurious velocity changes caused by temporal variations in ambient noise frequency content  

NASA Astrophysics Data System (ADS)

Ambient seismic noise cross-correlations are now being used to detect temporal variations of seismic velocity, which are typically on the order of 0.1 per cent. At this small level, temporal variations in the properties of noise sources can cause apparent velocity changes. For example, the spatial distribution and frequency content of ambient noise have seasonal variations due to the seasonal hemispherical shift of storms. Here, we show that if the stretching method is used to measure time-shifts, then the temporal variability of noise frequency content causes apparent velocity changes due to the changes in both amplitude and phase spectra caused by waveform stretching. With realistic seasonal variations of frequency content in the Los Angeles Basin, our numerical tests produce about 0.05 per cent apparent velocity change, comparable to what Meier et al. observed in the Los Angeles Basin. We find that the apparent velocity change from waveform stretching depends on time windows and station-pair distances, and hence it is important to test a range of these parameters to diagnose the stretching bias. Better understanding of spatiotemporal noise source properties is critical for more accurate and reliable passive monitoring.

Zhan, Zhongwen; Tsai, Victor C.; Clayton, Robert W.

2013-09-01

184

Analysis of group-velocity dispersion of high-frequency Rayleigh waves for near-surface applications  

USGS Publications Warehouse

The Multichannel Analysis of Surface Waves (MASW) method is an efficient tool to obtain the vertical shear (S)-wave velocity profile using the dispersive characteristic of Rayleigh waves. Most MASW researchers mainly apply Rayleigh-wave phase-velocity dispersion for S-wave velocity estimation with a few exceptions applying Rayleigh-wave group-velocity dispersion. Herein, we first compare sensitivities of fundamental surface-wave phase velocities with group velocities with three four-layer models including a low-velocity layer or a high-velocity layer. Then synthetic data are simulated by a finite difference method. Images of group-velocity dispersive energy of the synthetic data are generated using the Multiple Filter Analysis (MFA) method. Finally we invert a high-frequency surface-wave group-velocity dispersion curve of a real-world example. Results demonstrate that (1) the sensitivities of group velocities are higher than those of phase velocities and usable frequency ranges are wider than that of phase velocities, which is very helpful in improving inversion stability because for a stable inversion system, small changes in phase velocities do not result in a large fluctuation in inverted S-wave velocities; (2) group-velocity dispersive energy can be measured using single-trace data if Rayleigh-wave fundamental-mode energy is dominant, which suggests that the number of shots required in data acquisition can be dramatically reduced and the horizontal resolution can be greatly improved using analysis of group-velocity dispersion; and (3) the suspension logging results of the real-world example demonstrate that inversion of group velocities generated by the MFA method can successfully estimate near-surface S-wave velocities. ?? 2011 Elsevier B.V.

Luo, Y.; Xia, J.; Xu, Y.; Zeng, C.

2011-01-01

185

The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution  

NASA Astrophysics Data System (ADS)

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ˜150-200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

O'Donnell, J. P.; Adams, A.; Nyblade, A. A.; Mulibo, G. D.; Tugume, F.

2013-08-01

186

Surface waves in an incompressible fluid - Resonant instability due to velocity shear  

NASA Technical Reports Server (NTRS)

The effects of velocity shear on the resonance absorption of incompressible MHD surface waves are studied. It is found that there are generally values of the velocity shear for which the surface wave decay rate becomes zero. In some cases, the resonance absorption goes to zero even for very small velocity shears. It is also found that the resonance absorption can be strongly enhanced at other values of the velocity shear, so the presence of flows may be generally important for determining the effects of resonance absorption, such as might occur in the interaction of p-modes with sunspots. Resonances leading to instability of the global surface mode can exist, and instability can occur for velocity shears significantly below the Kelvin-Helmholtz threshold. These instabilities may play a role in the development or turbulence in regions of strong velocity shear in the solar wind or the earth's magnetosphere.

Hollweg, Joseph V.; Yang, G.; Cadez, V. M.; Gakovic, B.

1990-01-01

187

Phase and group velocities for Lamb waves in DOP-26 iridium alloy sheet  

SciTech Connect

The relatively coarse grain structure of iridium weldments limits the ultrasonic inspection of these structures to frequencies in the low megahertz range. As the material thickness is nominally 0.635 mm for clad vent set capsules, the low frequencies involved necessarily entail the generation of Lamb waves m the specimen. These waves are, of course, dispersive and detailed knowledge of both the phase and group velocities is required in order to determine accurately the location of flaws detected using Lamb waves. Purpose of this study is to elucidate the behavior of Lamb waves propagating in the capsule alloy and to quantify the velocities so that accurate flaw location is ensured. We describe a numerical technique for computing the phase velocities of Lamb waves (or of any other type of guided wave) and derive the group velocities from this information. A frequency-domain method is described for measuring group velocity when multiple Lamb modes are present and mutually interfering in the time domain, and experimental confirmation of the group velocity is presented for the capsule material.

Simpson, W.A.; McGuire, D.J.

1994-07-01

188

Comparison of Phase Velocities from Array Measurements of Rayleigh Waves Associated with Microtremor and Results Calculated from Borehole Shear-Wave Velocity Profiles  

Microsoft Academic Search

Shear-wave velocities (VS) are widely used for earthquake ground- motion site characterization. VS data are now largely obtained using borehole meth- ods. Drilling holes, however, is expensive. Nonintrusive surface methods are inex- pensive for obtaining VS information, but not many comparisons with direct borehole measurements have been published. Because different assumptions are used in data interpretation of each surface method

Hsi-Ping Liu; David M. Boore; William B. Joyner; David H. Oppenheimer; Richard E. Warrick; Wenbo Zhang; John C. Hamilton; Leo T. Brown

2000-01-01

189

S-wave velocity as an indicator of solid-liquid transition in clay  

NASA Astrophysics Data System (ADS)

Clayey landslides, which are widely spread all over the world, pose specific problems to territorial planners, owing to dramatic variations in kinematics. Indeed, the analysis of landslide movements in a clay-rich area like the Trièves plateau (French Western Alps) indicates that slow slope movements can suddenly accelerate or fluidize as a result of heavy and/or long-lasting rainfalls or loading. Previous rheometric tests performed on the Trièves clay revealed a thixotropic behavior of the clay with a highly pronounced viscosity bifurcation at a yield stress ?c. Below that stress, the material behaves like a solid, while it abruptly starts flowing when this yield stress is reached. This solid-liquid transition was investigated by measuring the variations of the shear wave velocity (Vs) in the Trièves clay during rheometric tests and flume tests. First, rheometric parallel-plate tests were performed at 3 different clay water contents (52%, 66% and 78%). Oscillatory stress tests were conducted during the experiments, allowing the shear modulus and Vs to be measured as a function of the shear stress level. Results revealed a dramatic Vs change at the same yield stress as for the viscosity bifurcation. When the stress is lower than the yield stress, Vs regularly increases with time up to a limit value. As soon as the yield stress is reached, Vs abruptly decreases to reach values of a few m/s in the fluidized clay. In order to investigate at a larger scale (approximately 1 m) the evolution of the shear wave velocity during the clay fluidization, flume tests were performed for two clay water contents (57 % and 68%). These experiments consist in progressively tilting a flume filled with a saturated clay layer (35cm wide, 60cm long and 10cm deep) until reaching the fluidization at a given slope. The Rayleigh wave velocity (VR), which is related to Vs, was continuously monitored using a piezometric source and 4 vertical component accelerometers placed at the surface of the clay layer. The Rayleigh wave propagation was reconstructed by cross correlating the source signal (50-700 Hz sweep) and the 4 recorded signals. The clay mass motion was measured, using 3 height sensors (vertical displacement) and a digital camera to follow the displacement of three superficial markers (colored pins). The tilt angle, which increased by 1 degree per minute, was given by an inclinometer placed on the flume. Signals were generated every 30 seconds. Results showed no variation of VR during the flume tilting. Just before that the mass moved at a critical angle, a rapid decrease in VR (between 4 and 7%) was observed on all the signals, evidencing a change in Vs at the base of the layer. Both experiments showed that Vs could be a valuable good indicator for rheological changes in clay. The seismic measurements during flume tests are in agreement with passive seismic monitoring results at the Pont Bourquin landslide (Switzerland), where a drop of 7% in VR was measured 4 days before a earth-slide earth-flow in 2010, which mobilized about 4,500 m3 of clay material.

Mainsant, Guénolé; Jongmans, Denis; Chambon, Guillaume; Larose, Eric; Baillet, Laurent

2013-04-01

190

The Velocity of Compressional Waves in Rocks to 10 Kilobars, Part 1  

Microsoft Academic Search

The velocity of compressional waves has been determined by measurement of travel time of pulses in specimens of rock at pressures to 10 kilobars ad room temperature. Most of the samples, mainly igneous and metamorphic rocks, furnished three specimens ori- ented at right angles to one another. The present paper gives experimental details, modal analyses, and numerical tables of velocity

Francis Birch

1960-01-01

191

Surface wave higher-mode phase velocity measurements using a roller-coaster-type algorithm  

Microsoft Academic Search

In order to solve a highly non-linear problem by introducing the smallest a priori information, we present a new inverse technique called the `roller coaster' technique and apply it to measure surface wave mode-branch phase velocities. The fundamental mode and the first six overtone parameter vectors, defined over their own significant frequency ranges, are smoothed average phase velocity perturbations along

Éric Beucler; Éléonore Stutzmann; Jean-Paul Montagner

2003-01-01

192

Regional P wave velocity structure of the Northern Cascadia Subduction Zone  

Microsoft Academic Search

This paper presents the first regional three-dimensional P wave velocity model for the Northern Cascadia Subduction Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and

K. Ramachandran; R. D. Hyndman; T. M. Brocher

2006-01-01

193

Ultrasound velocity measurement in long bones: Measurement method and simulation of ultrasound wave propagation  

Microsoft Academic Search

A new method for the measurement of ultrasound velocity in long bones is presented. The method can be applied in vitro as well as in vivo. It automatically corrects for the influence of soft tissue, such that the real velocity in bone is obtained. In a series of simulation experiments, hypotheses on the followed wave path were verified. A very

G. Lowet; G. Van der Perre

1996-01-01

194

Intracyclic Velocity Variation and Arm Coordination for Different Skilled Swimmers in the Front Crawl  

PubMed Central

The aim of this study was to examine whether the intracyclic velocity variation (IVV) was lower in elite swimmers than in beginner swimmers at various velocities, and whether differences may be related to arm coordination. Seven elite and nine beginner male swimmers swam front crawl at four different swimming velocities (maximal velocity, 75%, 85%, and 95% of maximal swimming velocity). The index of arm coordination (IDC) was calculated as the lag time between the propulsive phases of each arm. IVV was determined from the coefficient of variation of horizontal velocity within one stroke cycle. IVV for elite swimmers was significantly lower (26%) than that for beginner swimmers at all swimming velocities (p<0.01, 7.28 1.25% vs. 9.80 1.70%, respectively). In contrast, the IDC was similar between elite and beginner swimmers. These data suggest that IVV is a strong predictor of the skill level for front crawl, and that elite swimmers have techniques to decrease IVV. However, the IDC does not contribute to IVV differences between elite and beginner swimmers. PMID:25713666

Matsuda, Yuji; Yamada, Yosuke; Ikuta, Yasushi; Nomura, Teruo; Oda, Shingo

2014-01-01

195

Velocity renormalization of nodal quasiparticles in d-wave superconductors  

NASA Astrophysics Data System (ADS)

Gapless nodal quasiparticles emerge at a low-energy regime of high-Tc cuprate superconductors due to the dx2-y2 gap symmetry. We study the unusual renormalizations of the Fermi velocity vF and gap velocity v? of these quasiparticles close to various quantum critical points in a superconducting dome. Special attention is paid to the behavior of the velocity ratio, v?/vF, since it determines a number of observable quantities. We perform a renormalization-group analysis and show that the velocity ratio may vanish, approach unity, or diverge at different quantum critical points. The corresponding superfluid densities and critical temperatures are suppressed, slightly increased, or significantly enhanced. The effects of three types of static disorders, namely, random mass, random gauge potential, and random chemical potential, on the stability of the system are also addressed. An analogous analysis reveals that both random mass and random gauge potential are irrelevant. This implies that these fixed points of the velocity ratio are stable, and hence observable effects ignited by them are unchanged. However, the random chemical potential is marginal. As a result, these fixed points are broken, and thus, the instabilities of quantum phase transitions are triggered.

Wang, Jing

2013-02-01

196

Wave-induced velocities inside a model seagrass bed  

E-print Network

Laboratory measurements reveal the flow structure within and above a model seagrass meadow (dynamically similar to Zostera marina) forced by progressive waves. Despite being driven by purely oscillatory flow, a mean current ...

Luhar, Mitul

197

Compressional wave velocities in rocks at high temperatures and pressures, critical thermal gradients, and crustal low-velocity zones  

Microsoft Academic Search

The velocities of compressional waves have been determined for several igneous and metamorphic rocks to temperatures of 500°C at elevated confining pressures. At 2 kbar and between 25° and 300°C, temperature coefficients (?Vp\\/?T)p for many of the rocks range between -0.5×10-3 and -0.6×10-3 km s-1°C-1. At higher temperatures and 2 kbar the temperature coefficients show extreme variability, which is related

Nikolas I. Christensen

1979-01-01

198

Superconducting twin quarter wave resonator for acceleration of low velocity heavy ions  

Microsoft Academic Search

We have designed and fabricated a superconducting twin quarter wave resonator (Twin-QWR) made of niobium and copper for the acceleration of low velocity heavy ions. The resonator has two inner conductors and three acceleration gaps, which give a resonant frequency of 129.8MHz and an optimum beam velocity of 6% of the light velocity. Each inner conductor resonates like in a

H. Kabumoto; S. Takeuchi; M. Matsuda; N. Ishizaki; Y. Otokawa

2010-01-01

199

Longitudinal variations in planetary wave activity in the equatorial mesosphere  

NASA Astrophysics Data System (ADS)

Zonal and meridional winds in the equatorial mesosphere and lower thermosphere (65-98 km) measured at two sites separated by 94° in longitude are used to study the zonal structure of planetary-scale waves. The data were obtained with MF radars located at Pontianak (0°N, 109°E)and Christmas Island (2°N, 157°W). The data at Christmas Island were collected from January 1990 to December 1997 and the observations at Pontianak were made from November 1995 to July 1997. Power spectral techniques are used to study the amplitude and frequency variations of long-period oscillations as a function of height and time. A mean climatology of these variations taken from years 1990-1997 is presented. Strong peaks in zonal and meridional winds are found at tidal periods and for the quasi 2-day wave. Zonal spectra exhibit considerable power at periods of 3-10 days, with transient oscillations with periods near 3.5 day and 6.5 days being especially prominent. The 6.5-day wave is particularly strong during April and September. Examination of the phase differences obtained from cross-spectra between the two stations show that the 6.5-day wave is westward propagating with zonal wavenumber 1, while the 3.5 day wave is eastward propagating with wavenumber 1. The 6.5-day wave is identified as a manifestation of an unstable mode, while the 3.5-day wave is identified as an ultrafast Kelvin wave. There are significant longitudinal variations in the amplitudes and inferred momentum fluxes of the 3.5-day wave, amplitudes being larger in the Asian region than in the central Pacific.

Kovalam, S.; Vincent, R. A.; Reid, I. M.; Tsuda, T.; Nakamura, T.; Ohnishi, K.; Nuryanto, A.; Wiryosumarto, H.

1999-07-01

200

Shear velocity model for the Kyrgyz Tien Shan from joint inversion of receiver function and surface wave data  

NASA Astrophysics Data System (ADS)

The Tien Shan is the largest active intracontinental orogenic belt on Earth. To better understand the processes causing mountains to form at great distances from a plate boundary, we analyse passive source seismic data collected on 40 broad-band stations of the MANAS project (2005-2007) and 12 stations of the permanent KRNET seismic network to determine variations in crustal thickness and shear wave speed across the range. We jointly invert P- and S-wave receiver functions with surface wave observations from both earthquakes and ambient noise to reduce the ambiguity inherent in the images obtained from the techniques applied individually. Inclusion of ambient noise data improves constraints on the upper crust by allowing dispersion measurements to be made at shorter periods. Joint inversion can also reduce the ambiguity in interpretation by revealing the extent to which various features in the receiver functions are amplified or eliminated by interference from multiples. The resulting wave speed model shows a variation in crustal thickness across the range. We find that crustal velocities extend to ˜75 km beneath the Kokshaal Range, which we attribute to underthrusting of the Tarim Basin beneath the southern Tien Shan. This result supports the plate model of intracontinental convergence. Crustal thickness elsewhere beneath the range is about 50 km, including beneath the Naryn Valley in the central Tien Shan where previous studies reported a shallow Moho. This difference apparently is the result of wave speed variations in the upper crust that were not previously taken into account. Finally, a high velocity lid appears in the upper mantle of the Central and Northern part of the Tien Shan, which we interpret as a remnant of material that may have delaminated elsewhere under the range.

Gilligan, Amy; Roecker, Steven W.; Priestley, Keith F.; Nunn, Ceri

2014-10-01

201

Seismic Wave Velocity Heterogeneity and Discontinuity Topography of the D" Region beneath the Caribbean and Central America  

NASA Astrophysics Data System (ADS)

Triplication arrivals generated by shear wave interaction with a sharp velocity increase at the top of the D" region have been argued for the widespread presence of a D" velocity discontinuity beneath the Caribbean and Central America. Seismic tomography models have also shown strong lateral velocity heterogeneity in this region. To simultaneously investigate seismic velocity variations in the lowermost mantle and the D" discontinuity topography which trade off against each other, we model triplication waveforms from deep earthquakes in South America subduction zones and recorded by dense transportable USArray and permanent broadband stations in North America and Canada. The sourcestation configuration provides triplication arrivals which have the bottoming depths directly sampling D" in our study region. We divide sources and stations into a suite of linear transects. Then we employ a global search method which combines reflectivity synthetics and neighborhood algorithm (NA) to seek an optimal 1-D velocity model in the lowermost mantle around midway of each transect which best fits differential traveltimes (ScS-S, Sdiff-SKS and Scd-S) and triplication waveforms. All the differential times are corrected for contributions from heterogeneous mantle structure above D" using global tomography models. Though triplication shear waves emerge frequently between direct S and core-reflected ScS phases across the study area, as reported in previous studies, there are still a number of localized regions sampled by shear waves which contain no triplications. It implies that there exists strong lateral velocity heterogeneity and gradient in D" as well as significant topographic undulation of the D" discontinuity over the study region.

Ko, Y.; Kuo, B.; Hung, S.

2012-12-01

202

Earthq Sci (2010)23: 449-463 449 Three dimensional shear wave velocity structure of  

E-print Network

method from 8 s to 60 s. After that, Rayleigh wave group and phase velocity dispersion maps on 1° by 1° spatial grids are obtained at different periods. Finally, we invert these maps for the 3-D shear wave in the southeast part of Eurasia, China and its surrounding areas are geologically diverse and tec- tonically

Song, Xiaodong

203

Wave breaking in dispersion-decreasing fiber with normal group-velocity dispersion  

Microsoft Academic Search

We show wave breaking can occur in a dispersion-decreasing fiber with normal group-velocity dispersion preceding the parabolic pulse formation (PPF), and the distance where it happens can be described by two equations. This fact reveals that four-wave mixing also plays an important role in the process of PPF.

Ge Xia; Li Liu; Songzhan Li; Libing Zhou; Dejun Li

2009-01-01

204

Measurements of mantle wave velocities and inversion for lateral heterogeneity and anisotropy. II - Analysis by the single-station method  

NASA Astrophysics Data System (ADS)

In the present investigation, the single-station method reported by Brune et al. (1960) is utilized for an analysis of long-period Love(G) and Rayleigh(R) waves recorded on digital seismic networks. The analysis was conducted to study the lateral heterogeneity of surface wave velocities. The data set is examined, and a description is presented of the single-station method. Attention is given to an error analysis for velocity measurements, the estimation of geographical distribution of surface wave velocities, the global distribution of surface wave velocities, and the correlation between the surface wave velocities and the heat flow on the geoid. The conducted measurements and inversions of surface wave velocities are used as a basis to derive certain conclusions. It is found that the application of the single-station method to long-period surface waves recorded on digital networks makes it possible to reach an accuracy level comparable to great circle velocity measurements.

Nakanishi, I.; Anderson, D. L.

1984-08-01

205

Measurements of mantle wave velocities and inversion for lateral heterogeneity and anisotropy. II - Analysis by the single-station method  

NASA Technical Reports Server (NTRS)

In the present investigation, the single-station method reported by Brune et al. (1960) is utilized for an analysis of long-period Love(G) and Rayleigh(R) waves recorded on digital seismic networks. The analysis was conducted to study the lateral heterogeneity of surface wave velocities. The data set is examined, and a description is presented of the single-station method. Attention is given to an error analysis for velocity measurements, the estimation of geographical distribution of surface wave velocities, the global distribution of surface wave velocities, and the correlation between the surface wave velocities and the heat flow on the geoid. The conducted measurements and inversions of surface wave velocities are used as a basis to derive certain conclusions. It is found that the application of the single-station method to long-period surface waves recorded on digital networks makes it possible to reach an accuracy level comparable to great circle velocity measurements.

Nakanishi, I.; Anderson, D. L.

1984-01-01

206

Spatial variation of coda wave attenuation in northwestern Colombia  

NASA Astrophysics Data System (ADS)

One thousand seven hundred and eighty-six vertical-component, short-period observations of microearthquake codas from regional earthquakes recorded by 17 stations belonging to the National Seismological Network of Colombia were used to estimate seismic wave attenuation in Colombia. Local magnitudes range from 2.9 to 6.0 and only events occurring at hypocentral distances up to 255 km were considered for the analysis. The frequencies of interest lay between 1 and 19 Hz and the analysis was performed for each seismic station separately. Coda-wave attenuation (Q-1c) was estimated by means of a single-scattering method whereas the separation of intrinsic absorption (Q-1i) and scattering attenuation (Q-1s) from total attenuation (Q-1t) was performed using a multiple lapse time-window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers. A regionalization of the estimated Q0 (Qc at 1 Hz) values was performed and a contour map of seismic coda attenuation in Colombia is presented, where four zones with significant variations of attenuation related to different geological and tectonic characteristics can be observed. The highest attenuation is linked to the central and western regions (Q0 around 50 and 56) whereas a lower attenuation (Q0 around 69 and 67) is assigned to the northern and eastern regions. Results show that the Q-1 values are frequency dependent in the considered frequency range, and are approximated by a least-square fit to the power law Q-1(f) =Q-10(f/f0)-?. The exponents of the frequency dependence law ranged from ?= 0.65 to 1.01 for Q-1c, ?= 0.62 to 1.78 for Q-1i, ?= 0.28 to 1.49 for Q-1s, and ?= 0.53 to 1.67 for Q-1t. On the other hand, intrinsic absorption is found to dominate over scattering in the attenuation process for most of the stations and frequency bands analysed. Some discrepancies have been observed between the theoretical model and the observations for some frequency bands which indicate that it would be necessary to consider models for depth-dependent velocity structure and/or non-isotropic scattering patterns.

Vargas, Carlos A.; Ugalde, Arantza; Pujades, Lluís G.; Canas, José A.

2004-08-01

207

Exploitation of SAR data for measurement of ocean currents and wave velocities  

NASA Technical Reports Server (NTRS)

Methods of extracting information on ocean currents and wave orbital velocities from SAR data by an analysis of the Doppler frequency content of the data are discussed. The theory and data analysis methods are discussed, and results are presented for both aircraft and satellite (SEASAT) data sets. A method of measuring the phase velocity of a gravity wave field is also described. This method uses the shift in position of the wave crests on two images generated from the same data set using two separate Doppler bands. Results of the current measurements are pesented for 11 aircraft data sets and 4 SEASAT data sets.

Shuchman, R. A.; Lyzenga, D. R.; Klooster, A., Jr.

1981-01-01

208

Estimation of pseudo-2D shear-velocity section by inversion of high frequency surface waves  

USGS Publications Warehouse

A scheme to generate pseudo-2D shear-velocity sections with high horizontal resolution and low field cost by inversion of high frequency surface waves is presented. It contains six steps. The key step is the joint method of crossed correlation and phase shift scanning. This joint method chooses only two traces to generate image of dispersion curve. For Rayleigh-wave dispersion is most important for estimation of near-surface shear-wave velocity, it can effectively obtain reliable images of dispersion curves with a couple of traces. The result of a synthetic example shows the feasibility of this scheme. ?? 2005 Society of Exploration Geophysicists.

Luo, Y.; Liu, J.; Xia, J.; Xu, Y.; Liu, Q.

2006-01-01

209

Group velocity of extraordinary waves in superdense magnetized quantum plasma with spin-1/2 effects  

SciTech Connect

Based on the one component plasma model, a new dispersion relation and group velocity of elliptically polarized extraordinary electromagnetic waves in a superdense quantum magnetoplasma are derived. The group velocity of the extraordinary wave is modified due to the quantum forces and magnetization effects within a certain range of wave numbers. It means that the quantum spin-1/2 effects can reduce the transport of energy in such quantum plasma systems. Our work should be of relevance for the dense astrophysical environments and the condensed matter physics.

Li Chunhua; Ren Haijun; Yang Weihong [Department of Modern Physics, University of Science and Technology of China, 230026 Hefei (China); Wu Zhengwei [Department of Modern Physics, University of Science and Technology of China, 230026 Hefei (China); Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon (Hong Kong); Chu, Paul K. [Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon (Hong Kong)

2012-12-15

210

Variational Partitioned Runge-Kutta methods for Lagrangians linear in velocities  

E-print Network

In this paper we construct higher-order variational integrators for a class of degenerate systems described by Lagrangians that are linear in velocities. We analyze the geometry underlying such systems and develop the appropriate theory for variational integration. Our main observation is that the evolution takes place on the primary constraint and the 'Hamiltonian' equations of motion can be formulated as an index-1 differential-algebraic system. We also construct variational Runge-Kutta methods and analyze their properties. The general properties of Runge-Kutta methods depend on the 'velocity' part of the Lagrangian. If the 'velocity' part is also linear in the position coordinate, then we show that non-partitioned variational Runge-Kutta methods are equivalent to integration of the corresponding first-order Euler-Lagrange equations, which have the form of a Poisson system with a constant structure matrix, and the classical properties of the Runge-Kutta method are retained. If the 'velocity' part is nonlinear in the position coordinate, we observe a reduction of the order of convergence, which is typical of numerical integration of DAEs. We verify our results through numerical experiments for various dynamical systems.

Tomasz M. Tyranowski; Mathieu Desbrun

2014-01-30

211

Seasonal Variation in Conduction Velocity of Action Potentials in Squid Giant Axons  

E-print Network

Seasonal Variation in Conduction Velocity of Action Potentials in Squid Giant Axons JOSHUA J. C are not affected by rearing temperature (Treistman and Grant, 1993). The squid giant axon, long a model, Los Angeles, California 90095 Abstract. To determine whether the electrical properties of the squid

Bezanilla, Francisco

212

Seismic velocity variations along the rupture zone of the 1989 Loma Prieta earthquake, California  

E-print Network

Andreas fault zone since the 1906 San Francisco earthquake. The majority of the previous tomographic carried out to study the 1989 Mw6.9 Loma Prieta earthquake, the first major event to occur along the SanSeismic velocity variations along the rupture zone of the 1989 Loma Prieta earthquake, California G

Lin, Guoqing

213

Monitoring velocity variations in the crust using earthquake doublets: An application to the Calaveras fault, California  

Microsoft Academic Search

We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the frequency domain by cross correlation

G. Poupinet; V. L. Ellsworth; J. Frechet

1984-01-01

214

The chromospheric line-of-sight velocity variations in a solar microflare  

NASA Astrophysics Data System (ADS)

The variations of the chromospheric line-of-sight velocity in the active region NOAA 11024 are studied before, during, and after a solar microflare on 2009 July 4. At the day of the observations the main emergence phase was in this active region. The new emerging flux has interacted with the pre-existing magnetic field. The spectropolarimetric observations were carried out with the French-Italian THEMIS telescope (Spain, Tenerife). We used H? spectra of the high resolution obtained over 21 min. The spatial resolution was ?1 arcsec. The time interval between the spectra was 2.84 s. Doppler velocities were measured in the microflare location and its surroundings. We have revealed strong temporal variations of the line-of-sight velocity in the chromosphere. The velocities changed in the range -33-10 km/s. We revealed the velocity oscillations with the amplitude of 4-5 km/s. 12-14 min before the microflare both upward and downward motions with velocity values reaching about 20 km/s are found on the outer edge of the region studied. The amplitude of the oscillations increased. It may be the signature of the magnetic reconnection.

Leiko, U. M.; Kondrashova, N. N.

2015-02-01

215

A special relation between Young's modulus, Rayleigh-wave velocity, and Poisson's ratio.  

PubMed

Bayon et al. [(2005). J. Acoust. Soc. Am. 117, 3469-3477] described a method for the determination of Young's modulus by measuring the Rayleigh-wave velocity and the ellipticity of Rayleigh waves, and found a peculiar almost linear relation between a non-dimensional quantity connecting Young's modulus, Rayleigh-wave velocity and density, and Poisson's ratio. The analytical reason for this special behavior remained unclear. It is demonstrated here that this behavior is a simple consequence of the mathematical form of the Rayleigh-wave velocity as a function of Poisson's ratio. The consequences for auxetic materials (those materials for which Poisson's ratio is negative) are discussed, as well as the determination of the shear and bulk moduli. PMID:20000895

Malischewsky, Peter G; Tuan, Tran Thanh

2009-12-01

216

High resolution Rayleigh wave group velocity tomography in North China from ambient seismic noise  

NASA Astrophysics Data System (ADS)

This study presents the results of the Rayleigh wave group velocity tomography in North China performed using ambient seismic noise observed at 190 broadband and 10 very broad-band stations of the North China Seismic Array. All available vertical component time-series for the 14 months between 2007 January and 2008 February are cross-correlated to obtain empirical Rayleigh wave Green's functions that are subsequently processed, with the multiple filter method, to isolate the group velocity dispersion curves of the fundamental mode of the Rayleigh wave. Tomographic maps, with a grid spacing of 0.25° × 0.25°, are computed at periods of 4.5, 12, 20 and 28 s. The maps at short periods reveal lateral heterogeneity in the crust of North China, in good agreement with known geological and tectonic features. The North China Basin is imaged as a broad low velocity area, while the Taihangshan and Yanshan uplifts and Ordos block are imaged as high velocity zones, and the Quaternary intermountain basins show up as small low-velocity anomalies. A well-defined low velocity zone in the Beijing-Tianjin-Tangshan region is observed at 28 s period. The low velocity zone may be associated with the upwelling of hot mantle material. The group velocity maps at 4.5, 12 and 20 s are consistent with Bouguer gravity anomalies measured in the area of the Taihangshan fault, that cuts through the lower crust at least.

Fang, Lihua; Wu, Jianping; Ding, Zhifeng; Panza, G. F.

2010-05-01

217

Surface wave phase velocities of the Western United States from a two-station method  

NASA Astrophysics Data System (ADS)

We calculate two-station phase measurements using single-station measurements made on USArray Transportable Array data for surface waves at periods from 25 to 100 s. The phase measurements are inverted for baseline Love and Rayleigh wave phase velocity maps on a 0.5° × 0.5° grid. We make estimates of the arrival angle for each event at each station using a mini array method similar to beamforming, and apply this information to correct the geometry of the two-station measurements. These corrected measurements are inverted for an additional set of phase velocity maps. Arrival angles range from 0° to ±15°, and the associated corrections result in local changes of up to 4 per cent in the final phase velocity maps. We select our preferred models on the basis of the internal consistency of the measurements, finding that the arrival-angle corrections improve the two-station phase measurements, but that Love wave arrival-angle estimates may be contaminated by overtone interference. Our preferred models compare favourably with recent studies of the phase velocity of the Western United States. The corrected Rayleigh wave models achieve greater variance reduction than the baseline Rayleigh wave models, and the baseline Love wave models, which are more difficult to obtain, are robust and could be used in conjunction with the Rayleigh wave models to constrain radially anisotropic earth structure.

Foster, Anna; Ekström, Göran; Nettles, Meredith

2014-02-01

218

Near-surface Shear-wave Velocities using Passive and Active Surface-wave Methods in Chunchon, Korea  

NASA Astrophysics Data System (ADS)

To investigate near-surface shear-wave velocities (Vs) near an agricultural well in Chunchon, Korea, both passive and active surface-wave data were recorded. Ambient surface waves (microtremors) were recorded for 5 minutes at 10-ms sampling intervals using four 1-Hz vertical velocity sensors in triangular arrays with radii of 5, 10, 20, and 40 m. Synthetic surface waves, generated by a sledgehammer, were detected with a linear array of 24 4.5-Hz geophones separated at 2 m intervals and recorded for 2 s at a 2-ms sampling interval. Rayleigh wave phase-velocities were independently derived from the passive data using the Spatial Autocorrelation (SPAC) method and from the active data using the Multichannel Analysis of Surface Waves method. These methods yielded well-defined dispersion curves for frequencies of 7 to 19 Hz and 11 to 50 Hz, respectively. We combined these dispersion curves to better resolve the Vs structure and then derived an initial velocity model of 40 1-m thick layers using the 1/3-wavelength rule. This initial velocity model varied smoothly with depth except for an abrupt change of velocity at 2 m depth corresponding to the interface between the topsoil and an underlying gravelly sand layer. Additional information on interfaces was derived from refraction tomography using the sledgehammer-generated data and from the well located 5 m north of the SPAC array center. Slight modifications of the initially modeled Vs at the top and bottom of a weathered rock layer improved the match between the observed and modeled dispersion curves. The modeled Vs for the 2-m thick topsoil, 2-m thick gravelly sand, 1-m thick layer of cobbles, 9-m thick weathered bedrock, and the underlying soft bedrock were 231, 270, 388, 477, and 827 m/s, respectively.

Kim, K. Y.; Kim, W.; Park, Y.

2011-12-01

219

Detection of periodic variations in the vertical velocities of Galactic masers  

NASA Astrophysics Data System (ADS)

We have collected literature data on Galactic masers with trigonometric parallaxes measured by means of very-long-baseline interferometry (VLBI). We have obtained series of residual tangential (?Vcirc) and radial (?VR) velocities for 107 masers. Based on these series, we have re-determined the parameters of the Galactic spiral density wave using the method of spectral (periodogram) analysis. The tangential and radial perturbation amplitudes are f? = 6.0 ± 2.6 km s-1 and fR = 7.2 ± 2.2 km s-1, respectively; the perturbation wavelengths are ?? = 3.2 ± 0.5 kpc and ?R = 3.0 ± 0.6 kpc for a four-armed spiral model, m = 4. The phase of the Sun, ??, in the spiral density wave is -79° ± 14° and -199° ± 16° from the residual tangential and radial velocities, respectively. The most interesting result of this work is detecting a wave in vertical spatial velocity (W) versus distance R from the Galactic rotation axis. From spectral analysis, we have found the following characteristics for this wave: perturbation wavelength ?W = 3.4 ± 0.7 kpc and amplitude fW = 4.3 ± 1.2 km s-1.

Bobylev, V. V.; Bajkova, A. T.

2015-02-01

220

Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea  

NASA Astrophysics Data System (ADS)

The shear wave velocity structure of a wide angle seismic profile (OBS973-2) across Reed Bank in the southern continental margin of the South China Sea (SCS) is simulated by 2-D ray-tracing method, based on its previous P-wave model. This profile is 369-km-long and consists of fifteen three-component ocean bottom seismometers (OBS). The main results are as follows.(1) The model consists of seven layers and the shear wave velocity increases from 0.7 km/s at the top of sediment layer to 4.0 km/s in the lower crust. (2) The Moho depth decreases from 20-22 km at the Reed Bank to 9-11 km at the deep oceanic basin with the shear wave velocity of 4.2 km/s below the Moho. (3) The Vp/Vs ratio decreases with depth through the sedimentary layers, attributed to increased compaction and consolidation of the rocks. (4) In the continental upper crust (at model distance 90-170 km), S-wave velocity (2.5-3.2 km/s) is relatively low and Vp/Vs ratio (1.75-1.82) is relatively high compared with the other parts of the crust, corresponding to the lower P-wave velocity in the previous P-wave model and normal faults revealed by MCS data, indicating that a strong regional extensional movement had occurred during the formation process of the SCS at the Reed Bank area. (5) The S-wave structures indicate that Reed Bank crust has different rock compositions from that in the east section of the northern margin, denying the presence of conjugate relationship of Reed Bank with Dongsha islands. According to P-wave models and other data, we inferred that Reed Bank and Macclesfield were separated from the same continental crust during the rifting and break-up process.

Wei, Xiaodong; Ruan, Aiguo; Zhao, Minghui; Qiu, Xuelin; Wu, Zhenli; Niu, Xiongwei

2015-03-01

221

Fragmentation Speed, Elastic Wave Velocity and Fracture Velocity In Dome Magma  

Microsoft Academic Search

One of the most important parameters for the modeling of explosive eruptions is the fragmentation speed. In combination with the gas expansion, the fragmentation speed influences the effusion rate during an explosive eruption. We determined the statis- tically relevant variation of the density from pyroclastic depostits in the field (e.g. Kueppers et al, EGS 2002). This allows us to investigate

B. Scheu; O. Spieler; D. B. Dingwell

2002-01-01

222

Excitation of solitons by an external resonant wave with a slowly varying phase velocity  

SciTech Connect

A novel mechanism is proposed for the excitation of solitons in nonlinear dispersive media. The mechanism employs an external pumping wave with a varying phase velocity, which provides a continuous resonant excitation of a nonlinear wave in the medium. Two different schemes of a continuous resonant growth (continuous phase-locking) of the induced nonlinear wave are suggested. The first of them requires a definite time dependence of the pumping wave phase velocity and is relatively sensitive to the initial wave phase. The second employs the dynamic autoresonance effect and is insensitive to the exact time dependence of the pumping wave phase velocity. It is demonstrated analytically and numerically, for a particular example of a driven Korteweg-de Vries (KdV) equation with periodic boundary conditions, that as the nonlinear wave grows, it transforms into a soliton, which continues growing and accelerating adiabatically. A fully nonlinear perturbation theory is developed for the driven KdV equation to follow the growing wave into the strongly nonlinear regime and describe the soliton formation.

Aranson, I.; Meerson, B. [Hebrew Univ., Jerusalem (Israel). Racah Inst. of Physics; Tajima, Toshiki [Texas Univ., Austin, TX (United States)

1992-02-01

223

Phase velocity and interaction impedance measurements on slow-wave structures for high-power traveling-wave tubes  

Microsoft Academic Search

In this work some results of phase velocity and interaction impedance measurements made on a ring-bar slow-wave structure for a high-power traveling-wave tube are presented and discussed. The measurements were in range from 7 to 12 GHz. The experimental results obtained were compared to an analytical cold model based on the contra-wound helix theory and to 3D simulations done with

Daniel T. Lopes; Cláudio C. Motta

2008-01-01

224

S-Wave Velocity Structure of the Puli Basin, Taiwan, Using the Array Records of Microtremors  

NASA Astrophysics Data System (ADS)

The September 21, 1999 Chi-Chi earthquake induced strong shaking, resulting in severe damage in the Puli area. According to Huang and Tarng (2005), the collapse of many structures during the earthquake was very closely related to the site effects. The Shallow Shear-wave velocities have widely been used for earthquake ground-motion site characterization. Thus, the S-wave velocity structures of the Puli area are investigated using the array records of microtremors at 16 sites. The dispersion curves at these sites are calculated using the F-K method (Capon, 1969); then, the S-wave velocity structures at the Puli area are estimated by employing the surface wave inversion technique (Herrmann, 1991). If the S-wave velocity of bedrock is assumed to be 2000m/sec, the depths of the alluvium at the Puli area are between 360m (ZHO) and 810m (AIL, DAH). Besides, there are 3~4 distinct interfaces in the shallow velocity structure (0~1000m). The depth of the alluvium gradually increases from east to west and the deepest one appears at stations AIL and DAH. The results in this study are similar to those using the seismic exploration method (Wang et al., 2003; Huang, 2008) in the Puli area and the well-logging method at LAC.

Wu, C.; Huang, H.

2009-12-01

225

Noninvasive Method for Measuring Local Pulse Wave Velocity by Dual Pulse Wave Doppler: In Vitro and In Vivo Studies  

PubMed Central

Objectives To evaluate the validity and reproducibility of a noninvasive dual pulse wave Doppler (DPWD) method, which involves simultaneous recording of flow velocity of two independent sample volumes with a measurable distance, for measuring the local arterial pulse wave velocity (PWV) through in vitro and in vivo studies. Methods The DPWD mode of Hitachi HI Vision Preirus ultrasound system with a 5–13MHz transducer was used. An in vitro model was designed to compare the PWV of a homogeneous rubber tubing with the local PWV of its middle part measured by DPWD method. In the in vivo study, local PWV of 45 hypertensive patients (25 male, 49.8±3.1 years) and 45 matched healthy subjects (25 male, 49.3±3.0 years) were investigated at the left common carotid artery (LCCA) by DPWD method. Results In the in vitro study, the local PWV measured by DPWP method and the PWV of the homogeneous rubber tubing did not show statistical difference (5.16 ± 0.28 m/s vs 5.03 ± 0.15 m/s, p = 0.075). The coefficient of variation (CV) of the intra- and inter- measurements for local PWV were 3.46% and 4.96%, for the PWV of the homogeneous rubber tubing were 0.99% and 1.98%. In the in vivo study, a significantly higher local PWV of LCCA was found in the hypertensive patients as compared to that in healthy subjects (6.29±1.04m/s vs. 5.31±0.72m/s, P = 0.019). The CV of the intra- and inter- measurements in hypertensive patients were 2.22% and 3.94%, in healthy subjects were 2.07% and 4.14%. Conclusions This study demonstrated the feasibility of the noninvasive DPWD method to determine the local PWV, which was accurate and reproducible not only in vitro but also in vivo studies. This noninvasive echocardiographic method may be illuminating to clinical use. PMID:25786124

Wang, Zhen; Yang, Yong; Yuan, Li-jun; Liu, Jie; Duan, Yun-you; Cao, Tie-sheng

2015-01-01

226

Seismic waves velocities and anisotropy in serpentinized peridotites from Xigaze ophiolite: Abundance of serpentine in slow spreading ridge  

Microsoft Academic Search

The effect of serpentinization on seismic wave velocity and anisotropy has been analyzed in 6 peridotite samples of the Xigaze ophiolite, having harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P- and S-wave velocities are linearly correlated with serpentine content; ii) anisotropy of P- and S-waves decreases with increasing serpentinization (while in fresh

H. Horen; M. Zamora; G. Dubuisson

1996-01-01

227

RESEARCH PAPERS : Wave-theoretical inversion of teleseismic surface waves in a regional network: phase-velocity maps and a three-dimensional upper-mantle shear-wave-velocity model for southern Germany  

Microsoft Academic Search

Using teleseimic surface-wave data from 110 selected earthquakes recorded at 10broad-band stations in southern Germany we construct phase-velocity maps of Rayleigh waves for southern Germany. In a further step these maps are inverted for a three-dimensional model of the SV velocity of the upper 200 km of the mantle. We attempt to take into account the effect of heterogeneous structure

Wolfgang Friederich

1998-01-01

228

On Variational Methods in the Physics of Plasma Waves  

SciTech Connect

A fi rst-principle variational approach to adiabatic collisionless plasma waves is described. The focus is made on one-dimensional electrostatic oscillations, including phase-mixed electron plasma waves (EPW) with trapped particles, such as Bernstein-Greene-Kruskal modes. The well known Whitham's theory is extended by an explicit calculation of the EPW Lagrangian, which is related to the oscillation-center energies of individual particles in a periodic fi eld, and those are found by a quadrature. Some paradigmatic physics of EPW is discussed for illustration purposes. __________________________________________________

I.Y. Dodin

2013-03-08

229

Velocity Variations of the Kaskawulsh Glacier, Yukon Territory, 2009-2011  

NASA Astrophysics Data System (ADS)

Laser altimetry and satellite gravity surveys indicate that the St Elias Icefields are currently losing mass and are among the largest non-polar sea level contributors in the world. However, a poor understanding of glacier dynamics in the region is a major hurdle in evaluating regional variations in ice motion and the relationship between changing surface conditions and ice flux. This study combines in-situ dGPS measurements and advanced Radarsat-2 (RS-2) processing techniques to determine daily and seasonal ice velocities for the Kaskawulsh Glacier from summer 2009 to summer 2011. Three permanent dGPS stations were installed along the centreline of the glacier in 2009, with an additional permanent station on the South Arm in 2010. The Precise Point Positioning (PPP) method is used to process the dGPS data using high accuracy orbital reconstruction. RS-2 imagery was acquired on a 24-day cycle from January to March 2010, and from October to March 2010-2011 in a combination of ultra-fine and fine beam modes. Seasonal velocity regimes are readily identifiable in the dGPS results, with distinct variations in both horizontal velocity and vertical motion. The Spring Regime consists of an annual peak in horizontal velocity that corresponds closely with the onset of the melt season and progresses up-glacier, following the onset of melt at each station. The Summer Regime sees variable horizontal velocity and vertical uplift, superimposed on a long-term decline in motion. The Fall Regime sees a gradual slowing at all stations with little variation in horizontal velocity or vertical position. Rapid but short accelerations lasting up to 10 days were seen in the Winter regimes in both 2010 and 2011, occurring at various times throughout each regime. These events initiated at the Upper Station and progress down-glacier at propagation speeds up to 16,380 m day-1 and were accompanied by vertical uplift lasting for similar periods. Three velocity maps, one from the winter of 2010 and two from the fall/winter of 2011, produced from speckle tracking were validated by comparison with dGPS velocity, surface flow direction, and bedrock areas of zero motion, with an average velocity error of 2.0% and average difference in orientation of 4.3°.

Darling, Samantha

230

The velocity of sound behind strong shock waves in 2024 Al  

NASA Astrophysics Data System (ADS)

Rarefaction waves were produced by impacting a target with a thin plate. An optical technique was used to determine where the rarefaction from the back surface of the impactor overtook the shock wave induced in a step wedge target. Bromoform was placed on the front surface. When the shock reached the liquid it radiated steadily the rarefaction from the impactor overtakes it. The times when this occurred were used to determine where the rarefaction just overtook the shock in the target, and thus the sound velocity. The leading edge of this rarefaction wave travels at longitudinal sound velocity in solids. This velocity increases smoothly with pressure until shock heating causes the material to melt. The data indicate that melting on the Hugoniot of 2024 Al begins at about 125 GPa and is completed at 150 GPa.

McQueen, R. G.; Fritz, J. N.; Morris, C. E.

231

Velocity of sound behind strong shock waves in 2024 A1  

SciTech Connect

Rarefaction waves were produced by impacting a target with a thin plate. An optical technique was used to determine where the rarefaction from the back surface of the impactor overtook the shock wave induced in a step wedge target. Bromoform was placed on the front surface. When the shock reached the liquid it radiated steadily until the rarefaction from the impactor overtakes it. The times when this occurred were used to determine where the rarefaction just overtook the shock in the target, and thus the sound velocity. The leading edge of this rarefaction wave travels at longitudinal sound velocity in solids. This velocity increases smoothly with pressure until shock heating causes the material to melt. The data indicate that melting on the Hugoniot of 2024 Al begins at about 125 GPa and is completed at 150 GPa.

McQueen, R.G.; Fritz, J.N.; Morris, C.E.

1983-01-01

232

A shear wave velocity model of the European upper mantle from automated inversion of seismic shear and surface waveforms  

NASA Astrophysics Data System (ADS)

We present a new, S-velocity model of the European upper mantle, constrained by inversions of seismic waveforms from broad-band stations in Europe and surrounding regions. We collected seismograms for the years 1990-2007 from all permanent stations in Europe for which data were available. In addition, we incorporated data from temporary experiments. Automated multimode inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations were then solved for seismic velocity perturbations in the crust and mantle with respect to a 3-D reference model with a realistic crust. We present two versions of the model: one for the entire European upper mantle and another, with the highest resolution, focused on the upper 200 km of the mantle beneath western and central Europe and the circum Mediterranean. The mantle lithosphere and asthenosphere are well resolved by both models. Major features of the lithosphere-asthenosphere system in Europe and the Mediterranean are indentified. The highest velocities in the mantle lithosphere of the East European Craton (EEC) are found at about 150 km depth. There are no indications for a deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved as well. The locations of kimberlites correlate with reduced S-wave velocities in the shallow cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the northwestern margin of the EEC and may indicate erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath western Europe and between the Tornquist-Teisseyre Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere-asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the circum Mediterranean is found in regions of shallow asthenosphere and close to changes in the depth of the lithosphere-asthenosphere boundary. Very low velocities at shallow upper-mantle depths are present from eastern Turkey towards the Dead Sea transform fault system and Sinai, beneath locations of recent volcanism. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massif Central, Sinai and the Dead Sea, the Canary Islands and Iceland.

Legendre, C. P.; Meier, T.; Lebedev, S.; Friederich, W.; Viereck-Götte, L.

2012-10-01

233

Minimal position-velocity uncertainty wave packets in relativistic and non-relativistic quantum mechanics  

SciTech Connect

We consider wave packets of free particles with a general energy-momentum dispersion relation E(p). The spreading of the wave packet is determined by the velocity v={partial_derivative}{sub p}E. The position-velocity uncertainty relation {delta}x{delta}v{>=}1/2 |<{partial_derivative}{sub p}{sup 2}E>| is saturated by minimal uncertainty wave packets {phi}(p)=Aexp(-{alpha}E(p)+{beta}p). In addition to the standard minimal Gaussian wave packets corresponding to the non-relativistic dispersion relation E(p)=p{sup 2}/2m, analytic calculations are presented for the spreading of wave packets with minimal position-velocity uncertainty product for the lattice dispersion relation E(p)=-cos(pa)/ma{sup 2} as well as for the relativistic dispersion relation E(p)={radical}(p{sup 2}+m{sup 2}). The boost properties of moving relativistic wave packets as well as the propagation of wave packets in an expanding Universe are also discussed.

Al-Hashimi, M.H. [Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, Bern University, Sidlerstrasse 5, CH-3012 Bern (Switzerland)], E-mail: hashimi@itp.unibe.ch; Wiese, U.-J. [Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, Bern University, Sidlerstrasse 5, CH-3012 Bern (Switzerland)

2009-12-15

234

Nondestructive testing using stress waves: wave propagation in layered media  

E-print Network

by computer simulation programs. The results of this study are figures showing the variation of the apparent wave propagation velocity corresponding to the threshold frequency divided by the shear wave velocity of the medium as a function of Poisson's ratio...

Ortega, Jose Alberto

2013-02-22

235

Electromagnetic wave propagation with negative phase velocity in regular black holes  

SciTech Connect

We discuss the propagation of electromagnetic plane waves with negative phase velocity in regular black holes. For this purpose, we consider the Bardeen model as a nonlinear magnetic monopole and the Bardeen model coupled to nonlinear electrodynamics with a cosmological constant. It turns out that the region outside the event horizon of each regular black hole does not support negative phase velocity propagation, while its possibility in the region inside the event horizon is discussed.

Sharif, M., E-mail: msharif.math@pu.edu.pk; Manzoor, R., E-mail: rubabmanzoor9@yahoo.com [University of the Punjab, Department of Mathematics (Pakistan)

2012-12-15

236

Measuring the acoustic wave velocity and sample thickness using an ultrasonic transducer array  

NASA Astrophysics Data System (ADS)

We suggest a new method for determining the longitudinal and transverse acoustic wave velocities and sample thicknesses, which is based on the measurement and analysis of pulsed echo signals by an array of ultrasonic transducers. Analytical expressions relating the delay of signals detected by the array and the values of parameters to be determined are obtained within the framework of a ray model of the measuring system. Measurements on a reference sample have been performed. The values of ultrasonic wave velocities and sample thickness obtained using the proposed technique agree with the results of measurements using independent methods.

Titov, S. A.; Maev, R. G.; Bogachenkov, A. N.

2009-11-01

237

Shear wave velocities of Gulf Coast soils determined from crosshole and seismic cone penetration tests  

SciTech Connect

This paper provides a summary of shear wave velocity data obtained from Texas and Louisiana Gulf Coast soils using both crosshole test (CHT) and seismic cone penetration test (SCPT) methods. Both methods are described and a side-by-side comparison of the results from several sites where both methods were used is presented. The data collected were evaluated based on both soil type and geologic formation. Best-fit equations are listed for each material type relating shear wave velocity to various parameters.

Brettmann, T.; Gauer, R.C.; Yilmaz, R. [Fugro, Houston, TX (United States)

1996-11-01

238

Nonlinear pulse propagation and phase velocity of laser-driven plasma waves  

SciTech Connect

Laser evolution and plasma wave excitation by a relativistically-intense short-pulse laser in underdense plasma are investigated in the broad pulse limit, including the effects of pulse steepening, frequency red-shifting, and energy depletion. The nonlinear plasma wave phase velocity is shown to be significantly lower than the laser group velocity and further decreases as the pulse propagates owing to laser evolution. This lowers the thresholds for trapping and wavebreaking, and reduces the energy gain and efficiency of laser-plasma accelerators that use a uniform plasma profile.

Schroeder, Carl B.; Benedetti, Carlo; Esarey, Eric; Leemans, Wim

2011-03-25

239

Measurement of elastic modulus and ultrasonic wave velocity by piezoelectric resonator  

NASA Astrophysics Data System (ADS)

A piezoelectric ceramic resonator is used for the ‘electrical’ measurement of elastic properties, i.e. Young’s modulus and ultrasonic wave velocity in metallic materials. Piezoelectric response is precisely calculated for the piezoelectric ceramic ring fixed at the end of a metallic rod. The piezoelectric ring serves as both an actuator as well as a sensor. The experimental setup and method of measurement using higher overtones is explained in detail and practically demonstrated for a set of different metallic materials. Young’s moduli and ultrasonic wave velocities are measured within 3% relative error. The presented method is suitable for an advanced engineering class or physics laboratory training.

Erhart, Ji?í

2015-01-01

240

High resolution Rayleigh wave phase velocity tomography in northern North China  

NASA Astrophysics Data System (ADS)

This study presents the Rayleigh wave phase velocity tomographic results in northern North China. The data are from 190 broad-band and 10 very broad-band stations of the North China Seismic Array and 50 permanent stations nearby. All available teleseismic vertical component time-series are used to extract the phase velocity dispersion curves of the fundamental mode Rayleigh wave by interstation method. Tomographic maps are obtained at periods of 10, 15, 25 and 60 s with a grid spacing of 0.25°× 0.25°. The short-period phase velocity maps show good correlation with the geological and tectonic features. To be specific, lower velocities correspond to North China Basin and depression area whereas higher velocities are associated with Taihangshan and Yanshan uplifts. At 25 s, there are obvious low-velocity anomalies in Jizhong depression and Beijing-Tianjin-Tangshan region, indicating that ascendant low velocity channel may be formed beneath these areas and induce the velocity difference in the upper crust. The phase velocity map at 60 s reflects the upper-mantle information in the study area. High-velocity anomalies are observed at Yanshan blocks north to Zhangjiakou-Bohai seismic belt, suggesting that the materials are stable beneath these areas or the asthenosphere is at deeper location. Low-velocity anomalies are mainly south to the seismic belt, implying the asthenosphere is shallower and the materials are transformed by the open stretching rift trending NNE, resulting in many NNE-directed fault belts. These structural differences at depth may be controlled by the fault activity and strong tectonic movements.

Wang, Weilai; Wu, Jianping; Fang, Lihua

2012-04-01

241

A three-dimensional variational single-Doppler velocity retrieval method with simple conservation equation constraint  

Microsoft Academic Search

Summary  In this paper, a new three-dimensional variational analysis scheme capable of retrieving three-dimensional winds from single\\u000a Doppler observations of convective storms is developed. The method incorporates, in a single cost function, Doppler radar\\u000a observations, a background field, smoothness and mass continuity constraints, and the residual of reflectivity or radial velocity\\u000a conservation. By minimizing this cost function, an analysis with the

J. Gao; M. Xue; S.-Y. Lee; A. Shapiro; Q. Xu; K. K. Droegemeier

2006-01-01

242

Site response, shallow shear-wave velocity, and wave propagation at the San Jose, California, dense seismic array  

USGS Publications Warehouse

Ground-motion records from a 52-element dense seismic array near San Jose, California, are analyzed to obtain site response, shallow shear-wave velocity, and plane-wave propagation characteristics. The array, located on the eastern side of the Santa Clara Valley south of the San Francisco Bay, is sited over the Evergreen basin, a 7-km-deep depression with Miocene and younger deposits. Site response values below 4 Hz are up to a factor of 2 greater when larger, regional records are included in the analysis, due to strong surface-wave development within the Santa Clara Valley. The pattern of site amplification is the same, however, with local or regional events. Site amplification increases away from the eastern edge of the Santa Clara Valley, reaching a maximum over the western edge of the Evergreen basin, where the pre-Cenozoic basement shallows rapidly. Amplification then decreases further to the west. This pattern may be caused by lower shallow shear-wave velocities and thicker Quaternary deposits further from the edge of the Santa Clara Valley and generation/trapping of surface waves above the shallowing basement of the western Evergreen basin. Shear-wave velocities from the inversion of site response spectra based on smaller, local earthquakes compare well with those obtained independently from our seismic reflection/refraction measurements. Velocities from the inversion of site spectra that include larger, regional records do not compare well with these measurements. A mix of local and regional events, however, is appropriate for determination of site response to be used in seismic hazard evaluation, since large damaging events would excite both body and surface waves with a wide range in ray parameters. Frequency-wavenumber, plane-wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array. Conventional, high-resolution, and multiple signal characterization f-k power spectra and stacked slowness power spectra are compared. These spectra show surface waves generated/ scattered at the edges of the Santa Clara Valley and possibly within the valley at the western edge of the Evergreen basin.

Hartzell, S.; Carver, D.; Williams, R.A.; Harmsen, S.; Zerva, A.

2003-01-01

243

S-Band space traveling-wave tubes employing velocity resynchronization and collector depression  

Microsoft Academic Search

Two related traveling-wave tubes, both of which demonstrate in excess of 50 percent efficiency over the design bandwidth, are introduced. Low dielectric and conductive shield loading are combined with suitable velocity resynchronization techniques to provide effective beam-circuit wave interaction. Use is made of multi-stage depressed collectors to sort the electrons according to energy, thus recovering a portion of the spent

M. K. Schreba

1972-01-01

244

Are There Optical Solitary Wave Solutions in Linear Media with Group Velocity Dispersion?  

NASA Technical Reports Server (NTRS)

A generalized exact optical bright solitary wave solution in a three dimensional dispersive linear medium is presented. The most interesting property of the solution is that it can exist in the normal group-velocity-dispersion (GVD) region. In addition, another peculiar feature is that it may achieve a condition of 'zero-dispersion' to the media so that a solitary wave of arbitrarily small amplitude may be propagated with no dependence on is pulse width.

Li, Zhonghao; Zhou, Guosheng

1996-01-01

245

A universal wave spectrum for atmospheric temperature and velocity fluctuations in the stratosphere  

SciTech Connect

The first simultaneous power spectral densities of high resolution lower stratospheric temperature and horizontal/vertical velocity fluctuations are presented. Amplitudes are within a factor of 2 or 3 of other observations reported in literature. However, the measured amplitude ratios, which are absolute characteristics of the fluctuating wave field, disagree significantly (multiplicative factor up to 10) with predictions of the saturation theory. These results disagree with the concept of a universal spectrum controlled by saturated waves.

Cot, C.; Barat, J. (Service d'Aeronomie du CNRS, Verrieres le Buisson (France))

1990-09-01

246

Validity, Reproducibility, and Clinical Significance of Noninvasive Brachial-Ankle Pulse Wave Velocity Measurement  

Microsoft Academic Search

The present study was conducted to evaluate the validity and reproducibility of noninvasive brachial-ankle pulse wave velocity (baPWV) measurements and to examine the alteration of baPWV in patients with coro- nary artery disease (CAD). Simultaneous recordings of baPWV by a simple, noninvasive method and aortic pulse wave velosity (PWV) using a catheter tip with pressure manometer were performed in 41

Akira YAMASHINA; Hirofumi TOMIYAMA; Kazuhiro TAKEDA; Hideichi TSUDA; Tomio ARAI; Kenichi HIROSE; Yutaka KOJI; Saburoh HORI; Yoshio YAMAMOTO

2002-01-01

247

Length and activation dependent variations in muscle shear wave speed  

PubMed Central

Muscle stiffness is known to vary as a result of a variety of disease states, yet current clinical methods for quantifying muscle stiffness have limitations including cost and availability. We investigated the capability of shear wave elastography (SWE) to measure variations in gastrocnemius shear wave speed induced via active contraction and passive stretch. Ten healthy young adults were tested. Shear wave speeds were measured using a SWE transducer positioned over the medial gastrocnemius at ankle angles ranging from maximum dorsiflexion to maximum plantarflexion. Shear wave speeds were also measured during voluntary plantarflexor contractions at a fixed ankle angle. Average shear wave speed increased significantly from 2.6 m/s to 5.6 m/s with passive dorsiflexion and the knee in an extended posture, but did not vary with dorsiflexion when the gastrocnemius was shortened in a flexed knee posture. During active contractions, shear wave speed monotonically varied with the net ankle moment generated, reaching 8.3 m/s in the maximally contracted condition. There was a linear correlation between shear wave speed and net ankle moment in both the active and passive conditions; however, the slope of this linear relationship was significantly steeper for the data collected during passive loading conditions. The results show that SWE is a promising approach for quantitatively assessing changes in mechanical muscle loading. However, the differential effect of active and passive loading on shear wave speed makes it important to carefully consider the relevant loading conditions in which to use SWE to characterize in vivo muscle properties. PMID:23719230

DeWall, R J; Lee, K S; Thelen, D G

2013-01-01

248

Measurement of Wave Velocity in Cortical Bone by Micro-Brillouin Scattering Technique: Effect of Bone Tissue Properties  

NASA Astrophysics Data System (ADS)

Hypersonic wave velocity was measured in the cortical bone of bovine femur using a micro-Brillouin scattering technique. Using thin plate specimens, wave velocities propagating in the bone axis direction were measured. Next, focusing on the hydroxyapatite (HAp), which is one of the main components of bone, we estimated the relationship between wave velocity and HAp content. The decalcification caused a clear wave velocity decrease from 5.06×103 to 3.28×103 m/s, showing the strong effects of HAp on the elasticity of bone. The micro-Brillouin scattering technique would be helpful for the evaluation of bone characterization in a small area.

Fukui, Kenji; Takayanagi, Shinji; Suga, Daisuke; Matsukawa, Mami

2012-07-01

249

Prediction of Building Limestone Physical and Mechanical Properties by Means of Ultrasonic P-Wave Velocity  

PubMed Central

The aim of this study was to evaluate ultrasonic P-wave velocity as a feature for predicting some physical and mechanical properties that describe the behavior of local building limestone. To this end, both ultrasonic testing and compressive tests were carried out on several limestone specimens and statistical correlation between ultrasonic velocity and density, compressive strength, and modulus of elasticity was studied. The effectiveness of ultrasonic velocity was evaluated by regression, with the aim of observing the coefficient of determination r2 between ultrasonic velocity and the aforementioned parameters, and the mathematical expressions of the correlations were found and discussed. The strong relations that were established between ultrasonic velocity and limestone properties indicate that these parameters can be reasonably estimated by means of this nondestructive parameter. This may be of great value in a preliminary phase of the diagnosis and inspection of stone masonry conditions, especially when the possibility of sampling material cores is reduced. PMID:24511286

Concu, Giovanna; De Nicolo, Barbara; Valdes, Monica

2014-01-01

250

Three-dimensional P-wave velocity structure of Mt. Etna, Italy  

USGS Publications Warehouse

The three-dimensional P-wave velocity structure of Mt. Etna is determined to depths of 15 km by tomographic inversion of first arrival times from local earthquakes recorded by a network of 29 permanent and temporary seismographs. Results show a near-vertical low-velocity zone that extends from beneath the central craters to a depth of 10 km. This low-velocity region is coincident with a band of steeply-dipping seismicity, suggesting a magmatic conduit that feeds the summit eruptions. The most prominent structure is an approximately 8-km-diameter high-velocity body located between 2 and 12 km depth below the southeast flank of the volcano. This high-velocity body is interpreted as a remnant mafic intrusion that is an important structural feature influencing both volcanism and east flank slope stability and faulting.

Villasenor, A.; Benz, H.M.; Filippi, L.; De Luca, G.; Scarpa, R.; Patane, G.; Vinciguerra, S.

1998-01-01

251

Prediction of building limestone physical and mechanical properties by means of ultrasonic P-wave velocity.  

PubMed

The aim of this study was to evaluate ultrasonic P-wave velocity as a feature for predicting some physical and mechanical properties that describe the behavior of local building limestone. To this end, both ultrasonic testing and compressive tests were carried out on several limestone specimens and statistical correlation between ultrasonic velocity and density, compressive strength, and modulus of elasticity was studied. The effectiveness of ultrasonic velocity was evaluated by regression, with the aim of observing the coefficient of determination r(2) between ultrasonic velocity and the aforementioned parameters, and the mathematical expressions of the correlations were found and discussed. The strong relations that were established between ultrasonic velocity and limestone properties indicate that these parameters can be reasonably estimated by means of this nondestructive parameter. This may be of great value in a preliminary phase of the diagnosis and inspection of stone masonry conditions, especially when the possibility of sampling material cores is reduced. PMID:24511286

Concu, Giovanna; De Nicolo, Barbara; Valdes, Monica

2014-01-01

252

Using second-sound shock waves to probe the intrinsic critical velocity of liquid helium II  

NASA Technical Reports Server (NTRS)

A critical velocity truly intrinsic to liquid helium II is experimentally sought in the bulk fluid far from the apparatus walls. Termed the 'fundamental critical velocity,' it necessarily is caused by mutual interactions which operate between the two fluid components and which are activated at large relative velocities. It is argued that flow induced by second-sound shock waves provides the ideal means by which to activate and isolate the fundamental critical velocity from other extraneous fluid-wall interactions. Experimentally it is found that large-amplitude second-sound shock waves initiate a breakdown in the superfluidity of helium II, which is dramatically manifested as a limit to the maximum attainable shock strength. This breakdown is shown to be caused by a fundamental critical velocity. Secondary effects include boiling for ambient pressures near the saturated vapor pressure or the formation of helium I boundary layers at higher ambient pressures. When compared to the intrinsic critical velocity discovered in highly restricted geometries, the shock-induced critical velocity displays a similar temperature dependence and is the same order of magnitude.

Turner, T. N.

1983-01-01

253

Wavefield Analysis of Rayleigh Waves for Near-Surface Shear-Wave Velocity  

E-print Network

to generate accurate synthetic seismograms focusing on Rayleigh waves in presence of surface topography and internal discontinuities. With these solutions to forward modeling of Rayleigh waves, I evaluate the influence of surface topography to conventional...

Zeng, Chong

2011-05-18

254

Three-Dimensional Shear Wave Velocity Structure of the Peru Flat Slab Subduction Segment  

NASA Astrophysics Data System (ADS)

Recent studies focused on flat slab subduction segments in central Chile (L. S. Wagner, 2006) and Alaska (B. R. Hacker and G. A. Aber, 2012) suggest significant differences in seismic velocity structures, and hence, composition in the mantle wedge between flat and normal "steep" subducting slabs. Instead of finding the low velocities and high Vp/Vs ratios common in normal subduction zones, these studies find low Vp, high Vs, and very low Vp/Vs above flat slabs. This may indicate the presence of dry, cold material in the mantle wedge. In order to investigate the seismic velocities of the upper mantle above the Peruvian flat segment, we have inverted for 2D Rayleigh wave phase velocity maps using data from the currently deployed 40 station PULSE seismic network and some adjacent stations from the CAUGHT seismic network. We then used the sensitivity of surface waves to shear wave velocity structure with depth to develop a 3D shear wave velocity model. This model will allow us to determine the nature of the mantle lithosphere above the flat slab, and how this may have influenced the development of local topography. For example, dry conditions (high Vs velocities) above the flat slab would imply greater strength of this material, possibly making it capable of causing further inland overthrusting, while wet conditions (low Vs) would imply weaker material. This could provide some insight into the ongoing debate over whether the Fitzcarrald arch (along the northern most flank of the Altiplano) could be a topographical response to the subducted Nazca ridge hundred kilometers away from the trench (N. Espurt, 2012, P. Baby, 2005, V. A. Ramos, 2012) or not (J. Martinod, 2005, M. Wipf, 2008, T. Gerya, 2008).

Knezevic Antonijevic, S.; Wagner, L. S.; Beck, S. L.; Zandt, G.; Long, M. D.

2012-12-01

255

The characteristics and implications of Pn and Sn wave velocities beneath the offshore area of eastern Taiwan, Gagua Ridge and the West Philippine Basin  

NASA Astrophysics Data System (ADS)

Four large offshore events with the local magnitude from 5.0 to 6.0 in northeastern Taiwan were well recorded by total 28 short-period OBSs (Ocean Bottom Seismometer) which were deployed as a linear array at a station-to-station distance of about 15 km in the West Philippine Basin during the Leg 4 of TAIGER (TAiwan Integrated GEodynamics Research) MCS/OBS (Multi-Channel Seismic/Ocean Bottom Seismometer) experiment in 2009. Not only the 4 large events but also more than 40 earthquakes with the local magnitude larger than 3.0 occurred in the offshore region from northeastern Taiwan to southeastern Taiwan were recorded as well. The data provides a good opportunity for investigating the characteristics and discussing the implications of Pn and Sn wave velocities beneath the offshore area of eastern Taiwan, Gagua Ridge and the West Philippine Basin. The results support that: 1. The Pn and Sn wave velocities beneath the investigated area are 7.85 km/sec and 4.46 km/sec on average. 2. The Pn velocities display lateral variations, they are slower in the northern part and faster in the southern part in this area and the velocities can be from 7.8 to 8.2 km/sec. However, the Sn velocities do not display similar variations, it implies the change of Poisson's ratio; the cause may be temperature or fluid. 3. The variation from azimuth could be an effect to influence observed Pn wave velocity, it implies that the velocity structure for locating earthquakes which occur in this area must be more complicated than the one which is often used.

Huang, Y.; Wang, S.; Lee, C.

2013-12-01

256

Velocity Dispersion and Attenuation of Acoustic Waves in Granular Sedimentary Media.  

NASA Astrophysics Data System (ADS)

An experimental and theoretical investigation of the effects of stress, frequency, and clay content on compressional and shear wave velocities and attenuations has been conducted using tight gas sandstone samples. The ultrasonic pulse transmission technique (~ 1 MHz) was used to measure velocities and attenuations and calculate dynamic moduli of fully brine saturated samples with porosities from 3 to 11.9 percent and clay contents from 1 to 38 percent. Simultaneous measurements were carried out to record axial and radial deformation under a biaxial stress state in order to calculate the static elastic moduli. The static moduli were found to be 1 to 6 times smaller than the dynamic moduli under the stress state. The velocities measured at ultrasonic frequency were also compared to the sonic log velocities (~20 KHz) in order to investigate dispersion effects. The trend observed in P and S wave velocities in homogeneous intervals shows that clean sandstone velocities measured in the ultrasonic frequency range deviate systematically from the log derived velocities. Compressional and shear wave amplitude data exhibited a shift in peak frequency toward lower frequencies for clay rich samples as compared to clean samples showing the important role clays play in the dissipative behavior of sandstones. The deviations from the log derived velocities are correlatable in most cases to the clay content and dispersion. The presence of clay softens the rock grain contacts and causes larger contact area values compared to the values for nearly clean rock under the same applied load. The frame moduli of sedimentary rocks are strongly influenced by the properties of the grain contacts. A modified Hertz contact theory is presented for the self consistent calculation of contact deformation, equilibrium separation distance (film thickness) and contact area for two spherical asperities in contact and subjected to an external load. It is shown that surface forces, i.e. electrostatic repulsion, Born, structural, and Van der Waals forces can be incorporated into the contact deformation problem. These forces play an important role in determining seismic wave velocities and attenuations at low confining stresses. The computed equilibrium separation distances and contact radii were used to calculate velocities and attenuations as a function of frequency and compared with measured values for glass beads, Navajo, Berea, Obernkirchner and Fort Union sandstones. The velocities and attenuations calculated as functions of stress, frequency, fluid type and saturation are all in good agreement with reported experimental data.

Tutuncu, Azra Nur

257

Total-variation-based methods for gravitational wave denoising  

E-print Network

We describe new methods for denoising and detection of gravitational waves embedded in additive Gaussian noise. The methods are based on Total Variation denoising algorithms. These algorithms, which do not need any a priori information about the signals, have been originally developed and fully tested in the context of image processing. To illustrate the capabilities of our methods we apply them to two different types of numerically-simulated gravitational wave signals, namely bursts produced from the core collapse of rotating stars and waveforms from binary black hole mergers. We explore the parameter space of the methods to find the set of values best suited for denoising gravitational wave signals under different conditions such as waveform type and signal-to-noise ratio. Our results show that noise from gravitational wave signals can be successfully removed with our techniques, irrespective of the signal morphology or astrophysical origin. We also combine our methods with spectrograms and show how those can be used simultaneously with other common techniques in gravitational wave data analysis to improve the chances of detection.

Alejandro Torres; Antonio Marquina; José A. Font; José M. Ibáñez

2014-09-28

258

Crust and upper mantle heterogeneities in the southwest Pacific from surface wave phase velocity analysis  

NASA Astrophysics Data System (ADS)

Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8-100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283-294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well with most of previous studies: the tomographic imaging shows a large contrast between low and high phase velocities along the Solomon, New Hebrides and Fiji-Tonga trenches. The lowest phase velocity anomalies are distributed beneath northern and southern Fiji basins and the Lau basin (corresponding to the volume situated just above the dipping slabs), whereas the highest values are displayed beneath the Pacific plate and the eastern part of Indian plate downgoing under the North Fiji basin. At shorter periods, our results show that the phase velocity distributions are well correlated with the large structural crustal domains. The use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well-dispersed trains, allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important. The continental regions (Eastern Australia, New Guinea, Fiji islands and New Zealand) show low velocities which are likely due to thick continental crust, whereas the Tasmanian, D'Entrecasteaux, and the Northern and Southern Fiji basins are characterized by higher velocities suggesting thinner oceanic crust. Additional analysis including the anisotropic case and S-wave velocity inversion with depth is in progress.

Pillet, R.; Rouland, D.; Roult, G.; Wiens, D. A.

1999-02-01

259

Crustal thickness variation beneath the Romanian seismic network from Rayleigh wave dispersion and receiver function analysis  

NASA Astrophysics Data System (ADS)

Variations in crustal thickness in Romania where determined by joint inversion of P wave receiver functions (RFs) and Rayleigh wave group velocity dispersion. We present new models of shear wave velocity structure of the crust beneath Romanian broad band stations. The data set consist in more than 500 teleseismic earthquake with epicentral distance between 30° and 95°, magnitude greater than 6 and a signal-to-noise ratio greater than 3 for the P-wave pulse. Most epicenters are situated along the northern Pacific Rim and arrive with backazimuths (BAZs) between 0° and 135° at the Romanian seismic network. We combine receiver functions with fundamental-mode of the Rayleigh wave group velocities to further constrain the shear-wave velocity structure.To extract the group velocities we applied the Multiple Filter Technique analysis to the vertical components of the earthquakes recordings. This technique allowed us to identify the Rayleigh wave fundamental mode and to compute the dispersion curves of the group velocities at periods between 10 and 150 s allowing us to resolve shear wave velocities to a depth of 100 km. The time-domain iterative deconvolution procedure of Ligorr?a and Ammon (1999) was employed to deconvolve the vertical component of the teleseismic P waveforms from the corresponding horizontal components and obtain radial and transverse receiver functions at each broadband station. The data are inverted using a joint, linearized inversion scheme (Hermann, 2002) which accounts for the relative influence of each set of observations, and allows a trade-off between fitting the observations, constructing a smooth model, and matching a priori constraints. The results show a thin crust for stations located inside the Pannonian basin (28-30 km) and a thicker crust for those in the East European Platform (36-40 km). The stations within the Southern and Central Carpathian Orogen are characterized by crustal depths of ~35 km. For stations located in the Northern part of the Eastern Carpathians we found a crustal depth of 32 km. For two station located in the Apuseni Mountains the Moho discontinuity is replace by a transition zone extended between 36 to 40 km depth. For a station located in the Carpathians bent area we identify a double Moho (32 respectively 44 km depth) possible due to the Vrancea subduction process. For the crust of Moesian Platform we get higher values (~35 km) compare to those obtained from seismic refraction profile (VRANCEA'2001). The North Dobrogea crust reaches a thickness of about 44-46 km. For most of the stations the crust-mantle transition zone has a significant gradient, with velocity values varying from 3.8 to 4.7 km/s. Our results are compatible with results from previous studies.

Tataru, Dragos; Grecu, Bogdan; Zaharia, Bogdan

2014-05-01

260

Aortic Pulse Wave Velocity as a Marker of Cardiovascular Risk in Hypertensive Patients  

Microsoft Academic Search

Large artery damage is a major contributory factor to cardiovascular morbidity and mortality of patients with hypertension. Pulse wave velocity (PWV), a classic evaluation of arterial distensibility, has never been ascertained as a cardiovascular risk marker. To determine the factors influencing aortic PWV and the potential predictor role of this measurement, we studied a cohort of 710 patients with essential

Jacques Blacher; Roland Asmar; Saliha Djane; Gerard M. London; Michel E. Safar

261

An empirical method to estimate shear wave velocity of soils in the New Madrid seismic zone  

USGS Publications Warehouse

In this study, a set of charts are developed to estimate shear wave velocity of soils in the New Madrid seismic zone (NMSZ), using the standard penetration test (SPT) N values and soil depths. Laboratory dynamic test results of soil samples collected from the NMSZ showed that the shear wave velocity of soils is related to the void ratio and the effective confining pressure applied to the soils. The void ratio of soils can be estimated from the SPT N values and the effective confining pressure depends on the depth of soils. Therefore, the shear wave velocity of soils can be estimated from the SPT N value and the soil depth. To make the methodology practical, two corrections should be made. One is that field SPT N values of soils must be adjusted to an unified SPT N??? value to account the effects of overburden pressure and equipment. The second is that the effect of water table to effective overburden pressure of soils must be considered. To verify the methodology, shear wave velocities of five sites in the NMSZ are estimated and compared with those obtained from field measurements. The comparison shows that our approach and the field tests are consistent with an error of less than of 15%. Thus, the method developed in this study is useful for dynamic study and practical designs in the NMSZ region. Copyright ?? 1996 Elsevier Science Limited.

Wei, B.-Z.; Pezeshk, S.; Chang, T.-S.; Hall, K.H.; Liu, H.-P.

1996-01-01

262

Increase of shear wave velocity before the 1998 eruption of Merapi volcano (Indonesia)  

E-print Network

Increase of shear wave velocity before the 1998 eruption of Merapi volcano (Indonesia) U. Wegler,1 of the edifice of Merapi volcano (Java, Indonesia) before its eruption in 1998 by analyzing multiply scattered be used as a ``pressure-gauge'' for pressure changes inside of volcanoes, because increasing pressures

Snieder, Roel

263

The Velocity of Compressional Waves in Rocks to 10 Kilobars, Part 2  

Microsoft Academic Search

The measurements of the velocity of compressional waves up to 10 kilobars for some 250 specimens of rock, reported in part 1, are discussed with respect to the effects of porosity, alteration, anisotropy, and composition. The relations of isotropic elasticity are shown to be approximately valid for a number of examples. Reasonable agreement with theoretical values for quasi-isotropic aggregates is

Francis Birch

1961-01-01

264

ACE stimulated neural network for shear wave velocity determination from well logs  

NASA Astrophysics Data System (ADS)

Shear wave velocity provides invaluable information for geomechanical, geophysical, and reservoir characterization studies. However, measurement of shear wave velocity is time, cost and labor intensive. This study proposes a swift and exact methodology, called ACE stimulated neural network, for prediction of shear wave velocity from available well logs such that it will be able to surpass previous models. The proposed method is composed of two major parts: 1) transforming input/output data space to a higher correlated space using alternative condition expectation (ACE), and 2) making a neural network formulation in transformed data space. Transforming in the first step makes it easier for neural network to find the complicated underlying dependency of input/output data. Therefore, neural network will be able to develop an accurate and strong formulation between conventional well logs and shear wave velocity. The Propounded approach was successfully applied in one of the carbonate gas fields of Iran. A comparison between proposed model and previous models showed superiority of ACE stimulated neural network.

Asoodeh, Mojtaba; Bagheripour, Parisa

2014-08-01

265

Wave breaking in dispersion-decreasing fiber with normal group-velocity dispersion  

Microsoft Academic Search

We show wave breaking (WB) can occur in a dispersion decreasing fiber with normal group-velocity dispersion preceding the parabolic pulse formation (PPF), and the distance where it happens can be described by two equations. Based on the transformation of a nonlinear Schr dinger equation with the typical decreasing dispersion into the form of the uniform dispersion and “equivalent” gain, the

Ge Xia; Li Liu; Songzhan Li; Libing Zhou; Dejun Li

2009-01-01

266

Electrostatic ion-cyclotron waves driven by parallel velocity shear R. L. Merlinoa)  

E-print Network

Electrostatic ion-cyclotron waves driven by parallel velocity shear R. L. Merlinoa) Department cyclotron frequency propagating at large angles to the ambient magnetic field can be excited in a magnetized in determining the stability of shear-driven electrostatic ion-cyclotron EIC modes is also considered

Merlino, Robert L.

267

Stress dependent wave velocities in sedimentary rock cores: Why and why not?  

Microsoft Academic Search

Rocks exhibit stress dependent wave velocities when their constitutive behaviour shows nonlinear elasticity. The main physical mechanism responsible for stress dependency is opening or closure of cracks, which is particularly important in brittle rocks. Crack formation is also associated with breaking of grain contacts. We observe, by using synthetic sandstones formed under stress, a strong stress dependence during unloading (simulated

R. M. Holt; A.-K. Furre; P. Horsrud

1997-01-01

268

Near-surface characterization of a geotechnical site in north-east Missouri using shear-wave velocity measurements  

USGS Publications Warehouse

Shear-wave velocity (Vs) as a function of soil stiffness is an essential parameter in geotechnical characterization of the subsurface. In this study, multichannel analysis of surface wave (MASW) and downhole methods were used to map the shear-wave velocity-structure and depth to the bed-rock surface at a 125m ?? 125m geotechnical site in Missouri. The main objective was to assess the suitability of the site for constructing a large, heavy building. The acquired multichannel surface wave data were inverted to provide 1D shear-wave velocity profile corresponding to each shot gather. These 1D velocity profiles were interpolated and contoured to generate a suite of 2D shear-wave velocity sections. Integrating the shear-wave velocity data from the MASW method with the downhole velocity data and the available borehole lithologic information enabled us to map shear-wave velocity-structure to a depth on the order of 20m. The bedrock surface, which is dissected by a significant cut-and-fill valley, was imaged. The results suggest that the study site will require special consideration prior to construction. The results also demonstrate the successful use of MASW methods, when integrated with downhole velocity measurements and borehole lithologic information, in the characterization of the near surface at the geotechnical sites. ?? 2007 European Association of Geoscientists & Engineers.

Ismail, A.; Anderson, N.

2007-01-01

269

Shear wave anisotropy from aligned inclusions: ultrasonic frequency dependence of velocity and attenuation  

NASA Astrophysics Data System (ADS)

To understand their influence on elastic wave propagation, anisotropic cracked media have been widely investigated in many theoretical and experimental studies. In this work, we report on laboratory ultrasound measurements carried out to investigate the effect of source frequency on the elastic parameters (wave velocities and the Thomsen parameter ?) and shear wave attenuation) of fractured anisotropic media. Under controlled conditions, we prepared anisotropic model samples containing penny-shaped rubber inclusions in a solid epoxy resin matrix with crack densities ranging from 0 to 6.2 per cent. Two of the three cracked samples have 10 layers and one has 17 layers. The number of uniform rubber inclusions per layer ranges from 0 to 100. S-wave splitting measurements have shown that scattering effects are more prominent in samples where the seismic wavelength to crack aperture ratio ranges from 1.6 to 1.64 than in others where the ratio varied from 2.72 to 2.85. The sample with the largest cracks showed a magnitude of scattering attenuation three times higher compared with another sample that had small inclusions. Our S-wave ultrasound results demonstrate that elastic scattering, scattering and anelastic attenuation, velocity dispersion and crack size interfere directly in shear wave splitting in a source-frequency dependent manner, resulting in an increase of scattering attenuation and a reduction of shear wave anisotropy with increasing frequency.

de Figueiredo, J. J. S.; Schleicher, J.; Stewart, R. R.; Dayur, N.; Omoboya, B.; Wiley, R.; William, A.

2013-04-01

270

Subduction zones beneath Indonesia imaged by Rayleigh wave phase velocity tomography  

NASA Astrophysics Data System (ADS)

Situated at the junction of several tectonic plates including Indian-Australia, Eurasia, and Philippine Sea, the Indonesian archipelago is one of the most tectonically complex regions on earth with subductions, collisions and accretions occurring along and within its boundaries. A high-resolution lithospheric and upper mantle model, therefore, is needed to understand these complex processes beneath this region. We present a phase velocity model derived from teleseismic Rayleigh waves recorded at seismic stations in this region. We use the modified version of the two-plane wave tomography, in which the non-planar effects of surface wave propagation such as multipathing and scattering are accounted for by two plane wave interference and using of finite frequency kernels. We measure the amplitudes and phases at 16 individual periods ranging from 20s to 150s for the fundamental mode of Rayleigh waves at over 30 stations. 254 earthquakes are selected from global events greater than Ms 5.5 in the distance range of 25°- 150°. To account for the wavefield inconsistencies among stations for each earthquake due to the large scale of our study region, we divide the seismic array into 4 groups of stations in the two-plane wave parameter inversion. The phase velocity maps from our preliminary results show coherent features between adjacent periods. The most dominant structure in phase velocity maps for all periods is the strong fast-velocity belts beneath Sunda Trench, Java Trench, Timor Trough and the trenches around Celebes Sea, which shift gradually toward the subduction directions. The strength of the high velocity anomaly varies among trenches, likely suggesting the different age of subducting slabs. In addition, a velocity contrast in the middle of Borneo appears to mark the Lupar Line, a boundary between the stable Sundaland continental core and fragments of ophiolitic and Asian continental material accreted to Borneo during the Cretaceous. The 3-D shear wave structure derived from these 2-D phase velocity maps at different periods, which is in progress, certainly will sharpen the images of the complex subduction system, unraveling more geodynamic processes in this region.

Liu, F.; Yang, T.; Harmon, N.

2013-12-01

271

Developing regionalized models of lithospheric thickness and velocity structure across Eurasia and the Middle East from jointly inverting P-wave and S-wave receiver functions with Rayleigh wave group and phase velocities  

SciTech Connect

In this project, we are developing models of lithospheric structure for a wide variety of tectonic regions throughout Eurasia and the Middle East by regionalizing 1D velocity models obtained by jointly inverting P-wave and S-wave receiver functions with Rayleigh wave group and phase velocities. We expect the regionalized velocity models will improve our ability to predict travel-times for local and regional phases, such as Pg, Pn, Sn and Lg, as well as travel-times for body-waves at upper mantle triplication distances in both seismic and aseismic regions of Eurasia and the Middle East. We anticipate the models will help inform and strengthen ongoing and future efforts within the NNSA labs to develop 3D velocity models for Eurasia and the Middle East, and will assist in obtaining model-based predictions where no empirical data are available and for improving locations from sparse networks using kriging. The codes needed to conduct the joint inversion of P-wave receiver functions (PRFs), S-wave receiver functions (SRFs), and dispersion velocities have already been assembled as part of ongoing research on lithospheric structure in Africa. The methodology has been tested with synthetic 'data' and case studies have been investigated with data collected at an open broadband stations in South Africa. PRFs constrain the size and S-P travel-time of seismic discontinuities in the crust and uppermost mantle, SRFs constrain the size and P-S travel-time of the lithosphere-asthenosphere boundary, and dispersion velocities constrain average S-wave velocity within frequency-dependent depth-ranges. Preliminary results show that the combination yields integrated 1D velocity models local to the recording station, where the discontinuities constrained by the receiver functions are superimposed to a background velocity model constrained by the dispersion velocities. In our first year of this project we will (i) generate 1D velocity models for open broadband seismic stations in the western half of the study area (Eurasia and the Middle East) and (ii) identify well located seismic events with event-station paths isolated to individual tectonic provinces within the study area and collect broadband waveforms and source parameters for the selected events. The 1D models obtained from the joint inversion will then be combined with published geologic terrain maps to produce regionalized models for distinctive tectonic areas within the study area, and the models will be validated through full waveform modeling of well-located seismic events recorded at local and regional distances.

Julia, J; Nyblade, A; Hansen, S; Rodgers, A; Matzel, E

2009-07-06

272

Superconducting twin quarter wave resonator for acceleration of low velocity heavy ions  

NASA Astrophysics Data System (ADS)

We have designed and fabricated a superconducting twin quarter wave resonator (Twin-QWR) made of niobium and copper for the acceleration of low velocity heavy ions. The resonator has two inner conductors and three acceleration gaps, which give a resonant frequency of 129.8 MHz and an optimum beam velocity of 6% of the light velocity. Each inner conductor resonates like in a coaxial quarter-wave line resonator. The resonator was designed to have a separable structure so that we could treat the inner conductor's part fully made of high purity niobium apart from the outer conductor made of niobium and copper. We obtained an acceleration field gradient of 5.8 MV/m at an RF power input of 4 W.

Kabumoto, H.; Takeuchi, S.; Matsuda, M.; Ishizaki, N.; Otokawa, Y.

2010-01-01

273

Generation of electromagnetic waves in the very low frequency band by velocity gradient  

SciTech Connect

It is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector k{sup ¯}=kc/?{sub pe} (normalized by the electron skin depth) and the obliqueness, k{sub ?}/k{sub ||}, where k{sub ?,||} are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when k{sup ¯}?1 and k{sub ?}?k{sub ||} and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics.

Ganguli, G., E-mail: Gurudas.Ganguli@nrl.navy.mil; Tejero, E.; Crabtree, C.; Amatucci, W. [Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375-5346 (United States)] [Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375-5346 (United States); Rudakov, L. [Icarus Research Inc., P.O. Box 30780, Bethesda, Maryland 20824-0780 (United States)] [Icarus Research Inc., P.O. Box 30780, Bethesda, Maryland 20824-0780 (United States)

2014-01-15

274

Seismic wave propagation through a low-velocity nuclear rubble zone  

SciTech Connect

A 2-dimensional finite-difference code has been developed as part of the Hydroacoustic Nuclear Proliferation Monitoring Project (ST639). This code is used to model seismic and acoustic wave propagation in the Earth`s crust and oceanic water column. As a demonstration, we investigate the propagation of seismic energy through a low-velocity region corresponding to the rubble zone created by an underground nuclear explosion. The geometry of the simulated source and surface recording instruments is representative of a seismic refraction survey. The purpose of this numerical simulation is to determine perturbations in the seismic wave field due to the low velocity region (nuclear cavity) can be detected at the surface. Wave energy passing through the cavity is noticeably delayed, and is easily observed on cross-sections of the P and S wave fields. Synthetic seismograms recorded at the surface demonstrate an amplitude reduction and phase shift for those stations corresponding to ray-paths through the cavity. A significant fraction of energy is reflected off the low-velocity zone, and propagates as backward traveling waves.

Larsen, S.; Harris, D.

1993-10-01

275

A reappraisal of surface wave group velocity tomography in the Subantarctic Scotia Sea and surrounding ridges  

NASA Astrophysics Data System (ADS)

A reappraisal of surface wave tomography in the remote Scotia Sea region and surrounding ridges is presented. New group velocity dispersion curves were obtained from local and regional earthquakes recorded at permanent Antarctic stations from 2001 to 2013 and used to update the measurements reported by Vuan et al. (2000). Rayleigh and Love group velocity maps for periods ranging from 15 to 50 s were retrieved using a tomographic inversion. The group velocity anomalies are clearly associated with the major crustal and upper mantle features of the Antarctic, Scotia and South American plates. The updated dataset allows for considerable decrease of the correlation length of the crustal heterogeneities that can be resolved, especially in the west Scotia Sea, central Scotia Sea and Bransfield Basin. Surface wave tomography results were compared with CRUST 1.0 group velocity maps and revealed specific areas where more detailed information is made available by our regional study. In particular, low group velocity anomalies of the Bransfield Strait rifting and continental fragments that are detached from the Antarctic Peninsula and spreading along the South Scotia ridge are not shown by the reference CRUST 1.0 model. A comparison between the average seismic velocities beneath the west and central Scotia Sea shows that both have an oceanic-type structure; however, the crust of the central sea is thicker (12-14 km) and slower than that of the 20 Ma old western sea.

Vuan, A.; Sugan, M.; Plasencia Linares, M. P.

2014-12-01

276

Second sound shock waves and critical velocities in liquid helium 2. Ph.D. Thesis  

NASA Technical Reports Server (NTRS)

Large amplitude second-sound shock waves were generated and the experimental results compared to the theory of nonlinear second-sound. The structure and thickness of second-sound shock fronts are calculated and compared to experimental data. Theoretically it is shown that at T = 1.88 K, where the nonlinear wave steepening vanishes, the thickness of a very weak shock must diverge. In a region near this temperature, a finite-amplitude shock pulse evolves into an unusual double-shock configuration consisting of a front steepened, temperature raising shock followed by a temperature lowering shock. Double-shocks are experimentally verified. It is experimentally shown that very large second-sound shock waves initiate a breakdown in the superfluidity of helium 2, which is dramatically displayed as a limit to the maximum attainable shock strength. The value of the maximum shock-induced relative velocity represents a significant lower bound to the intrinsic critical velocity of helium 2.

Turner, T. N.

1979-01-01

277

Condition assessment of PC tendon duct filling by elastic wave velocity mapping.  

PubMed

Imaging techniques are high in demand for modern nondestructive evaluation of large-scale concrete structures. The travel-time tomography (TTT) technique, which is based on the principle of mapping the change of propagation velocity of transient elastic waves in a measured object, has found increasing application for assessing in situ concrete structures. The primary aim of this technique is to detect defects that exist in a structure. The TTT technique can offer an effective means for assessing tendon duct filling of prestressed concrete (PC) elements. This study is aimed at clarifying some of the issues pertaining to the reliability of the technique for this purpose, such as sensor arrangement, model, meshing, type of tendon sheath, thickness of sheath, and material type as well as the scale of inhomogeneity. The work involved 2D simulations of wave motions, signal processing to extract travel time of waves, and tomography reconstruction computation for velocity mapping of defect in tendon duct. PMID:24737961

Liu, Kit Fook; Chai, Hwa Kian; Mehrabi, Nima; Yoshikazu, Kobayashi; Shiotani, Tomoki

2014-01-01

278

Condition Assessment of PC Tendon Duct Filling by Elastic Wave Velocity Mapping  

PubMed Central

Imaging techniques are high in demand for modern nondestructive evaluation of large-scale concrete structures. The travel-time tomography (TTT) technique, which is based on the principle of mapping the change of propagation velocity of transient elastic waves in a measured object, has found increasing application for assessing in situ concrete structures. The primary aim of this technique is to detect defects that exist in a structure. The TTT technique can offer an effective means for assessing tendon duct filling of prestressed concrete (PC) elements. This study is aimed at clarifying some of the issues pertaining to the reliability of the technique for this purpose, such as sensor arrangement, model, meshing, type of tendon sheath, thickness of sheath, and material type as well as the scale of inhomogeneity. The work involved 2D simulations of wave motions, signal processing to extract travel time of waves, and tomography reconstruction computation for velocity mapping of defect in tendon duct. PMID:24737961

Liu, Kit Fook; Mehrabi, Nima; Yoshikazu, Kobayashi; Shiotani, Tomoki

2014-01-01

279

Variational stereo imaging of oceanic waves with statistical constraints.  

PubMed

An image processing observational technique for the stereoscopic reconstruction of the waveform of oceanic sea states is developed. The technique incorporates the enforcement of any given statistical wave law modeling the quasi-Gaussianity of oceanic waves observed in nature. The problem is posed in a variational optimization framework, where the desired waveform is obtained as the minimizer of a cost functional that combines image observations, smoothness priors and a weak statistical constraint. The minimizer is obtained by combining gradient descent and multigrid methods on the necessary optimality equations of the cost functional. Robust photometric error criteria and a spatial intensity compensation model are also developed to improve the performance of the presented image matching strategy. The weak statistical constraint is thoroughly evaluated in combination with other elements presented to reconstruct and enforce constraints on experimental stereo data, demonstrating the improvement in the estimation of the observed ocean surface. PMID:23807444

Gallego, Guillermo; Yezzi, Anthony; Fedele, Francesco; Benetazzo, Alvise

2013-11-01

280

Estimation of Atterberg limits and bulk mass density of an expansive soil from P-wave velocity measurements  

Microsoft Academic Search

This brief technical note reports the relationship between P-wave velocity and the Atterberg limits and bulk mass density\\u000a of an expansive soil from the Derince region of Turkey. Reasonably good correlations were found, which were improved when\\u000a the relationship was between P-wave velocity divided by water content.

Cengiz Kurtulus; Fadime Sertcelik; M. Mucella Canbay; ?brahim Sertcelik

2010-01-01

281

STRESS WAVE VELOCITY AND DYNAMIC MODULUS OF ELASTICITY OF YELLOW-POPLAR RANGING FROM 100 TO 10 PERCENT MOISTURE CONTENT  

Microsoft Academic Search

Moisture content has a significant impact on mechanical properties of wood. In recent years, stress wave velocity has been used as an in situ and non-destructive method for determining the stiffness of wooden elements. The objective of this study was to determine what effect moisture content has on stress wave velocity and dynamic modulus of elasticity. Results indicated significant relationships

Jody D. Gray; Shawn T. Grushecky; James P. Armstrong

282

Constraints on the deep thermal structure of the Dharwar craton, India, from heat flow, shear wave velocities, and mantle xenoliths  

Microsoft Academic Search

We have used constraints from seismic shear wave vertical velocity profiles, geothermobarometry estimates on mantle xenoliths, and surface heat flux and heat production measurements to analyze the thermal regime of the deep lithosphere beneath India. In the Dharwar craton of southern India, the shear wave velocity gradient in the mantle, as well as xenolith geothermobarometry data, suggests a low mantle

Sukanta Roy; Jean-Claude Mareschal

2011-01-01

283

Intima-media thickness of carotid artery and aortic pulse wave velocity as determinants of cerebral blood flow velocity.  

PubMed

The current study aims to check the relationship between parameters derived from brachial blood pressure, the carotid artery intima-media thickness (IMT), pulse wave velocity (PWV) and mean cerebral blood flow velocity (mCBFV) in the middle cerebral artery (MCA). In consecutive adult outpatients we recorded the brachial systolic blood pressure (SBP), diastolic blood pressure (DBP), pulse pressure (PP), mean arterial blood pressure (MAP), PWV and IMT. mCBFV was assessed using Doppler ultrasound probe applied to the transtemporal window. The mean±s.d. age of 165 patients (50% women) was 56.7±11.8 years. Women and men differed significantly in SBP, PP, total cholesterol and mCBFV. Age (r=-0.44, P<0.001) and BMI (r=-0.25, P<0.01) were significantly and reversely related to mCBFV. Compared with healthy individuals, hypertensive (P<0.05) and diabetic (P<0.01) patients had lower mCBFV. IMT and PWV were related to mCBFV (IMT, r=-0.36; P<0.001, and PWV, r=-0.34; P<0.001). After adjustment for possible confounders, the relationship between mCBFV and PWV did not retain statistical significance (P=0.54). However, the relationship between mCBFV and IMT remained statistically significant (P=0.02). The association between lower CBFV and higher IMT may constitute a link between increased IMT and risk of cerebrovascular events. PMID:24304708

Kwater, A; G?sowski, J; Grodzicki, T

2014-06-01

284

On the velocity variation in atmospheric pressure plasma plumes driven by positive and negative pulses  

SciTech Connect

To better understand the variation in the ''plasma bullet'' velocity, the dynamics of an atmospheric pressure plasma plume driven by positive and negative pulses are investigated in detail. It is found that, before the plasma exits the nozzle, the plasma propagates at a speed of about 30 km/s for both positive and negative pulses. As soon as the plasma exits the nozzle, the plasma propagation speed increases dramatically for both cases. The peak velocity for the case of the positive pulse is much higher than that of the negative pulse, it is approximately 150 km/s and 70 km/s, respectively. According to the optical emission spectra, the acceleration behavior of the plasma bullet when it exits the nozzle is due to the increase in the N{sub 2}{sup +} concentration.

Xiong, Z.; Lu, X.; Xian, Y.; Jiang, Z.; Pan, Y [College of Electrical and Electronic Engineering, HuaZhong University of Science and Technology, WuHan, Hubei 430074 (China)

2010-11-15

285

Determination of elastic anisotropy of rocks from P- and S-wave velocities: numerical modelling and lab measurements  

NASA Astrophysics Data System (ADS)

The most common type of waves used for probing anisotropy of rocks in laboratory is the direct P wave. Information potential of the measured P-wave velocity, however, is limited. In rocks displaying weak triclinic anisotropy, the P-wave velocity depends just on 15 linear combinations of 21 elastic parameters, called the weak-anisotropy parameters. In strong triclinic anisotropy, the P-wave velocity depends on the whole set of 21 elastic parameters, but inversion for six of them is ill-conditioned and these parameters are retrieved with a low accuracy. Therefore, in order to retrieve the complete elastic tensor accurately, velocities of S waves must also be measured and inverted. For this purpose, we developed a lab facility which allows the P- and S-wave ultrasonic sounding of spherical rock samples in 132 directions distributed regularly over the sphere. The velocities are measured using a pair of P-wave sensors with the transmitter and receiver polarized along the radial direction and using two pairs of S-wave sensors with the transmitter and receiver polarized tangentially to the spherical sample in mutually perpendicular directions. We present inversion methods of phase and ray velocities for elastic parameters describing general triclinic anisotropy. We demonstrate on synthetic tests that the inversion becomes more robust and stable if the S-wave velocities are included. This applies even to the case when the velocity of the S waves is measured in a limited number of directions and with a significantly lower accuracy than that of the P wave. Finally, we analyse velocities measured on a rock sample from the Outokumpu deep drill hole, Finland. We present complete sets of elastic parameters of the sample including the error analysis for several levels of confining pressure ranging from 0.1 to 70 MPa.

Svitek, Tomáš; Vavry?uk, Václav; Lokají?ek, Tomáš; Petružálek, Mat?j

2014-12-01

286

Combining different data to constrain the anisotropic shear-wave velocity structure of the Earth's mantle.  

NASA Astrophysics Data System (ADS)

Over the last 10 years, dozens of new stations of the Global Seismographic Network have been deployed in remote and previously uninstrumented regions. The seismograms recorded at these sites provide new constraints on the structure of the Earth's mantle. We investigate the effect of including new measurements of long-period body and mantle-wave waveforms in the tomographic inversion for the anisotropic shear-wave velocity structure of the mantle. To ensure good radial resolution throughout the mantle, we combine the waveform data with measurements of Rayleigh- and Love-wave phase velocities and a diverse data set of absolute and differential body-wave travel times. To further improve the ray-path coverage we test the consistency of the SS-S and ScS-S differential travel times (Bolton, 1996) with our travel-time data and include them in the inversion. All waveform, surface-wave and body-wave travel-time measurements are corrected according to the global crustal model CRUST 2.0 (Bassin et al., 2000). We parameterize the mantle model using 362 spherical splines and 14 radial B-splines and minimize both horizontal and vertical gradients to regularize the inverse problem. To alleviate the effect of an arbitrary choice between a continuous and split model across the 670-km discontinuity, we define a split parameterization and control the change across the discontinuity using norm damping. Finally, we focus our attention on the capability of the combined data set to resolve the anisotropic structure in the mantle parameterized in terms of the velocities of horizontally- and vertically-polarized shear waves.

Kustowski, B.; Ekström, G.; Dziewo?ski, A. M.

2004-12-01

287

Variational structure of inverse problems in wave propagation and vibration  

SciTech Connect

Practical algorithms for solving realistic inverse problems may often be viewed as problems in nonlinear programming with the data serving as constraints. Such problems are most easily analyzed when it is possible to segment the solution space into regions that are feasible (satisfying all the known constraints) and infeasible (violating some of the constraints). Then, if the feasible set is convex or at least compact, the solution to the problem will normally lie on the boundary of the feasible set. A nonlinear program may seek the solution by systematically exploring the boundary while satisfying progressively more constraints. Examples of inverse problems in wave propagation (traveltime tomography) and vibration (modal analysis) will be presented to illustrate how the variational structure of these problems may be used to create nonlinear programs using implicit variational constraints.

Berryman, J.G.

1995-03-01

288

Line formation in AGB atmospheres including velocity effects. Molecular line profile variations of long period variables  

NASA Astrophysics Data System (ADS)

Context. Towards the end of the evolutionary stage of the asymptotic giant branch (AGB) the atmospheres of evolved red giants are considerably influenced by radial pulsations of the stellar interiors and developing stellar winds. The resulting complex velocity fields severely affect molecular line profiles (shapes, time-dependent shifts in wavelength, multiple components) observable in near-infrared spectra of long period variables. Time-series high-resolution spectroscopy allows us to probe the atmospheric kinematics and thereby study the mass loss process. Aims: With the help of model calculations the complex line formation process in AGB atmospheres was explored with the focus on velocity effects. Furthermore, we aimed for atmospheric models which are able to quantitatively reproduce line profile variations found in observed spectra of pulsating late-type giants. Methods: Models describing pulsation-enhanced dust-driven winds were used to compute synthetic spectra under the assumptions of chemical equilibrium and LTE. For this purpose, we used molecular data from line lists for the considered species and solved the radiative transfer in spherical geometry including the effects of velocity fields. Radial velocities (RV) derived from Doppler-shifted (components of) synthetic line profiles provide information on the gas velocities in the line-forming region of the spectral features. In addition, we made use of radial optical depth distributions to give estimates for the layers where lines are formed and to illustrate the effects of velocities in the line formation process. Results: Assuming uniform gas velocities for all depth points of an atmospheric model we estimated the conversion factor between gas velocities and measured RVs to p = ugas/RV ? 1.2-1.5. On the basis of dynamic model atmospheres and by applying our spectral synthesis codes we investigated in detail the finding that various molecular features in AGB spectra originate at different geometrical depths of the very extended atmospheres of these stars. We show that the models are able to quantitatively reproduce the characteristic line profile variations of lines sampling the deep photosphere (CO ?v = 3, CN) of Mira variables and the corresponding discontinuous, S-shaped RV curve. The global velocity fields (traced by different features) of typical long-period variables are also realistically reproduced. Possible reasons for discrepancies concerning other modelling results (e.g. CO ?v = 2 lines) are outlined. In addition, we present a model showing variations of CO ?v = 3 line profiles comparable to observed spectra of semiregular variables and discuss that the non-occurence of line doubling in these objects may be due to a density effect. Conclusions: The results of our line profile modelling are another indication that the dynamic models studied here are approaching a realistic representation of the outer layers of AGB stars with or without mass loss.

Nowotny, W.; Höfner, S.; Aringer, B.

2010-05-01

289

Noninvasive assessment of vocal fold mucosal wave velocity using color doppler imaging.  

PubMed

The vibratory movement of the vocal folds (VF) plays an important role in normal function of phonation. We developed a noninvasive technique to quantify the human mucosal wave velocity (MWV) in vivo using color Doppler imaging (CDI). During phonation, the motion of mucosa-air interface generates a unique pattern of US color artefacts that assist the identification of true VF location. An in vitro study using a vibrating string phantom was conducted to investigate how the CDI displayed a vibrating soft tissue at high frequency. The vibrating amplitude, frequency, mass density and the acoustic impedance of the soft tissues were found to dominate the formation of color artefacts. Based on the model of finite string with fixed ends, we estimated the mean MWV for 10 adult volunteers (6 men, 4 women, ages 34 +/- 5 years) with normal VF function. The mean MWVs for the men were found to vary from 2.1 to 10 m/s in a frequency range of 85 to 310 Hz at their comfortable pitch and intensity, and the women typically had higher MWVs that varied from 5.0 to 16.5 m/s in a frequency range of 180 to 480 Hz. The MWV increased linearly with the frequency and there was no observable difference in mucosa stiffness due to the effect of gender. The variation in MWV as it propagates vertically can be seen from the color and shape of the artefacts. The VF polyp resulted in abnormal MWV and different CDI vibratory artefacts. The CDI artefacts provide insight on the dynamics of mucosa structure during phonation, and the method presented is promising for noninvasive monitoring of laryngeal functions clinically. PMID:11750743

Shau, Y W; Wang, C L; Hsieh, F J; Hsiao, T Y

2001-11-01

290

Seismic wave velocity of rocks in the Oman ophiolite: constraints for petrological structure of oceanic crust  

NASA Astrophysics Data System (ADS)

Evaluation of rock velocities and comparison with velocity profiles defined by seismic refraction experiments are a crucial approach for understanding the petrological structure of the crust. In this study, we calculated the seismic wave velocities of various types of rocks from the Oman ophiolite in order to constrain a petrological structure of the oceanic crust. Christensen & Smewing (1981, JGR) have reported experimental elastic velocities of rocks from the Oman ophiolite under oceanic crust-mantle conditions (6-430 MPa). However, in their relatively low-pressure experiments, internal pore-spaces might affect the velocity and resulted in lower values than the intrinsic velocity of sample. In this study we calculated the velocities of samples based on their modal proportions and chemical compositions of mineral constituents. Our calculated velocities represent the ‘pore-space-free’ intrinsic velocities of the sample. We calculated seismic velocities of rocks from the Oman ophiolite including pillow lavas, dolerites, plagiogranites, gabbros and peridotites at high-pressure-temperature conditions with an Excel macro (Hacker & Avers 2004, G-cubed). The minerals used for calculations for pillow lavas, dolerites and plagiogranites were Qtz, Pl, Prh, Pmp, Chl, Ep, Act, Hbl, Cpx and Mag. Pl, Hbl, Cpx, Opx and Ol were used for the calculations for gabbros and peridotites. Assuming thermal gradient of 20° C/km and pressure gradient of 25 MPa/km, the velocities were calculated in the ranges from the atmospheric pressure (0° C) to 200 MPa (160° C). The calculation yielded P-wave velocities (Vp) of 6.5-6.7 km/s for the pillow lavas, 6.6-6.8 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.9-7.5 km/s for the gabbros and 8.1-8.2 km/s for the peridotites. On the other hand, experimental results reported by Christensen & Smewing (1981, JGR) were 4.5-5.9 km/s for the pillow lavas, 5.5-6.3 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.5-7.7 km/s for the gabbros and 6.3-7.9 km/s for the peridotites. Although the two results are broadly comparable to each other for plagiogranites and gabbros, the calculated velocities are considerably higher than the experimental ones for pillow lavas, dolerites and peridotites. The discrepancy for the pillow lavas and dolerites can be attributed to the presence of pore-spaces in the experimental samples. On the other hand, serpentinization of peridotite samples likely resulted in lower velocities in experiments than in calculation. We compared our results with Vp structure of the oceanic crust and mantle (White et al. 1992, JGR). The calculated Vp of peridotites and gabbros are comparable to those of mantle and layer-3, respectively. The calculated Vp of dolerites is comparable to layer-3 and considerably higher than layer-2 velocities. However, recent deep drilling results (Holes 504B and 1256D) indicate the seismic layer-2 of oceanic crust mainly composed of dolerites, which is consistent with the experimental P-wave velocities of dolerites (Christensen & Smewing, 1981, JGR). These results imply that the velocity structure of seismic layer-2 reflects the distribution of pore-spaces in the upper oceanic crust.

Saito, S.; Ishikawa, M.; Shibata, S.; Akizuki, R.; Arima, M.; Tatsumi, Y.; Arai, S.

2010-12-01

291

Velocity ratio variations in the source region of earthquake swarms in NW Bohemia obtained from arrival time double-differences  

NASA Astrophysics Data System (ADS)

Crustal earthquake swarms are an expression of intensive cracking and rock damaging over periods of days, weeks or month in a small source region in the crust. They are caused by longer lasting stress changes in the source region. Often, the localized stressing of the crust is associated with fluid or gas migration, possibly in combination with pre-existing zones of weaknesses. However, verifying and quantifying localized fluid movement at depth remains difficult since the area affected is small and geophysical prospecting methods often cannot reach the required resolution. We apply a simple and robust method to estimate the velocity ratio between compressional (P) and shear (S) waves (vP/vS-ratio) in the source region of an earthquake swarm. The vP/vS-ratio may be unusual small if the swarm is related to gas in a porous or fractured rock. The method uses arrival time difference between P and S waves observed at surface seismic stations, and the associated double differences between pairs of earthquakes. An advantage is that earthquake locations are not required and the method seems lesser dependent on unknown velocity variations in the crust outside the source region. It is, thus, suited for monitoring purposes. Applications comprise three natural, mid-crustal (8-10 km) earthquake swarms between 1997 and 2008 from the NW-Bohemia swarm region. We resolve a strong temporal decrease of vP/vS before and during the main activity of the swarm, and a recovery of vP/vS to background levels at the end of the swarms. The anomalies are interpreted in terms of the Biot-Gassman equations, assuming the presence of oversaturated fluids degassing during the beginning phase of the swarm activity.

Dahm, Torsten; Fischer, Tomas

2014-02-01

292

Regional variations in upper mantle compressional velocities beneath southern California 1. Post-shock temperatures: Their experimental determination, calculation, and implications, 2.. Ph.D. Thesis  

NASA Technical Reports Server (NTRS)

The compressional velocity within the upper mantle beneath Southern California is investigated through observations of the dependence of teleseismic P-delays at all stations of the array on the distance and azimuth to the event. The variation of residuals with azimuth was found to be as large as 1.3 sec at a single station; the delays were stable as a function of time, and no evidence was found for temporal velocity variations related to seismic activity in the area. These delays were used in the construction of models for the upper mantle P-velocity structure to depths of 150 km, both by ray tracing and inversion techniques. The models exhibit considerable lateral heterogeneity including a region of low velocity beneath the Imperial Valley, and regions of increased velocity beneath the Sierra Nevada and much of the Transverse Ranges. The development is described of a technique for the experimental determination of post-shock temperatures, and its application to several metals and silicates shocked to pressures in the range 5 to 30 GPa. The technique utilizes an infra-red radiation detector to determine the brightness temperature of the free surface of the sample after the shock wave has passed through it.

Raikes, S. A.

1978-01-01

293

Mass variation governed by the universe expansion velocity and the cosmic acceleration  

E-print Network

Much effort has been made in trying to solve, or at least evade, the inconsistencies that emerge from general relativity as the framework for a cosmological model. The extradimensional models rise as superb possibilities on this regard. In this work I present Wesson's Space-Time-Matter theory of gravity cosmological solutions. A relation between mass variation at cosmological scales and the expansion velocity of the universe is obtained. Such a novelty on Space-Time-Matter theory of gravity predicts a transition from a decelerated to an accelerated phase of the universe expansion.

Pedro H. R. S. Moraes

2015-02-10

294

Effect of phase transitions on compressional-wave velocities in the Earth's mantle.  

PubMed

The velocities of seismic waves in the Earth are governed by the response of the constituent mineral assemblage to perturbations in pressure and stress. The effective bulk modulus is significantly lowered if the pressure of the seismic wave drives a volume-reducing phase transformation. A comparison between the amount of time required by phase transitions to reach equilibrium and the sampling period thus becomes crucial in defining the softening and attenuation of compressional waves within such a two-phase zone. These phenomena are difficult to assess experimentally, however, because data at conditions appropriate to the Earth's deep interior are required. Here we present synchrotron-based experimental data that demonstrate softening of the bulk modulus within the two-phase loop of olivine-ringwoodite on a timescale of 100 s. If the amplitude of the pressure perturbation and the grain size are scaled to those expected in the Earth, the compressional-wave velocities within the discontinuities at 410, 520 and, possibly, 660 km are likely to be significantly lower than otherwise expected. The generalization of these observations to aluminium-controlled phase transitions raises the possibility of large velocity perturbations throughout the upper 1,000 km of the mantle. PMID:18719587

Li, Li; Weidner, Donald J

2008-08-21

295

In vivo noninvasive method for measuring local wave velocity in femoral arteries of pig  

NASA Astrophysics Data System (ADS)

We have proposed generating a bending wave in the arterial wall using ultrasound radiation force and measuring the wave velocity along the arterial wall [Zhang et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 52, 642-652 (2005)]. Here, we report the results of in vivo studies on pigs. The pig was anesthetized, and a micromanometer tip catheter was inserted into the femoral artery to measure luminal pressure. A water bath was created on the animal's groin to allow unimpeded access of the ultrasound beams to the femoral artery. The femoral artery was first located using a 13-MHz linear-array transducer. Then, a vibro-acoustography image was obtained to ensure precise positioning of the excitation force relative to the artery. The artery was excited by the force transducer and the resulting vibration of the arterial wall was measured by a sensing Doppler transceiver. Measured wave velocity was 3.1 m/s at 300 Hz. With this new method wave velocity over a distance of 5 mm, and therefore stiffness of arteries, can be measured locally and non-invasively. Measurement time is short in a few tens of milliseconds, which allows pressure dependence and pharmacological effect on the wall properties to be measured at different cardiac times.

Zhang, Xiaoming; Kinnick, Randall; Pislaru, Cristina; Fatemi, Mostafa; Greenleaf, James

2005-09-01

296

Evaluation of interlayer interfacial stiffness and layer wave velocity of multilayered structures by ultrasonic spectroscopy.  

PubMed

An ultrasonic evaluation procedure for the interlayer interfacial normal stiffness and the intralayer longitudinal wave velocity of multilayered plate-like structures is proposed. Based on the characteristics of the amplitude reflection spectrum of ultrasonic wave at normal incidence to a layered structure with spring-type interlayer interfaces, it is shown that the interfacial normal stiffness and the longitudinal wave velocity in the layers can be simultaneously evaluated from the frequencies of local maxima and minima of the spectrum provided that all interfaces and layers have the same properties. The effectiveness of the proposed procedure is investigated from the perspective of the sensitivity of local extremal frequencies of the reflection spectrum. The feasibility of the proposed procedure is also investigated when the stiffness of each interface is subjected to small random fluctuations about a certain average value. The proposed procedure is applied to a 16-layered cross-ply carbon-fiber-reinforced composite laminate. The normal stiffness of resin-rich interfaces and the longitudinal wave velocity of plies in the thickness direction evaluated from the experimental reflection spectrum are shown to be consistent with simple theoretical estimations. PMID:24993205

Ishii, Yosuke; Biwa, Shiro

2014-07-01

297

The on-axis phase and group velocities of plane wave diffracted by a small circular aperture  

Microsoft Academic Search

Based on the Rayleigh–Sommerfeld diffraction integral equation, the on-axis phase and group velocities of the pulsed plane wave diffracted by a small circular aperture have been investigated, respectively. It is shown that the diffraction results in the superluminality of the phase and group velocities at some certain on-axis points in vacuum, and the group velocity is even infinity.

Guoquan Zhou; Shaomin Wang; Daomu Zhao; Jinxin Xu

2003-01-01

298

Radial velocity variations of the pulsating subdwarf B star PG 1605+072  

E-print Network

We present an analysis of high-speed spectroscopy of the pulsating subdwarf B star PG 1605+072. Periodic radial motions are detected at frequencies similar to those reported for photometric variations in the star, with amplitudes of up to 6 km/s. Differences between relative strengths for given frequency peaks for our velocity data and previously measured photometry are probably a result of shifting of power between modes over time. Small differences in the detected frequencies may also indicate mode-shifting. We report the detection of line-shape variations using the moments of the cross correlation function profiles. It may be possible to use the moments to identify the star's pulsation modes.

Vincent M. Woolf; C. Simon Jeffery; Donald L. Pollacco

2001-11-12

299

A physical model study of converted wave amplitude variation in a reservoir of systematically aligned vertical fractures  

NASA Astrophysics Data System (ADS)

The existence of fractures not only provides spaces for the residence of oils and gases reside, but it also creates pathways for migration. Characterizing a fractured reservoir thus becomes an important subject and has been widely studied by exploration geophysicists and drilling engineers. In seismic anisotropy, a reservoir of systematically aligned vertical fractures (SAVF) is often treated as a transversely isotropic medium (TIM) with a horizontal axis of symmetry (HTI). Subjecting to HTI, physical properties vary in azimuth. P-wave reflection amplitude, which is susceptible to vary in azimuth, is one of the most popular seismic attributes which is widely used to delineate the fracture strike of an SAVF reservoir. Instead of going further on analyzing P-wave signatures, in this study, we focused on evaluating the feasibility of orienting the fracture strike of an SAVF reservoir using converted (C-) wave amplitude. For a C-wave is initiated by a downward traveling P-wave that is converted on reflection to an upcoming S-wave; the behaviors of both P- and S-waves should be theoretically woven in a C-wave. In our laboratory work, finite offset reflection experiments were carried out on the azimuthal plane of a HTI model at two different offset intervals. To demonstrate the azimuthal variation of C-wave amplitude in a HTI model, reflections were acquired along the principal symmetry directions and the diagonal direction of the HTI model. Inheriting from phenomenon of S-wave splitting in a transversely isotropic medium (TIM), P-waves get converted into both the fast (S1) and slow (S2) shear modes at all azimuths outside the vertical symmetry planes, thus producing split PS-waves (PS1 and PS2). In our laboratory data, the converted PS1- (C1-) wave were observed and identified. As the azimuth varies from the strike direction to the strike normal, C1-wave amplitude exhibits itself in a way of weakening and can be view from the common-reflection-point (CRP) gathers. Therefore, in conjunction with the azimuthal velocity and the amplitude variations in the P-wave and the azimuthal polarization of the S-wave, the azimuthal variation of C-wave amplitude which is experimentally demonstrated could be considered as a valuable seismic attribute in orienting the fracture strike of a SAVF reservoir. (Key words: converted wave, transversely isotropic medium, physical modeling, amplitude, fracture)

Chang, C.; Sun, L.; Lin, C.; Chang, Y.; Tseng, P.

2013-12-01

300

Discovery of a surface wave velocity anomaly in the West Sea of South Korea  

NASA Astrophysics Data System (ADS)

Imaging of Rayleigh- and Love-wave velocities is very important in detecting geophysical anomalies within the earth. Surface wave velocity imaging studies using ambient noise have provided enhanced and detailed images of velocity anomalies for sedimentary basins, hotspots, and volcanoes in various regions of the Earth (Yang et al., 2008). Cross-correlations of ambient noises observed from the Korea Meteorological Administration (KMA) seismic network provide the short-period Rayleigh-and Love-wave dispersion characteristics of the Korean Peninsula (Cho et al., 2007). Signal whitening and multiple-filter analysis are used to equalise power in signals from different times before noise processing, such as cross-correlation and stacking to extract group velocities from the estimated Green's functions, which are then used to image the spatially varying dispersion at periods between 1 and 5 s. The analysis method and data used in this paper are the same as those of Cho et al. (2007) except for the addition of the dataset of a new station, HUK. However, this paper notes that Rayleigh- and Love-wave velocity images in short periods show a very different group velocity image for the north-eastern area of the HUK station because additional data was analysed. This velocity anomaly corresponds with the residual anomaly of gravity tomography obtained in prior studies (Yu and Min, 2005; Kim and Oh, 2007). Our results show that a fracture zone concerning the Permo-Triassic collision (Choi et al., 2006; Kwon et al., 2009) exists below the north-eastern sea of the HUK station. In addition, recent studies (de Ridder and Dellinger, 2011; de Ridder and Biondi, 2013; Mordret et al., 2011, 2013a, 2013b, 2013c; Bussat and Kugler, 2011) regarding ambient noise tomography in hydro-carbon fields show that the anomaly might have resulted from the hydro-carbon reservoir. In the near future, the ambient noise tomography (ANT) method can replace seismic survey dominantly using body waves to find oil and gas reservoirs.

Cho, Kwang Hyun

2014-01-01

301

S-wave velocity images of the Dead Sea Basin provided by ambient seismic noise  

NASA Astrophysics Data System (ADS)

Based on passive seismic interferometry applied to ambient seismic noise recordings between station pairs belonging to a small-scale array, we have obtained shear wave velocity images of the uppermost materials that make up the Dead Sea Basin. We extracted empirical Green's functions from cross-correlations of long-term recordings of continuous data, and measured inter-station Rayleigh wave group velocities from the daily correlation functions for positive and negative correlation time lags in the 0.1-0.5 Hz bandwidth. A tomographic inversion of the travel times estimated for each frequency is performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved. Subsequently, the velocity-frequency curves are inverted to obtain S-wave velocity images of the study area as horizontal depth sections and longitude- and latitude-depth sections. The results, which are consistent with other previous ones, provide clear images of the local seismic velocity structure of the basin. Low shear velocities are dominant at shallow depths above 3.5 km, but even so a spit of land with a depth that does not exceed 4 km is identified as a salt diapir separating the low velocities associated with sedimentary infill on both sides of the Lisan Peninsula. The lack of low speeds at the sampling depth of 11.5 km implies that there are no sediments and therefore that the basement is near 10-11 km depth, but gradually decreasing from south to north. The results also highlight the bowl-shaped basin with poorly consolidated sedimentary materials accumulated in the central part of the basin. The structure of the western margin of the basin evidences a certain asymmetry both whether it is compared to the eastern margin and it is observed in north-south direction. Infill materials down to ˜8 km depth are observed in the hollow of the basin, unlike what happens in the north and south where they are spread beyond the western Dead Sea shore.

Badal, José; Chen, Yun; Chourak, Mimoun; Stankiewicz, Jacek

2013-10-01

302

S-wave velocity profiles near the Hayward fault obtained by active and passive surface wave methods  

NASA Astrophysics Data System (ADS)

We measured S-wave velocity (Vs) profiles at five sites the Hayward (Fig.1) using active and passive surface wave methods. The methods are multichannel analysis of surface waves using an active source (MASW), a passive surface wave method using geophones in a linear array (MAM), and a two-station spatial autocorrelation method (2ST-SPAC) using long-period accelerometers. Maximum array size varied from 345 to 565 m depending on the site. Minimum frequency and corresponding maximum wavelength ranged from 0.6 to 2 Hz and 500 to 1500 m, depending on the site. Phase velocities obtained from the three methods were combined to produce a single dispersion curve for each site. A nonlinear inversion consisting of a least squares method and a genetic algorithm was used to estimate Vs profiles from the dispersion curves to a depth of 400 to 700 m depending on the site. Vs profiles (Fig.2) show significant differences across the Hayward fault. On the west side of the fault (Southgate Park to Cemetery), there is a low velocity layer at the surface, with Vs less than 400 m/s to a depth of 40 to 90 m. A thick intermediate velocity layer with Vs ranging from 400 to 1200 m/s lies beneath the low velocity layer. Bedrock with Vs greater than 1200 m/s was measured at depths greater than 450 m. On the east side of the Fault (CSU East Bay), thickness of the low velocity layer (Vs < 400 m/s) is less than 5 m, and depth to bedrock (Vs > 1200 m/s) is less than 170 m. The results of this investigation are generally consistent with existing borehole Vs logs and other geophysical investigations, and indicate that bedrock depth changes approximately 300 m across the fault. To evaluate the effect of a significant change of bedrock depth on surface ground motion due to earthquakes, a representative Vs cross section perpendicular to the fault was constructed and theoretical amplification including two-dimensional structure was calculated using a viscoelastic finite-difference method. Calculation results show that the low frequency (1 to 3 Hz) component of ground motion is locally amplified on the west side of the fault because of the effect of two-dimensional structure.

Hayashi, K.; Craig, M.; Aql, R.; Shuler, S.

2013-12-01

303

TEMPERATURE VARIATION OF ULTRASONIC ATTENUATION AND PHASE VELOCITY IN VO2 AND V2O3 CRYSTALS  

E-print Network

'atténuation ultrasonore et de la vitesse de phase près de la transition de phase de VO2 et V2O3. Dans la phase haute transitions of VO2 and V2O3. We show that there exists a variation of the attenuation and of the velocityL-25 TEMPERATURE VARIATION OF ULTRASONIC ATTENUATION AND PHASE VELOCITY IN VO2 AND V2O3 CRYSTALS J

Boyer, Edmond

304

Higher Brachial-Ankle Pulse Wave Velocity Is Associated with More Advanced Carotid Atherosclerosis in End-Stage Renal Disease  

Microsoft Academic Search

Brachial-ankle pulse wave velocity is a new measure of arterial stiffness. We examined whether higher brachial-ankle pulse wave velocity is associated with more advanced carotid atherosclerosis and left ventricular hypertrophy in patients with end-stage renal disease, and whether this effect would be mediated by the influence of wave reflection on central arterial pressure. In 68 patients with end stage renal

Masanori Munakata; Junko Sakuraba; Jun Tayama; Takashi Furuta; Akira Yusa; Tohru Nunokawa; Kaoru Yoshinaga; Takayoshi Toyota

2005-01-01

305

Measurements of the changing wave velocities of sand during the formation and dissociation of disseminated methane hydrate  

NASA Astrophysics Data System (ADS)

The formation and dissociation of methane hydrate within sediment can lead to large changes in wave velocities, which provide valuable insights into the processes involved in hydrate formation. These are of practical importance in geophysical characterization, as well as developing strategies for the future exploitation of methane hydrates. This paper presents changes in wave velocity, measured during hydrate formation, and subsequent dissociation, using the resonant column apparatus. Hydrate was formed under "drained" and "undrained" conditions. Drained specimens had free access to methane during formation, while for undrained specimens, methane content was fixed. Hydrate formation and dissociation were induced by changing the specimen temperature under constant effective stress. In excess of 20 determinations of shear wave and flexural wave velocity were carried out over a 9 h period, both during hydrate formation and dissociation. This time was sufficient to record almost all of the changes in wave velocity within a specimen. The exothermic nature of hydrate formation was clearly seen in the form of spikes in temperature measured at the base of the specimens. For all specimens, the relationship between wave velocity and degree of hydrate saturation was nonlinear and significantly different during formation and dissociation. The patterns observed suggest that hydrate morphology not only is important in controlling the ultimate wave velocities, at the end of formation, but has a significant impact on the rates of change of wave velocities during formation and dissociation. A conceptual model is presented to explain differences in observed behavior during formation and dissociation.

Sultaniya, A. K.; Priest, J. A.; Clayton, C. R. I.

2015-02-01

306

Normal-Mode and Free-Air Gravity Constraints on Lateral Variations in Velocity and Density of Earth's Mantle  

Microsoft Academic Search

With the use of a large collection of free-oscillation data and additional con- straints imposed by the free-air gravity anomaly, lateral variations in shear velocity, compressional velocity, and density within the mantle; dynamic to- pography on the free surface; and topography on the 660-km discontinuity and the core-mantle boundary were determined. The velocity models are consistent with existing models based

Miaki Ishii; Jeroen Tromp

1999-01-01

307

Phase velocity and interaction impedance on slow-wave structures for power traveling-wave tubes  

Microsoft Academic Search

In this work, some results from a theoretical cold analysis of the ring-bar slow-wave structure are shown. The ring-bar structure is suitable to be used in high-power traveling-wave tubes. Thus, an investigation was done in order to understand the properties of this structure and why it is better than single helices when a TWT is operated at high-voltage levels. The

Daniel T. Lopes; Cláudio C. Motta

2008-01-01

308

Lateral variations of coda wave attenuation in the Alps  

NASA Astrophysics Data System (ADS)

We explore lateral variations of coda wave attenuation in the French Alps and surrounding regions. The area of investigation extends from the Rhine Graben in the north, to the northern Apennine Range in the south, and includes the Eastern and Western Alps. Following the classical work of Aki and Chouet (1975), coda wave attenuation has been characterized by measuring the coda quality factor of short-period S waves (Qc). We have selected about 2000 weak to moderate earthquakes, with magnitudes ranging from 3 to 5. Waveform data recorded by permanent seismic networks have been collected at the ORFEUS data center through the ArcLink protocol. Qc has been measured in five frequency bands [1-2], [2-4], [4-8], [8-16], [16-32] Hz, by applying a simple linear regression to the smooth energy envelopes of seismograms in the time domain. Various choices of coda window length (Lw), and coda onset time (tw, as measured from the origin time) have been tested to ensure that our measurements are free from any systematic effects of lapse-time dependence in the range of epicentral distance considered. The optimal choice, which simultaneously maximizes the geographical coverage and minimizes the measurement biases, is obtained for Lw=50s and tw=70s, for epicentral distances smaller than 180 km. The map of Qc is obtained by discretizing the Alpine region into pixels of dimension (20km x 20km). For each source/receiver pair, the estimated value of Qc is distributed along the direct ray path. An average over all paths that cross an individual pixel is performed to obtain the local value of Qc. A spatial smoothing over an area covering a square of 9 pixels is subsequently applied. The maps of Qc display strong lateral variations of attenuation in the Alpine area. At all frequencies, the ratio between the lowest and largest value of Qc is typically larger than 2. The attenuation pattern is complex but relatively independent of frequency. A notable exception is a low attenuation region located between Torino and Geneva, which is clearly visible in the 1-2 Hz frequency band and disappears at higher frequencies. Some geological formations such as the Upper Rhine Graben and the eastern Alps show up clearly on the maps and systematically exhibit lower attenuation than the Po Valley and the Apennines. The French Alps are characterized by an attenuation gradient increasing from the north-west to the south-east. The typical scale of the spatial variations of the coda quality factor is of the order of 100km, which suggests rapid lateral variation of attenuation properties in the crust.

Mayor, Jessie; Calvet, Marie; Margerin, Ludovic; Traversa, Paola

2014-05-01

309

Flute mode waves near the lower hybrid frequency excited by ion rings in velocity space  

NASA Technical Reports Server (NTRS)

Discrete emissions at the lower hybrid frequency are often seen on the S3-3 satellite. Simultaneous observation of perpendicularly heated ions suggests that these ions may provide the free energy necessary to drive the instability. Studies of the dispersion relation for flute modes excited by warm ion rings in velocity space show that waves are excited with real frequencies near the lower hybrid frequency and with growth rates ranging from about 0.01 to 1 times the ion cyclotron frequency. Numerical results are therefore consistent with the possibility that the observed ions are the free energy source for the observed waves.

Cattell, C.; Hudson, M.

1982-01-01

310

DETERMINATION OF ELASTIC WAVE VELOCITY AND RELATIVE HYPOCENTER LOCATIONS USING REFRACTED WAVES. I. METHODOLOGY.  

USGS Publications Warehouse

An arrival time difference method utilizing refracted arrivals from earthquakes in a homogeneous, layered earth model has been developed for the simultaneous determination of near-source (in situ) velocity and relative locations of earthquakes. The method is particularly applicable when analyzig data from arrays in which most of the recording stations are far (i. e. , several focal depths) from a group of events. This iterative scheme locates earthquakes relative to a master event and performs an inversion for in situ velocity using a generalized inverse-least squares estimation procedure.

Shedlock, Kaye M.; Roecker, Steven W.

1985-01-01

311

Lithospheric structure beneath Eastern Africa from joint inversion of receiver functions and Rayleigh wave velocities  

NASA Astrophysics Data System (ADS)

Crust and upper mantle structure beneath eastern Africa has been investigated using receiver functions and surface wave dispersion measurements to understand the impact of the hotspot tectonism found there on the lithospheric structure of the region. In the first part of this thesis, I applied H-kappa stacking of receiver functions, and a joint inversion of receiver functions and Rayleigh wave group velocities to determine the crustal parameters under Djibouti. The two methods give consistent results. The crust beneath the GEOSCOPE station ATD has a thickness of 23+/-1.5 km and a Poisson's ratio of 0.31+/-0.02. Previous studies give crustal thickness beneath Djibouti to be between 8 and 10 km. I found it necessary to reinterprete refraction profiles for Djibouti from a previous study. The crustal structure obtained for ATD is similar to adjacent crustal structure in many other parts of central and eastern Afar. The high Poisson's ratio and Vp throughout most of the crust indicate a mafic composition, suggesting that the crust in Afar consists predominantly of new igneous rock emplaced during the late synrift stage where extension is accommodated within magmatic segments by diking. In the second part of this thesis, the seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities to obtain new constraints on the thermal structure of the lithosphere. Crustal structure from the joint inversion for Ethiopia and Djibouti is similar to previously published models. Beneath the Main Ethiopian Rift (MER) and Afar, the lithospheric mantle has a maximum shear wave velocity of 4.1-4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ˜100-125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau appears to have been thinned by ˜30-50 km and the maximum shear wave velocity reduced by ˜0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the lithosphere beneath the Ethiopian Plateau can be explained by a plume model, if a plume rapidly thinned the lithosphere by ˜30--50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the lithosphere since then. About 45-65% of the 1-1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed lithospheric structure. In the final part of this thesis, the shear-wave velocity structure of the crust and upper mantle beneath Kenya has been obtained from a joint inversion of receiver functions, and Rayleigh wave group and phase velocities. The crustal structure from the joint inversion is consistent with crustal structure published previously by different authors. The lithospheric mantle beneath the East African Plateau in Kenya is similar to the lithosphere under the East African Plateau in Tanzania. Beneath the Kenya Rift, the lithosphere extends to a depth of at most ˜75 km. The lithosphere under the Kenya Plateau is not perturbed when compared to the highly perturbed lithosphere beneath the Ethiopian Plateau. On the other hand, the lithosphere under the Kenya Rift is perturbed as compared to the Kenya Plateau or the rest of the East African Plateau, but is not as perturbed as the lithosphere beneath the Main Ethiopian Rift or the Afar. Although Kenya and Ethiopia have similar uplift and rifting histories, they have different volcanic histories. Much of Ethiopia has been affected by the Afar Flood Basalt volcanism, which may be the cause of this difference in lithospheric structure between these two regions.

Dugda, Mulugeta Tuji

312

Influence of pressure on permeability and elastic wave velocities in macro- and micro-fractured rock  

NASA Astrophysics Data System (ADS)

Long-term integrity is required for many sub-surface structures excavated in the rock mass; such as repositories for radioactive wastes and caverns for the storage of liquid petroleum gas and natural gas. An essential requirement for long-term integrity is to retard the migration of fluids into and through these structures. In crustal rocks, fracture and pore networks provide the principal pathways for fluid flow. However, if fractures and pores are closed or sealed, the migration of fluids can be retarded. Thus it is important to study the process of fracture and pore closure. Here, we report changes in the fluid permeability and P- and S-wave velocities of intact, macro-fractured, and micro-fractured rock sample subjected to elevated effective pressures. In order to investigate the influence of pressure on the closure of macro-fractures and micro-fractures, we used a rock sample with no visible pre-existing cracks and very low initial permeability. For this reason, Seljadur basalt (SB) from Iceland was chosen as the sample material. SB is a fresh, columnar-jointed, intrusive basalt with a porosity of 4 % and no visible microcracks. Permeability was measured in a servo-controlled permeameter using the steady-state flow method. The permeameter is equipped with transducers that allow the simultaneous measurement of P- and S-wave velocities. The wave velocities were measured by the ultrasonic transmission method. Measurements of permeability and elastic wave velocities were first made on intact samples. The Brazil test technique was then used to split the samples in half to provide macro-fractured sample for further testing. Measurments of permeability and elastic wave velocities were then made on the macro-fractured sample in order to investigate the effect of fracture closure. Then, we heated the macro-fractured sample at 800 degree Celsius in order to produce micro-fractures in the sample and conducted further measurements of permeability and wave velocities. It was shown that the permeability of intact SB was low and remains essentially constant over the whole effective pressure range. By contrast, the permeability of the macro-fractured SB was initially much higher, but decreased clearly as effective pressure was increased and the fracture became closed. The permeability of the macro- and micro-fractured SB also decreased with increasing the pressure. The permeability of the macro- and micro-fractured SB was similar to that of the macro-fractured SB under low pressure. This result indicates that the open macro-fractures dominate the permeability under low pressure. On the other hand, the permeability of the macro-and micro-fractured SB was higher than that of the macro-fractured SB under high pressure. This indicates that the macro-fractures with low aspect ratio close easily and open micro-fractures with high aspect ratio become dominant to the permeability with increasing the pressure. The difference in the wave velocities between intact, macro-fractured, and macro- and micro-fractured samples decreased with increasing effective pressure, tracking the closure of the fractures. Specifically, the increase of S-wave velocity vibrating nornal to the macro-fractures was clear with increasing the pressure, which indicates the closure of fracture. Overall, results in this study demonstrate the importance of the closure of fractures to increase the shielding ability of rock and to retard the migration of fluids into and through structres in a rock mass.

Meredith, Philip; Nara, Yoshitaka; Yoneda, Tetsuro; Kaneko, Katsuhiko

2010-05-01

313

Stress- and damage-induced changes in coda wave velocities in concrete  

NASA Astrophysics Data System (ADS)

Diffuse ultrasonic measurements are used to monitor progressive stress-induced damage in a number of concrete specimens. Each specimen was subjected to uniaxial compression, applied in small steps over one or several load cycles. At each step, the loading was held constant and a series of ultrasonic measurements parallel and perpendicular to the loading were obtained. The measurements were taken during both the loading and unloading phases. The time scale of the signals was chosen such that the diffuse ultrasonic regime could be recorded. Using Coda Wave Interferometry (CWI), the corresponding changes in the velocity of diffuse ultrasonic waves were monitored. The evolution of the slope of the CWI velocity-stress curve with the increased level of damage in concrete is demonstrated.

Shokouhi, P.

2013-01-01

314

Ultrasound P-wave velocities and amplitudes during triaxial deformation of three sandstone varieties: the chances to monitor different states of internal drainage  

NASA Astrophysics Data System (ADS)

Time-dependent variations in velocities and amplitudes of elastic waves are among the most important sources of information to monitor natural or induced subsurface processes at transient states of drainage. Laboratory experiments provide valuable information on the characteristic properties of elastic wave propagation associated with different mechanical states during deformation, in particular with respect to the role of variations in microstructure and effective stress. Triaxial deformation characteristics of dry and saturated sandstone samples (Ruhr sandstone, Wilkeson sandstone, Fontainebleau sandstone) at various confining pressures up to 150 MPa and strain rates ranging from 10-7 and 10-3 s-1 were investigated by simultaneously recorded ultrasound signals of P-waves propagating parallel to the applied deviatoric stress with the aim to monitor different states of internal drainage. During brittle deformation velocities and amplitudes initially increased, exhibited a maximum prior to failure and decreased, whilst the magnitude of variations decreased with increasing confining pressure and strongly differed between the three sandstones. Differences between the deformation characteristics of dry and saturated samples at equivalent effective pressures document the onset and magnitude of dilatancy hardening of saturated samples associated with the transition from effectively drained to undrained conditions at strain rates consistent with estimates from actual measurements of hydraulic properties. Monitoring strain-rate dependent variations of internal drainage with ultrasound P-waves was conspicuously limited for the three sandstones varieties due to an inherent ambiguity of velocities under the applied pressure and saturation conditions. However, the actual occurrence of insufficient drainage was associated with characteristic variations in velocity and amplitude. Effectively undrained conditions during deformation accompanied a reduced axial strain at maximum amplitudes compared to dry experiments at otherwise constant experimental conditions. The decrease in amplitudes at lower strains is likely attributed to pore overpressure inducing stable crack propagation, when hydraulic diffusion is too low to compensate for a reduction in pore space during the initial elastic compaction. Also, the reduction in velocity prior to failure was less pronounced for saturated samples under effectively undrained conditions than observed for dry and drained saturated samples documenting local pore pressure diminution during inelastic dilation resulting in dilatancy hardening. Critical strain rates for internal drainage independently deduced from properties of ultrasound waves agreed within an order of magnitude with critical strain rates estimated from both hydraulic and mechanical properties.

Duda, M.; Renner, J.

2012-04-01

315

Determination of shallow shear wave velocity profiles in the Cologne, Germany area using ambient vibrations  

Microsoft Academic Search

We have used both single-station and array methods to determine shallow shear velocity site profiles in the vicinity of the city of Cologne, Germany from ambient vibration records. Based on fk-analysis we assume that fundamental-mode Rayleigh waves dominate the analysed wavefield in the frequency range of 0.7-2.2 Hz. According to this view a close relation exists between H\\/V spectral ratios

Frank Scherbaum; Klaus-G. Hinzen; Matthias Ohrnberger

2003-01-01

316

Azimuthal anisotropy of the Pg-wave velocity in hypocentral volumes of NW Croatia  

Microsoft Academic Search

Based on a large set of arrival times of the Pg phase reported by local and regional stations, we estimate azimuthal anisotropy of the Pg-wave velocity in focal volumes of the upper crust in NW Croatia. The method is based on analyses of the azimuthal dependence\\u000a of ratios of cumulative differences of arrival times and travel paths between foci of

Marijan Herak; Davorka Herak; Josip Stipcevic

2009-01-01

317

Correction of beam-steering effects in low-velocity superconducting quarter-wave cavities  

Microsoft Academic Search

Superconducting cavities presently used for acceleration of ions in velocity range 0.01c to 0.3c (where c is the speed of light) are based on quarter-wave resonators. Currently there are several design proposals in nuclear physics laboratories for application of this type of cavity for acceleration of light and heavy ions. The operating frequencies of the cavities range from 50 to

P. N. Ostroumov; K. W. Shepard

2001-01-01

318

Correction of beam-steering effects in low-velocity superconducting quarter-wave cavities  

Microsoft Academic Search

Superconducting cavities presently used for acceleration of ions in velocity range ~0.01c to 0.3c (where c is the speed of light) are based on quarter-wave resonators. Currently there are several design proposals in nuclear physics laboratories for application of this type of cavity for acceleration of light and heavy ions. The operating frequencies of the cavities range from ~50 to

P. N. Ostroumov; K. W. Shepard

2001-01-01

319

Pulse Wave Velocity Is Associated with Metabolic Syndrome Components in CAPD Patients  

Microsoft Academic Search

Background: Overall, the proportion of deaths due to cardiovascular disease (CVD) reached 40–50% in dialysis patients. Pulse wave velocity (PWV) reflects arterial stiffness and may provide an integrated index of vascular status and CVD risk. Individual components of the metabolic syndrome (MetS) are well-established cardiovascular risk factors. Thus we conducted a cross-sectional study in continuous ambulatory peritoneal dialysis (CAPD) patients

Xing-wei Zhe; Jing Zeng; Xin-kui Tian; Wei Chen; Yue Gu; Li-tao Cheng; Hui-min Chen; Jonas Axelsson; Bengt Lindholm; Tao Wang

2008-01-01

320

Effect of disorder on slow light velocity in optical slow-wave structures  

Microsoft Academic Search

Slow-wave optical structures such as coupled photonic crystal cavities, coupled microresonators, and similar coupled-resonator optical waveguides are being proposed for slowing light because of the nature of their dispersion relationship. Since the group velocity becomes small, slow light and enhanced light-matter interaction may be observed at the edges of the waveguiding band. We derive a model of the effects of

Shayan Mookherjea; Andrew Oh

2007-01-01

321

Electromagnetic waves with negative phase velocity in Schwarzschild-de Sitter spacetime  

E-print Network

The propagation of electromagnetic plane waves with negative phase velocity (NPV) is considered in Schwarzschild-(anti-)de Sitter spacetime. It is demonstrated that NPV propagation occurs in Schwarzschild-de Sitter spacetime at lower values of the cosmological constant than is the case for de Sitter spacetime. Furthermore, we report that neither is NPV propagation observed in Schwarzschild-anti-de Sitter spacetime, nor is it possible outside the event horizon of a Schwarzschild blackhole.

Tom G. Mackay; Akhlesh Lakhtakia; Sandi Setiawan

2005-08-23

322

Estimation of local pulse wave velocity using arterial diameter waveforms: Experimental validation in sheep  

NASA Astrophysics Data System (ADS)

Increased arterial stiffness is associated with an increased risk of cardiovascular events. Estimation of arterial stiffness using local pulse wave velocity (PWV) promises to be very useful for noninvasive diagnosis of arteriosclerosis. In this work we estimated in an instrumented sheep, the local aortic pulse wave velocity using two sonomicrometry diameter sensors (separated 7.5 cm) according to the transit time method (PWVTT) with a sampling rate of 4 KHz. We simultaneously measured aortic pressure in order to determine from pressure-diameter loops (PWVPDLoop), the "true" local aortic pulse wave velocity. A pneumatic cuff occluder was implanted in the aorta in order to compare both methods under a wide range of pressure levels. Mean pressure values ranged from 47 to 101 mmHg and mean proximal diameter values from 12.5. to 15.2 mm. There were no significant differences between PWVTT and PWVPDLoop values (451±43 vs. 447±48 cm/s, p = ns, paired t-test). Both methods correlated significantly (R = 0.81, p<0.05). The mean difference between both methods was only -4±29 cm/s, whereas the range of the limits of agreement (mean ± 2 standard deviation) was -61 to +53 cm/s, showing no trend. In conclusion, the diameter waveforms transit time method was found to allow an accurate and precise estimation of the local aortic PWV.

Graf, S.; Craiem, D.; Barra, J. G.; Armentano, R. L.

2011-12-01

323

Prediction of rocks thermal conductivity from elastic wave velocities, mineralogy and microstructure  

NASA Astrophysics Data System (ADS)

While knowledge on Thermal Conductivity (TC) of rocks is of interest in many fields, determining this property remains challenging. In this paper, a modelling approach for TC prediction from Elastic Wave Velocity (EWV) measurements is reported. To this end, a new effective TC model for a typical sedimentary rock is introduced that explicitly accounts for the presence of pores, pressure-sensitive microcracks (or grain contacts) and formation fluids. A model of effective elasticity is also devised for this same rock that links its microstructural characteristics to the velocity of elastic waves. The two models are based on the same effective medium approach and involve the same microstructural parameters. A workflow based on this explicit modelling approach is devised that allows for the prediction of the TC of a reservoir rock using (i) the elastic waves velocities, (ii) the dominant mineral content and (iii) the bulk porosity. This workflow is validated using experimental data reported in the literature for dry and water-saturated Fontainebleau and Berea sandstones. The datasets include measurements of TC and EWV as a function of effective pressure. In addition, it is shown that the dependence of TC on the rock microstructure is formally and practically similar to that of EWV. It is also demonstrated that the accuracy of TC predictions from EWV increases with effective pressure (burial depth). The underlying assumptions and limitations of the present approach together with the effect of burial are discussed.

Pimienta, Lucas; Sarout, Joel; Esteban, Lionel; Piane, Claudio Delle

2014-05-01

324

Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion  

NASA Astrophysics Data System (ADS)

The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure, we first improved the linearized time-domain receiver function inversion method. The continental Moho is relatively shallow (˜ 30 km) at the coastline of the Sea of Japan and at the Seto Inland Sea, and becomes deeper-greater than 40 km-around 35°N and 133.8°E. Near the Seto Inland Sea, a low-velocity layer of thickness 10 km lies under the continental Moho. This low-velocity layer corresponds to the subducting oceanic crust of the Philippine Sea plate. The oceanic Moho continues to descend from south to northwest and exhibits complicated ridge and valley features. The oceanic Moho runs around 25 km beneath the Pacific coast and 45 km beneath the Seto Inland Sea, and it extends to at least to 34.5°N. The depth variation of the Moho discontinuities is in good qualitative agreement with the concept of isostasy. From the configurations of both the continental and oceanic Mohos, we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of Shikoku and that a mantle wedge may exist beneath the western and eastern part of Shikoku. The southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake.

Shiomi, Katsuhiko; Obara, Kazushige; Sato, Haruo

2006-06-01

325

Compositional variation of density and seismic velocities in natural peridotites at STP conditions: Implications for seismic imaging  

E-print Network

Compositional variation of density and seismic velocities in natural peridotites at STP conditions natural spinel- and garnet-peridotite samples (n = 133) at standard temperature (T ) and pressure (P) (STP-members of peridotitic minerals, the variation of elastic moduli with Mg# at STP conditions holds at elevated P and T

Lee, Cin-Ty Aeolus

326

Magnetic Activity–related Radial Velocity Variations in Cool Stars: First Results from the Lick Extrasolar Planet Survey  

Microsoft Academic Search

The discovery of the radial velocity ( ) signatures of planets around several solar-like stars highlights the v rimportance of exploring the sources of variations intrinsic to the stars themselves. We study the stars in the v rLick planetary survey for variations related to stellar activity: the rotation of starspots and convective inhomv rogeneities and their temporal evolution. We study

Steven H. Saar; R. Paul Butler; Geoffrey W. Marcy

1998-01-01

327

Synthetic seismograms for a synthetic Earth: long-period P- and S-wave traveltime variations can be explained by temperature alone  

NASA Astrophysics Data System (ADS)

Current interpretations of seismic observations typically argue for significant chemical heterogeneity being present in the two large low shear velocity provinces under Africa and the Pacific. Recently, however, it has been suggested that large lateral temperature variations in the lowermost mantle resulting from a strong thermal gradient across D? may provide an alternative explanation. In case of a high heat flux from the core into the mantle, the magnitude of shear wave velocity variations in tomographic models can be reconciled with isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong core heating has been tested in a consistent manner only against tomographic S-wave velocity models, but not against P-wave velocity models. Here, we explore a new approach to assess geodynamic models and test the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P and S waves. Using a spectral element method, we simulate 3-D global wave propagation for periods down to 10 s in synthetic 3-D elastic structures derived from a geodynamic model. Seismic heterogeneity is predicted by converting the temperature field of a high-resolution mantle circulation model (MCM) into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow against seismic data. The statistics of long-period body wave traveltime observations show a markedly different behaviour for P and S waves: the standard deviation of P-wave delay times stays almost constant with turning depth, whereas that of the S-wave delay times increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations computed for the isochemical MCM reproduce these different trends. This is not expected from a ray-theoretical point of view and highlights the importance of finite-frequency effects. Most importantly, the large lateral temperature variations in the lower mantle related to strong core heating are able to explain most of the standard deviation of observed P- and S-wave delay times. This is a strong indication that seismic heterogeneity in the lower mantle is likely dominated by thermal variations on the length scales relevant for long-period body waves.

Schuberth, Bernhard S. A.; Zaroli, Christophe; Nolet, Guust

2012-03-01

328

Variation in the dispersion of axisymmetric waves in infinite circular rods with crystallographic wire texture  

SciTech Connect

This paper presents the solution to the frequency equation for a number of polycrystalline, textured circular rods having transverse isotropy. The effective, second-order elastic stiffness tensors were estimated using the recursive general Hill arithmetic mean (GHAM). The velocity dispersion curves for a number of combinations of materials and crystallographic fiber or wire textures were calculated and the variation due to texture displayed. At large wavelengths, the velocity dispersion of fiber textured materials exhibits a lowest-order axisymmetric mode which varies only with the directional Poisson[close quote]s ratios in a manner similar to that of isotropic aggregates. In this wavelength regime, the waves propagate nondispersively at the wave speed, C[sub 0], as dictated by the directional Young[close quote]s modulus. At wavelengths smaller than the rod radius, the dispersion curves were more influenced by the full anisotropy of the wire textures. At these wavelengths, the dispersion curves for the anisotropic materials deviated significantly from those of the isotropic materials and one another with the higher axisymmetric vibration modes exhibiting extreme differences. This deviation is a function of the single crystal anisotropy and nature of the wire textures. < --[AN] -->

Mason, T.A. (Materials Science and Technology Division, MS G755, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States))

1999-09-01

329

Standard practice for measuring the ultrasonic velocity in polyethylene tank walls using lateral longitudinal (LCR) waves  

E-print Network

1.1 This practice covers a procedure for measuring the ultrasonic velocities in the outer wall of polyethylene storage tanks. An angle beam lateral longitudinal (LCR) wave is excited with wedges along a circumferential chord of the tank wall. A digital ultrasonic flaw detector is used with sending-receiving search units in through transmission mode. The observed velocity is temperature corrected and compared to the expected velocity for a new, unexposed sample of material which is the same as the material being evaluated. The difference between the observed and temperature corrected velocities determines the degree of UV exposure of the tank. 1.2 The practice is intended for application to the outer surfaces of the wall of polyethylene tanks. Degradation typically occurs in an outer layer approximately 3.2-mm (0.125-in.) thick. Since the technique does not interrogate the inside wall of the tank, wall thickness is not a consideration other than to be aware of possible guided (Lamb) wave effects or reflection...

American Society for Testing and Materials. Philadelphia

2011-01-01

330

Attenuation and velocity structure from diffuse coda waves: Constraints from underground array data  

NASA Astrophysics Data System (ADS)

An analysis of coda waves excited in the 0.2-20 Hz frequency band and recorded by the underground array Underseis (central Italy) has been performed to constrain both seismic attenuation at regional scale and velocity structure in the Mount Gran Sasso area. Attenuation was estimated with the MLTWA method, and shows a predominance of scattering phenomena over intrinsic absorption. The values of Qi and Qs are compatible with other estimates obtained in similar tectonic environments. Array methods allowed for a detailed study of the propagation characteristics, demonstrating that earthquake coda at frequencies greater than about 6 Hz is composed of only body waves. Coherence and spectral characteristics of seismic waves measured along the coda of local and regional earthquakes indicate that the wavefield becomes fully diffuse only in the late coda. The frequency-dependent energy partitioning between horizontal and vertical components has been also estimated and compared with synthetic values computed in a layered half-space under the diffuse field assumption. This comparison confirms that, for frequencies higher than 6 Hz, the coda appears as a sum of body waves coming from all directions while, in the low frequency range (0.2-2 Hz), the observations can be well explained by a coda wavefield composed of an equipartition mixture of surface and body waves traveling in a multiple-layered medium. A Monte-Carlo inversion has been performed to obtain a set of acceptable velocity models of the upper crust. The present results show that a broadband coda wavefield recorded in an underground environment is useful to constrain both the regional attenuation and the velocity structure of the target area, thereby complementing the results of classical array analysis of the wavefield.

Galluzzo, Danilo; La Rocca, Mario; Margerin, Ludovic; Del Pezzo, Edoardo; Scarpa, Roberto

2015-03-01

331

Line-profile variations in radial-velocity measurements. Two alternative indicators for planetary searches  

NASA Astrophysics Data System (ADS)

Aims: We introduce two methods to identify false-positive planetary signals in the context of radial-velocity exoplanet searches. The first is the bi-Gaussian cross-correlation function fitting (and monitoring of the parameters derived from it), and the second is the measurement of asymmetry in radial-velocity spectral line information content, Vasy. We assess the usefulness of each of these methods by comparing their results with those delivered by current indicators. Methods: We make a systematic analysis of the most used common line profile diagnosis, Bisector Inverse Slope and Velocity Span, along with the two proposed ones. We evaluate all these diagnosis methods following a set of well-defined common criteria and using both simulated and real data. We apply them to simulated cross-correlation functions that are created with the program SOAP and which are affected by the presence of stellar spots. We consider different spot properties on stars with different rotation profiles and simulate observations as obtained with high-resolution spectrographs. We then apply our methodology to real cross-correlation functions, which are computed from HARPS spectra, for stars with a signal originating in activity (thus spots) and for those with a signal rooted on a planet. Results: We demonstrate that the bi-Gaussian method allows a more precise characterization of the deformation of line profiles than the standard bisector inverse slope. The calculation of the deformation indicator is simpler and its interpretation more straightforward. More importantly, its amplitude can be up to 30% larger than that of the bisector span, allowing the detection of smaller-amplitude correlations with radial-velocity variations. However, a particular parametrization of the bisector inverse slope is shown to be more efficient on high-signal-to-noise data than both the standard bisector and the bi-Gaussian. The results of the Vasy method show that this indicator is more effective than any of the previous ones, being correlated with the radial-velocity with more significance for signals resulting from a line deformation. Moreover, it provides a qualitative advantage over the bisector, showing significant correlations with RV for active stars for which bisector analysis is inconclusive. Conclusions: We show that the two indicators discussed here should be considered as standard tests to check for the planetary nature of a radial-velocity signal. We encourage the usage of different diagnosis as a way of characterizing the often elusive line profile deformations.

Figueira, P.; Santos, N. C.; Pepe, F.; Lovis, C.; Nardetto, N.

2013-09-01

332

SAVANI2: towards a waveform-based image of shear-velocity variations underneath Europe embedded in a global model  

NASA Astrophysics Data System (ADS)

In a recent study (Auer et al. 2013, in revision) we have devised a novel tomography approach to image radially anisotropic shear-velocity variations in the Earth's mantle. By applying our tomography toolbox to a comprehensive compilation of surface-wave phase delays from fundamental modes up to the 6th overtone and cross-correlation traveltimes of major body-wave phases, we derived the multi-resolution tomography model SAVANI, which is one of the first whole-mantle models of radial S-wave anisotropy. Here we illustrate the first steps towards the second iteration of our model ("SAVANI2"), in which we define Europe and the surrounding regions as the target area for a higher-resolution regional revision of our initial model. To this end, we augment our global database with additional teleseismic and regional broadband measurements recorded within the last five years. We download raw waveforms from the Orfeus and IRIS data centers in a fully automated way with a python based toolbox and extract multiple-frequency traveltime delays in the period range between 5 and 25 s employing the method of Sigloch et al. (2006). Furthermore, we replace the crustal model CRUST2.0 with its successor CRUST1. Importantly, waveform observations will be interpreted using Fréchet sensitivity kernels computed with AxiSEM (Nissen-Meyer et al., 2007), which is an efficient visco-elastic spectral element solver for axisymmetric background models. The main idea behind SAVANI2 is to keep semi-approximate (ray) theory where appropriate (global long-wavelength structure, surface wave dispersion), but to revert to a full-waveform interpretation where necessary (regional scale, non-geometrical wave phenomena). Our hybrid approach to waveform inversion has multi-scale capabilities and is essentially equivalent to the first iteration step of a Gauss-Newton type inverse problem, thus allowing full access to the model resolution matrix. The set of algorithms we are developing represent a straightforward manner to stepwise improve upon a global background model by updating the tomographic system whenever new data becomes available.

Auer, Ludwig; Boschi, Lapo; van Driel, Martin; Becker, Thorsten; Nissen-Meyer, Tarje; Sigloch, Karin; Hosseini-zad, Kasra; Giardini, Domenico

2014-05-01

333

Velocity variations in Volcán de Colima associated with the 2003 MW = 7.4 Tecomán earthquake, Mexico  

NASA Astrophysics Data System (ADS)

We present measurements of temporal variations of seismic velocity at Volcán de Colima using seismic noise correlation techniques. About 13 years of continuous recording, mainly from the four short-period stations of the monitoring network, were processed. The daily cross-correlation functions obtained between pairs of stations present high degree of stability over the period of study. This allowed the use of the stretching method to estimate relative velocity variations in the structure. After correcting for the effect of variations of the sampling frequency of the recording system, small velocity fluctuations are obtained that are poorly correlated with the effusive and explosive activity of the volcano. However, a large velocity drop of 1 to 2 % occurred on January 22, 2003 during the MW = 7.4 Tecomán earthquake the epicentre of which is located offshore at about 100 km SW from the volcano. The velocity progressively recovered its initial value in about 6 years. No similar velocity variations could be detected for stations pairs outside the volcanic structure and closer to the rupture zone. Interestingly, the effusive activity that began on May 2001 stopped 10 days after the earthquake and the velocity drop.

Lesage, P.; Reyes-Dávila, G.; Arámbula-Mendoza, R.

2012-04-01

334

Structure of the Crust beneath Cameroon, West Africa, from the Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions  

SciTech Connect

The Cameroon Volcanic Line (CVL) is a major geologic feature that cuts across Cameroon from the south west to the north east. It is a unique volcanic lineament which has both an oceanic and a continental sector and consists of a chain of Tertiary to Recent, generally alkaline volcanoes stretching from the Atlantic island of Pagalu to the interior of the African continent. The oceanic sector includes the islands of Bioko (formerly Fernando Po) and Sao Tome and Principe while the continental sector includes the Etinde, Cameroon, Manengouba, Bamboutos, Oku and Mandara mountains, as well as the Adamawa and Biu Plateaus. In addition to the CVL, three other major tectonic features characterize the region: the Benue Trough located northwest of the CVL, the Central African Shear Zone (CASZ), trending N70 degrees E, roughly parallel to the CVL, and the Congo Craton in southern Cameroon. The origin of the CVL is still the subject of considerable debate, with both plume and non-plume models invoked by many authors (e.g., Deruelle et al., 2007; Ngako et al, 2006; Ritsema and Allen, 2003; Burke, 2001; Ebinger and Sleep, 1998; Lee et al, 1994; Dorbath et al., 1986; Fairhead and Binks, 1991; King and Ritsema, 2000; Reusch et al., 2010). Crustal structure beneath Cameroon has been investigated previously using active (Stuart et al, 1985) and passive (Dorbath et al., 1986; Tabod, 1991; Tabod et al, 1992; Plomerova et al, 1993) source seismic data, revealing a crust about 33 km thick at the south-western end of the continental portion of the CVL (Tabod, 1991) and the Adamawa Plateau, and thinner crust (23 km thick) beneath the Garoua Rift in the north (Stuart et al, 1985) (Figure 1). Estimates of crustal thickness obtained using gravity data show similar variations between the Garoua rift, Adamawa Plateau, and southern part of the CVL (Poudjom et al., 1995; Nnange et al., 2000). In this study, we investigate further crustal structure beneath the CVL and the adjacent regions in Cameroon using 1-D shear wave velocity models obtained from the joint inversion of Rayleigh wave group velocities and P-receiver functions for 32 broadband seismic stations. From the 1-D shear wave velocity models, we obtain new insights into the composition and structure of the crust and upper mantle across Cameroon. After briefly reviewing the geological framework of Cameroon, we describe the data and the joint inversion method, and then interpret variations in crustal structure found beneath Cameroon in terms of the tectonic history of the region.

Tokam, A K; Tabod, C T; Nyblade, A A; Julia, J; Wiens, D A; Pasyanos, M E

2010-02-18

335

Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions  

NASA Astrophysics Data System (ADS)

Using seismic profiles of triplicated waveforms, we show significant lateral variations in the SH velocity (Vs) structure of the transition zone (TZ) beneath Korea and adjacent regions. Beneath Sakhalin, we detected a high Vsanomaly (˜2%) limited to middle regions of the TZ (mid-TZ), and a largeVsjump across the 660-km discontinuity. A similar jump inVs also occurs beneath the northern portion of the North China Craton (NCC). Beneath Korea, a high Vs anomaly (˜2%) in the lower TZ is inferred, accompanied by a relatively small Vsjump across the 660-km discontinuity, which is depressed by about ˜15-20 km. The deep structure under the eastern part of northeast China (NEC) also includes a slightVsanomaly (˜1%) in the lower TZ but does not exhibit significant depression of the 660-km discontinuity. Compared with previous study, our observations reveal strong regional variations of the TZ structure on a relatively short scale. These variations most likely reflect the geometrical distribution of the subducting northwest Pacific plate. Our results suggest that the subducting slab dips across the mid-TZ under Sakhalin, and becomes flattened atop of the 660-km discontinuity beneath Korea, while only the tip of the slab reaches the lower TZ beneath the NEC. The TZ beneath the NCC does not show evidence of the slab stagnation.

Zhang, Ruiqing; Wu, Qingju; Li, Yonghua; Romanowicz, Barbara

2012-09-01

336

Generation of lower hybrid and whistler waves by an ion velocity ring distribution  

SciTech Connect

Using fully kinetic simulations in two and three spatial dimensions, we consider the generation and nonlinear evolution of lower hybrid waves produced by a cold ion ring velocity distribution in a low beta plasma. We show that the initial development of the instability is very similar in two and three dimensions and not significantly modified by electromagnetic effects, consistent with linear theory. At saturation, the level of electric field fluctuations is a small fraction of the background thermal energy; the electric field and corresponding density fluctuations consist of long, field-aligned striations. Energy extracted from the ring goes primarily into heating the background ions and the electrons at comparable rates. The initial growth and saturation of the magnetic components of the lower hybrid waves are related to the electric field components, consistent with linear theory. As the growing electric field fluctuations saturate, parallel propagating whistler waves develop by the interaction of two lower hybrid waves. At later times, these whistlers are replaced by longer wavelength, parallel propagating whistlers that grow through the decay of the lower hybrid fluctuations. Wave matching conditions demonstrate these conversion processes of lower hybrid waves to whistler waves. The conversion efficiency (=ratio of the whistler wave energy to the energy in the saturated lower hybrid waves) is computed and found to be significant ({approx}15%) for the parameters of the three-dimensional simulation (and even larger in the two-dimensional simulation), although when normalized in terms of the initial kinetic energy in the ring ions the overall efficiency is very small (<10{sup -4}). The results are compared with relevant linear and nonlinear theory.

Winske, D.; Daughton, W. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2012-07-15

337

Comparison of an Oscillometric Method with Cardiac Magnetic Resonance for the Analysis of Aortic Pulse Wave Velocity  

PubMed Central

Objectives Pulse wave velocity (PWV) is the proposed gold-standard for the assessment of aortic elastic properties. The aim of this study was to compare aortic PWV determined by a recently developed oscillometric device with cardiac magnetic resonance imaging (CMR). Methods PWV was assessed in 40 volunteers with two different methods. The oscillometric method (PWVOSC) is based on a transfer function from the brachial pressure waves determined by oscillometric blood pressure measurements with a common cuff (Mobil-O-Graph, I.E.M. Stolberg, Germany). CMR was used to determine aortic PWVCMR with the use of the transit time method based on phase-contrast imaging at the level of the ascending and abdominal aorta on a clinical 1.5 Tesla scanner (Siemens, Erlangen, Germany). Results The median age of the study population was 34 years (IQR: 24–55 years, 11 females). A very strong correlation was found between PWVOSC and PWVCMR (r = 0.859, p < 0.001). Mean PWVOSC was 6.7 ± 1.8 m/s and mean PWVCMR was 6.1 ± 1.8 m/s (p < 0.001). Analysis of agreement between the two measurements using Bland-Altman method showed a bias of 0.57 m/s (upper and lower limit of agreement: 2.49 m/s and -1.34 m/s). The corresponding coefficient of variation between both measurements was 15%. Conclusion Aortic pulse wave velocity assessed by transformation of the brachial pressure waveform showed an acceptable agreement with the CMR-derived transit time method. PMID:25612307

Feistritzer, Hans-Josef; Reinstadler, Sebastian J.; Klug, Gert; Kremser, Christian; Seidner, Benjamin; Esterhammer, Regina; Schocke, Michael F.; Franz, Wolfgang-Michael; Metzler, Bernhard

2015-01-01

338

Ultrasonic wave velocity measurement in small polymeric and cortical bone specimens  

NASA Technical Reports Server (NTRS)

A system was refined for the determination of the bulk ultrasonic wave propagation velocity in small cortical bone specimens. Longitudinal and shear wave propagations were measured using ceramic, piezoelectric 20 and 5 MHz transducers, respectively. Results of the pulse transmission technique were refined via the measurement of the system delay time. The precision and accuracy of the system were quantified using small specimens of polyoxymethylene, polystyrene-butadiene, and high-density polyethylene. These polymeric materials had known acoustic properties, similarity of propagation velocities to cortical bone, and minimal sample inhomogeneity. Dependence of longitudinal and transverse specimen dimensions upon propagation times was quantified. To confirm the consistency of longitudinal wave propagation in small cortical bone specimens (< 1.0 mm), cut-down specimens were prepared from a normal rat femur. Finally, cortical samples were prepared from each of ten normal rat femora, and Young's moduli (Eii), shear moduli (Gij), and Poisson ratios (Vij) were measured. For all specimens (bone, polyoxymethylene, polystyrene-butadiene, and high-density polyethylene), strong linear correlations (R2 > 0.997) were maintained between propagation time and distance throughout the size ranges down to less than 0.4 mm. Results for polyoxymethylene, polystyrene-butadiene, and high-density polyethylene were accurate to within 5 percent of reported literature values. Measurement repeatability (precision) improved with an increase in the wave transmission distance (propagating dimension). No statistically significant effect due to the transverse dimension was detected.

Kohles, S. S.; Bowers, J. R.; Vailas, A. C.; Vanderby, R. Jr

1997-01-01

339

Ion Bernstein waves in a plasma with a kappa velocity distribution  

NASA Astrophysics Data System (ADS)

Using a Vlasov-Poisson model, a numerical investigation of the dispersion relation for ion Bernstein waves in a kappa-distributed plasma has been carried out. The dispersion relation is found to depend significantly on the spectral index of the ions, ?i, the parameter whose smallness is a measure of the departure from thermal equilibrium of the distribution function. Over all cyclotron harmonics, the typical Bernstein wave curves are shifted to higher wavenumbers (k) if ?i is reduced. For waves whose frequency lies above the lower hybrid frequency, ?LH, an increasing excess of superthermal particles (decreasing ?i) reduces the frequency, ?peak, of the characteristic peak at which the group velocity vanishes, while the associated kpeak is increased. As the ratio of ion plasma to cyclotron frequency (?pi/?ci) is increased, the fall-off of ? at large k is smaller for lower ?i and curves are shifted towards larger wavenumbers. In the lower hybrid frequency band and harmonic bands above it, the frequency in a low-?i plasma spans only a part of the intraharmonic space, unlike the Maxwellian case, thus exhibiting considerably less coupling between adjacent bands for low ?i. It is suggested that the presence of the ensuing stopbands may be a useful diagnostic for the velocity distribution characteristics. The model is applied to the Earth's plasma sheet boundary layer in which waves propagating perpendicularly to the ambient magnetic field at frequencies between harmonics of the ion cyclotron frequency are frequently observed.

Nsengiyumva, F.; Mace, R. L.; Hellberg, M. A.

2013-10-01

340

Aspherical heterogeneity of the mantle from phase velocities of mantle waves  

NASA Technical Reports Server (NTRS)

Long-period surface waves are used to map the lateral heterogeneity of the upper 100-600 km of the mantle. There is good correlation of velocity with surface tectonics and heat flow. Convergence regions are generally slow for Love waves and fast for Rayleigh waves. Back arc basins have slower than average shallow mantle. Some island arcs show evidence of fast material at greater depth. Deep-seated slow anomalies underlie the Red Sea-Afar region of north-east Africa, western North American-northern East Pacific Rise, Indian Ocean triple junction and the Tasman Sea-Campbell Plateau regions. The fastest regions are in the north central Australia-New Guinea and the South Atlantic.

Nakanishi, I.; Anderson, D. L.

1984-01-01

341

The nonlinear theory of slow-wave electron cyclotron masers with inclusion of the beam velocity spread  

SciTech Connect

The nonlinear theory of slow-wave electron cyclotron masers (ECM) with an initially straight electron beam is developed. The evolution equation of the nonlinear beam electron energy is derived. The numerical studies of the slow-wave ECM efficiency with inclusion of Gaussian beam velocity spread are presented. It is shown that the velocity spread reduces the interaction efficiency. -- Highlights: •The theory of slow-wave electron cyclotron masers is considered. •The calculation of efficiency under the resonance condition is presented. •The efficiency under Gaussian velocity spreads has been obtained.

Kong, Ling-Bao, E-mail: konglingbao@gmail.com [School of Science, Beijing University of Chemical Technology, Beijing 100029 (China) [School of Science, Beijing University of Chemical Technology, Beijing 100029 (China); Beijing Key Laboratory of Environmentally Harmful Chemicals Assessment, Beijing University of Chemical Technology, Beijing 100029 (China); Wang, Hong-Yu [School of Physics, Anshan Normal University, Anshan 114005 (China)] [School of Physics, Anshan Normal University, Anshan 114005 (China); Hou, Zhi-Ling, E-mail: houzl@mail.buct.edu.cn [School of Science, Beijing University of Chemical Technology, Beijing 100029 (China) [School of Science, Beijing University of Chemical Technology, Beijing 100029 (China); Beijing Key Laboratory of Environmentally Harmful Chemicals Assessment, Beijing University of Chemical Technology, Beijing 100029 (China); Jin, Hai-Bo [School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081 (China)] [School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081 (China); Du, Chao-Hai [Institute of Electronics, Chinese Academy of Sciences, Beijing 100190 (China)] [Institute of Electronics, Chinese Academy of Sciences, Beijing 100190 (China)

2013-12-15

342

Interpretation of phase velocity measurements of wind-generated surface waves  

NASA Technical Reports Server (NTRS)

Differences in experimental results for surface gravity wave phase speeds obtained by cross spectra and slope-height spectra are compared. It is shown that, for a simple two-dimensional model containing both dispersive and non-dispersive waves, the observed differences can be explained readily. The slope-height technique measures a weighted average of the wavenumber squared; for frequencies sufficiently large compared with the dominant frequency, the computed phase-speed variation with frequency is representative of the wave having the larger wavenumber - the dispersive wave. For the cross-spectral method, it is shown that the small-probe-separation assumption usually employed is not valid for frequencies much larger than the dominant frequency, except at the singular point where both waves have exactly equal spectral densities and the phase function represents an average of the two modes. For all other cases, the phase function approaches that of the wave having the larger spectral density, and essentially ignores the presence of the other wave, even if both modes are relatively close in their contributions to the total spectral density.

Dudis, J. J.

1981-01-01

343

Changes in elastic wave velocity and rock microstructure due to basalt-CO2-water reactions  

NASA Astrophysics Data System (ADS)

The chemical interaction between carbon dioxide, water, and basalt is a common process in the earth, which results in the dissolution of primary minerals that later precipitate as alteration minerals. This occurs naturally in volcanic settings, but more recently basalts have been suggested as reservoirs for sequestration of anthropogenic CO2. In both the natural and man-made cases, rock-fluid reactions lead to the precipitation of carbonates. Here, we quantify changes in ultrasonic wave speeds, associated with changes in the frame of whole-rock basalts, as CO2 and basalt react. After 30weeks of reactions and carbonate precipitation, the ultrasonic wave speed in dry basalt samples increases between 4% and 20% and permeability is reduced by up to an order of magnitude. However, porosity decreases only by 2% to 3%. The correlation between significant changes in wave speed and permeability indicates that a precipitate is developing in fractures and compliant pores. Thin sections, XRF-loss on ignition, and water chemistry confirm this observation. This means time-lapse seismic monitoring of a CO2-water-basalt system cannot assume invariance of the rock frame, as typically done in fluid substitution models. We conclude that secondary mineral precipitation causes a measurable change in the velocities of elastic waves in basalt-water-CO2 systems, suggesting that seismic waves could be used to remotely monitor future CO2 injection sites. Although monitoring these reactions in the field with seismic waves might be complicated due to the heterogeneous nature of basalt, quantifying the elastic velocity changes associated with rock alteration in a controlled laboratory experiment forms an important step toward field-scale seismic monitoring.

Adam, Ludmila; Wijk, Kasper; Otheim, Thomas; Batzle, Michael

2013-08-01

344

Changes in Elastic Wave Velocity and Rock Microstructure due to Basalt-CO2-Water Reactions  

NASA Astrophysics Data System (ADS)

The chemical interaction between carbon dioxide, water and basalt is a common process in the earth, which results in the dissolution of primary minerals that later precipitate as alteration minerals. This occurs naturally in volcanic settings, but more recently basalts have been suggested as reservoirs for sequestration of anthropogenic CO2. In both the natural and man-made case, rock-fluid reactions lead to the precipitation of carbonates. Here, we quantify changes in ultrasonic wave speeds, associated with changes in the frame of whole-rock basalt samples, as CO2 and basalt react. After 30 weeks of reactions and carbonate precipitation, the ultrasonic wave speed in dry basalt samples increases between 4% and 20% and permeability is reduced by up to an order of magnitude. However, porosity decreases only by 2% to 3%. The correlation between significant changes in wave speed and permeability indicates that precipitate is developing in fractures and compliant pores. Thin sections, XRF-Loss On Ignition and water chemistry confirm this. Our findings show that time-lapse seismic monitoring of a CO2-water-basalt system cannot assume invariance of the rock frame, as typically done in fluid substitution models. We conclude that secondary mineral precipitation causes a measurable change in the velocities of elastic waves in basalt-water-CO2 systems, suggesting that seismic waves could be used to remotely monitor future CO2 injection sites. Although monitoring these reactions in the field with seismic waves might be complicated due to the heterogeneous nature of basalt, quantifying the elastic velocity changes associated with rock alteration in a controlled laboratory experiment forms an important step toward field-scale seismic monitoring.

Adam, L.; van Wijk, K.; Otheim, L. T.; Batzle, M. L.

2013-12-01

345

Strong seismic wave scattering in the low-velocity anomaly associated with subduction of oceanic plate  

NASA Astrophysics Data System (ADS)

Analyses of dense seismic records in Kanto, Japan, revealed distinct pulse broadening and peak delay of high-frequency S waves at central Chiba. These phenomena are observed at frequency range of 1-8 Hz and exist only for ray paths passing through the low-velocity (LV) zone at depth of 20-40 km beneath northwestern Chiba. To obtain a more detailed understanding of these phenomena, we conducted 2-D and 3-D finite difference method simulations of seismic wave propagation using a realistic heterogeneous structure model. Through numerous simulations we demonstrated that strong seismic scattering, due to localized strong small-scale heterogeneities in the LV zone and in the oceanic crust, is a major cause of strong pulse broadening and peak delay of high-frequency S waves. After comparing simulation results with observations, the most preferable small-scale velocity heterogeneity in the LV zone is characterized by a Gaussian power spectral density function (PSDF) with correlation distance a of 1-2 km and rms value ? = 0.07-0.09, superposed on a background exponential PSDF (a = 3 km, ? = 0.07). Assuming strong velocity heterogeneities, observed amplitude decay at Chiba is also well explained by strong scattering attenuation in the LV zone. Because the LV zone, which has been reported by seismic tomography studies, is interpreted as being constructed by the dehydration of the subducting oceanic crust of the Philippine Sea Plate, strong small-scale velocity heterogeneity in the LV zone may be related to the random distribution of fluid in this volume.

Takemura, Shunsuke; Yoshimoto, Kazuo

2014-05-01

346

Non-triggered quantification of central and peripheral pulse-wave velocity  

PubMed Central

Purpose Stiffening of the arteries results in increased pulse-wave velocity (PWV), the propagation velocity of the blood. Elevated aortic PWV has been shown to correlate with aging and atherosclerotic alterations. We extended a previous non-triggered projection-based cardiovascular MR method and demonstrate its feasibility by mapping the PWV of the aortic arch, thoraco-abdominal aorta and iliofemoral arteries in a cohort of healthy adults. Materials and Methods The proposed method "simultaneously" excites and collects a series of velocity-encoded projections at two arterial segments to estimate the wave-front velocity, which inherently probes the high-frequency component of the dynamic vessel wall modulus in response to oscillatory pressure waves. The regional PWVs were quantified in a small pilot study in healthy subjects (N = 10, age range 23 to 68 yrs) at 3T. Results The projection-based method successfully time-resolved regional PWVs for 8-10 cardiac cycles without gating and demonstrated the feasibility of monitoring beat-to-beat changes in PWV resulting from heart rate irregularities. For dul-slice excitation the aliasing was negligible and did not interfere with PWV quantification. The aortic arch and thoracoabdominal aorta PWV were positively correlated with age (p < 0.05), consistent with previous reports. On the other hand, the PWV of the iliofemoral arteries showed decreasing trend with age, which has been associated with the weakening of muscular arteries, a natural aging process. Conclusion The PWV map of the arterial tree from ascending aorta to femoral arteries may provide additional insight into pathophysiology of vascular aging and atherosclerosis. PMID:22188972

2011-01-01

347

Control of periodic variations in Saturn's magnetosphere by compressional waves  

NASA Astrophysics Data System (ADS)

Many of the periodic variations observed in Saturn's magnetosphere can be linked directly to the presence of a rotating pattern of field-aligned currents that link the northern and southern ionospheres with each other and with the magnetosphere. Such a current system is incorporated in a magnetohydrodynamic simulation that has previously been shown to reproduce many of the observed periodic properties of the system. Here the simulation is used to investigate a range of phenomena that can be attributed to the effects of compressional waves launched from the rotating current sources. The compressional waves are found to drive the flapping of the plasma sheet and the expansion and contraction of the magnetopause in each rotation period. Because the compressional perturbations weaken as they rotate from morning to evening around the dayside of the magnetosphere, the boundary develops a strong morning-evening asymmetry. A fit to the shape is provided that may be useful in further investigation of magnetopause properties, but there is already evidence of the proposed asymmetry in the observations of Clarke et al. (2010a).

Kivelson, Margaret Galland; Jia, Xianzhe

2014-10-01

348

Shear wave velocity structure beneath southern California: Mantle at crustal depths beneath the Borderlands  

NASA Astrophysics Data System (ADS)

We use Rayleigh waves to invert for the shear velocity structure beneath the Borderlands of southern California. Fundamental mode Rayleigh waves from 101 teleseismic events at a complete range of azimuths were recorded by a maximum of 102 stations, including stations belonging to the USArray. This ray coverage gives us an unprecedentedly well-resolved view of the lower crust and upper mantle structure beneath the Borderlands. We observe a high velocity anomaly beneath the Gulf of Santa Catalina and the northern end of the San Diego Trough that dips slightly to the west toward the southern portion of the Santa Catalina Basin. This anomaly is on the order of 15-20% faster than typical mid-crustal velocities in the southern California region. The magnitude of this fast anomaly requires that it be derived from a mantle source. Southern California has undergone a complex tectonic history, closely associated with the subduction of the Farallon plate system, the rotation of the Western Transverse Ranges, and the subsequent extension that lead to the formation of the basins of the Borderlands. Taking the magnitude of the velocity anomaly and the tectonic history of the region into account, we interpret this fast anomaly to represent mantle material that has risen to crustal levels associated with the formation of a metamorphic core complex in the Borderlands. Previous studies reference the extension that occurred in response to the rotation of the Western Transverse Ranges and the subsequent emplacement of the Catalina Schist as mechanisms for the formation of a metamorphic core complex in the Borderlands region. However, we see no sign of low velocities in the deeper mantle that characterize other areas in the region that are thought to have experienced upwelling.

Rau, C. J.; Forsyth, D. W.; Wang, Y.

2009-12-01

349

Alternative dust-ion acoustic waves in a magnetized charge varying dusty plasma with nonthermal electrons having a vortex-like velocity distribution  

NASA Astrophysics Data System (ADS)

Alternative localized dust-ion acoustic waves are investigated in a magnetized charge varying dusty plasma with nonthermal electrons having a vortex-like velocity distribution. The correct non-Maxwellian charging currents are obtained based on the well-known orbit limited motion theory. Following the standard reductive perturbation technique, a Schamel-Zakharov Kuznetsov Burgers (S-ZKB) equation is derived. It is shown that due to an interplay between trapping and nonthermality, our dusty plasma model may support solitary as well as shock waves the main quantities (phase velocity, amplitude and width) of which are drastically influenced by trapping, nonthermality and charge variation. Due to the flexibility provided by the outlined distribution function (two concepts of non isothermality), we stress that our model should provide a good fit of the space observations.

Hadjaz, Idir; Tribeche, Mouloud

2014-06-01

350

Dust-acoustic solitary waves and double layers in a magnetized dusty plasma with nonthermal ions and dust charge variation  

SciTech Connect

The effect of nonthermal ions and variable dust charge on small-amplitude nonlinear dust-acoustic (DA) waves is investigated. It is found that both compressive and rarefactive solitons exist and depend on the nonthermal parameter a. Using a reductive perturbation theory, a Zakharov-Kuznetsov (ZK) equation is derived. At critical value of a, a{sub c}, a modified ZK equation with third- and fourth-order nonlinearities, is obtained. Depending on a, the solution of the evolution equation reveals whether there is coexistence of both compressive and rarefactive solitary waves or double layers (DLs) with the possibility of their two kinds. In addition, for certain plasma parameters, the solitary wave disappears and a DL is expected. The variation of dust charge number, wave velocity, and soliton amplitude and its width against system parameters is investigated for the DA solitary waves. It is shown that the incorporation of both the adiabatic dust-charge variation and the nonthermal distributed ions modifies significantly the nature of DA solitary waves and DA DLs. The findings of this investigation may be useful in understanding the ion acceleration mechanisms close to the Moon and also enhances our knowledge on pickup ions around unmagnetized bodies, such as comets, Mars, and Venus.

El-Taibany, W.F.; Sabry, R. [Department of Physics, Faculty of Science-Damietta, Mansoura University, Damietta El-Gedida, P.O. 34517 (Egypt)

2005-08-15

351

AN EXPERIMENTAL STUDY OF SHOCK WAVES RESULTING FROM THE IMPACT OF HIGH VELOCITY MISSILES ON ANIMAL TISSUES  

PubMed Central

The spark shadowgram method of studying shock waves is described. It has been used to investigate the properties of such waves produced by the impact of a high velocity missile on the surface of water. The method can be adapted for study of behavior of shock waves in tissue by placing the tissue on a water surface or immersing it in water. Spark shadowgrams then reveal waves passing from tissue to water or reflected from tissue surfaces. Reflection and transmission of shock waves from muscle, liver, stomach, and intestinal wall are compared with reflection from non-living surfaces such as gelatin gel, steel, plexiglas, cork, and air. Because of its heterogeneous structure, waves transmitted by tissue are dispersed and appear as a series of wavelets. When the accoustical impedance (density x wave velocity) of a medium is less than that in which the wave is moving, reflection will occur with inversion of the wave; i.e., a high pressure wave will become a low pressure wave. This inversion occurs at an air surface and is illustrated by shadowgrams of reflection from stomach wall, from a segment of colon filled with gas, and from air-filled rubber balloons. Bone (human skull and beef ribs) shows good reflection and some transmission of shock waves. When steel is directly hit by a missile, clearly visible elastic waves pass from metal to water, but a similar direct hit on bone does not result in elastic waves strong enough to be detected by a spark shadowgram. PMID:19871617

Harvey, E. Newton; McMillen, J. Howard

1947-01-01

352

Fast simulated annealing inversion of surface waves on pavement using phase-velocity spectra  

USGS Publications Warehouse

The conventional inversion of surface waves depends on modal identification of measured dispersion curves, which can be ambiguous. It is possible to avoid mode-number identification and extraction by inverting the complete phase-velocity spectrum obtained from a multichannel record. We use the fast simulated annealing (FSA) global search algorithm to minimize the difference between the measured phase-velocity spectrum and that calculated from a theoretical layer model, including the field setup geometry. Results show that this algorithm can help one avoid getting trapped in local minima while searching for the best-matching layer model. The entire procedure is demonstrated on synthetic and field data for asphalt pavement. The viscoelastic properties of the top asphalt layer are taken into account, and the inverted asphalt stiffness as a function of frequency compares well with laboratory tests on core samples. The thickness and shear-wave velocity of the deeper embedded layers are resolved within 10% deviation from those values measured separately during pavement construction. The proposed method may be equally applicable to normal soil site investigation and in the field of ultrasonic testing of materials. ?? 2006 Society of Exploration Geophysicists.

Ryden, N.; Park, C.B.

2006-01-01

353

Pn wave velocity and anisotropy beneath Pamir and its adjacent regions  

NASA Astrophysics Data System (ADS)

As the western end point of continental collision between the Indian and Eurasian plates, Pamir is an ideal place to research uplifting mechanisms in the Tibetan plateau. In this study, 141 644 Pn arrivals were used to obtain seismic wave velocities and anisotropy in the uppermost mantle beneath Pamir and its adjacent regions by performing tomographic inversion of Pn travel times. The data were selected from multiple databases, including ISC/EHB, the Annual Bulletin of Chinese Earthquakes, and regional bulletins of Xinjiang. The tomography results reveal significant features with high resolution and correlate well with geological structures. The main results are as follows: (1) The Pn wave velocities are particularly high in the old stable blocks such as Tarim basin, Indian plate and Tajik basin, while the low Pn velocities always lie in tectonically active regions like the western Tibetan plateau, Pamir, Tianshan and Hindu Kush. (2) Strong Pn anisotropy is found beneath the Indian-Eurasian collision zone; its direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression stress and relative crustal movement. The result is probably caused by the pure shear deformation in the uppermost mantle of the collision zone. (3) A geodynamic continent-continent collision model is proposed to show anisotropy and collision mechanisms between the Indian plate and the Tarim and Tajik basins.

Feng, Biao; Pei, Shunping

2012-12-01

354

An explicit relation for the apparent phase velocity of Rayleigh waves in a vertically heterogeneous elastic half-space  

NASA Astrophysics Data System (ADS)

This paper presents the mathematical derivation of an explicit relation for the apparent (or effective) phase velocity of Rayleigh waves in a vertically heterogeneous, isotropic elastic half-space for harmonic excitation. As a kinematical feature, the apparent phase velocity captures the superposition, in a spatial Fourier series, of the individual modes of propagation of Rayleigh waves and describes the speed of propagation of a composite waveform generated by a vertically oscillating point load. The relation, which is a function of the distance from the source, frequency and depth, depends explicitly on the modal phase and group velocities of Rayleigh waves, and their corresponding wavenumbers and eigenfunctions, which can be computed directly from the solution of the Rayleigh-wave eigenproblem. A practical scenario for the application of the notion of apparent Rayleigh-wave phase velocity is the modelling of the dispersion curve in the well-known surface wave measurement methods `spectral analysis of surface waves' (SASW) and `multichannel analysis of surface waves' (MASW). Apart from a theoretical motivation, the availability in surface wave testing of an explicit formula for the calculation of the apparent Rayleigh-wave phase velocity may lead to the development of a new class of inversion algorithms capable of taking into account the influence of all the modes of surface wave propagation. To demonstrate the exactness of the explicit relation, the predicted values of apparent phase velocity are compared to those computed synthetically from a numerical simulation of SASW and MASW testing for three case studies, which show both single as well as multiple mode dominance effects.

Lai, Carlo G.; Mangriotis, Maria-Daphne; Rix, Glenn J.

2014-11-01

355

Aortic pulse wave velocity and the degree of atherosclerosis in the elderly: a pathological study based on 304 autopsy cases  

Microsoft Academic Search

Introduction:Studies examining the correlation between aortic pulse wave velocity (PWV) and atherosclerosis have reported conflicting results. The present paper verifies this correlation by conducting autopsy examination of elderly subjects.

Motoji Sawabe; Ryutaro Takahashi; Satoru Matsushita; Toshio Ozawa; Tomio Arai; Akihiko Hamamatsu; Ken-ichi Nakahara; Kouji Chida; Hiroshi Yamanouchi; Shigeo Murayama; Noriko Tanaka

2005-01-01

356

Spatial Parallelism of a 3D Finite Difference, Velocity-Stress Elastic Wave Propagation Code  

SciTech Connect

Finite difference methods for solving the wave equation more accurately capture the physics of waves propagating through the earth than asymptotic solution methods. Unfortunately. finite difference simulations for 3D elastic wave propagation are expensive. We model waves in a 3D isotropic elastic earth. The wave equation solution consists of three velocity components and six stresses. The partial derivatives are discretized using 2nd-order in time and 4th-order in space staggered finite difference operators. Staggered schemes allow one to obtain additional accuracy (via centered finite differences) without requiring additional storage. The serial code is most unique in its ability to model a number of different types of seismic sources. The parallel implementation uses the MP1 library, thus allowing for portability between platforms. Spatial parallelism provides a highly efficient strategy for parallelizing finite difference simulations. In this implementation, one can decompose the global problem domain into one-, two-, and three-dimensional processor decompositions with 3D decompositions generally producing the best parallel speed up. Because i/o is handled largely outside of the time-step loop (the most expensive part of the simulation) we have opted for straight-forward broadcast and reduce operations to handle i/o. The majority of the communication in the code consists of passing subdomain face information to neighboring processors for use as ''ghost cells''. When this communication is balanced against computation by allocating subdomains of reasonable size, we observe excellent scaled speed up. Allocating subdomains of size 25 x 25 x 25 on each node, we achieve efficiencies of 94% on 128 processors. Numerical examples for both a layered earth model and a homogeneous medium with a high-velocity blocky inclusion illustrate the accuracy of the parallel code.

MINKOFF,SUSAN E.

1999-12-09

357

Spatial parallelism of a 3D finite difference, velocity-stress elastic wave propagation code  

SciTech Connect

Finite difference methods for solving the wave equation more accurately capture the physics of waves propagating through the earth than asymptotic solution methods. Unfortunately, finite difference simulations for 3D elastic wave propagation are expensive. The authors model waves in a 3D isotropic elastic earth. The wave equation solution consists of three velocity components and six stresses. The partial derivatives are discretized using 2nd-order in time and 4th-order in space staggered finite difference operators. Staggered schemes allow one to obtain additional accuracy (via centered finite differences) without requiring additional storage. The serial code is most unique in its ability to model a number of different types of seismic sources. The parallel implementation uses the MPI library, thus allowing for portability between platforms. Spatial parallelism provides a highly efficient strategy for parallelizing finite difference simulations. In this implementation, one can decompose the global problem domain into one-, two-, and three-dimensional processor decompositions with 3D decompositions generally producing the best parallel speedup. Because I/O is handled largely outside of the time-step loop (the most expensive part of the simulation) the authors have opted for straight-forward broadcast and reduce operations to handle I/O. The majority of the communication in the code consists of passing subdomain face information to neighboring processors for use as ghost cells. When this communication is balanced against computation by allocating subdomains of reasonable size, they observe excellent scaled speedup. Allocating subdomains of size 25 x 25 x 25 on each node, they achieve efficiencies of 94% on 128 processors. Numerical examples for both a layered earth model and a homogeneous medium with a high-velocity blocky inclusion illustrate the accuracy of the parallel code.

Minkoff, S.E.

1999-12-01

358

Velocity field, surface profile and curvature resolution of steep and short free-surface waves  

NASA Astrophysics Data System (ADS)

On steep, millimeter-scale, 2D water waves, surface profile, and subsurface velocity field are measured with high-spatio-temporal resolution. This allows resolving surface vorticity, which is captured in the surface boundary layer and compared with its direct computation from interface curvature and velocity. Data are obtained with a combination of high-magnification time-resolved particle image velocimetry (PIV) and planar laser-induced fluorescence. The latter is used to resolve the surface profile and serves as a processing mask for the former. PIV processing schemes are compared to optimize accuracy locally, and profilometry data are treated to obtain surface curvature. This diagnostic enables new insights into free-surface dynamic, in particular, wave growth and surface vorticity generation, for flow regimes not studied previously. The technique is demonstrated on a high-speed water jet discharging in quiescent air at a Reynolds number of 4.8 × 104. Shear-layer instability below the surface leads to streamwise traveling waves with wavelength ? ~ 2 mm and steepness , where a is the crest to trough amplitude. Flow structures are resolved at these scales by recording at 16 kHz with a magnification of 4.

André, Matthieu A.; Bardet, Philippe M.

2014-04-01

359

Variations of the velocity contrast and rupture properties of M6 earthquakes along the Parkfield section of the San Andreas fault  

NASA Astrophysics Data System (ADS)

We investigate the seismic velocity contrast across the San Andreas fault (SAF) in the Parkfield area using fault zone head waves (FZHW) that propagate along the bimaterial fault interface and direct P waves. We systematically analyse large data sets of near-fault waveforms recorded by several seismic networks over the period 1984-2005. Clear FZHW are observed at many stations on the NE side of the fault in the creeping section of the SAF north of Middle Mountain (MM). This indicates the presence of a sharp bimaterial interface and that the NE side of the fault has lower seismic velocities in that region. The obtained P-wave velocity contrast is about 5-10 per cent north of MM, and it systematically decreases to 0-2 per cent near Gold Hill (GH). The along-strike variations of the velocity contrast are consistent with geological observations of a sliver of high-velocity rock immediately to the NE of the SAF near GH, associated with the GH fault, and existing 3-D seismic tomography results. The obtained imaging results offer an explanation for the mixed rupture directions of the M6-type Parkfield earthquakes. The strong velocity contrast around MM is expected to produce a preferred propagation direction to the SE for earthquakes that nucleate near MM (e.g. the 1934 and 1966 Parkfield earthquakes). In contrast, the near-zero velocity contrast and multiple fault branches near GH imply that earthquakes that nucleate near GH (e.g. the 2004 Parkfield earthquake) are not expected to have a preferred propagation direction to the SE, and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities. The observed systematic reduction of the velocity contrast along the SAF from NW of MM to SE of GH provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the Parkfield section and propagate to the SE, and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the NW.

Zhao, Peng; Peng, Zhigang; Shi, Zheqiang; Lewis, Michael A.; Ben-Zion, Yehuda

2010-02-01

360

Estimating the influence of stress on elastic wave velocities from measurements made at different scales on sandstone facies  

NASA Astrophysics Data System (ADS)

Knowing how stress influences velocity is crucial for seismic reservoir monitoring because seismic reflection signatures are sensitive to the dynamic processes related to production or injection. In this thesis, I address this issue by conducting research to quantify the influence of stress on elastic wave velocities at different measurement scales. I compare the influence of stress on elastic wave velocities measured from borehole sonic data, time-lapse multicomponent surface seismic data, and laboratory core experiments. To make this comparison, I develop a methodology by combining a series of existing techniques to estimate the influence of stress on shear-wave velocity acquired from crossed-dipole borehole sonic data. By combining the crossed-dipole radial profile of shear wave velocity with the redistribution of stress caused by the presence of a borehole, I establish a relationship between velocity and stress from borehole measurements made within the reservoir. Previously, information about the influence of stress on elastic wave velocity has been primarily obtained through laboratory measurements. To estimate the magnitude of far-field principal stresses at the reservoir level in the subsurface I first integrate the formation bulk density well log to approximate the stress exerted by the overburden. Then, I use radial profiles of shear-wave slowness obtained from borehole sonic data to estimate the far-field principal horizontal stresses. Estimates of the far-field principal stresses are needed to model the stress redistribution caused by the drillout of a borehole. To study the influence that borehole stress redistribution has on shear-wave velocity, I combine the borehole stresses that influence a vertically propagating fast shear wave and the radial profile of the fast shear wave velocity. I use an excess compliance rock physics model to extrapolate the borehole shear wave velocity/stress relationship to predict the influence stress has on compressional wave velocity. Time-lapse multicomponent surface seismic data were recorded to monitor a water alternating gas (CO2) injection scheme for enhanced oil recovery. Interpretations of the time-lapse seismic data, in conjunction with a reservoir simulation model, indicate regions of increased reservoir pore pressure. To simulate the reservoir conditions during a water alternating gas injection scheme, I conducted laboratory core experiments in which I measured compressional and shear-wave velocities as a function of confining pressure, pore pressure, fluid type (which included CO2 in the gas and supercritical phase), and lithology. The shear wave velocity from the borehole sonic data and the time-lapse multicomponent surface seismic data show a very similar response to changes in stress. The laboratory experiments indicate the core velocity measurements are less sensitive to a change in stress than both the borehole and seismic data. Borehole sonic data provide an alternative and, in this case, a more accurate method than laboratory core measurements for estimating stress-induced changes in shear-wave velocity within a reservoir. An accurate rock-physics relationship between shear-wave velocity and effective stress is required for a quantitative interpretation of pore pressure changes from multicomponent time-lapse seismic data.

Wandler, Aaron Vallejo

361

Technique for Measurements of Elastic Wave Velocities and Thickness of Solid Plate from Access on Only One Side  

NASA Astrophysics Data System (ADS)

A novel method is proposed for the simultaneous measurement of longitudinal and transverse wave velocities and the thickness of isotropic solid plates from access on only one side. Line contact transducers were placed on the same side of the specimen, and waveforms were acquired at various distances between the transducers. Longitudinal wave velocity and the thickness of the specimen can be simultaneously determined from the relationship between the transit time of the longitudinal wave mode and the ultrasonic beam path. A least-squares fitting method was employed to increase the accuracy of measurements. Transverse wave velocity also can be determined from the transit time of the mode-converted transverse wave and the corresponding beam path determined by Snell’s law. Experimental results for several specimens of different materials as well as different thicknesses show good agreement with those measured by conventional methods.

Kim, Young H.; Song, Sung-Jin; Lee, Jeong-Ki

2005-07-01

362

Elastic-wave velocity in marine sediments with gas hydrates: Effective medium modeling  

USGS Publications Warehouse

We offer a first-principle-based effective medium model for elastic-wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame; (b) hydrate becomes a component of the solid phase, modifying the elasticity of the frame. The goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) consistent with estimates obtained from resistivity, chlorinity and evolved gas data. Copyright 1999 by the American Geophysical Union.

Helgerud, M.B.; Dvorkin, J.; Nur, A.; Sakai, A.; Collett, T.

1999-01-01

363

Effects of Ambient Velocity Shear on Nonlinear Internal Waves and Associated Mixing at the Columbia River Plume Front  

E-print Network

37 Effects of Ambient Velocity Shear on Nonlinear Internal Waves and Associated Mixing that in the presence of strong ambient velocity shear, the maximum amplitude of the NLIW structure function appear well) project. The effects of ambient shear on NLIW dynamic characteristics can be analyzed using a high

Hickey, Barbara

364

Combined ultrasonic elastic wave velocity and microtomography measurements at high pressures  

NASA Astrophysics Data System (ADS)

Combined ultrasonic and microtomographic measurements were conducted for simultaneous determination of elastic property and density of noncrystalline materials at high pressures. A Paris-Edinburgh anvil cell was placed in a rotation apparatus, which enabled us to take a series of x-ray radiography images under pressure over a 180° angle range and construct accurately the three-dimensional sample volume using microtomography. In addition, ultrasonic elastic wave velocity measurements were carried out simultaneously using the pulse reflection method with a 10° Y-cut LiNbO3 transducer attached to the end of the lower anvil. Combined ultrasonic and microtomographic measurements were carried out for SiO2 glass up to 2.6 GPa and room temperature. A decrease in elastic wave velocities of the SiO2 glass was observed with increasing pressure, in agreement with previous studies. The simultaneous measurements on elastic wave velocities and density allowed us to derive bulk (Ks) and shear (G) moduli as a function of pressure. Ks and G of the SiO2 glass also decreased with increasing pressure. The negative pressure dependence of Ks is stronger than that of G, and as a result the value of Ks became similar to G at 2.0-2.6 GPa. There is no reason why we cannot apply this new technique to high temperatures as well. Hence the results demonstrate that the combined ultrasonic and microtomography technique is a powerful tool to derive advanced (accurate) P-V-Ks-G-(T) equations of state for noncrystalline materials.

Kono, Yoshio; Yamada, Akihiro; Wang, Yanbin; Yu, Tony; Inoue, Toru

2011-02-01

365

Combined ultrasonic elastic wave velocity and microtomography measurements at high pressures  

SciTech Connect

Combined ultrasonic and microtomographic measurements were conducted for simultaneous determination of elastic property and density of noncrystalline materials at high pressures. A Paris-Edinburgh anvil cell was placed in a rotation apparatus, which enabled us to take a series of x-ray radiography images under pressure over a 180{sup o} angle range and construct accurately the three-dimensional sample volume using microtomography. In addition, ultrasonic elastic wave velocity measurements were carried out simultaneously using the pulse reflection method with a 10{sup o} Y-cut LiNbO{sub 3} transducer attached to the end of the lower anvil. Combined ultrasonic and microtomographic measurements were carried out for SiO{sub 2} glass up to 2.6 GPa and room temperature. A decrease in elastic wave velocities of the SiO{sub 2} glass was observed with increasing pressure, in agreement with previous studies. The simultaneous measurements on elastic wave velocities and density allowed us to derive bulk (K{sub s}) and shear (G) moduli as a function of pressure. K{sub s} and G of the SiO{sub 2} glass also decreased with increasing pressure. The negative pressure dependence of K{sub s} is stronger than that of G, and as a result the value of K{sub s} became similar to G at 2.0-2.6 GPa. There is no reason why we cannot apply this new technique to high temperatures as well. Hence the results demonstrate that the combined ultrasonic and microtomography technique is a powerful tool to derive advanced (accurate) P-V-Ks-G-(T) equations of state for noncrystalline materials.

Kono, Yoshio; Yamada, Akihiro; Wang, Yanbin; Yu, Tony; Inoue, Toru (Ehime U); (UC)

2011-09-16

366

Combined ultrasonic elastic wave velocity and microtomography measurements at high pressures  

SciTech Connect

Combined ultrasonic and microtomographic measurements were conducted for simultaneous determination of elastic property and density of noncrystalline materials at high pressures. A Paris-Edinburgh anvil cell was placed in a rotation apparatus, which enabled us to take a series of x-ray radiography images under pressure over a 180 deg. angle range and construct accurately the three-dimensional sample volume using microtomography. In addition, ultrasonic elastic wave velocity measurements were carried out simultaneously using the pulse reflection method with a 10 deg. Y-cut LiNbO{sub 3} transducer attached to the end of the lower anvil. Combined ultrasonic and microtomographic measurements were carried out for SiO{sub 2} glass up to 2.6 GPa and room temperature. A decrease in elastic wave velocities of the SiO{sub 2} glass was observed with increasing pressure, in agreement with previous studies. The simultaneous measurements on elastic wave velocities and density allowed us to derive bulk (K{sub s}) and shear (G) moduli as a function of pressure. K{sub s} and G of the SiO{sub 2} glass also decreased with increasing pressure. The negative pressure dependence of K{sub s} is stronger than that of G, and as a result the value of K{sub s} became similar to G at 2.0-2.6 GPa. There is no reason why we cannot apply this new technique to high temperatures as well. Hence the results demonstrate that the combined ultrasonic and microtomography technique is a powerful tool to derive advanced (accurate) P-V-K{sub s}-G-(T) equations of state for noncrystalline materials.

Kono, Yoshio; Yamada, Akihiro; Inoue, Toru [Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577 (Japan); Wang Yanbin; Yu, Tony [GeoSoilEnviroCARS, Center for Advanced Radiation Sources, University of Chicago, 5640 S, Ellis Avenue, Chicago, Illinois 60637 (United States)

2011-02-15

367

Velocity lag of solid particles in oscillating gases and in gases passing through normal shock waves  

NASA Technical Reports Server (NTRS)

The velocity lag of micrometer size spherical particles is theoretically determined for gas particle mixtures passing through a stationary normal shock wave and also for particles embedded in an oscillating gas flow. The particle sizes and densities chosen are those considered important for laser Doppler velocimeter applications. The governing equations for each flow system are formulated. The deviation from Stokes flow caused by inertial, compressibility, and rarefaction effects is accounted for in both flow systems by use of an empirical drag coefficient. Graphical results are presented which characterize particle tracking as a function of system parameters.

Maxwell, B. R.; Seasholtz, R. G.

1974-01-01

368

Improved formula for continuous-wave measurements of ultrasonic phase velocity  

NASA Technical Reports Server (NTRS)

An improved formula for continuous-wave ultrasonic phase velocity measurements using contact transducers is derived from the transmission line theory. The effect of transducer-sample coupling bonds is considered for measurements of solid samples even though it is often neglected because of the difficulty of accurately determining the bond thickness. Computer models show that the present formula is more accurate than previous expressions. Laboratory measurements using contacting transducers with the present formula are compared to measurements using noncontacting (hence effectively correction-free) capacitive transducers. The results of the experiments verify the validity and accuracy of the new formula.

Chern, E. J.; Cantrell, J. H., Jr.; Heyman, J. S.

1981-01-01

369

Acoustic Fields of Bulk Acoustic Waves Excited by Phase Velocity Scanning of Laser Interference Fringes  

NASA Astrophysics Data System (ADS)

Directivity and generation efficiency of transverse and longitudinal bulk acoustic waves (BAWs) excited by phase velocity scanning (PVS) of laser interference fringes were theoretically investigated. Formulas were derived for the BAW generation based on scanning interference fringes (SIF) which were produced by intersecting two coherent laser beams with different frequencies on an opaque specimen. It was deduced that the amplitude of the BAW was proportional to the product of the scanning length of the SIF and the directivity pattern due to the thermoelastic line source. Unidirectionality of the BAW generated by the SIF was theoretically verified in an ideal state.

Nishino, Hideo; Tsukahara, Yusuke; Takemoto, Mikio; Yamanaka, Kazushi

2002-04-01

370

Optical pin apparatus for measuring the arrival time and velocity of shock waves and particles  

DOEpatents

An apparatus for the detection of the arrival and for the determination of the velocity of disturbances such as shock-wave fronts and/or projectiles. Optical pins using fluid-filled microballoons as the light source and an optical fiber as a link to a photodetector have been used to investigate shock-waves and projectiles. A microballoon filled with a noble gas is affixed to one end of a fiber-optic cable, and the other end of the cable is attached to a high-speed streak camera. As the shock-front or projectile compresses the microballoon, the gas inside is heated and compressed producing a bright flash of light. The flash of light is transmitted via the optic cable to the streak camera where it is recorded. One image-converter streak camera is capable of recording information from more than 100 microballoon-cable combinations simultaneously.

Benjamin, R.F.

1983-10-18

371

Optical pin apparatus for measuring the arrival time and velocity of shock waves and particles  

DOEpatents

An apparatus for the detection of the arrival and for the determination of the velocity of disturbances such as shock-wave fronts and/or projectiles. Optical pins using fluid-filled microballoons as the light source and an optical fiber as a link to a photodetector have been used to investigate shock-waves and projectiles. A microballoon filled with a noble gas is affixed to one end of a fiber-optic cable, and the other end of the cable is attached to a high-speed streak camera. As the shock-front or projectile compresses the microballoon, the gas inside is heated and compressed producing a bright flash of light. The flash of light is transmitted via the optic cable to the streak camera where it is recorded. One image-converter streak camera is capable of recording information from more than 100 microballoon-cable combinations simultaneously.

Benjamin, Robert F. (315 Rover Blvd., Los Alamos, NM 87544)

1987-01-01

372

Optical pin apparatus for measuring the arrival time and velocity of shock waves and particles  

SciTech Connect

An apparatus is disclosed for the detection of the arrival and for the determination of the velocity of disturbances such as shock-wave fronts and/or projectiles. Optical pins using fluid-filled microballoons as the light source and an optical fiber as a link to a photodetector have been used to investigate shock-waves and projectiles. A microballoon filled with a noble gas is affixed to one end of a fiber-optic cable, and the other end of the cable is attached to a high-speed streak camera. As the shock-front or projectile compresses the microballoon, the gas inside is heated and compressed producing a bright flash of light. The flash of light is transmitted via the optic cable to the streak camera where it is recorded. One image-converter streak camera is capable of recording information from more than 100 microballoon-cable combinations simultaneously. 3 figs.

Benjamin, R.F.

1987-03-10

373

Large-scale Observations of a Subauroral Polarization Stream by Midlatitude SuperDARN Radars: Instantaneous Longitudinal Velocity Variations  

NASA Technical Reports Server (NTRS)

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

Clausen, L. B. N.; Baker, J. B. H.; Sazykin, S.; Ruohoniemi, J. M.; Greenwald, R. A.; Thomas, E. J.; Shepherd, S. G.; Talaat, E. R.; Bristow, W. A.; Zheng, Y.; Coster, A. J.

2012-01-01

374

First high resolution P wave velocity structure beneath Tenerife Island, (Canary Islands, Spain)  

NASA Astrophysics Data System (ADS)

3D velocity structure distribution has been imaged for first time using high resolution traveltime seismic tomography of the active volcano of Tenerife Island (Canary Islands, Spain). It is located in the Atlantic Ocean. In this island is situated the Teide stratovolcano (3718 m high) that is part of the Cañadas-Teide-Pico Viejo volcanic complex. Las Cañadas is a caldera system more than 20 kilometers wide where at least four distinct caldera processes have been identified. Evidence for many explosive eruptions in the volcanic complex has been found; the last noticeable explosive eruption (sub-plinean) occurred at Montaña Blanca around 2000 years ago. During the last 300 years, six effusive eruptions have been reported, the last of which took place at Chinyero Volcano on 18 November 1909. In January 2007, a seismic active experiment was carried out as part of the TOM-TEIDEVS project. About 6850 air gun shots were fired on the sea and recorded on a dense local seismic land network consisting of 150 independent (three component) seismic stations. The good quality of the recorded data allowed identifying P-wave arrivals up to offsets of 30-40 km obtaining more than 63000 traveltimes used in the tomographic inversion. The images have been obtained using ATOM-3D code (Koulakov, 2009). This code uses ray bending algorithms in the ray tracing for the forward modelling and in the inversion step it uses gradient methods. The velocity models show a very heterogeneous upper crust that is usual in similar volcanic environment. The tomographic images points out the no-existence of a magmatic chamber near to the surface and below Pico Teide. The ancient Las Cañadas caldera borders are clearly imaged featuring relatively high seismic velocity. Moreover, we have found a big low velocity anomaly in the northwest dorsal of the island. The last eruption took place in 1909 in this area. Furthermore, in the southeast another low velocity anomaly has been imaged. Several resolution and accuracy tests were carried out to quantify the reliability of the final velocity models. Checkerboard tests show that the well-resolved are located up to 6-8 km depth. Also we carried out synthetic tests in which we successfully reproduce single anomalies observed in the velocity models. Especially we have study carefully the low velocity anomalies found in the NW and SE, which have been recovered successfully. The jack-knife technique have been used and our results are stable if we remove the 50% of the data for different stations, but if we reject all the data for some stations, the velocity models can change. These tests assure the uniqueness of the first 3D velocity model that characterizes the internal structure of the Tenerife Island. As main conclusions of our work we can remark: a) This is the first 3-D velocity image of the area; b) we have observed low velocity anomalies near to surface that could be associated to the presence of magma, water reservoirs and volcanic landslides; c) high velocity anomalies could be related to ancient volcanic episodes or basement structures; d) our results could help to resolve many questions relate to the evolution of the volcanic system, as the presence or not of big landslides, calderic explosions or others; e) this image is a very important tool to improve the knowledge of the volcanic hazard, and therefore volcanic risk. We would like to highlight the importance of take into account the risk of eruption in other areas besides Pico Teide-Las Cañadas system.

Garcia-Yeguas, Araceli; Ivan, Koulakov; Ibañez Jesus, M.; Valenti, Sallarès.

2010-05-01

375

Ion Bernstein waves in a plasma with a kappa velocity distribution  

SciTech Connect

Using a Vlasov-Poisson model, a numerical investigation of the dispersion relation for ion Bernstein waves in a kappa-distributed plasma has been carried out. The dispersion relation is found to depend significantly on the spectral index of the ions, ?{sub i}, the parameter whose smallness is a measure of the departure from thermal equilibrium of the distribution function. Over all cyclotron harmonics, the typical Bernstein wave curves are shifted to higher wavenumbers (k) if ?{sub i} is reduced. For waves whose frequency lies above the lower hybrid frequency, ?{sub LH}, an increasing excess of superthermal particles (decreasing ?{sub i}) reduces the frequency, ?{sub peak}, of the characteristic peak at which the group velocity vanishes, while the associated k{sub peak} is increased. As the ratio of ion plasma to cyclotron frequency (?{sub pi}/?{sub ci}) is increased, the fall-off of ? at large k is smaller for lower ?{sub i} and curves are shifted towards larger wavenumbers. In the lower hybrid frequency band and harmonic bands above it, the frequency in a low-?{sub i} plasma spans only a part of the intraharmonic space, unlike the Maxwellian case, thus exhibiting considerably less coupling between adjacent bands for low ?{sub i}. It is suggested that the presence of the ensuing stopbands may be a useful diagnostic for the velocity distribution characteristics. The model is applied to the Earth's plasma sheet boundary layer in which waves propagating perpendicularly to the ambient magnetic field at frequencies between harmonics of the ion cyclotron frequency are frequently observed.

Nsengiyumva, F.; Mace, R. L.; Hellberg, M. A. [School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4000 (South Africa)] [School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4000 (South Africa)

2013-10-15

376

Impact Velocity as a Source of Variations in Crater Depth on Mercury  

NASA Astrophysics Data System (ADS)

High-resolution images and altimetry of 115 craters obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission have been analyzed to further the understanding of the formation and subsequent modification on impact craters on Mercury. Measurements derived from altimetry include crater depth (d), rim height, central peak/ring height, and crater wall/terrace width. Images have been used to derive crater diameter (D) and to assess crater degradation state as well as the nature of any observed impact melt. Analyses of the measurements tie these geometrical parameters to the crater characteristics seen in images. An important first observation is that the freshest craters on Mercury are not necessarily the deepest, as is typically inferred in most studies of d/D ratios. Frequently, the heights of the central peak are also smaller. A good example is provided by a comparison of two peak-ring craters of similar size on Mercury: Atget (D = 102 km) and Hokusai (D = 93 km). Hokusai is extremely fresh and has few superposed craters, broadly extensive craters rays, and distal secondaries. Hokusai also possesses substantial amounts of visible impact melt and evidence for a rampart-like structure reminiscent of ejecta observed on Mars. Although not markedly altered by subsequent impacts, Atget crater is slightly older and more degraded. Atget ejecta show no obvious evidence for melt ponds in the ejecta as seen at Hokusai. The younger Hokusai has a smaller d/D value of 0.022 (d = 2.1 km) than Atget's ratio of 0.032 (d = 3.3 km). In other examples, shallower fresh craters also display a central structure with less relief than comparable but less fresh craters of similar size. The Hokusai-Atget comparison suggests that a shallower depth may be associated with a greater volume of impact melt, but calculations indicate that this increased melt volume is probably not directly responsible for the lesser depth. The observed variation in impact melt volume, as well as the difference in crater depth, could alternatively be the result of differences in impact angle, as has been proposed for craters on the Moon, but no oblique impact-angle morphologies are apparent in the data associated with these and other examples. Structural effects that might be associated with the presence of different central structures, such as a central peak ring, are unlikely to be the cause of the observed depth difference because both Atget and Hokusai have peak rings. Target variations are also unlikely to be important, because both craters are in similar volcanic terrain. The observed greater melt generated at Hokusai than at Atget is best explained by the broad range of impact velocities expected on Mercury, which can range from 15 to 75 km/s. A larger impact velocity will not only generate more impact melt volume, an indicator of higher impact velocity, but from laboratory data a higher velocity will also produce shallower transient craters that can form shallower final craters.

Barnouin, O. S.; Susorney, H. C.; Ernst, C. M.; Neumann, G. A.; Johnson, C. L.; Balcerski, J.; Hauck, S. A.

2012-12-01

377

The thin section rock physics: Modeling and measurement of seismic wave velocity on the slice of carbonates  

NASA Astrophysics Data System (ADS)

This paper discusses a new approach for investigating the seismic wave velocity of rock, specifically carbonates, as affected by their pore structures. While the conventional routine of seismic velocity measurement highly depends on the extensive laboratory experiment, the proposed approach utilizes the digital rock physics view which lies on the numerical experiment. Thus, instead of using core sample, we use the thin section image of carbonate rock to measure the effective seismic wave velocity when travelling on it. In the numerical experiment, thin section images act as the medium on which wave propagation will be simulated. For the modeling, an advanced technique based on artificial neural network was employed for building the velocity and density profile, replacing image's RGB pixel value with the seismic velocity and density of each rock constituent. Then, ultrasonic wave was simulated to propagate in the thin section image by using finite difference time domain method, based on assumption of an acoustic-isotropic medium. Effective velocities were drawn from the recorded signal and being compared to the velocity modeling from Wyllie time average model and Kuster-Toksoz rock physics model. To perform the modeling, image analysis routines were undertaken for quantifying the pore aspect ratio that is assumed to represent the rocks pore structure. In addition, porosity and mineral fraction required for velocity modeling were also quantified by using integrated neural network and image analysis technique. It was found that the Kuster-Toksoz gives the closer prediction to the measured velocity as compared to the Wyllie time average model. We also conclude