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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

The group velocity variation of Lamb wave in fiber reinforced composite plate.  

PubMed

Experimentally measured Lamb wave group velocities in composite materials with anisotropic characteristics are not the same as the theoretical group velocities which is calculated with the Lamb wave dispersion equation. This discrepancy arises from the fact that the angle between the group velocity direction and the phase velocity direction in anisotropic materials exists. Wave propagation in a composite material with anisotropic characteristics should be considered with respect to magnitude correction in addition to direction correction. In this study, S0 mode phase velocity dispersion curves are depicted with the variation of degree with respect to the fiber direction using a Lamb wave dispersion relation in the unidirectional, bidirectional, and quasi-isotropic composite plates. Slowness surface is sketched by the reciprocal value of the phase velocity curves. The magnitude and direction of the group velocity could be calculated from the slowness surface. The recalculated group velocities with consideration of the magnitude and direction from the slowness surface are compared with experimentally measured group velocities. The proposed method shows good agreements with theoretical and experimental results. PMID:17881029

Rhee, Sang-Ho; Lee, Jeong-Ki; Lee, Jung-Ju

2007-12-01

3

Lateral variation of shallow S-wave velocity structure in south Taiwan revealed from Rayleigh wave analysis for TAIGER explosion  

NASA Astrophysics Data System (ADS)

Wide-angle reflections across north and south Taiwan were held on in 2008 for the TAIGER project. Each transects consist of hundreds of Texan instruments with vertical component sensor, extending from the west Costal Plain to the Central Range. Focus on south Taiwan, the obvious surface waves generated by S1 and S2 explosions provide good dataset to investigate detailed shear wave structure and lateral variation along the transect. The signals generated by S1 explosion significantly show separated Rayleigh wave trains by the station within about 20 km distance. The spectrums show the period band of Rayleigh wave trains both range from 0.5 to 2.0 sec and larger amplitude of later wave train. To realize the characteristic of the Rayleigh waves, we used multiple-filter analysis to obtain group velocity dispersions in this study. The analyses show that group velocities of the Rayleigh waves are extremely low and thus regarded as the results from the soft sediment in the Coastal Plain. The wave trains are regarded as higher-mode and fundamental-mode Rayleigh waves based on waveform comparison with broadband stations and theoretical modeling. By contrast, only fundamental-mode Rayleigh wave was generated by S2 explosion. Overall, the group velocities at each station increase significantly from west to east. The inversed shear-wave velocity models from average group velocity dispersion illustrate low velocities and thus request high Poisson’s ratio of 0.3 ~0.4 to coincide with P wave velocity model from P arrival studies. Such information of near-surface structure would be very important to understand the site effects, which would result in destructive ground shaking.

Lai, Y.; Huang, B.; Yen, H.; Okaya, D. A.; Wang, C.; Wu, F. T.

2010-12-01

4

Global Love wave phase velocity variation and its significance to plate tectonics  

NASA Astrophysics Data System (ADS)

Global Love wave phase velocity variation was constructed for periods between 80 and 200 s by using approximately 9000 paths from 971 earthquakes (with M ? 5.5). The data set was from GDSN and GEOSCOPE networks between 1980 and 1987. We examined both the spherical harmonic expansion method and the block parameterization method. With a simple, constant damping parameter approach, synthetic tests showed that more accurate results were obtained by the block parameterization method than by the spherical harmonic expansion method. We adopted the block inversion method with a (nearly) equal area block (5° × 5° near the equator) discretization. The general pattern of the resulting maps were consistent with previous global and local studies. With a discretization of 5° by 5°, the maps were detailed enough to test some plate tectonic models. For the Pacific, Atlantic and Indian Oceans, surface-wave velocities increased smoothly to plate ages older than 100 Ma. Simple forward modeling showed that seismic phase velocity variation with a continuous thickening of lithosphere up to about 150 Ma fits the present observation, disagreeing with the model deduced from the heat flow and ocean depth data, which change variations at about 60-80 Ma. The seismic phase velocity variations in different oceans showed systematic differences at younger ages, and convergence beyond 100 Ma. The difference at younger ages implies a failure of scaling derived from a simple thermal boundary layer model for oceanic plates. Age-seismic phase velocity relationships on each side of ridges were also examined and asymmetric velocity variations were found, which suggests differences in thermal states from one side of the ridge to the other. In order to further examine the thermal state of the lithosphere, average age-phase velocity relations were established for each ocean, and subtracted from phase velocity variation maps. The results indicated broad, low-velocity regions in the south Pacific (super-swell region), the south and west Indian Ocean, and high-velocity regions east of the East Pacific Rise and in the north to northeast Indian Ocean. The results reflect the asymmetry of phase velocity variation about ridges. There is some correlation between hot-spot locations and low-velocity anomalies, but additional, large-scale thermal anomalies exist under old oceanic plate.

Zhang, Yu-Shen; Tanimoto, Toshiro

1991-04-01

5

Variation of fundamental mode Rayleigh wave group velocity dispersion in Iran and the surrounding region  

NASA Astrophysics Data System (ADS)

We present group velocity dispersion results from a study of regional fundamental mode Rayleigh waves propagating across Iran and the surrounding region. Data for these measurements come from field deployments within Iran by the University of Cambridge (UK) and the Universite Joseph-Fourier (FRA) in conjunction with International Institute of Earthquake Engineering and Seismology (Iran), within Oman by the Universite Pierre et Marie Curie-Paris (FRA), in addition to data from IRIS and Geofone. 1D path-averaged dispersion measurements have been made for ~800 source-receiver paths using multiple filter analysis. We combine these observations in a tomographic inversion to produce group velocity images between 15 and 60~s period. Because of the dense path coverage, these images have substantially higher lateral resolution for this region than is currently available from global and regional group velocity studies. We observe variations in short-period group velocity which is consistent with the surface geology. Low group velocity (2.45-2.55~km/s) at short periods (15-20~s) is observed beneath the south Caspian Basin, northern Iran, the Persian Gulf, the Zagros, the Makran, northern Afghanistan and southern Pakistan. Somewhat higher group velocity (2.60-2.70~km/s) at these periods occurs in central Iran. At intermediate periods (30-40~s) group velocities over most of the region are low (2.90-3.10~km/s) compared to Arabia. At longer periods (50-60~s) group velocities remain low (3.35-3.45~km/s) over most of Iran but there is a suggestion of higher group velocities beneath the northern and central Zagros.

Rham, D.; Priestley, K.; Tatar, M.; Paul, A.; Hatzfeld, D.; Radjaee, A.; Nowrouzi, G.; Kaviani, A.; Tiberi, C.

2005-12-01

6

Short wave-length variation of seismic velocity anomalies associated with stagnant slab  

NASA Astrophysics Data System (ADS)

When mineral physicists discuss mantle properties associated with stagnant slab referring to seismological studies, they focus on long wave-length characteristics provided by seismic tomography studies. In this sense the gap between seismological studies and mineral physics is still large to explore mantle properties. On the other hand, triplicated regional body waves are highly sensitive to the transition zone structure and have ample information regarding seismic velocity anomaly and the discontinuity depth associated with stagnant cold slab. We present results of regional waveform modeling using data from deep focus earthquakes (Mw?5.5, depth?300 km) that occurred in the Kurile to Izu-Bonin subduction zones during the period of 1990 to 2005. Here the range of regional distance is considered up to 32 deg where triplication can be observed from a deep focus event due to the high velocity anomaly (HVA) and depression of the discontinuity depth. After examining the data quality, regional waveform data of about 50 events were selected for analysis. The body waves which sampled the region with apparent stagnant slab strongly, can be modeled by model M3.11 (HVA in the transition zone with depression of the discontinuity to 690 km, Tajima and Grand, 1998), model M2.0 (HVA similar to M3.11 but without broad depression of the discontinuity) or a model with their slight modifications if not by iasp91 (Kennett and Engdahl, 1991). Structural variation in the transition zone has been determined in terms of seismic model M3.11 for the Kuriles subduction zone, M2.0 for the region beneath the Japan sea and the northeastern rim of the Eurasian plate or others. The structural boundaries are sharp and distinct showing steep gradient of velocity anomaly between the regions of different structure. The variation of the discontinuity depths indicates lateral variation of temperature beneath the flattened slab. We also found anomalously broadened P waveforms that propagated through the vicinity of structural boundaries. We postulate that the broadened P waves are SV-to-P converted waves in a narrow zone of low velocity anomaly (or a high Poisson's ratio of ~ 0.32) associated with stagnant slab.

Nakagawa, T.; Tajima, F.

2006-12-01

7

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

8

Analytic study of traveling-wave velocity variation in line-focusing schemes for plasma x-ray lasers.  

PubMed

Efficient amplification of coherent short-wavelength pulses along a plasma gain column requires traveling-wave excitation with the sweep velocity matched to the signal group velocity. Through simulations incorporating the gain dynamics of the system, we show that the group velocity is not constant but increases monotonically along the line focus due to strong saturation. We demonstrate a line-focusing configuration that results in traveling wave excitation with the sweep velocity well matched to the spatially varying group velocity. Moreover, we show through numerical simulations that the improved velocity matching yields a significant improvement in signal amplification. PMID:24922210

Jia, Fei; Staub, Felix; Siegrist, Michael; Balmer, Jürg E

2014-05-20

9

The variation with T sub e and T sub i of the velocity of unstable ionospheric two-stream waves  

SciTech Connect

It is generally accepted that unstable ionospheric plasma waves moving at the ion-acoustic velocity (two-stream waves) are responsible for the so-called type 1 VHF radar echoes commonly observed at equatorial and auroral latitudes. These same waves apparently are also the source of type 4 echoes, which have sharply peaked spectra with unusually large Doppler shifts and are seen at auroral latitudes during sufficiently disturbed conditions. But how, exactly, is the observed Doppler shift, or equivalently the ion-acoustic velocity C{sub s}, related to the electron and ion temperatures The expression usually quoted, with occasional caveats, is the isothermal result C{sub s}{sup 2}=K(T{sub e}+T{sub i})/m{sub i}. The validity of the isothermal assumption has not been of much concern until recently, when the first simultaneous independent measurements of the temperatures and C{sub s} were made in Scandinavia. The authors argue here that, in fact, the electrons should usually be treated as adiabatic, with three degrees of freedom, while the ions may or may not be adiabatic (with only one degree of freedom), depending upon the temperatures, the altitude, and the radar frequency. In other words, the ion effects generally should be calculated kinetically. The differences between the two models in the computed wave velocity are substantial ({approximately}20-40%). A comparison between European Incoherent Scatter (EISCAT) temperatures and wave velocities measured with the Cornell University Portable Radar Interferometer (CUPRI) shows good agreement with the model given here.

Farley, D.; Providakes, J. (Cornell Univ., Ithaca, NY (USA))

1989-11-01

10

Joint Inversion of Receiver Functions and Surface Wave Group Velocities from the MANAS data set to Determine Custal Thickness Variations in theTien Shan  

NASA Astrophysics Data System (ADS)

The Tien Shan is the largest active intracontinental orgogenic belt on the Earth. To better understand the processes causing mountains to form in this location distant from a plate boundary, we analyze passive source seismic data collected on 40 broad band stations of the MANAS project (2005-2007) to determine variations in crustal thickness and wavespeed across the range. The linear MANAS array transects the Tien Shan just to the east of the Talas Fergana fault and extends from the Tarim Basin north over the Kokshal Range and across the Naryn Valley to the Kyrgyz Range and the Kazakh Shield. This data set has a denser station spacing (~10 km) than that available in previous studies. We combine P- and S-wave receiver functions with surface wave observations from both earthquakes and ambient noise analysis to reduce the ambiguity inherent in the images obtained from the techniques applied individually. In particular, fundamental-mode surface-wave dispersion observations are sensitive to absolute wavespeed averages rather than contrasts, while receiver functions are primarily sensitive to wavespeed contrasts and vertically integrated travel times rather than absolute wavespeeds. Moreover, analysis of the ambient noise allows dispersion measurements at shorter periods which improves constraints for the upper crust. We jointly invert P- and S-wave receiver functions, fundamental mode Rayleigh wave group velocity determined from 1.75 years of continuous seismic ambient noise for periods 4-28s, and group velocity data for periods 10-70s from the surface wave study of Acton et al. (2010). The resulting crustal model show a strong variation in the Moho depth across the range. We find the thickest crust (~60 km) beneath the Kokshal range, while that beneath the Naryn Valley, in the middle of the Tien Shan is thin (~45 km) and is of similar thickness to that beneath the Tarim Basin and Kazakh shield. This suggests a lack of crustal shortening, or shortening of a previously thinned crust, in the middle of the range.

Gilligan, A.; Priestley, K. F.; Roecker, S. W.

2012-12-01

11

Numerical Values of Acoustic Surface Wave Velocities.  

National Technical Information Service (NTIS)

The velocity of acoustic surface waves on solids is calculated in terms of the values of the transverse wave velocities for Poisson's ratios from 0.0001 to 0.5000 in steps smaller than 0.0004. (Author)

W. G. Mayer G. B. Lamers

1966-01-01

12

Eurasian surface wave tomography: Group velocities  

NASA Astrophysics Data System (ADS)

This paper presents the results of a study of the dispersion characteristics of broadband fundamental surface waves propagating across Eurasia. The study is broader band, displays denser and more uniform data coverage, and demonstrates higher resolution than previous studies of Eurasia performed on this scale. In addition, the estimated group velocity maps reveal the signatures of geological and tectonic features never before displayed in similar surface wave studies. We present group velocity maps from 20 s to 200 s period for Rayleigh waves and from 20 s to 125 s for Love waves. Broadband waveform data from about 600 events from 1988 through 1995 recorded at 83 individual stations across Eurasia have produced about 9000 paths for which individual dispersion curves have been estimated. Dispersion curves from similar paths are clustered to reduce redundancy, to identify outliers for rejection, and to assign uncertainty estimates. On average, measurement uncertainty is about 0.030-0.040 km/s and is not a strong function of frequency. Resolution is estimated from "checker-board" tests, and we show that average resolutions across Eurasia range from 5° to 7.5° but degrade at periods above about 100 s and near the periphery of the maps. The estimated maps produce a variance reduction relative to the Preliminary Reference Earth Model (PREM) of more than 90% for Rayleigh waves below 60 s period but reduce to about 70% between 80 and 200 s period. For Love waves, variance reductions are similar, being above 90% for most periods below 100 s and falling to 70% at 150 s. Synthetic experiments are presented to estimate the biases that theoretical approximations should impart to the group velocity maps, in particular source group time shifts, azimuthal anisotropy, and systematic event mislocations near subducting slabs. The most significant problems are probably caused by azimuthal anisotropy, but above 100 s the effect of source group time shifts may also be appreciable. These effects are probably below the signal levels that we interpret here, however. Many known geological and tectonic structures are observed in the group velocity maps. Of particular note are the signatures of sedimentary basins, continental flood basalts, variations in crustal thickness, backarc spreading, downgoing slabs, and continental roots. Comparison of the estimated group velocity maps with those predicted by CRUST5.1/S16B30 is qualitatively good, but there are significant differences in detail which provide new information that should help to calibrate future crustal and upper mantle models of Eurasia.

Ritzwoller, Michael H.; Levshin, Anatoli L.

1998-03-01

13

Anisotropic shear velocity structure beneath the Eastern Lau back-arc Spreading Center from joint Rayleigh and Love wave ambient noise tomography - Implications for the variation of mantle wedge melt distributions  

NASA Astrophysics Data System (ADS)

We analyze Rayleigh and Love wave dispersion extracted from ambient noise correlation of OBS data to obtain a 3D shear velocity model beneath the Eastern Lau Spreading Center (ELSC). The ELSC is a 400-km-long back-arc spreading center lying close to the Tonga trench in the southwestern Pacific. Seafloor morphology, crustal thickness and lava composition data show systematic variations along the ridge as the ridge migrates away from the volcanic arc front to the north, indicating a decreasing influence of the subducting slab. Phase velocities for Rayleigh and Love waves were obtained from inter-station coherences of ambient noise recorded at 50 OBS stations. The phase velocity measurements were then inverted for anisotropic phase velocity maps and subsequently inverted for a 3D shear velocity model. Two separate low velocity zones (LVZs) are imaged beneath the back-arc basin. The shallow LVZ lies beneath the Tofua Volcanic Arc closer to the trench. The deeper LVZ underneath the back-arc spreading center widens and shifts laterally with respect to the ridge as the ridge migrates away from the volcanic arc. The two LVZs overlap to the south of 21 S but become distinct to the north, indicating that the northern ELSC segment is supplied by a different source of melt than the southern segment. Anisotropic Rayleigh wave velocity maps show fast directions sub-parallel to the trench, indicating possible along-trench mantle flow beneath the spreading center. (a) Bathymetry map of study area and locations of cross-sections shown in (b). (b) Seismic shear velocity variation at cross-sections shown in figure 1. The top three are perpendicular to the ridge and the bottom one is along the ridge.

Zha, Y.; Webb, S. C.; Wiens, D. A.; Blackman, D. K.; Dunn, R. A.; Conder, J. A.; Menke, W. H.

2013-12-01

14

Radial Velocity Variations of K Giant Stars  

NASA Astrophysics Data System (ADS)

We have observed a sample of 182 K giants at Lick Observatory over the past 2.5 years, using the Hamilton Spectrograph in conjunction with the Iodine Cell. This yields radial velocities with precisions of about 5-7 m/s. The stars in our sample have been selected from the Hipparcos Catalogue and are believed to be bona-fide single stars. Two thirds of the stars show radial velocity dispersions of less than about 50 m/s, with a peak at about 20 m/s. Some stars show short-term variations on the order of days that seem to be rather random, whereas other stars show long-term trends of the order of years. The radial velocity curves of some of the stars suggest periodic variations with periods of the order of 1-2 years, which could be due either to non-radial pulsations or the rotational modulation of surface features. We find that redder K giants in general show larger radial velocity variations, whereas we do not find a clear correlation between radial velocity variations and photometric variability.

Mitchell, D. S.; Frink, S.; Quirrenbach, A.; Fischer, D. A.

2001-12-01

15

Surface Wave Velocity of Crosslinked Polyacrylate Gels  

NASA Astrophysics Data System (ADS)

Surface wave velocities of crosslinked polyacrylate hydrogelswere measured as a function of water content with differentcompositions of sodium polyacrylate (NaPA) and polyacrylic acid (PAA).The water content and composition dependencies of the surface wavevelocity were discussed.

Matsuoka, Tatsuro; Kinouchi, Wataru; ShinobuKoda, ShinobuKoda; Nomura, Hiroyasu

1999-05-01

16

Elastic wave velocities of Apollo 14, 15, and 16 rocks  

NASA Technical Reports Server (NTRS)

Elastic wave velocities of two Apollo 14 rocks, 14053 and 14321, three Apollo 15 rocks, 15058, 15415, and 15545, and one Apollo 16 rock 60315 have been determined at pressures up to 10 kb. For sample 14321, the variation of the compressional wave velocities with temperature has been measured over the temperature range from 27 to 200 C. Overall elastic properties of these samples except sample 15415 are very similar to those of Apollo 11, 12, and 14 rocks and are concordant with Toksoz et al.'s (1972) interpretation that lunar upper crust is of basaltic composition. Temperature derivative of the P wave velocity for sample 14321 is a half to one order of magnitude larger than that for single crystalline minerals. This suggests that the seismic velocity in the lunar crust may be affected significantly by the temperature distribution.

Mizutani, H.; Newbigging, D. F.

1973-01-01

17

Latitudinal Variation of Solar Wind Velocity  

NASA Astrophysics Data System (ADS)

Single station solar wind velocity measurements using the Ooty Radio Telescope (ORT) in India (operating at 327 MHz) are reported for the period August 1992 to August 1993. Interplanetary scintillation (IPS) observations on a large number of compact radio sources covering a latitudinal range of ±80° were used to derive solar wind velocities using the method of fitting a power law model to the observed IPS spectra. The data shows a velocity versus heliographic latitude pattern which is similar to that reported by Rickett and Coles (1991) for the 1981 1982 period. However, the average of the measured equatorial velocities are higher, being about 470 km s-1 compared to their value of 400 km s-1. The distribution of electron density variations (?N e ) between 50R? and 90R? was also determined and it was found that ?N e was about 30% less at the poles as compared to the equator.

Ananthakrishnan, S.; Balasubramanian, V.; Janardhan, P.

1995-04-01

18

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

19

Wave measurements using GPS velocity signals.  

PubMed

This study presents the idea of using GPS-output velocity signals to obtain wave measurement data. The application of the transformation from a velocity spectrum to a displacement spectrum in conjunction with the directional wave spectral theory are the core concepts in this study. Laboratory experiments were conducted to verify the accuracy of the inversed displacement of the surface of the sea. A GPS device was installed on a moored accelerometer buoy to verify the GPS-derived wave parameters. It was determined that loss or drifting of the GPS signal, as well as energy spikes occurring in the low frequency band led to erroneous measurements. Through the application of moving average skill and a process of frequency cut-off to the GPS output velocity, correlations between GPS-derived, and accelerometer buoy-measured significant wave heights and periods were both improved to 0.95. The GPS-derived one-dimensional and directional wave spectra were in agreement with the measurements. Despite the direction verification showing a 10° bias, this exercise still provided useful information with sufficient accuracy for a number of specific purposes. The results presented in this study indicate that using GPS output velocity is a reasonable alternative for the measurement of ocean waves. PMID:22346618

Doong, Dong-Jiing; Lee, Beng-Chun; Kao, Chia Chuen

2011-01-01

20

Dissecting Lagrangian Velocities and Ramp Waves  

NASA Astrophysics Data System (ADS)

Lagrangian velocities are typically the observational quantity for compression wave experiments that probe material response at high-pressure. Depending on the experimental compression methodology (high-explosive to laser), a wide range of strain rates are possible from 10^3 to 10^8 s-1 yielding unique insight into non-linear wave propagation. We have been developing a method to understand the thermodynamic response of a material subjected to these high-pressure, compression waves. As part of that effort, we have found that the relationship between Lagrangian acceleration and the velocity offers insight into the material response and helps to dissect the velocity for subsequent analysis related to equation of state and non-elastic response mechanisms. We shall discuss this methodology and provide several examples.

Orlikowski, Daniel; Minich, Roger

2011-06-01

21

Soil liquefaction evaluation using shear wave velocity  

Microsoft Academic Search

A reasonably good relationship between shear wave velocity (SWV) and standard penetration resistance (SPT) of granular soils in agreement with previous studies was obtained from field tests. A similar correlation between SWV and cone penetration resistance of granular soils was also obtained. Using Seed's Standard Penetration Test (SPT)-based soil liquefaction charts, new charts of soil liquefaction evaluation based on SWV

Kamil Kayabali

1996-01-01

22

Seismic Wave Velocities in Rocks I : Modeling Heterogeneous Media and their Elastic Wave Velocities  

Microsoft Academic Search

On the basis of the viewpoint that rocks are typical random heterogeneous media consisting of different minerals and micro-cracks, modeling techniques for predicting seismic wave veloci- ties in crystalline rocks are reviewed. The term seismic velocity is sometimes unclear in its phys- ical meaning in real media that contain heterogeneity and anisotropy. The physical meaning of seismic velocity is first

Osamu NISHIZAWA

23

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

24

Velocity plateaus in traveling-wave electrophoresis.  

PubMed

One-dimensional models are used to study traveling-wave electrophoresis, a tunable method for separating charged analytes. A traveling-electrode model reveals the mechanism for longitudinal oscillations. A stationary-electrode model explains the origin of mode-locked plateaus in the average velocity, predicts devil's staircases with nested Farey sequences, and reduces to a continuum sinusoidal model in the high electrode-density limit. PMID:23214624

Correll, Robert; Edwards, Boyd F

2012-10-01

25

Beyond group velocity - Wave velocity and signal velocity. I The group-velocity operator in the absence of attenuation  

NASA Astrophysics Data System (ADS)

Radiation-field propagation in a dispersive medium is investigated theoretically, considering the heuristic case of negligeable attenuation. The wave-packet and wave-front approaches to the metrology of group velocity are reviewed, and a new approach based on a realistic and accesible criterion and free from limiting assumptions about the form of the emitted signal or its spectrum is developed, expanding the analysis of wave moments (Baird, 1972; Bradford, 1976) to encompass field source, medium, and detector. The formalism and methodology of the theory of the spatiotemporal signal and its representations is employed, defining a wave center and a signal center as the spatial and temporal markers of the field, respectively, and characterizing their motions by a wave velocity, and a signal velocity, both dependent on emitted-signal form and medium properties. The nonattenuation case is examined in detail.

Bonnet, G.

1983-10-01

26

Love wave phase velocity in the Atlantic upper mantle  

NASA Astrophysics Data System (ADS)

Much of what is currently known about the dynamics of the oceanic upper mantle, including temperature structure, composition, melt content, and deformation, comes from regional seismic models of the Pacific basin. Well-known differences between the Pacific and the Atlantic, including the much slower seafloor spreading rates and plate velocities in the Atlantic, suggest that the dynamic processes differ beneath these two basins. A regional seismic study of the Atlantic basin will allow us to better quantify and model the differences between the Atlantic and Pacific basins. We have measured a new data set of phase delays for fundamental-mode Love waves traversing the Atlantic upper mantle. The measurements are made using the generalized seismological data functional approach of Gee and Jordan (1992) in the period range 50-125 s. The measurements are derived from waveforms generated by earthquakes with Mw > 5.6 that occurred within or on the margins of the Atlantic basin during 1992 - 2012. To avoid contamination of mostly-oceanic paths by continental lithosphere, we consider only seismic stations located within or on the margins of the basin. We investigate, through experiments with synthetic seismograms, the effect of higher-mode interference on the fundamental-mode data set. The phase-delay observations are used to investigate lateral variations in Love wave phase velocity in the Atlantic basin with two approaches. One, phase velocity is assumed to vary only as a function of seafloor age, and a pure-path approach is employed to determine age-dependent velocities. Two, we solve for 2-D phase-velocity maps in order to capture phase-velocity variations that cannot be incorporated into the underlying age dependence. Through comparison with an equivalent data set of Rayleigh wave phase-delay observations, we investigate the strength of radial anisotropy in the Atlantic upper mantle.

Friedman, D. P.; Dalton, C. A.; Gaherty, J. B.

2013-12-01

27

Shear wave velocity model in Iceland from ambient noise and teleseismic Rayleigh wave tomography  

NASA Astrophysics Data System (ADS)

Iceland is a place of great geophysical interest due to its location at a hotspot and in the Mid-Atlantic spreading ridge. Despite numerous geophysics studies, disagreements still exist on crustal and mantle structure beneath Iceland. This research aims to construct a new shear wave model of Iceland by combining Rayleigh wave tomography from ambient seismic noise and teleseismic data recorded at the HOTSPOT experiment. Rayleigh wave phase velocities at periods from 5 to 25 s were obtained from ambient noise tomography. These short period phase velocities were combined with the longer period maps from teleseismic data by Li and Detrick [2006] for inverting 3-D SV wave velocities. Shear wave velocity in the shallow and mid crust shows a broad distribution of low velocity anomalies in central and southern Iceland and the lower crust map presents a more concentrated low velocity anomaly mainly beneath the Iceland hotspot. The low velocities in the crust are largely due to partial melt and high temperature associated with the hotspot activity. The variation of low velocity anomaly implies that melt is produced or accumulated in the lower crust under the hotspot and transported to nearby volcanic and rift zones at shallow depths. In the mantle, low velocity anomalies are present in areas that correlate well with the current and past spreading ridge locations rather than beneath the hotspot. In contrast, a relative high velocity anomaly is imaged below 80 km at the hotspot, which could reflect mantle residual after melt extraction or anisotropy caused by vertical flow in the plume conduit.

Li, A.; Azevedo, L. C.; Fu, Y. V.; Yuan, D.

2013-12-01

28

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

NASA Astrophysics Data System (ADS)

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 a representative suite of sam- ples that cover the density range. Two types of experiments have been performed. (1) Fragmentation speeds have been determined at constant differential pressure (DP) us- ing the fragmentation bomb. (2) Elastic wave velocities have been determined using a cubic anvil press. There is an empirical relationship between elastic wave veloci- ties and fracture velocity. From our measurements and this relationship we derive the fracture propagation speed for dome magmas and its correlation with fragmentation speed, as a function of vesicularity. In a case study of Unzen dacite, initial results indi- cate a negative correlation between fragmentation and elastic wave speeds. In general we find that both a DP reduction and a density increase cause a reduction in frag- mentation speed reduction. A density increase (porosity decrease) during an eruption may (a) reduce the potential (gas expansion) energy available for acceleration and (b) decrease the fragmentation layer velocity. In combination these effects could lead to a pulsation or even cessation of the eruption. Furtherwere, local density variations in the dome or conduit could cause a complex geometrical progression of the fragmentation front.

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

29

Depth resolution of earthquakes in Central Asia by moment tensor inversion of long-period Rayleigh waves: Effects of phase velocity variations across eurasia and their calibration  

Microsoft Academic Search

The moment tensor inversion method for long-period Rayleigh waves is discussed from the point of view of event-depth resolution. In this method, depth is usually determined by finding the minima in appropriate least squares residuals versus depth curves. We show that depth cannot be resolved if the data are sampled only at one frequency and a choice of frequencies spanning

Barbara Romanowicz

1981-01-01

30

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

Microsoft Academic Search

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.

Ryan J. DeWall; Tomy Varghese; Ernest L. Madsen

2011-01-01

31

Simultaneous inversion for 3-D variations in shear and bulk velocity in the mantle  

Microsoft Academic Search

Until recently, most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in P- or S-wave velocities. The S-wave velocity variations are larger and hence provide stronger signal on long-period waveforms. The direct P travel times, being the first arrivals, on the other hand, are most frequently reported in the International Seismological Centre (ISC) Bulletins.

Wei-Jia Su; Adam M. Dziewonski

1997-01-01

32

Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves  

Microsoft Academic Search

The shear-wave (S-wave) velocity of near-surface ma- terials (soil, rocks, pavement) and its effect on seismic- wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Rayleigh-wave phase velocity of a layered-earth model is a function of frequency and four groups of earth prop- erties: P-wave velocity, S-wave velocity, density, and thickness of layers. Analysis of the

Jianghai Xia; Richard D. Miller; Choon B. Park

1999-01-01

33

Seismic coda wave envelope in depth-dependent S wave velocity structure  

Microsoft Academic Search

Coda waves are considered to be composed of scattered waves. For simplicity, previous models to generate coda waves have often assumed spatial uniformity of seismic wave velocity. To account for a depth-dependent S wave velocity structure, energy densities of coda waves are synthesized using a Monte Carlo simulation method, based on the multiple isotropic scattering assumption for media consisting of

Mitsuyuki Hoshiba

1997-01-01

34

Seismic Wave Velocities and Earth Structure on The African Continent.  

National Technical Information Service (NTIS)

Phase and group velocities of seismic surface waves were determined for the African continent. Rayleigh wave phase velocities range from approximately 3.90 to 4.20 km/sec in the period range of 30.0 to 63.0 sec. Group velocities of fundamental mode Love a...

F. Gumper P. W. Pomeroy

1969-01-01

35

Doppler velocity dealiasing with millimeter wave radar RHI data  

Microsoft Academic Search

Millimeter wave radar has a unique advantage on detecting cloud microphysical structure. However, it is easy to cause the velocity aliasing due to its short wavelength. Velocity dealiasing is an effective means of detecting wind field information with Doppler radar. This paper reviews the cause of velocity folding and proposed a human-machine interaction method aimed at millimeter wave radar in

Lixue Song; Ming Wei

2010-01-01

36

The energy injection into waves with a zero group velocity  

Microsoft Academic Search

The frequency maxima of electron cyclotron harmonic (ECH) waves provide strong responses to sounding in various plasma regimes in the solar system. The frequency maxima correspond to waves for which the group velocity and thus the energy propagation velocity in the plasma frame of reference is zero. A particle-in-cell (PIC) code is employed to show that the propagation of wave

M. E. Dieckmann; S. C. Chapman; A. Ynnerman; G. Rowlands

1999-01-01

37

Shear Wave Velocity Estimation by Virtual Sensing Array Spectrum Analysis  

Microsoft Academic Search

Tissue displacement by continuous shear wave propagation successfully gives local wavelength, which depends on the local velocity of the shear wave. Our goal for velocity distribution estimation in future clinical applications is to achieve less than 10% error and less than 5 mm spatial resolution. However, this has been difficult to achieve owing to the presence of multiple shear waves

Takashi Miwa; Raj Kumar Parajuli; Ryosuke Tomizawa; Yoshiki Yamakoshi

2011-01-01

38

Variation in ejecta size with ejection velocity  

NASA Astrophysics Data System (ADS)

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.

1987-07-01

39

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

40

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

41

Optically Recording Velocity Interferometer System (ORVIS) For Subnanosecond Particle-Velocity Measurements in Shock Waves.  

National Technical Information Service (NTIS)

An optically recording velocity interferometer system (ORVIS) has been developed to measure particle velocity with subnanosecond resolution for shock waves in condensed matter. The fringe pattern of a wide-angle Michelson interferometer was focused as a s...

D. D. Bloomquist S. A. Sheffield

1982-01-01

42

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

43

Low Velocity Waves in Slowly Rotating Glass Tubes  

NASA Astrophysics Data System (ADS)

Over the past 2 years I have been placing a thin layer of dust with a gradation of particle sizes into mostly 2.5 cm diameter glass and pyrex tubes 120 cm long, closed at one end with a smaller diameter spout on the other end for changing the dust and sealing off tubes with a vacuum. I rotated the tubes (near 1rev/6s) level on upside down castors with cloth belts driven by geared down shaded pole motors separately mounted or driven with an air source for tubes mounted with air bearings. I attempted to eliminate sound as a factor. I found that the dust immediately tended to move to produce patterns that seemed to indicate the presence of standing waves of several frequencies. For example if I used 480 cm/s as a wave velocity I calculated frequencies, from spacings between centers of structures, like 80, 54.4, 60, 96, 44.8, 40, 120, 48, etc. These frequencies correspond to common frequencies that I found in plants for the past 10 years. The 480 cm/s came from a velocity that I found earlier for certain W-waves traveling outside of plants. Note that the patterns vary with time, with diurnal, longer, and shorter variations. See the Wagner web page.

Wagner, Orvin

1998-03-01

44

Porosity estimation based on seismic wave velocity at shallow depths  

NASA Astrophysics Data System (ADS)

Seismic wave velocity and porosity are used for the estimation of dynamic behaviors in the Earth, including seismicity and liquefaction. To increase the resolution of subsurface observations, seismic wave velocity and porosity can be combined in a compound method. To this end, in this paper, we utilize and rearrange the Wood, Gassmann, and Foti methods - three techniques commonly used to estimate porosity based on seismic wave velocity at shallow depths. Seismic wave velocity is obtained by a field velocity probe using the horizontal transmission technique. Porosity calculated using the Gassmann method shows the highest reliability considering observed porosity criteria. The sensitivities of each method are compared using the error norm. Results show that the Gassmann method has low sensitivity for calculating porosity, whereas the Wood and Foti methods have high sensitivity. Consequently, the Gassmann method is recommended for estimating porosity at shallow depths when using measured elastic wave velocity.

Lee, Jong-Sub; Yoon, Hyung-Koo

2014-06-01

45

Mantle P-wave velocity structure beneath the Hawaiian hotspot  

NASA Astrophysics Data System (ADS)

Three-dimensional images of P-wave velocity structure beneath the Hawaiian Islands, obtained from a network of seafloor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a high-velocity anomaly in the shallow upper mantle that is parabolic in map view. Low velocities continue downward to the mantle transition zone between 410 and 660 km depth and extend into the topmost lower mantle, although the resolution of lower mantle structure from this data set is limited. Comparisons of inversions with separate data sets at different frequencies suggest that contamination by water reverberations is not markedly biasing the P-wave imaging of mantle structure. Many aspects of the P-wave images are consistent with independent tomographic images of S-wave velocity in the region, but there are some differences in upper mantle structure between P-wave and S-wave velocities. Inversions without station terms show a southwestward shift in the location of lowest P-wave velocities in the uppermost mantle relative to the pattern for shear waves, and inversions with station terms show differences between P-wave and S-wave velocity heterogeneity in the shallow upper mantle beneath and immediately east of the island of Hawaii. Nonetheless, the combined data sets are in general agreement with the hypothesis that the Hawaiian hotspot is the result of an upwelling, high-temperature plume. The broad upper-mantle low-velocity region beneath the Hawaiian Islands may reflect the diverging "pancake" at the top of the upwelling zone; the surrounding region of high velocities could represent a downwelling curtain; and the low-velocity anomalies southeast of Hawaii in the transition zone and topmost lower mantle are consistent with predictions of plume tilt.

Wolfe, Cecily J.; Solomon, Sean C.; Laske, Gabi; Collins, John A.; Detrick, Robert S.; Orcutt, John A.; Bercovici, David; Hauri, Erik H.

2011-03-01

46

Velocity-locked solitary waves in quadratic media.  

PubMed

We demonstrate experimentally the existence of three-wave resonant interaction solitary triplets in quadratic media. Stable velocity-locked bright-dark-bright spatial solitary triplets, determined by the balance between the energy exchange rates and the velocity mismatch between the interacting waves, are excited in a KTP crystal. PMID:20366477

Baronio, Fabio; Conforti, Matteo; De Angelis, Costantino; Degasperis, Antonio; Andreana, Marco; Couderc, Vincent; Barthélémy, Alain

2010-03-19

47

Velocity measurements for turbulent nonseparated flow over solid waves  

Microsoft Academic Search

Laser-Doppler velocimetry measurements have been made of nonseparated velocity fields over solid sinusoidal wavy surfaces. Time-averaged velocity and turbulent intensity data are given. The measurements were conducted over waves on the bottom wall of a rectangular channel with a cross section of twenty four by two inches, the two inch dimension being vertical. Two sets of velocity data were obtained

K. A. Frederick; T. J. Hanratty

1985-01-01

48

An Examination of Rayleigh Wave Phase Velocities, South Shetland Islands  

Microsoft Academic Search

We examine the crustal and upper mantle structure of Bransfield Strait, the South Shetland Islands, and the Antarctic Peninsula using data from the Seismic Experiment in Patagonia and Antarctica (SEPA). We use Rayleigh wave phase velocity dispersion measurements from 20 teleseismic events to determine the interstation phase velocities at periods between 16 and 120 seconds. Maps of the velocities indicate

S. D. Robertson Maurice; D. A. Wiens; J. F. Lawrence

2003-01-01

49

Wave velocities in a pre-stressed anisotropic elastic medium  

Microsoft Academic Search

Modified Christoffel equations are derived for three-dimensional wave propagation in a general anisotropic medium under initial\\u000a stress. The three roots of a cubic equation define the phase velocities of three quasi-waves in the medium. Analytical expressions\\u000a are used to calculate the directional derivatives of phase velocities. These derivatives are, further, used to calculate the\\u000a group velocities and ray directions of

M. D. Sharma; Neetu Garg

2006-01-01

50

Crustal Structure of Greenland and the Western North Atlantic Using Surface Wave Group Velocities  

NASA Astrophysics Data System (ADS)

We present the results of a Rayleigh wave study where we characterize the structure of the crust and upper lithospheric mantle across Greenland using regional earthquakes from the mid-Atlantic Ridge and northern Canada. Vertical-component seismograms from January 1999 to May 2009 were analyzed to compute group velocities. Rayleigh wave group velocities for periods 5 to 90 seconds were obtained by applying a multiple filter analysis method to the fundamental mode. These dispersion curves are similar to those from previous Greenland models for certain paths, particularly at intermediate and longer periods. Group velocities at short periods are typically faster than those for continental reference models for the majority of paths studied. An tomographic inversion was made to produce maps of isotropic group velocity variation across Greenland. 1D group velocity dispersion curves were extracted from the maps and inverted for shear wave velocity structure. Three profiles were analyzed; 2 continental and 1 oceanic. Group velocity maps show clearly the difference between continental and oceanic crust at shorter periods. A circular low-velocity anomaly, surrounded by high velocities, is observed in central Greenland and may be related to variations in crustal thickness in this region. The shear wave models show high velocities (up to 10% above global reference models) in the crust and uppermost mantle for the southern part of Greenland, likely associated with the Archean craton. A very low-velocity structure is also present in the northeast part of the study region, and is consistent with results from larger-scale surface wave studies. This structure likely represents the 20 km thick sedimentary sequence in the continental shelf of this area. The Moho depth is not well constrained by the inversion due to starting-model dependence; however the models suggest significant variation in crustal thickness across Greenland and its surroundings.

Darbyshire, F. A.; Joyal, G.; Dahl-Jensen, T.; Larsen, T.

2011-12-01

51

The Height Variation of Granular and Oscillatory Velocities.  

National Technical Information Service (NTIS)

Previous observations of spatially-resolved vertical velocity variations in ten lines of Fe I spanning the height range 0 < or = h < or = 1000 km are re-analyzed using velocity weighting functions. The amplitudes and scale heights of granular and oscillat...

R. C. Canfield

1976-01-01

52

INVERSION OF PHASE VELOCITIES OF SURFACE WAVES WITH HIGH MODES  

Microsoft Academic Search

Elastic properties of near-surface materials and their effects on seismic wave propagation are of fundamental interest in groundwater, engineering, and environmental studies. As an example, Imai and Tonouchi (1982) studied compressional (P) and shear (S) wave velocities in an embankment, and also in alluvial, diluvial, and Tertiary layers, showing that S-wave velocities (Vs) in such deposits correspond to the N-value

Jianghai Xia; Richard D. Miller; Choon B. Park

1999-01-01

53

Phase relationships between total electron content variations, Doppler velocity oscillations and geomagnetic pulsations  

Microsoft Academic Search

The phase relationship between variations of ionospheric total electron content (TEC) and ground-level ULF geomagnetic pulsations has been examined for the advection and compression mechanisms. To determine the causal mechanism several earlier studies have examined the phase difference between oscillations of Doppler velocity in ionospherically reflected radio waves and simultaneous ULF geomagnetic pulsations. In most instances it was found that

J. Y. Liu; F. T. Berkey

1994-01-01

54

Phase relationships between total electron content variations, Doppler velocity oscillations and geomagnetic pulsations  

Microsoft Academic Search

The phase relationship between variations of ionospheric total electron content (TEC) and ground-level ULF geomagnetic pulsations has been examined for the advection and compression mechanisms. To determine the causal mechanism, several earlier studies have examined the phase difference between oscillations of Doppler velocity in ionospherically reflected radio waves and simultaneous ULF geomagnetic pulsations. In most instances it was found that

J. Y. Liu; F. T. Berkey

1994-01-01

55

Group velocity of cylindrical guided waves in anisotropic laminate composites.  

PubMed

An explicit expression for the group velocity of wave packets, propagating in a laminate anisotropic composite plate in prescribed directions, is proposed. It is based on the cylindrical guided wave asymptotics derived from the path integral representation for wave fields generated in the composites by given localized sources. The expression derived is theoretically confirmed by the comparison with a known representation for the group velocity vector of a plane guided wave. Then it is experimentally validated against laser vibrometer measurements of guided wave packets generated by a piezoelectric wafer active sensor in a composite plate. PMID:24437754

Glushkov, Evgeny; Glushkova, Natalia; Eremin, Artem; Lammering, Rolf

2014-01-01

56

Detailed P- and S-wave Velocity Models Along the LARSE II Transect, Southern California  

NASA Astrophysics Data System (ADS)

Structural details of the crust determined from P-wave velocity models can be improved with S-wave velocity models, and S-wave velocities are needed for model-based predictions of strong ground motion in southern California. We picked P- and S-wave travel times for refracted phases from explosive-source shots gathers of the Los Angeles Region Seismic Experiment, Phase II (LARSE II), and we developed refraction velocity models from these picks using two different inversion algorithms. Vp/Vs ratios were calculated from the resulting P- and S-wave models where both models are constrained by ray coverage. The two P-wave velocity models are compared to each other and to results from forward modeling. Generally, the P-wave inverse and forward models agree well for velocities lower than 5.0 km/s but only broadly agree with each other for velocities above 5.0 km/s. Similarly, the S-wave inverse models agree well with each other for velocities lower than 2.5 km/s but only broadly agree for velocities higher than 2.5 km/s. The most prominent structures in our S-wave models are two north-dipping low-velocity zones in the Central Transverse Ranges that we interpret as faults. These low-velocity zones differ somewhat between the two inversion models, but the Vp/Vs models (one model for each technique) show these features to be remarkably similar. Interestingly, both Vp/Vs models have several features that are not visible in either the P- or S-wave models alone. Two of these features (relatively high Vp/Vs ratios) occur in the vicinity of wells that bottom in "granitic" rocks and we interpret these high Vp/Vs ratios to indicate that the granitic rocks are highly fractured or even brecciated. Finally, to evaluate the Southern California Earthquake Center (SCEC) Community Velocity Model (CVM), which predicts Vs based on the Vp model, we compare data from our Vp and Vp/Vs models to empirical formulas that relate P- to S-wave velocities (see Brocher, 2005). These empirical curves provide an adequate average relationship between Vp and Vs, but our model Vs varies as much as ± 20 % for Vp ? 5.0 km/s and as much as ± 35 % for Vp ? 5.0 km/s. This large variation in the predicted S- wave velocity demonstrates the value of determining Vs independently from Vp.

Murphy, J. M.; Fuis, G. S.; Ryberg, T.; Lutter, W. J.; Catchings, R. D.; Goldman, M. R.

2008-12-01

57

Blood characteristics effect on pulse wave velocity.  

PubMed

PWV, a surrogate marker for vascular stiffness, can be also expressed by the Bramwell-Hill equation. The effect of blood density to PWV has been ignored, because variation of blood density is assumed to be negligible. In some clinical situation, blood density could be changed, and blood density as a mechanical property of blood flow might affect to PWV. While the elastic property plays an important role in determining the wave propagation in an elastic tube, our assumption is that there might be some relation between blood flow and vascular wall, and that the characteristics of blood flow might influence PWV. This study was objected to investigate the role of mechanical and hemorheologic parameters on PWV in subjects with cardiovascular disease. We have measured and analyzed the PWV, hemorheologic parameters, and other clinical parameters in 814 patients with coronary arterial disease scheduled for coronary angiography. There is no commercial method for measuring whole blood density. So, we defined the density score, which is sum of hemoglobin and total protein. And the hemorheologic parameters were measured within 4 hours after sampling by automated microfluidic hemorheometer. And the effect of all the clinical and hemorheologic parameter on PWV was analyzed by multiple linear regression analysis. Many clinical parameters including age and blood pressure, high shear WBV and ESR as hemorheologic parameters, and density score were correlated well with ba-PWV. However, many clinical variables, high shear WBV and ESR lost the independent significance on multivariable regression analysis. Only age, SBP, and density score were independent variables (p < 0.001). In conclusion, density score as a mechanical property of blood might be suggested as an independent variable influencing PWV in addition to age and blood pressure, but hemorheologic parameters, such as RBC deformability, aggregation, and whole blood viscosity do not affect PWV independently. PMID:23445636

Kim, Jong Youn; Yoon, Jihyun; Cho, Minhee; Lee, Byoung-Kwon; Karimi, Ali; Shin, Sehyun

2013-01-01

58

Comparative study of methodologies for pulse wave velocity estimation.  

PubMed

Arterial stiffness, estimated by pulse wave velocity (PWV), is an independent predictor of cardiovascular mortality and morbidity. However, the clinical applicability of these measurements and the elaboration of reference PWV values are difficult due to differences between the various devices used. In a population of 50 subjects aged 20-84 years, we compared PWV measurements with three frequently used devices: the Complior and the PulsePen, both of which determine aortic PWV as the delay between carotid and femoral pressure wave and the PulseTrace, which estimates the Stiffness Index (SI) by analyzing photoplethysmographic waves acquired on the fingertip. PWV was measured twice by each device. Coefficient of variation of PWV was 12.3, 12.4 and 14.5% for PulsePen, Complior and PulseTrace, respectively. These measurements were compared with the reference method, that is, a simultaneous acquisition of pressure waves using two tonometers. High correlation coefficients with the reference method were observed for PulsePen (r = 0.99) and Complior (r = 0.83), whereas for PulseTrace correlation with the reference method was much lower (r = 0.55). Upon Bland-Altman analysis, mean differences of values +/- 2s.d. versus the reference method were -0.15 +/- 0.62 m/s, 2.09 +/- 2.68 m/s and -1.12 +/- 4.92 m/s, for PulsePen, Complior and Pulse-Trace, respectively. This study confirms the reliability of Complior and PulsePen devices in estimating PWV, while the SI determined by the PulseTrace device was found to be inappropriate as a surrogate of PWV. The present results indicate the urgent need for evaluation and comparison of the different devices to standardize PWV measurements and establish reference values. PMID:18528411

Salvi, P; Magnani, E; Valbusa, F; Agnoletti, D; Alecu, C; Joly, L; Benetos, A

2008-10-01

59

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

60

Explanation for Malischewsky's approximate expression for the Rayleigh wave velocity.  

PubMed

An approach for obtaining approximations of the Rayleigh wave velocity created by the principle of least squares is introduced. In view of this approach, Malischewsky's approximation of the Rayleigh wave velocity for Poisson ratios v in the set of [-1, 0.5] proposed quite recently is explained. It is shown that Malischewsky's approximation obtained by trial and error is (almost) identical with the one established by this approach. PMID:16919306

Vinh, Pham Chi; Malischewsky, Peter G

2006-12-01

61

Aortic pulse wave velocity in renal transplant patients  

Microsoft Academic Search

Aortic pulse wave velocity in renal transplant patients.BackgroundIn subjects with end-stage renal disease (ESRD) undergoing hemodialysis, aortic pulse wave velocity (PWV) is increased independently of blood pressure level and mostly is a strong predictor of cardiovascular risk. Few studies on this subject have been performed in renal transplant patients.MethodsAortic PWV was determined noninvasively in 106 patients with kidney transplantation and

SOLA AOUN BAHOUS; ANTOINE STEPHAN; WADAD BARAKAT; JACQUES BLACHER; ROLAND ASMAR; MICHEL E SAFAR; Profressor Michel E. Safar

2004-01-01

62

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

63

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.

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, Andre G.; Abecasis, Goncalo 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

64

Longitudinal variation and waves in Jupiter's south equatorial wind jet  

NASA Astrophysics Data System (ADS)

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 increasing with distance from the disturbance. The chevrons move with velocities near the maximum wind jet velocity of ˜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 = 75-100) 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 ˜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°N and 7.5°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.; Rogers, John H.; Gierasch, Peter J.; Choi, David; Allison, Michael D.; Adamoli, Gianluigi; Mettig, Hans-Joerg

2012-04-01

65

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

66

Long-term radial velocity variations of Omicron And  

NASA Astrophysics Data System (ADS)

The result of radial velocity measures of the star Omicron And from high dispersion plates relative to the years 1964, 1966, 1975 and 1976 are given. The averaged values of the 1964 and 1966 plates confirm the existence of the minimum in the trend of the radial velocities and fit well the curve obtained in the hypothesis of periodic long-term radial velocity variations with the period of 23.5 years (Fracassini et al., 1977). Preliminary orbital elements in the hypothesis of a long period spectroscopic binary system have been tentatively drawn.

Pastori, L.; Antonello, E.; Fracassini, M.; Pasinetti, L. E.

1982-08-01

67

Experimental Study of the Drift Velocity in Progressive Waves.  

National Technical Information Service (NTIS)

The second order drift component of the velocity field under progressive water waves is the subject of this experimental work. It is an Eulerian representation of the mass transport phenomenon in water waves. In the present study the primary attention was...

L. Zhan W. Dursthoff T. Bruce C. A. Greated

1992-01-01

68

Coda Wave Interferometry for Estimating Nonlinear Behavior in Seismic Velocity  

Microsoft Academic Search

In coda wave interferometry, one records multiply scattered waves at a limited number of receivers to infer changes in the medium over time. With this technique, we have determined the nonlinear dependence of the seismic velocity in granite on temperature and the associated acoustic emissions. This technique can be used in warning mode, to detect the presence of temporal changes

Roel Snieder; Alexandre Grêt; Huub Douma; John Scales

2002-01-01

69

Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves  

USGS Publications Warehouse

The shear-wave (S-wave) velocity of near-surface materials (soil, rocks, pavement) and its effect on seismic-wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Rayleigh-wave phase velocity of a layered-earth model is a function of frequency and four groups of earth properties: P-wave velocity, S-wave velocity, density, and thickness of layers. Analysis of the Jacobian matrix provides a measure of dispersion-curve sensitivity to earth properties. S-wave velocities are the dominant influence on a dispersion curve in a high-frequency range (>5 Hz) followed by layer thickness. An iterative solution technique to the weighted equation proved very effective in the high-frequency range when using the Levenberg-Marquardt and singular-value decomposition techniques. Convergence of the weighted solution is guaranteed through selection of the damping factor using the Levenberg-Marquardt method. Synthetic examples demonstrated calculation efficiency and stability of inverse procedures. We verify our method using borehole S-wave velocity measurements.Iterative solutions to the weighted equation by the Levenberg-Marquardt and singular-value decomposition techniques are derived to estimate near-surface shear-wave velocity. Synthetic and real examples demonstrate the calculation efficiency and stability of the inverse procedure. The inverse results of the real example are verified by borehole S-wave velocity measurements.

Xia, J.; Miller, R. D.; Park, C. B.

1999-01-01

70

Influence of cementation and permeability on wave velocities in poorly consolidated sands  

Microsoft Academic Search

The role of cementation, porosity, permeability and consolidation on acoustic properties of unconsolidated sands and poorly consolidated sandstones has been investigated. Partially and fully water saturated Ottawa sand and poorly consolidated sandstone samples were measured in a bench-top ultrasonic assembly in permafrost conditions at several ice saturation states to determine the variations in wave velocities associated with the degree of

Azra N. Tutuncu; Jack Dvorkin; Amos Nur

1997-01-01

71

Shear-wave attenuation and velocity studies in southeastern Asia  

NASA Astrophysics Data System (ADS)

Models of shear-wave Q (Qmu) have been obtained for southeastern Asia using two methods. The first method inverts attenuation coefficients of the fundamental Rayleigh mode obtained using a standard two-station technique. The second method matches theoretical amplitude spectra for the fundamental and higher-mode Rayleigh waves computed for previously obtained velocity and assumed Qmu models, and earthquakes with known source depths and focal mechanisms, to observed spectra. The latter method provides much better regional coverage than the first and allows us to map lateral variations of Qmu at various levels in the crust and uppermost mantle. For the single-station, multi-mode method, I assumed an Earth model consisting of three layers, layer 1 being 10 km, layer 2 being 20 km, and layer 3 being 30 km in thickness. Qmu in layer 1 achieves lowest values (about 40) in the southern part of the Tibetan Plateau and in the Tarim basin and is highest (about 250) in southeastern China. The Qmu map of layer 2 indicates that the highest Qmu values (about 150) he in the central part of China and in parts of the Sino-Korean platform. The lowest Q mu value (about 50) occurs in Tibet and the Pamir thrust system. Layer 2 exhibits an overall increase in Qmu going from south to north. For layer 3 the resolution of crustal variations in Qmu, is poorer than layers 1 and 2. Available results, however, indicate that Qmu, is highest (about 180) under southern Mongolia and the Tarim basin, somewhat lower (100) beneath the southern portion of the Baikal Rift, and lowest (80) under the Pamir thrust system.

Jemberie, Alemayehu Lakew

72

Anisotropic parameter estimation using velocity variation with offset analysis  

NASA Astrophysics Data System (ADS)

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.

2013-09-01

73

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

74

Relationship of D'' structure with the velocity variations near the inner-core boundary  

NASA Astrophysics Data System (ADS)

Variations in regional differential times between PKiKP (i) and PKIKP (I) have been attributed to hemispheric P-velocity variations of about 1% in the upper 100 km of the inner core (referred to as HIC). The top of the inner core appears relatively fast beneath Asia where D'' is also fast. An alternative interpretation could be the lateral variation in P velocity at the lowermost outer core (HOC) producing the same differential times. To resolve this issue, we introduce the diffracted PKP phase near the B caustic (Bdiff) in the range of 139-145° epicenter distances, and the corresponding differential times between Bdiff and PKiKP and PKIKP as observed on broadband arrays. Due to the long-wavelength nature of Bdiff, we scaled the S-wave tomography model with k values (k ? dlnVs/dlnVp) to obtain large-scale P-wave velocity structure in the lower mantle as proposed by earlier studies. Waveform synthetics of Bdiff constructed with small k's predict complex waveforms not commonly observed, confirming the validity of large scaling factor k. With P-velocity in lower mantle constrained at large scale, the extra travel-time constraint imposed by Bdiff helps to resolve the HOC-HIC issue. Our preliminary results suggest k > 2 for the lowermost mantle and support HIC hypothesis. An important implication is that there appears to be a relationship of D'' velocity structures with the structures near the inner core boundary via core dynamics.

Luo, Sheng-Nian; Ni, Sidao; Helmberger, Don

2002-06-01

75

A search for rapid radial velocity variations in ? Circini  

NASA Astrophysics Data System (ADS)

We present rapid radial velocity measurements (?~55 m/s) for the roAp star ? Cir made using a molecular iodine gas absorption cell. A period analysis places an upper limit of 60 m/s for the peak-to-peak amplitude of any radial velocity variations in the frequency range 20 to 375 days^-1^. Phasing the measurements to the known photometric period of 6.83 minutes results in an upper limit of 36 m/s (peak-to-peak) for the radial velocity amplitude of the pulsations. This yields an upper limit to the ratio of radial velocity amplitude to photometric amplitude (2K/{DELTA}m_B_) of about 10 km/s/mag, substantially less than was found for two other roAp stars by other investigators.

Hatzes, A. P.; Kuerster, M.

1994-05-01

76

Tomographic imaging of P wave velocity structure beneath the region around Beijing  

NASA Astrophysics Data System (ADS)

The three-dimension crustal and upper mantle structures in the region around Beijing were studied by seismic tomography. We used the P wave arrival times from local and teleseismic events. These events were recorded by 250 stations of the North China Seismic Array and 108 stations of the Beijing Telemetry Seismic Network. 118 869 P wave arrivals from 10 285 local events and 12 189 P wave arrivals from 107 teleseismic events were used in the inversion. We obtained the 3-D P wave velocity structure of the crust and upper mantle with the horizontal resolution of 0.3° in the studied region. The tomographic imaging shows the remarkably heterogeneous velocity variation. The velocity anomalies are in well agreement with the geological structure in the shallow crust. The different relationships between seismic activities and velocity anomalies may imply the different seismogenic structure and mechanism. Beneath the Moho under Taihangshan mountain and Yanshan mountain, we found the high velocity anomalies deep to 120 km and 200 km, respectively. The deep high velocity zone may be explained by the existence of the mountain root under Yanshan mountain. The high velocity anomalies in the upper mantle of North China basin may be the relics of the de-rooting from the former craton mantle lithosphere.

Ding, Zhifeng; Zhou, Xiaofeng; Wu, Yan; Li, Guiyin; Zhang, Hong

2009-08-01

77

Activity-Induced Radial Velocity Variation of M Dwarf Stars  

NASA Astrophysics Data System (ADS)

Stellar magnetic activity manifests itself in a variety of ways including starspots-cool, dark regions on the stellar surface. Starspots can cause variations (`jitter') in spectral line-profiles which can mimic the radial velocity (RV) variations caused by an orbiting planet, or create RV noise that can drown out a planetary signature. Cool, low-mass M dwarf stars can be highly active, which can make detection of potentially habitable planets around these stars difficult. We investigate radial velocity variations caused by different activity (spot) patterns on M dwarf stars in order to determine the limits of detectability for small planets orbiting active M dwarfs. We report on our progress toward the aim of answering the following questions: What types of spot patterns are realistic for M dwarf stars? What effect will spots have on M dwarf RV measurements? Can jitter from M dwarf spots mimic planetary signals? What is the ideal observing wavelength to reduce M dwarf jitter?

Andersen, Jan Marie; Korhonen, Heidi

2014-04-01

78

Shear wave velocity structure over the eastern Indian subcontinent  

NASA Astrophysics Data System (ADS)

Fundamental-mode surface wave generated by twelve earthquakes, which occurred in northeastern India and nearby regions and were recorded at the Shillong (SHL) and Chiengmai (CHG) seismic stations are used to determine the shear wave velocity structure beneath these regions. The Frequency Time Analysis method is used to determine the group velocities for periods ranging from 4 to 50 s for fundamental-mode Rayleigh and Love waves. A high shear wave velocity (4.2-4.7 km s -1) is estimated in the lower 30-34 km of the crust. Crustal thickness is found to vary from 36 to 56 km with an overlying 2-4 km sedimentary thickness of shear wave velocity of 2.55-2.75 km s -1. The shear wave velocity in the upper 10 km of crust is 3.5-3.7 km s -1 below the sedimentary layer. The inferred high shear velocity for the lower crust beneath these regions suggests an oceanic affinity and they are different from the Indian shield structure. The lower crust must have an oceanic origin as derived from the reminiscent of a certain oceanic lithosphere. The available data do not permit us to estimate the upper mantle structure precisely; however, it shows a higher upper mantle shear velocity of 5.1-5.3 km s -1. This may be indicative of active tectonism beneath these regions in the uppermost mantle. The Moho boundary is not sharply defined here. The increase in crustal thickness may be due to the collision of the Indian and Eurasian plates in the north and subduction of the Indian plate beneath the Burmese Arc in the east.

Singh, D. D.

1994-02-01

79

Wave velocities in sandstones from elastic network simulations  

SciTech Connect

An elastic network model is used to calculate compressional and shear-wave velocities in a model sandstone. The model consists of mineral components and pores distributed at random on a regular cubic lattice. Springs are used to represent compressional, shear, and rotational forces between neighboring grains. The authors have used the network model to calculate the dependence of wave velocities on porosity and clay content in sandstones. The calculated velocities are a linear function of both porosity and total clay content. The coefficients in this bilinear relationship are in quantitative agreement with published experimental correlations for sandstone velocities. This result provides theoretical support for the linear velocity-porosity-clay content relationship commonly assumed in empirical correlations. The rotational forces between grains are necessary to obtain quantitative agreement with experimental data. Wave velocities calculated using the continuum self-consistent imbedding approximation agree with the elastic network for quartz/water mixtures, but not for quartz/clay mixtures. This disagreement demonstrates the importance of a granular elastic model with rotational forces for calculating velocities in rocks with multiple mineral components.

Gist, G.A.; Thompson, A.H. (Exxon Production Research Co., Houston, TX (United States)); Katz, A.J. (Texas Instruments, Dallas, TX (United States)); Berry, M.J. II (Harvard Univ., Cambridge, MA (United States). Dept. of Physics)

1993-03-01

80

Shear velocity structure beneath south-eastern Tibet from Rayleigh wave tomography  

NASA Astrophysics Data System (ADS)

We analyze fundamental mode Rayleigh waves that are recorded at 51 stations in southeast Tibet. Phase velocity variations are obtained at 15 periods from 20 to 143 s using the two-plane-wave inversion technique and are inverted for 3-D shear velocity structure. Our S wave velocity model reveals a pronounced low-velocity zone in the middle and lower crust, consistent with previous observations in other part of Tibet. The low velocity layer is probably associated with partial melt of crustal material due to crustal thickening, and/or high temperature from shear heating and asthenosphere upwelling. Negative anomalies in the crust are imaged at 92 degree, correlating with the location of a north-south rift zone. In the upper mantle, a fast mantle lid is present beneath most of southeastern Tibet to a depth of ~120 km. High velocity anomalies down to ~260 km are observed in the west and east parts of the area, reflecting the subducted Indian lithosphere. A vertical low velocity column is observed in the middle of the region between 94 and 97 degrees from lower crust to the mid upper mantle. It indicates mantle upwelling of asthenosphere through a slab window that could be formed by the vertical tear of the subducted Indian lithosphere at the eastern syntaxis resulted from the significant change of the subducting direction. The opening of this slab window and the consequently mantle upwelling must be partially responsible for the low velocity layer in the crust and the rifts along 92 degree.

Fu, Y. V.; Li, A.; Chen, Y. J.

2009-12-01

81

Imaging Rayleigh wave attenuation and phase velocity in the western and central United States  

NASA Astrophysics Data System (ADS)

The EarthScope USArray provides an opportunity to obtain detailed images of the continental upper mantle at an unprecedented scale. The majority of mantle models derived from USArray data to date contain spatial variations in seismic-wave speed; however, little is known about the attenuation structure of the North American upper mantle. Joint interpretation of seismic attenuation and velocity models can improve upon the interpretations based only on velocity, and provide important constraints on the temperature, composition, melt content, and volatile content of the mantle. We jointly invert Rayleigh wave phase and amplitude observations for phase velocity and attenuation maps for the western and central United States using USArray data. This approach exploits the amplitudes' sensitivity to velocity and the phase delays' sensitivity to attenuation. The phase and amplitude data are measured in the period range 20--100 s using a new interstation cross-correlation approach, based on the Generalized Seismological Data Functional algorithm, that takes advantage of waveform similarity at nearby stations. The Rayleigh waves are generated from 670 large teleseismic earthquakes that occurred between 2006 and 2012, and measured from all available Transportable Array stations. We consider two separate and complementary approaches for imaging attenuation variations: (1) the Helmholtz tomography (Lin et al., 2012) and (2) two-station path tomography. Results obtained from the two methods are contrasted. We provide a preliminary interpretation based on the observed relationship between Rayleigh wave attenuation and phase velocity.

Bao, X.; Dalton, C. A.; Jin, G.; Gaherty, J. B.

2013-12-01

82

Tomographic imaging of local earthquake delay times for three-dimensional velocity variation in Western Washington  

Microsoft Academic Search

Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the Puget Sound region of Western Washington. The 37,500 square km region is represented by nearly cubic blocks of 5 km per side. P-wave arrival time observations from 4,387 crustal earthquakes, with depths of 0 to 40 km, were used as sources producing

Jonathan M. Lees; Roberts S. Crosson

1990-01-01

83

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

84

Minimizers with discontinuous velocities for the electromagnetic variational method.  

PubMed

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 Liénard-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. PMID:20866898

De Luca, Jayme

2010-08-01

85

Minimizers with discontinuous velocities for the electromagnetic variational method  

NASA Astrophysics Data System (ADS)

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 Liénard-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

2010-08-01

86

Elastic wave velocities and thermal diffusivities of Apollo 14 rocks.  

NASA Technical Reports Server (NTRS)

The compressional- and shear-wave velocities of Apollo 14 lunar rocks 14311,50 and 14313,27 as functions of pressure up to 10 kb and the thermal diffusivity of sample 14311,50 over the temperature range 100 to 550 K have been measured. Both samples 14311 and 14313 are polymict fragmental rocks. The overall elastic and anelastic behavior of the Apollo 14 samples are similar to those of Apollo 11 and 12 samples; low velocity and low Q at pressures below 1 kb and rapid increase of velocity and Q with pressure are also typical of the Apollo 14 rocks. The available data of P- and S-wave velocities of lunar rocks show that Birch's law holds for the lunar rocks. The thermal diffusivity of a lunar rock in vacuum is found to be significantly lower than that in air at one atmospheric pressure.

Mizutani, H.; Fujii, N.; Hamano, Y.; Osako, M.

1972-01-01

87

SHEAR WAVE VELOCITY FIELD FROM SURFACE WAVES TO DETECT ANOMALIES IN THE SUBSURFACE  

Microsoft Academic Search

ABSTRACT Mapping the bedrock surface at depths ranging from 1 m to as deep as 30 m, identifying potential fracture zones within bedrock, and delineating dissolution\\/potential subsidence features can be effectively done,in a ,variety of near-surface settings using the shear wave ,velocity field calculated by the ,multi- channel,analysis of surface,waves ,(MASW) method. A 2-D shear wave ,velocity field accurate to

Richard D. Miller; Jianghai Xia; Choon Byong Park; Julian Ivanov

88

Impact of Phase Transitions on P Wave Velocities  

SciTech Connect

In regions where a high pressure phase is in equilibrium with a low pressure phase, the bulk modulus defined by the P-V relationship is greatly reduced. Here we evaluate the effect of such transitions on the P wave velocity. A model, where cation diffusion is the rate limiting factor, is used to project laboratory data to the conditions of a seismic wave propagating in the two-phase region. We demonstrate that for the minimum expected effect there is a significant reduction of the seismic velocity, as large as 10% over a narrow depth range.

D Weidner; L Li

2011-12-31

89

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

90

Lateral variations in the upper mantle velocity structure in the northwestern pacific margin  

NASA Astrophysics Data System (ADS)

The upper mantle velocity structure in various island arc-marginal sea regions of the northwestern Pacific has been studied in detail to a depth of about 640 km from the analysis of P- and S-wave travel times data of 363 earthquakes with focal depths varying from 35 to 640 km. Wave velocities were obtained at the depths of foci by using Kaila's (1969) analytical method in several units of the northwestern Pacific margin: (1) Honshu-Izu Bonin-Japan Sea, (2) Kyushu-Shikoku, (3) Hokkaido, (4) Okhotsk Sea, (5) southern Kurile Islands, (6) northern Kurile Islands, (7) Kamchatka, (8) Ryukyu Islands, and (9) Taiwan-Luzon. There is substantial evidence for large lateral variations, of the order of 8-10%, in both P- and S-wave velocity structure to about 250 km depth within the subducting lithospheric slab in the northwestern Pacific margin. The P velocity determined at 40 km depth, is found to be about 8.20 km/sec in the southern Kurile Islands 7.90 km/sec in the northern Kurile Islands, and the Japanese Is., 7.80 km/sec in Kamchatka and 8.05 km/sec in the Ryukyu Islands, and Taiwan-Luzon regions. The S velocity determined at 40 km depth, in the southern Kurile Is., is also found to be relatively higher, about 4.60 km/sec, compared to all other regions in the northwestern Pacific margin where it is varying from 4.30 to 4.45 km/sec. P and S velocities determined to about 185 km depth in the southern Kurile Islands region are found to be 3-5% higher than those obtained at comparable depths in the Hokkaido island, northern Kurile Islands and Kamchatka. P. velocities, determined to a depth of about 255 km, in the Ryukyu and Taiwan-Luzon regions are quite similar and are about 5% higher on the average, than the nearly constant P velocity of 7.88 km/sec found in the Kyushu and Shikoku islands of the southwest Japan. In the southwest Japan region, both P and S wave velocities are found to remain constant to almost 255 km depth. This might be due to the presence of a large magma chamber resulting also in the extensive volcanic activity observed in southwest Japan (e.g. Sakurazima volcano). The P- and S-wave velocity-depth functions, determined in the depth range of 290-640 km, in the Okhotsk Sea region, reveal a sharp first-order velocity discontinuity at 390 km depth. The observed velocity jumps are 9.2% for P and 5.6% for S waves in this region. In the central Japan region also, comprising the Honshu and the Japan Sea, there is a sharp first-order velocity discontinuity at depths of 365 km for P and 345 km for S waves with associated velocity jumps of 8.6% for P and 4.8% for S waves. The S velocities below this discontinuity to about 640 km depth in the central Japan and the Okhotsk sea regions are found to be considerably lower. The entire area, comprising the high heat-flow regions of the marginal seas of Japan and Okhotsk, may be a high temperature/high attenuation zone extending to 640 km depth. There is no evidence for the presence of a significant low-velocity layer in the northwestern Pacific margin.

Kaila, K. L.; Krishna, V. G.

1985-03-01

91

Shear-wave velocity of surficial geologic sediments in Northern California: Statistical distributions and depth dependence  

USGS Publications Warehouse

Shear-wave velocities of shallow surficial geologic units were measured at 210 sites in a 140-km2 area in the greater Oakland, California, area near the margin of San Francisco Bay. Differences between average values of shear-wave velocity for each geologic unit computed by alternative approaches were in general smaller than the observed variability. Averages estimated by arithmetic mean, geometric mean, and slowness differed by 1 to 8%, while coefficients of variation ranged from 14 to 25%. With the exception of the younger Bay mud that underlies San Francisco Bay, velocities of the geologic units are approximately constant with depth. This suggests that shear-wave velocities measured at different depths in these surficial geologic units do not need to be normalized to account for overburden stress in order to compute average values. The depth dependence of the velocity of the younger Bay mud most likely is caused by consolidation. Velocities of each geologic unit are consistent with a normal statistical distribution. Average values increase with geologic age, as has been previously reported. Velocities below the water table are about 7% less than those above it. ?? 2005, Earthquake Engineering Research Institute.

Holzer, T. L.; Bennett, M. J.; Noce, T. E.; Tinsley, III, J. C.

2005-01-01

92

P- and S-wave velocity measurements on saturated siliceous conglomerates: determination of frame moduli  

NASA Astrophysics Data System (ADS)

Reservoir conditions, such as the pore pressure and the fluid saturation levels, will change during the production of fluids from the earth. These changes will also influence the seismic wave properties of the rock. In order to better understand its seismic response, compressional and shear wave velocities were measured on a series of low porosity (less than 10%) conglomerates under different confining and pore pressures under both dry and water saturated conditions. P- and S-wave velocities were simultaneously measured using standard pulse transmission methods. In all cases, the velocities increased dramatically with effective confining pressures to 60 MPa. As expected, saturated P-wave velocities were always greater than those under `dry' (i.e. pore space under vacuum) conditions. Conventional assumptions leading from Gassmann's relations suggest that the S-wave velocity would drop; however, in this study the S-wave velocity increased after saturation. The contradiction of the observed data with the theory has been attributed to a number of mechanisms, such as viscous coupling, the reduction in free surface energy, and frequency dispersion due to local flow of the fluid in the microcracks. The pore geometry, the microcracks and the clay content are among the most important factors influencing the seismic properties of these rocks. These geological factors are characterized though thin section, Scanning Electron Microscope (SEM), and Hg-porosimetry. The stress sensitive intra- and inter-grain cracks observed through these images play an important role in the velocity pressure relationship. The variation of the Vp/Vs and Poisson's ratios as functions of the effective pressure are also shown.

He, T.; Schmitt, D. R.

2005-12-01

93

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.; Centre for Petroleum Geoscience; CO2 Sequestration

2011-12-01

94

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

95

Velocity measurements for a turbulent nonseparated flow over solid waves  

Microsoft Academic Search

The laser-Doppler velocimeter was used to obtain measurements of the streamwise velocity over solid sinusoidal waves of small enough amplitude that a nonseparated flow existed. The measurements provide a critical test for Reynolds stress closure models since they are particularly sensitive to happenings in the viscous wall region (y+ < 40), for which present theories are of uncertain accuracy. The

K. A. Frederick; T. J. Hanratty

1988-01-01

96

Studies on the determination of shear wave velocity in sands  

Microsoft Academic Search

Laboratory investigations such as resonant column, cyclic triaxial, and torsional shear tests are usually conducted on undisturbed and reconstituted sand samples in order to determine their shear modulus, which can be used to compute shear wave velocity. However, these methods are extremely cumbersome and indirect, they employ cost-intensive instrumentation, and they require trained manpower. These problems can be overcome by

P. P. Bartake; D. N. Singh

2007-01-01

97

The velocity structure and anisotropy of the central and southern Atlantic lithosphere from surface wave studies  

NASA Astrophysics Data System (ADS)

Beneath the south east Atlantic Ocean previous tomographic work shows that velocities at depths greater than 100km are faster than those predicted using simple velocity - temperature conversions and the widely accepted plate model. Furthermore, at these depths, variations in velocity continue to show a correlation with age. Here, we present preliminary results from two surface wave techniques, to investigate in detail the structure and dynamics of the upper mantle under the central and southern Atlantic Ocean. Group velocity dispersion curves for oceanic paths have been obtained using the Multi Filter Technique for approximately 10,000 paths between periods of 20s and 100s. The path-average group velocities are then combined within a tomographic inversion, to obtain the regional variations in velocity. These short to intermediate period group velocities have depth sensitivities from the surface to approximately 90km, constraining the focus to velocity variations within the lithosphere rather than the whole lithosphere-asthenosphere system. There is a clear distinction at all periods between oceanic and continental velocities. Velocity variations beneath the ocean define key features such as a region of low velocity beneath the ridge at all periods, most strongly picked out in the south of the region. In the south east, where there is the most optimal path coverage, we see a correlation between the age of lithosphere and increase in velocity at longer periods. Following the group velocity analysis, the same data have been inverted for shear velocity structure using a waveform inversion technique. Path-average shear velocity is computed for each data and again incorporated into a tomographic inversion. Periods from 50-160s are included in the waveform inversion, thus giving a focus on the deeper lithospheric structure and variations at the lithosphere-asthenosphere boundary. Additionally, we present results which incorporate azimuthal anisotropy in the tomographic inversions. From comparisons of the anisotropic results at a variety of periods and depths, we provide further information on the link between dynamic flow within the asthenosphere, and deformation of the overlying plate.

McCarthy, E. S.; Fishwick, S.; England, R. W.

2012-12-01

98

Long-term radial velocity variations in some Be stars  

NASA Astrophysics Data System (ADS)

The radial velocities of EW Lac, 28 Tau, zeta Tau, KX And, KY And, CX Dra, and 88 Her are presented on long time-scales to study long-term variations and their eventual correlation with spectroscopic behavior, and to further predictions of shell phases. EW Lac shows long-term variations with a period of 40.3 yr, which may be of dynamical nature. The curve of RV for 28 Tau shows long-period variations that may be periodical, and in zeta Tau, the occurrence of another activity phase from 1908-1920 is suggested, in which the RV, V/R and the appearance of the spectra seem similar to those following the year 1958. Useful indications for KX and KY And are given.

Antonello, E.; Fracassini, M.; Pasinetti, L. E.; Pastori, L.

1982-04-01

99

Passive monitoring of temporal, coseismic, velocity variations at the ocean floor  

NASA Astrophysics Data System (ADS)

Passive techniques are now widely used for seismic imaging, for example ambient noise tomography. A recent development of passive techniques, namely the Passive Image Interferometry, allows the monitoring of temporal variations of seismic velocities from the continuous recording of ambient noise. The sensitivity of the method is as high as 0.01% in relative velocity changes. Such a high sensitivity is reached by using the (multiply-) scattered waves reconstructed in the noise cross-correlation functions in the surface-wave coda. In our study we utilized passive image interferometry to detect changes in velocity before/after a Mw 6.0 earthquake at the Gofar transform fault at the East Pacific Rise. The data set consists of a year of continuous recording at 15 ocean bottom seismometers (OBS) located in the vicinity of the fault. A velocity drop of 0.1% is measured at the time of the earthquake. Preliminary results suggest that the temporal variation is localized within the fault zone. Coseismic variations are usually interpreted as a consequence of the stress release on the fault. Multi-component analysis (3 components seismometers plus accelerometers) allows to make independent measurements of the velocity variations, and further improve the resolution. We note that the method is insensitive to clock errors, which is of particular interest when working with OBS, and can also be used to measure and correct for clock errors, if any. The sensitivity of the method, along with its low cost and its continuous and real time nature, make it a promising tool for monitoring geological structures at different scales.

Gouedard, P.; Collins, J. A.; McGuire, J. J.; van der Hilst, R. D.

2010-12-01

100

Validation and Reproducibility of Aortic Pulse Wave Velocity as Assessed With Velocity-Encoded MRI  

Microsoft Academic Search

Purpose: To validate magnetic resonance imaging (MRI) assessment of aortic pulse wave velocity (PWVMRI) with PWV determined from invasive intra-aortic pressure measurements (PWVINV) and to test the reproducibility of the measurement by MRI. Materials and Methods: PWVMRI Was compared with PWVINV in 18 nonconsecutive patients scheduled for catheterization for suspected coronary artery disease. Reproducibility of PWVMRI was tested in 10

Heynric B. Grotenhuis; Jos J. M. Westenberg; Paul Steendijk; Geest van der R. J; Jaap Ottenkamp; Jeroen J. Bax; J. Wouter Jukema; Albert de Roos

2009-01-01

101

Traveling waves in an optimal velocity model of freeway traffic.  

PubMed

Car-following models provide both a tool to describe traffic flow and algorithms for autonomous cruise control systems. Recently developed optimal velocity models contain a relaxation term that assigns a desirable speed to each headway and a response time over which drivers adjust to optimal velocity conditions. These models predict traffic breakdown phenomena analogous to real traffic instabilities. In order to deepen our understanding of these models, in this paper, we examine the transition from a linear stable stream of cars of one headway into a linear stable stream of a second headway. Numerical results of the governing equations identify a range of transition phenomena, including monotonic and oscillating travelling waves and a time- dependent dispersive adjustment wave. However, for certain conditions, we find that the adjustment takes the form of a nonlinear traveling wave from the upstream headway to a third, intermediate headway, followed by either another traveling wave or a dispersive wave further downstream matching the downstream headway. This intermediate value of the headway is selected such that the nonlinear traveling wave is the fastest stable traveling wave which is observed to develop in the numerical calculations. The development of these nonlinear waves, connecting linear stable flows of two different headways, is somewhat reminiscent of stop-start waves in congested flow on freeways. The different types of adjustments are classified in a phase diagram depending on the upstream and downstream headway and the response time of the model. The results have profound consequences for autonomous cruise control systems. For an autocade of both identical and different vehicles, the control system itself may trigger formations of nonlinear, steep wave transitions. Further information is available [Y. Sugiyama, Traffic and Granular Flow (World Scientific, Singapore, 1995), p. 137]. PMID:11308709

Berg, P; Woods, A

2001-03-01

102

Shear wave velocity imaging using transient electrode perturbation: phantom and ex vivo validation.  

PubMed

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

DeWall, Ryan J; Varghese, Tomy; Madsen, Ernest L

2011-03-01

103

Calculation of Rayleigh-wave phase velocities due to models with a high-velocity surface layer  

NASA Astrophysics Data System (ADS)

Rayleigh-wave phase velocities have been utilized to determine shear (S)-wave velocities in near-surface geophysics since early 1980s. One of the key steps is to calculate theoretical dispersion curves of an earth model. When the S-wave velocity of the surface layer is higher than some of the layers below, however, the Rayleigh-wave phase velocity in a high-frequency range calculated by existing algorithms approaches the lowest S-wave velocity among the layers above the half-space, rather than a value related to the S-wave velocity of the surface layer. According to our numerical modeling results based on wave equation, trends of the Rayleigh-wave dispersive energy approach about a 91% of the S-wave velocity of the surface layer at a high-frequency range when its wavelength is much shorter than the thickness of the surface layer, which cannot be fitted by a dispersion curve calculated by existing algorithms. We propose a method to calculate Rayleigh-wave phase velocities of models with a high-velocity surface layer by considering its penetration depth. We build a substituted model that only contains the layer with the lowest S-wave velocity among the layers above the half-space and the layers above it. We use the substituted model to replace the original model to calculate phase velocities when the Rayleigh-wave wavelength is not long enough to penetrate the lowest S-wave velocity layer. Several synthetic models are used to verify fitness between the dispersion curve calculated by our proposed method and the trend of the highest dispersive energy. Examples of inversion also demonstrate high accuracy of using our method as the forward calculation method during the inversions.

Pan, Yudi; Xia, Jianghai; Gao, Lingli; Shen, Chao; Zeng, Chong

2013-09-01

104

P, S wave velocity model of the crust and upper most mantle of Albania region  

NASA Astrophysics Data System (ADS)

This paper describes the one-dimensional (1D) velocity model computed by VELEST in the SEISAN seismic analysis system, inverting re-picked P-wave and S-wave arrival times recorded during 2002-2006 by the Albanian, Montenegro, Thessalonica and Macedonia seismic networks. The re-picked data yield P-wave and S-wave velocities proved to be more suitable compared to bulletin data for this detailed inversion study. Seismic phases recorded by the Albania seismic network and integrated with data from the Montenegro, Thessalonica and Macedonia networks are used to prepare the Albanian seismic bulletin. Earthquake hypocenters from the Albanian bulletins have also location errors that are negligible for civil protection purposes, large scale seismotectonic analyses and more accurate hypocentral determinations which are necessary for detailed seismotectonic and geodynamic studies. It was noted that the smoothness of the velocity variation increased with depth. A velocity of 5.5 km/s was calculated for the upper crust, 6.1 km/s was calculated for the middle crust and 6.9 km/s was computed for the lower crust. P wave velocity was 7.85 km/s at depth of 50 km and for the upper mantle it is 8.28 km/s. Using the improved velocity model, the earthquakes which occurred in Albania in the past 5 years were able to be relocated, achieving constrained hypocentral determinations for events in Albania. The interpretation of the 1 D velocity models infers interesting features of the deep structure of Albania. These results represent an important step towards more detailed seismotectonic analyses.

Ormeni, Rrapo

2011-01-01

105

Predicting S-Wave Velocities for Unconsolidated Sediments at Low Effective Pressure.  

National Technical Information Service (NTIS)

Accurate S-wave velocities for shallow sediments are important in performing a reliable elastic inversion for gas hydrate-bearing sediments and in evaluating velocity models for predicting S-wave velocities, but few S-wave velocities are measured at low e...

M. W. Lee

2010-01-01

106

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

107

Tomographic imaging of local earthquake delay times for three-dimensional velocity variation in western Washington  

NASA Astrophysics Data System (ADS)

Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the Puget Sound region of Western Washington. The 37,500 square km region is represented by nearly cubic blocks of 5 km per side. P-wave arrival time observations from 4,387 crustal earthquakes, with depths of 0 to 40 km, were used as sources producing 36,865 rays covering the target region. A conjugate gradient method (LSQR) is used to invert the large, sparse system of equations. To diminish the effects of noisy data, the Laplacian is constrained to be zero within horizontal layers, providing smoothing of the model. The resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure. Results of the inversion are correlated with some known geologic features and independent geophysical measurements. High P-wave velocities along the eastern flank of the Olympic Peninsula are interpreted to reflect the subsurface extension of Crescent terrane. Low velocities beneath the Puget Sound further to the east are inferred to reflect thick sediment accumulations. The Crescent terrane appears to extend beneath Puget Sound, consistent with its interpretation as a major accretionary unit. In the southern Puget Sound basin, high velocity anomalies at depths of 10-20 km are interpreted as Crescent terrane and are correlated with a region of low seismicity. Near Mt. Rainier, high velocity anomalies may reflect buried plutons.

Lees, Jonathan M.; Crosson, Robert S.

1990-04-01

108

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

NASA Astrophysics Data System (ADS)

Shear (S)-wave velocity is a key property of near-surface materials and is the fundamental parameter for many environmental and engineering geophysical studies. Directly acquiring accurate S-wave velocities from a seismic shot gather is usually difficult due to the poor signal-to-noise ratio. The relationship between Rayleigh-wave phase velocity and frequency has been widely utilized to estimate the S-wave velocities in shallow layers using the multichannel analysis of surface waves (MASW) technique. Hence, Rayleigh wave is a main focus of most near-surface seismic studies. Conventional dispersion analysis of Rayleigh waves assumes that the earth is laterally homogeneous and the free surface is horizontally flat, which limits the application of surface-wave methods to only 1D earth models or very smooth 2D models. In this study I extend the analysis of Rayleigh waves to a 2D domain by employing the 2D full elastic wave equation so as to address the lateral heterogeneity problem. I first discuss the accurate simulation of Rayleigh waves through finite-difference method and the boundary absorbing problems in the numerical modeling with a high Poisson's ratio (> 0.4), which is a unique near-surface problem. Then I develop an improved vacuum formulation 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 dispersion analysis in 2D and 3D domains by numerical investigations. At last I examine the feasibility of inverting waveforms of Rayleigh waves for shallow S-wave velocities using a genetic algorithm. Results of the study show that Rayleigh waves can be accurately simulated in near surface using the improved vacuum formulation. Spurious reflections during the numerical modeling can be efficiently suppressed by the simplified multiaxial perfectly matched layers. The conventional MASW method can tolerate gentle topography changes with insignificant errors. Finally, many near-surface features with strong lateral heterogeneity such as dipping interfaces, faults, and tunnels can be imaged by the waveform inversion of Rayleigh waves for shallow S-wave velocities. This thesis consists of four papers that are either published (chapter 1) or in review (chapter 2, 3, and 4) for consideration of publication to peer-refereed journals. Each chapter represents a paper, and therefore inadvertently there will be a certain degree of overlap between chapters (particularly for the introduction parts, where references to many common papers occur).

Zeng, Chong

109

Investigation of surface wave amplitudes in 3-D velocity and 3-D Q models  

NASA Astrophysics Data System (ADS)

It has been long recognized that seismic amplitudes depend on both wave speed structures and anelasticity (Q) structures. However, the effects of lateral heterogeneities in wave speed and Q structures on seismic amplitudes has not been well understood. We investigate the effects of 3-D wave speed and 3-D anelasticity (Q) structures on surface-wave amplitudes based upon wave propagation simulations of twelve globally-distributed earthquakes and 801 stations in Earth models with and without lateral heterogeneities in wave speed and anelasticity using a Spectral Element Method (SEM). Our tomographic-like 3-D Q models are converted from a velocity model S20RTS using a set of reasonable mineralogical parameters, assuming lateral perturbations in both velocity and Q are due to temperature perturbations. Surface-wave amplitude variations of SEM seismograms are measured in the period range of 50--200 s using boxcar taper, cosine taper and Slepian multi-tapers. We calculate ray-theoretical predictions of surface-wave amplitude perturbations due to elastic focusing, attenuation, and anelastic focusing which respectively depend upon the second spatial derivative (''roughness'') of perturbations in phase velocity, 1/Q, and the roughness of perturbations in 1/Q. Both numerical experiments and theoretical calculations show that (1) for short-period (~ 50 s) surface waves, the effects of amplitude attenuation due to 3-D Q structures are comparable with elastic focusing effects due to 3-D wave speed structures; and (2) for long-period (> 100 s) surface waves, the effects of attenuation become much weaker than elastic focusing; and (3) elastic focusing effects are correlated with anelastic focusing at all periods due to the correlation between velocity and Q models; and (4) amplitude perturbations are depend on measurement techniques and therefore cannot be directly compared with ray-theoretical predictions because ray theory does not account for the effects of measurement techniques. We calculate 3-D finite-frequency sensitivity of surface-wave amplitude to perturbations in wave speed and anelasticity (Q) which fully account for the effects of elastic focusing, attenuation, anelastic focusing as well as measurement techniques. We show that amplitude perturbations calculated using wave speed and Q sensitivity kernels agree reasonably well with SEM measurements and therefore the sensitivity kernels can be used in a joint inversion of seismic phase delays and amplitudes to simultaneously image high resolution 3-D wave speed and 3-D Q structures in the upper mantle.

Ruan, Y.; Zhou, Y.

2010-12-01

110

Do variations of the ambient noise wavefield reflect on measured seismic velocity changes?  

NASA Astrophysics Data System (ADS)

One way of monitoring changes in seismic wave velocities is by following short-term variations in noise correlation functions calculated on short time windows. However, difficulties may arise when the spatial distribution of noise sources evolves with time as well. The relationship between the measured seismic velocity fluctuations and the time-evolution of the spatial distribution of the noise wavefield is investigated through beamforming analysis. We show why it is important to monitor the evolution of the ambient noise wavefield when one wants to assess the reliability of measurements of velocity changes. Finally, a way of determining a noise source orientation using only a co-located measurement of translational and rotational motion is presented.

Hadziioannou, C.; Larose, E. F.; Roux, P.; Campillo, M.; Wassermann, J. M.; Igel, H.

2011-12-01

111

Optically Recording Velocity Interferometer System (ORVIS) for subnanosecond particle-velocity measurements in shock waves  

NASA Astrophysics Data System (ADS)

An optically recording velocity interferometer system (ORVIS) has been developed to measure particle velocity with subnanosecond resolution for shock waves in condensed matter. The fringe pattern of a wide-angle Michelson interferometer was focused as a set of dots on the slit of a high speed streak camera, resulting in a continuous record of the fringe position as a function of time. This technique was employed to measure the particle velocity of a witness foil in a series of experiments to study the nature of detonation-driven shock waves. The present experiments demonstrated a time resolution of about 300 ps, and we believe that 20 ps resolution may be achievable with this technique. The improvement in time resolution of up to two orders of magnitude over current diagnostic techniques will be an aid in the study of several aspects of shock wave phenomena such as shock front thickness, detonation wave theory, fast relaxation at an impact plane, and fine structure in shock fronts associated with chemical reaction.

Bloomquist, D. D.; Sheffield, S. A.

112

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

113

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

114

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

115

Group velocity and characteristic wave curves of Lamb waves in composites: Modeling and experiments  

Microsoft Academic Search

The propagation characteristics of Lamb waves in composites, with emphasis on group velocity and characteristic wave curves, are investigated theoretically and experimentally. In particular, the experimental study focuses on the existence of multiple higher-order Lamb wave modes that can be observed from piezoelectric sensors by the excitation of ultrasonic frequencies. Using three-dimensional (3-D) elasticity theory, the exact dispersion relations governed

Lei Wang; F. G. Yuan

2007-01-01

116

Compositional variation versus partial melting: What is the cause of low velocity and high conductivity?  

NASA Astrophysics Data System (ADS)

The causes for low velocity, high conductivity regions of Earth (asthenosphere in the upper mantle, some regions of the D" layer at the bottom of the mantle) are controversial. I will discuss two possible causes: anomalies in chemical composition and presence of liquids (e.g., partial melt). Based on the thermodynamics and the physics of melt generation and migration, I will discuss the difficulties of partial melt model to explain geophysical anomalies. A brief review will be presented about the hydrogen-based model for geophysical anomalies including a new theory on the relationship between isotope diffusion and conductivity, and the role of grain-boundary sliding to affect seismic wave velocities. I conclude that the variation in hydrogen content is the most plausible explanation for the anomalies in the upper mantle and the transition zone. However, hydrogen model unlikely explains the observed very large velocity reduction in the ultra-low velocity regions in the D" layer. Fe-enrichment is a possible cause for low velocity and high conductivity. However, difficulties with this model are (i) the core is under-saturated with oxygen and hence the mantle next to the core must be depleted with FeO, and (ii) previously proposed mechanisms of Fe penetration are inefficient. A new finding in my lab shows that FeO-depletion at the bottom of the mantle likely promotes the penetration of molten iron leading to the low velocity and high conductivity.

Karato, Shun-ichiro

2013-04-01

117

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

118

Direct Estimation of the Rayleigh Wave Phase Velocity in Microtremors  

NASA Astrophysics Data System (ADS)

The mictotremor survey method (MSM) is one of the techniques used to estimate the shear-wave velocity structure of sedimentary layers. In the MSM, inverse analysis of the dispersion of the phase velocity is used for the velocity modeling. The passive nature of the MSM brings time/cost advantages over other active survey techniques, and it has been used for the strong motion prediction in the sedimentary plains in Japan (Matsuoka et al., 2003).The Spatial autocorrelation (SPAC) method (Aki, 1957), where records from a circular array of evenly spaced sensors and a central sensor is analyzed, is one of the commonly used measurement/analysis techniques in the MSM. In this method, the directional average of complex coherence functions (CCFs) between the records at the center and the circumference is equal to the Bessel function of the first kind of zero order J0(?r/c) (?: angular frequency, r: radius of circular array, c: phase velocity), and, hence, the phase velocities can be estimated. However, in practice, the requirement for equally spaced sensor arrangement in the SPAC often restricts the observation sites especially in urban areas.The authors have newly derived a method to directly estimate J0(?r/c) and the phase velocity with fewer restrictions on the sensor arrangements. In this method (the Direct Estimation Method: DEM), the Bessel function J0(?r/c) is obtained as a solution of the simultaneous equations of the CCFs. We have also found on theoretical grounds that the sensor arrangement in the DEM is much more flexible than the SPAC. In practice sensors on the circumference can be set at almost any place as long as the overall arrangement has a symmetry. A field experiment has confirmed that the phase velocity determined from the DEM agreed with the phase velocity by the SPAC method.

Shiraishi, H.; Matsuoka, T.; Asanuma, H.

2005-12-01

119

A Bayesian approach to the detection of temporal changes in P wave velocity  

NASA Astrophysics Data System (ADS)

On the basis of Akaike's Bayesian information criterion (ABIC), a new method is proposed for detecting a temporal change in a seismic velocity in a source region. The method of joint hypocenter determination was modified in order to determine a seismic velocity in a source layer as a function of time together with hypocenters and station corrections. Arrival times of initial waves of shallow earthquakes in a small area are analyzed in this method. The smoothness of the estimated temporal variation in the velocity is guaranteed by the introduction of a prior distribution of the parameter. The hyperparameter of the prior distribution of the velocity, the reading error of arrival times, and the initial velocity in the source layer are chosen to minimize ABIC. This procedure was applied to the 1983 eastern Yamanashi M = 6.0 earthquake in central Japan. We analyzed P arrival times of 374 earthquakes observed at 12 stations in the network of the National Research Center for Disaster Prevention by dividing the whole period (from October 1981 to May 1987) into 12 six-month subperiods. Calculating ABICs for different combinations of the three parameters above, we searched for the minimum value of ABIC and found two minima. The first one corresponds to a model of a constant velocity in time, and the other corresponds to a model of a variable velocity with 5% velocity change at maximum. However, since ABIC in the former is 10 smaller than that in the latter, the former constant velocity model is statistically more suitable than the latter. Furthermore, generating artificial data with the same reading errors as the actual data, we used computer simulation to examine the lower limit of the velocity change detectable for this data set. In conclusion, the velocity in the source region is 6.24±0.18 km/s, and the velocity change exceeding 6-7% at maximum did not exist during the 6 years before and after the M = 6.0 earthquake.

Hurukawa, Nobuo; Imoto, Masajiro

1989-02-01

120

Temporal change in shallow subsurface P- and S-wave velocities and S-wave anisotropy inferred from coda wave interferometry  

NASA Astrophysics Data System (ADS)

Recent progresses in theoretical and observational researches on seismic interferometry reveal the possibility to detect subtle change in subsurface seismic structure. This high sensitivity of seismic interferometry to the medium properties may thus one of the most important ways to directly observe the time-lapse behavior of shallow crustal structure. Here, using the coda wave interferometry, we show the co-seismic and post-seismic changes in P- and S-wave velocities and S-wave anisotropy associated with the 2011 off the Pacific coast of Tohoku earthquake (M9.0). In this study, we use the acceleration data recorded at KiK-net stations operated by NIED, Japan. Each KiK-net station has a borehole whose typical depth is about 100m, and two three-component accelerometers are installed at the top and bottom of the borehole. To estimate the shallow subsurface P- and S-wave velocities and S-wave anisotropy between two sensors and their temporal change, we select about 1000 earthquakes that occurred between 2004 and 2012, and extract body waves propagating between borehole sensors by computing the cross-correlation functions (CCFs) of 3 x 3 component pairs. We use frequency bands of 2-4, 4-8, 8-16 Hz in our analysis. Each averaged CCF shows clear wave packets traveling between borehole sensors, and their travel times are almost consistent with those of P- and S-waves calculated from the borehole log data. Until the occurrence of the 2011 Tohoku earthquake, the estimated travel time at each station is rather stable with time except for weak seasonal/annual variation. On the other hand, the 2011 Tohoku earthquake and its aftershocks cause sudden decrease in the S-wave velocity at most of the KiK-net stations in eastern Japan. The typical value of S-wave velocity changes, which are measured by the time-stretching method, is about 5-15%. After this co-seismic change, the S-wave velocity gradually recovers with time, and the recovery continues for over one year following the logarithm of the lapse time. At some stations, the estimated P-wave velocity also shows co-seismic velocity decrease and subsequent gradual recovery. However, the magnitude of estimated P-wave velocity change is much smaller than that of S-wave, and at the other stations, the magnitude of P-wave velocity change is smaller than the resolution of our analysis. Using the CCFs computed from horizontal components, we also determine the seismic anisotropy in subsurface structure, and examine its temporal change. The estimated strength of anisotropy strength shows co-seismic increase at most of stations where co-seismic velocity change is detected. Nevertheless, the direction of anisotropy after the 2011 Tohoku earthquake stays about the same as before. These results suggest that, in addition to the change in pore pressure and corresponding decrease in the rigidity, the change in the aspect ratio of pre-existing subsurface fractures/micro-crack may be another key mechanism causing the co-seismic velocity change in shallow subsurface structures.

Yamamoto, M.; Nishida, K.; Takeda, T.

2012-12-01

121

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

122

Shear Wave Velocity Structure of the Pampean Flat Slab Region from Ambient Noise Tomography  

NASA Astrophysics Data System (ADS)

The South American Cordillera formed by the subduction of the Nazca plate beneath South America. While this is often considered a ‘typical’ compressive upper plate subduction zone, there are several along-strike variations in both the nature of subduction and the style of deformation. From 30° to 32° S the Nazca plate flattens out at 100 km depth for ~300 km before resuming a steeper angle of subduction. Flat slab subduction shutoff of arc magmatism and caused deformation to migrate inboard into the Sierras Pampeanas. While flat slab subduction has had a profound impact on the regions’s tectonics, the presence of preexisting features related to the rifting of Gondwanaland and the accretion of terranes have also had a large impact on deformation. We use ambient noise tomography (ANT) to calculate regional shear wave velocities to better understand the tectonic development of the Pampean flat slab region. ANT utilizes the cross correlation of seismic noise to approximate the Green’s function between two seismic stations. Using this technique, we measure Rayleigh wave phase velocities at periods between 8 and 30 seconds, allowing us to measure shear wave velocities down to 40 km depth. Initial tomography results show a strong correlation between phase velocity and basin structure. Fast phase velocities at the 10 second period correlate with the Sierra de Pie de Palo, Sierra de Valle Fertil in the west and the Sierras de Cordoba in the east, while slow velocities correlate with the Bermejo and Cuyo basins. At longer periods (beyond 20 seconds), there is a pattern of slow phase velocities in the west beneath the Precordillera and the high Andes while fast phase velocities are present in the east beneath the Sierras Pampeanas. These fast velocities most likely reflect faster mid- to lower crustal velocities and a shallower Moho. To further our interpretation we inverted phase velocities to calculate regional shear wave structure. At shallow depths (< 15 km) the pattern of fast bedrock exposures and slow basins observed in phase velocities is also found in shear velocities. The fastest upper crustal velocities are observed beneath Sierra de Pie de Palo. These velocities are consistent with the idea that this range is a sliver of rock that was accreted at Grenville ages to the Precordillera block to form the Cuyania composite terrane. At 20 to 35 km depth, faster shear velocities are observed beneath the Pampian block than beneath the Cuyania and Chilenia terranes. These slow velocities observed below the high Andes and Precordillera most likely represent a change in lithology and can potentially explain why shortening has largely been focused there. Beneath 40 km depth, mantle shear wave velocities are observed within the eastern Sierras Pampeanas and mid- to lower crustal velocities are observed beneath the Sierras Pampeanas. This observation reflects the thicker crustal root supporting the high Andes. Based on these observations we conclude that terrane boundaries play a significant role in controlling the location and magnitude of deformation within the Pampean flat slab region.

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

2010-12-01

123

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

124

Velocities of Compressional and Shear Waves in Marine Sediments Determined In Situ from a Research Submersible  

Microsoft Academic Search

In situ measurements of the velocity and attenuation of compressional waves, and velocities of Stoneley waves from which shear-wave velocities were computed, were made at three stations in the sea floor off San Diego, California, from a research submersible; a fourth station was occupied by divers. Sediment types and water depths ranged from medium sand at 20 meters to clayey

E. L. Hamilton; H. P. Bucker; D. L. Keir; J. A. Whitney

1970-01-01

125

Measurement of directional wave spectrum with a modular acoustic velocity sensor  

Microsoft Academic Search

Directional wave spectra, valuable to describe the sea-state, can be computed as the correlation of horizontal velocity with pressure measured at a single point. Pressure was used instead of the vertical component of velocity for the correlation since the deployment height of 70 cm above the bottom attenuated the vertical wave velocity component. A Modular Acoustic Velocity Sensor (MAVS) with

Albert J. Williams; Eugene A. Terray

2000-01-01

126

Temporal variations in seismic velocities obtained from seismic tomography in Southwest Iceland  

NASA Astrophysics Data System (ADS)

Local earthquake (LE) seismic tomography is generally performed assuming the seismic velocity structure is stationary with time. Seismic velocities are in turn controlled by mechanical properties that may change with time, because of temporal variations of e.g. temperature, pressure, and fluid saturation. The reasons not to consider temporal variations in the seismic velocities when performing LE tomography should therefore mainly be attributed to a shortage of seismic stations and events. During the year 2000 more than 110,000 earthquakes occurred in the area of the Reykjanes Volcanic Zone (RVZ) and the South Iceland Seismic Zone (SISZ). This seismicity rate is much larger than normal, and is related to the two magnitude 6 earthquakes that occurred there in June 2000. Events were selected and divided into suitable time windows and inverted in sequence. The starting P- and S-wave seismic velocity models were based on data from 1973-98, and are considered to be very robust. Within the region considered well-resolved, variations occur mainly in four regions. Two of those coincide with two volcanic systems. One of them is the Hengill volcanic area, which consists of an active central volcano within a 70 - 80 km long fissure system. North of the volcano is the Nesjavellir hydrothermal field, in which a 400MWth geothermal power plant is operated. The changes in Vp/Vs ratio we observe in this region are gradual. For example, a decrease lasting approximately one year is observed, starting in March 1999. These observations are correlated with temperature observations from the hydrothermal wells. The other two regions where we observe variations with time are located within the SISZ, near the two large June 2000 earthquake hypocenters. Instead of a gradual change, a sudden increase is observed in the near source region after the large earthquakes. Work that involves relative locations of earthquakes is ongoing, something which we expect will refine the models further.

Tryggvason, A.; Bödvarsson, R.; Björnsson, G.; Roberts, R. G.

2005-12-01

127

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

128

Laboratory velocities and attenuation of p-waves in limestones during freeze-thaw cycles  

SciTech Connect

The velocity and the attenuation of compressional P-waves, measured in the laboratory at ultrasonic frequencies during a series of freezing and thawing cycles, are used as a method for predicting frost damage in a bedded limestone. Pulse transmission and spectral ratio techniques are used to determine the P-wave velocities and the attenuation values relative to an aluminum reference samples with very low attenuation. Limestone samples were water saturated under vacuum conditions, jacketed with rubber sleeves, and immersed in an antifreeze bath (50 percent methanol solution). They were submitted to repeated 24-hour freezing and thawing cycles simulating natural environment conditions. During the freeze/thaw cycles, P-wave velocities and quality factor Q diminished rapidly in thawed rock samples, indicating modification of the pore space. Measurements of crack porosity were conducted by hydrostatic compression tests on cubic rock samples that had been submitted to these freeze/thaw cycles. These measurements are used as an index of crack formation. The hydrostatic compression tests confirmed the phases of rock damage that were shown by changes in the value of Q. Furthermore, comparison between Q values and crack porosity demonstrate that the variations of P-wave attenuation are caused by the creation of new cracks and not by the enlargement of pre-existing cracks.

Remy, J.M.; Bellanger, M.; Homand-Etienne, F. (Lab. de Geomecanique de Nancy, Vandoeuvre-les-Nancy (France))

1994-02-01

129

Crustal and uppermost mantle S-wave velocity structure beneath the Japanese islands from seismic ambient noise tomography  

NASA Astrophysics Data System (ADS)

In this study, the crustal and uppermost mantle shear wave velocities beneath the Japanese islands have been determined by inversion from seismic ambient noise tomography using data recorded at 75 Full Range Seismograph Network of Japan broad-band seismic stations, which are uniformly distributed across the Japanese islands. By cross-correlating 2 yr of vertical component seismic ambient noise recordings, we are able to extract Rayleigh wave empirical Green's functions, which are subsequently used to measure phase velocity dispersion in the period band of 6-50 s. The dispersion data are then inverted to yield 2-D tomographic phase velocity maps and 3-D shear wave velocity models. Our results show that the velocity variations at short periods (˜10 s), or in the uppermost crust, correlate well with the major known surface geological and tectonic features. In particular, the distribution of low-velocity anomalies shows good spatial correlation with active faults, volcanoes and terrains of sediment exposure, whereas the high-velocity anomalies are mainly associated with the mountain ranges. We also observe that large upper crustal earthquakes (5.0 ? M ? 8.0, depth ? 25 km) mainly occurred in low-velocity anomalies or along the boundary between low- and high-velocity anomalies, suggesting that large upper crustal earthquakes do not strike randomly or uniformly; rather they are inclined to nucleate within or adjacent to low-velocity areas.

Guo, Zhi; Gao, Xing; Shi, Heng; Wang, Weiming

2013-04-01

130

New Hybridized Surface Wave Approach for Geotechnical Modeling of Shear Wave Velocity at Strong Motion Recording Stations  

NASA Astrophysics Data System (ADS)

Strong motion recording (SMR) networks often have little or no shear wave velocity measurements at stations where characterization of site amplification and site period effects is needed. Using the active Spectral Analysis of Surface Waves (SASW) method, and passive H/V microtremor method we have investigated nearly two hundred SMR sites in California, Alaska, Japan, Australia, China and Taiwan. We are conducting these studies, in part, to develop a new hybridized method of site characterization that utilizes a parallel array of harmonic-wave sources for active-source SASW, and a single long period seismometer for passive-source microtremor measurement. Surface wave methods excel in their ability to non-invasively and rapidly characterize the variation of ground stiffness properties with depth below the surface. These methods are lightweight, inexpensive to deploy, and time-efficient. They have been shown to produce accurate and deep soil stiffness profiles. By placing and wiring shakers in a large parallel circuit, either side-by-side on the ground or in a trailer-mounted array, a strong in-phase harmonic wave can be produced. The effect of arraying many sources in parallel is to increase the amplitude of waves received at far-away spaced seismometers at low frequencies so as to extend the longest wavelengths of the captured dispersion curve. The USGS system for profiling uses this concept by arraying between two and eight electro-mechanical harmonic-wave shakers. With large parallel arrays of vibrators, a dynamic force in excess of 1000 lb can be produced to vibrate the ground and produce surface waves. We adjust the harmonic wave through a swept-sine procedure to profile surface wave dispersion down to a frequency of 1 Hz and out to surface wave-wavelengths of 200-1000 meters, depending on the site stiffness. The parallel-array SASW procedure is augmented using H/V microtremor data collected with the active source turned off. Passive array microtremor data reveal the natural and resonance characteristics of the ground by capturing persistent natural vibrations. These microtremors are the result of the interaction of surface waves arriving from distant sources and the stiffness structure of the site under investigation. As such, these resonance effects are effective in constraining the layer thicknesses of the SASW shear wave velocity structure and aid in determining the depth of the deepest layer. Together, the hybridized SASW and H/V procedure provides a complete data set for modeling the geotechnical aspects of ground amplification of earthquake motions. Data from these investigations are available at http://walrus.wr.usgs.gov/geotech.

Kayen, R.; Carkin, B.; Minasian, D.

2006-12-01

131

Helioseismic measurements in the solar envelope using group velocities of surface waves  

NASA Astrophysics Data System (ADS)

At intermediate- and high-degree l, solar p and f modes can be considered as surface waves. Using variational principle, we derive an integral expression for the group velocities of the surface waves in terms of adiabatic eigenfunctions of normal modes, and address the benefits of using group-velocity measurements as a supplementary diagnostic tool in solar seismology. The principal advantage of using group velocities, when compared with direct analysis of the oscillation frequencies, comes from their smaller sensitivity to the uncertainties in the near-photospheric layers. We address some numerical examples where group velocities are used to reveal inconsistencies between the solar models and the seismic data. Further, we implement the group-velocity measurements to the calibration of the specific entropy, helium abundance Y, and heavy-element abundance Z in the adiabatically stratified part of the solar convective envelope, using different recent versions of the equation of state. The results are in close agreement with our earlier measurements based on more sophisticated analysis of the solar oscillation frequencies. These results bring further support to the downward revision of the solar heavy-element abundances in recent spectroscopic measurements.

Vorontsov, S. V.; Baturin, V. A.; Ayukov, S. V.; Gryaznov, V. K.

2014-07-01

132

Laser probing system for the accurate detection of surface acoustic wave phase velocities  

NASA Astrophysics Data System (ADS)

The present work demonstrates a high-resolution technique for the optical detection of the phase and amplitude of high frequency surface acoustic waves (SAWs). The test setup incorporates a mode-locked picosecond laser, harmonic mixing, and coherent detection, and it allows not only the measurement of the surface wave field but also the direct determination of the phase velocity. A measurement bandwidth in excess of 2 GHz is achieved. The maximum scan length was 4 cm. As a substrate, LiNbO3 has been used for the test measurements. Minimum detectable surface displacement and dynamic range were 1 pm/Hz1/2 and 40 dB, respectively. The method enables the determination of the phase velocity with a resolution of 1.5 (DOT) 10-5 in dependence of crystal cut, temperature, and frequency. An additional feature is that phase velocity values can be obtained with spatial resolution, i.e., velocity variation effects along the propagation path can be evaluated. We found that the assumption that the SAW velocity is constant across the whole device surface is not valid in general.

Weigel, Robert; Holm, Andreas; Soelkner, Gerald; Ruile, Werner; Russer, Peter

1994-06-01

133

Hamiltonians and variational principles for Alfvén simple waves  

NASA Astrophysics Data System (ADS)

The evolution equations for the magnetic field induction B with the wave phase for Alfvén simple waves are expressed as variational principles and in the Hamiltonian form. The evolution of B with the phase (which is a function of the space and time variables) depends on the generalized Frenet-Serret equations, in which the wave normal n (which is a function of the phase) is taken to be tangent to a curve X, in a 3D Cartesian geometry vector space. The physical variables (the gas density, fluid velocity, gas pressure and magnetic field induction) in the wave depend only on the phase. Three approaches are developed. One approach exploits the fact that the Frenet equations may be written as a 3D Hamiltonian system, which can be described using the Nambu bracket. It is shown that B as a function of the phase satisfies a modified version of the Frenet equations, and hence the magnetic field evolution equations can be expressed in the Hamiltonian form. A second approach develops an Euler-Poincaré variational formulation. A third approach uses the Frenet frame formulation, in which the hodograph of B moves on a sphere of constant radius and uses a stereographic projection transformation due to Darboux. The equations for the projected field components reduce to a complex Riccati equation. By using a Cole-Hopf transformation, the Riccati equation reduces to a linear second order differential equation for the new variable. A Hamiltonian formulation of the second order differential equation then allows the system to be written in the Hamiltonian form. Alignment dynamics equations for Alfvén simple waves give rise to a complex Riccati equation or, equivalently, to a quaternionic Riccati equation, which can be mapped onto the Riccati equation obtained by stereographic projection.

Webb, G. M.; Hu, Q.; le Roux, J. A.; Dasgupta, B.; Zank, G. P.

2012-01-01

134

Three-Dimensional Velocity Structure in Southern California from Teleseismic Surface Waves and Body Waves.  

NASA Astrophysics Data System (ADS)

Analysis of teleseismic waves generated by large earthquakes worldwide across the Southern California TriNet Seismic Broadband Array has yielded high quality measurements of both surface waves and body waves. Rayleigh waves and Love waves were previously analyzed using a spectral fitting technique (Tanimoto. and Prindle-Sheldrake, GRL 2002; Prindle-Sheldrake and Tanimoto, submitted to JGR), producing a three-dimensional S-wave velocity structure. Features in our velocity structure show some regional contrasts with respect to the starting model (SCEC 2.2), which has detailed crustal structure, but laterally homogeneous upper mantle structure. The most prominent of which is a postulated fast velocity anomaly located west of the Western Transverse Ranges that could be related to a rotated remnant plate from Farallon subduction. Analysis indicates that, while Rayleigh wave data are mostly sensitive to mantle structure, Love wave data require some modifications of crustal structure from SCEC 2.2 model. Recent advances in our velocity structure focus on accommodation of finite frequency effect, and the addition of body waves to the data. Thus far, 118 events have been analyzed for body waves. A simple geometrical approach is used to represent the finite frequency effect in phase velocity maps. Due to concerns that, for seismic phases between 10-100 seconds, structure away from the ray theoretical is also sampled by a propagating surface wave, we have adopted a technique which examines a normal mode formula in its asymptotic limit (Tanimoto, GRL 2003 in press). An ellipse, based on both distance from source to receiver and wavelength, can be used to approximate the effect on the structure along the ray path and adjacent structure. Three models were tested in order to select the appropriate distribution within the ellipse; the first case gives equal weight to all blocks within the ellipse; case 2 incorporates a Gaussian function which falls off perpendicular to the ray path, allowing the amplitude to peak at the receiver; case 3 is the same as case 2, yet removes the effect of the peak at the receiver. A major improvement is that the locale under consideration has expanded due to the effect of ray paths spreading over a larger area than the ray theoretical. Comparison of the three techniques yields very similar results, and all techniques show an exceptional correlation to the ray theoretical phase velocity maps. After analyzing our data in terms of the finite frequency effect, we find that little change has occurred as a result of employing this technique other than expanding our region of study. P-wave measurements were obtained from the data set for 118 events. Preliminary results show systematic patterns. We have successfully measured 30 S-wave events which we plan to incorporate into our velocity structure. Our goal is to proceed with a joint inversion of P-waves, S-waves and Surface waves for a collective Southern California velocity structure.

Prindle-Sheldrake, K. L.; Tanimoto, T.

2003-12-01

135

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

136

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

NASA Astrophysics Data System (ADS)

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

137

Predicting S-wave velocities for unconsolidated sediments at low effective pressure  

USGS Publications Warehouse

Accurate S-wave velocities for shallow sediments are important in performing a reliable elastic inversion for gas hydrate-bearing sediments and in evaluating velocity models for predicting S-wave velocities, but few S-wave velocities are measured at low effective pressure. Predicting S-wave velocities by using conventional methods based on the Biot-Gassmann theory appears to be inaccurate for laboratory-measured velocities at effective pressures less than about 4-5 megapascals (MPa). Measured laboratory and well log velocities show two distinct trends for S-wave velocities with respect to P-wave velocity: one for the S-wave velocity less than about 0.6 kilometer per second (km/s) which approximately corresponds to effective pressure of about 4-5 MPa, and the other for S-wave velocities greater than 0.6 km/s. To accurately predict S-wave velocities at low effective pressure less than about 4-5 MPa, a pressure-dependent parameter that relates the consolidation parameter to shear modulus of the sediments at low effective pressure is proposed. The proposed method in predicting S-wave velocity at low effective pressure worked well for velocities of water-saturated sands measured in the laboratory. However, this method underestimates the well-log S-wave velocities measured in the Gulf of Mexico, whereas the conventional method performs well for the well log velocities. The P-wave velocity dispersion due to fluid in the pore spaces, which is more pronounced at high frequency with low effective pressures less than about 4 MPa, is probably a cause for this discrepancy.

Lee, Myung W.

2010-01-01

138

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

139

Phase velocity tomography of surface waves using ambient noise cross correlation and array processing  

NASA Astrophysics Data System (ADS)

Continuous recordings of ambient seismic noise across large seismic arrays allows a new type of processing using the cross-correlation technique on broadband data. We propose to apply double beamforming (DBF) to cross correlations to extract a particular wave component of the reconstructed signals. We focus here on the extraction of the surface waves to measure phase velocity variations with great accuracy. DBF acts as a spatial filter between two distant subarrays after cross correlation of the wavefield between each single receiver pair. During the DBF process, horizontal slowness and azimuth are used to select the wavefront on both subarray sides. DBF increases the signal-to-noise ratio, which improves the extraction of the dispersive wave packets. This combination of cross correlation and DBF is used on the Transportable Array (USArray), for the central U.S. region. A standard model of surface wave propagation is constructed from a combination of the DBF and cross correlations at different offsets and for different frequency bands. The perturbation (phase shift) between each beam and the standard model is inverted. High-resolution maps of the phase velocity of Rayleigh and Love waves are then constructed. Finally, the addition of azimuthal information provided by DBF is discussed, to construct curved rays that replace the classical great-circle path assumption.

Boué, Pierre; Roux, Philippe; Campillo, Michel; Briand, Xavier

2014-01-01

140

Phase Velocities of Long-Period Surface Waves and Structure of the Upper Mantle, 1, Great-Circle Love and Rayleigh Wave Data  

Microsoft Academic Search

New long-period dispersion data are obtained from the surface waves generated by the Alaska earthquake of March 28, 1964, and recorded at Isabella, Kipapa, and Stutt- gart. Digital techniques were used to isolate phases and determine spectrums over the period band 80 to 670 seconds. Available phase velocity data are now accurate enough to permit us to discuss regional variations

M. Nafi Toksöz; Don L. Anderson

1966-01-01

141

S-wave velocity structure in southwest China from surface wave tomography and receiver functions  

NASA Astrophysics Data System (ADS)

Using the surface wave records of 504 teleseismic events at 50 temporal and 92 permanent seismic stations in southwest China, we extracted the phase velocity dispersion curves with interstation correlation method, and obtained the phase velocity maps at 10, 15, 25, 40, 60 and 75 s with a grid space of 0.5×0.5 from surface wave tomography. Meanwhile, we obtained the S-wave velocity structures beneath three profiles from the joint inversion of receiver functions and surface waves. From the maps at short periods (10 and 15 s) and long periods (40, 60 and 75 s), different distribution features of high velocity zones (HVZs for short) and low velocity zones (LVZs for short) are shown in the study area: HVZs at short periods are shown in the Panzhihua - Emeishan region, Sichuan basin and Weixi-Lijiang region, surrounding the LVZs from Songpan-Ganzi block to the east of Lijiang where there are significant higher elevations; whereas HVZs at long periods are shown in the Weixi-Lijiang region, Panzhihua-Chuxiong basin and Kunming-Tonghai region and forming a line in the center part of the study area, and the fast polarization directions of the shear wave from SKS analysis on the two sides of the line change significantly. These phenomena indicate plateau material flow can be blocked in two different depth intervals and leads to different horizontal extensions. From the maps at long periods and the structures along the profiles, LVZs are shown in the upper mantle beneath rapid slip fault zones, such as Anninghe - Zemuhe - Xiaojiang fault zone, Red River fault zone and Xiaojinhe fault zone, implying these faults are deep penetrating faults. Figure (a-f) Rayleigh wave phase velocity maps at 10, 15, 25,40,60 and 75 s with a resolution of 100 km. The black lines represent faults. The red points represent M?6 earthquakes. The colour scale changes in different panels. Figure (g) Distribution of the seismic stations and regional tectonic features in the study area. Figure (h-j) The S wave velocity structures beneath the profiles AA', BB' and CC' from the joint inversion of receiver functions and surface waves. The results at different stations are vertically projected to the profiles.

Wang, W.; Wu, J.; Fang, L.; Lai, G.; Yang, T.

2013-12-01

142

Lithospheric Velocity Structure Along the Dead Sea Transform From the Joint Inversion of P- and S-Wave Receiver Functions and Dispersion Velocities  

NASA Astrophysics Data System (ADS)

The Dead Sea Transform (DST) is a ~1000 km long fault system that separates the Arabian and African plates and links the active spreading center of the Red Sea with the continental collision zone in the Zagros mountains. Independent thermo-mechanical modeling had suggested that a mantle plume intruding from the south might have eroded the lithosphere East of the DST, but recent results from a number of temporary seismic deployments in the area have found little variation in lithospheric thickness to support it and proposed that such geodynamic processes might have operated on both sides of the DST. Those lithospheric thickness estimates have been obtained through the stack and migration of Sp conversions in S-receiver functions, which assumes a global background velocity model to calibrate the depth-scale that might not represent mantle structure under the DST accurately. In this study, we obtain lithospheric and sub-lithospheric S-velocity structure under several permanent broadband stations along the DST from the joint inversion of P- and S-wave receiver functions and tomographic surface-wave dispersion velocities. The joint inversion approach simultaneously models S-P times and Ps amplitudes in P-receiver functions, P-S times and Sp amplitudes in S-receiver functions, and fundamental-mode, Rayleigh-wave group velocities, providing local 1D models of S-velocity under individual recording stations. Our velocity models display interesting variations in crustal thickness along the DST, with values under 40 km in the southern portions and values well over 40 km more to the North, near Lebanon. The models also display S-velocity values in the lithospheric mantle that are generally slow, in the 4.3-4.4 km/s range, sometimes overlaying a relatively shallow asthenospheric low velocity channel, above 150 km depth.

Julia, J.; Nyblade, A.; Pasyanos, M. E.; Matzel, E.; Rodgers, A.; Al-Amri, A. M.

2012-12-01

143

Lateral Crustal Velocity Variations across the Andean Foreland in San Juan, Argentina from the JHD Analysis and 3D P and S Velocity inversion  

NASA Astrophysics Data System (ADS)

Lateral crustal velocity variations across the Andean Foreland in San Juan Argentina are explored by joint hypocentral determination (JHD) analysis and 3D velocity inversion. JHD results show consistent positive station corrections beneath Precordillera and negative station corrections beneath Pie de Palo, corresponding to regions of low and high velocity, respectively. These observations are supported by the results from the 3D velocity inversion. A 20% increase in velocity is observed from the Precordilleras in the west to Pie de Palo in the east. The tomography result also reveals a narrow east dipping and NNE trending high velocity anomalous zone bisecting the southern half of Pie de Palo. This anomalous zone was previously identified by a magnetic study and was interpreted to represent the structure corresponding to the Grenvillian Precordillera-Pie de Palo tectonic boundary zone. Finally, P and S station corrections are calculated from the synthetic travel time obtained by using the resultant 3D P- and S- wave velocity model. The observed pattern and magnitude of the P- and S-wave station corrections are recovered successfully from the synthetic calculation, indicating that the resultant 3D velocity model is close to the real earth structure in the Andean Foreland region. Relocation of all intermediate events from the flat subducting slab using this newly acquired 3D velocity model shows a significant change in the slab geometry. The relocated hypocenter distribution is more clustered than previous studies obtained using a 1D model. The slab is simply flat and it resumes a normal subduction angle towards the east of the study area.

Asmerom, B. B.; Chiu, J.; Pujol, J.; Smalley, R.

2010-12-01

144

The velocity structure of the upper mantle of europe from the ambient noise surface wave tomography  

NASA Astrophysics Data System (ADS)

The method for surface wave tomography based on the records of ambient seismic noise (Ambient Noise Tomography, ANT) is applied to the data from the East European and West European stations. In order to reduce the effects of the earthquakes at long periods, the cross correlation functions were calculated for the time interval of 2001-2003, when distinct clusters of the earthquakes were absent. Using the local dispersion curves in the range of 10-100 s, we reconstructed the vertical velocity sections at the nodes of the 3° × 3° grid. On the basis on these curves, we calculated the horizontal distributions of S-velocity variations in the upper mantle in the depth interval of 75-275 km and the vertical velocity sections along the profiles across the Vrancea zone and the region of the Baltic and Ukrainian shields. The velocity distribution in the Vrancea zone confirms the subduction of the ancient oceanic plate from the east westwards and the detachment of its bottom part, as hypothesized by some authors. Beneath the Baltic Shield lithosphere, there is a low-velocity zone, which can be interpreted as the asthenospheric layer. It is noted that the velocity distributions beneath the Baltic and Ukrainian shields are similar, which probably points to the genetic relationship between these two structures.

Yanovskaya, T. B.; Lyskova, E. L.

2013-09-01

145

Direct estimation of the Rayleigh wave phase velocity in microtremors  

NASA Astrophysics Data System (ADS)

We present a new estimation technique using a circular array for the Rayleigh wave phase velocity in microtremors. This technique allows us to employ more flexible sensor arrangements than that of Aki's spatial autocorrelation(SPAC) technique. One of the features is that the array output is represented by simultaneous equations consisting of complex coherence functions(CCFs). A CCF consists of two components:the Bessel function J0(?r/c)(?:angular frequency, r:radius, c:phase velocity) and an error term that varies with the azimuth of the sources. An analysis of the SPAC operation has demonstrated that the azimuthal averaging of the CCFs reduces the error term, and the average value can be approximated to J0(?r/c). This result suggests the phase velocity can be determined by extracting J0(?r/c) from the CCFs. Therefore, we tried to directly estimate J0(?r/c) without the azimuthal averaging and developed a solution method referred to as the direct estimation method(DEM).

Shiraishi, Hidetaka; Matsuoka, Tatsuro; Asanuma, Hiroshi

2006-09-01

146

Magnetospheric Electron-Velocity-Distribution Function Information from Wave Observations  

NASA Astrophysics Data System (ADS)

The electron-velocity-distribution function was determined to be highly non-Maxwellian and more appropriate to a kappa distribution, with ? ? 2.0, near magnetic midnight in the low-latitude magnetosphere just outside a stable plasmasphere during extremely quiet geomagnetic conditions. The kappa results were based on sounder-stimulated Qn plasma resonances using the Radio Plasma Imager (RPI) on the IMAGE satellite; the state of the plasmasphere was determined from IMAGE/EUV observations. The Qn resonances correspond to the maximum frequencies of Bernstein-mode waves that are observed between the harmonics of the electron cyclotron frequency in the frequency domain above the upper-hybrid frequency. Here we present the results of a parametric investigation that included suprathermal electrons in the electron-velocity-distribution function used in the plasma-wave dispersion equation to calculate the Qn frequencies for a range of kappa and fpe/fce values for Qn resonances from Q1 to Q9. The Qn frequencies were also calculated using a Maxwellian distribution and they were found to be greater than those calculated using a kappa distribution with the frequency differences increasing with increasing n for a fixed ? and with decreasing ? for a fixed n. The calculated fQn values have been incorporated into the RPI BinBrowser software providing a powerful tool for rapidly obtaining information on the nature of the magnetospheric electron-velocity-distribution function and the electron number density Ne. This capability enabled accurate (within a few percent) in-situ Ne determinations to be made along the outbound orbital track as IMAGE moved away from the plasmapause. The extremely quiet geomagnetic conditions allowed IMAGE/EUV-extracted counts to be compared with the RPI-determined orbital-track Ne profile. The comparisons revealed remarkably similar Ne structures.

Benson, R. F.; Vinas, A. F.; Osherovich, V. A.; Fainberg, J.; Purser, C. M.; Adrian, M. L.; Galkin, I. A.; Reinisch, B. W.

2013-12-01

147

Magnetospheric electron-velocity-distribution function information from wave observations  

NASA Astrophysics Data System (ADS)

The electron-velocity-distribution function was determined to be highly non-Maxwellian and more appropriate to a kappa distribution, with ? ? 2.0, near magnetic midnight in the low-latitude magnetosphere just outside a stable plasmasphere during extremely quiet geomagnetic conditions. The kappa results were based on sounder-stimulated Qn plasma resonances using the Radio Plasma Imager (RPI) on the IMAGE satellite; the state of the plasmasphere was determined from IMAGE/EUV observations. The Qn resonances correspond to the maximum frequencies of Bernstein-mode waves that are observed between the harmonics of the electron cyclotron frequency in the frequency domain above the upper-hybrid frequency. Here we present the results of a parametric investigation that included suprathermal electrons in the electron-velocity-distribution function used in the plasma-wave dispersion equation to calculate the Qn frequencies for a range of kappa and fpe/fce values for Qn resonances from Q1 to Q9. The Qn frequencies were also calculated using a Maxwellian distribution, and they were found to be greater than those calculated using a kappa distribution with the frequency differences increasing with increasing n for a fixed ? and with decreasing ? for a fixed n. The calculated fQn values have been incorporated into the RPI BinBrowser software providing a powerful tool for rapidly obtaining information on the nature of the magnetospheric electron-velocity-distribution function and the electron number density Ne. This capability enabled accurate (within a few percent) in situ Ne determinations to be made along the outbound orbital track as IMAGE moved away from the plasmapause. The extremely quiet geomagnetic conditions allowed IMAGE/EUV-extracted counts to be compared with the RPI-determined orbital-track Ne profile. The comparisons revealed remarkably similar Ne structures.

Benson, Robert F.; ViñAs, Adolfo F.; Osherovich, Vladimir A.; Fainberg, Joseph; Purser, Carola M.; Adrian, Mark L.; Galkin, Ivan A.; Reinisch, Bodo W.

2013-08-01

148

Average shear-wave velocity structure of the Kamchatka peninsula from the dispersion of surface waves  

NASA Astrophysics Data System (ADS)

An average shear-wave velocity structure has been estimated for the path between the Kamchatka Isthmus and Petropavlovsk-Kamchatski. It is obtained from the Monte Carlo inversion of the Rayleigh and Love wave group velocity dispersion curves measured using broad-band seismograms of events in Northern Kamchatka recorded by the IRIS station PET in Petropavlovsk-Kamchatski. The Moho interface was found at a depth of 35±5 km and the Konrad one at 18±4 km. An important feature of the found structure is a low velocity in the upper mantle. This result is coherent with the recent and present-% day volcanic activity in Kamchatka. Synthetic long period seismograms computed for the obtained structure are in good agreement with observed ones.

Shapiro, N. M.; Gorbatov, A. V.; Gordeev, E.; Dominguez, J.

2000-09-01

149

Modeling Compressional and Shear Wave Velocities of Unconsolidated Sediments in the Vadose Zone  

NASA Astrophysics Data System (ADS)

Recent advances in seismic surveys have provided ways to image shallow structure in highly attenuating soils and near-surface rock. Applications include using surface wave methods to find tunnels; mapping landfills with seismic refraction methods; finding faults, the water table, or other strong heterogeneities using seismic reflection surveys. Recent improvements in laboratory ultrasonic measurement techniques have provided reliable data on compressional and shear wave velocities in soils at low pressures analogous to the top few meters to tens of meters of the subsurface. The availability of these data points the way for development of interpretation methods that may allow seismologists to obtain more information from their data in the future. For environmental applications, improvements to interpretation methods could lead to reliable detection of second-order features such as changes in soil saturation, presence of dense non-aqueous phase liquids, or changes in clay content. Traditional modeling techniques developed in the oil industry are optimized for consolidated materials at pressures and depths greater than those typical for environmental applications. The shallow subsurface velocities are highly nonlinear and the soils are highly attenuating, and these characteristics must be considered when modeling velocities (and also when processing seismic data). Various factors, including grain contact roughness, location of clay with respect to sand grains, location of fluid in the partially-saturated case, loose vs. dense packing, all affect the compressional and shear wave velocities and attenuation and their pressure (depth) dependence. Care must be taken when applying effective medium theories or grain-contact theories to model shallow soil velocity behavior. Unconsolidated materials at low pressures show much greater variation in compressional vs. shear wave properties than consolidated materials and high-pressure applications would show. Despite the modeling difficulties, progress in seismic interpretation may be made if data are available for both compressional and shear waves. Examples of pitfalls and methods for successful soil velocity modeling will be presented using lab and field velocity data from the literature, for sands, silty sands, and other unconsolidated materials. The goal is to develop techniques to improve interpretation of seismic data from the vadose zone. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-ENG-48 and supported specifically by the DOE Environmental Management Science Program. >http://www.llnl.gov/ees/esd/expgeoph/Berge/EMSP/

Berge, P. A.

2001-12-01

150

New constraints on the arctic crust and uppermost mantle: surface wave group velocities, P n, and S n  

NASA Astrophysics Data System (ADS)

We present the results of a study of surface wave dispersion across the Arctic region (>60°N) and compare the estimating group velocity maps with new maps of the body wave phases P n and S n. Data recorded at about 250 broadband digital stations from several global and regional networks were used to obtain Rayleigh and Love wave group velocity measurements following more than 1100 events with magnitudes Ms>5.0 that occurred in the northern hemisphere from 1977 to 1998. These measurements were used to construct both isotropic and 2 ? azimuthally anisotropic group velocity maps from 15 to 200 s period. As elsewhere in the world, the observed maps display the signatures of sedimentary and oceanic basins, crustal thickness variations, and upper mantle anomalies under both continents and oceans. We also present P n and S n maps produced from a groomed data set of travel times from the ISC and NEIC bulletins. The long period group velocity maps correlate well with P n and S n velocities. Finally, at long wavelengths, the estimated 2 ? azimuthal anisotropy in Rayleigh wave group velocity correlates well with the azimuthal anisotropy in phase velocity obtained in a global scale study of Trampert and Woodhouse. Because attempts to improve the resolution to regional scales change both the amplitude and the pattern of the inferred azimuthal anisotropy, caution should be exercised in interpreting the anisotropy maps.

Levshin, A. L.; Ritzwoller, M. H.; Barmin, M. P.; Villaseñor, A.; Padgett, C. A.

2001-04-01

151

Spatial Parallelism of a 3D Finite Difference, Velocity-Stress Elastic Wave Propagation Code  

Microsoft Academic Search

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

Susan E. Minkoff; SUSAN E

1999-01-01

152

Wave velocity dispersion and attenuation in media exhibiting internal oscillations  

NASA Astrophysics Data System (ADS)

Understanding the dynamical and acoustical behavior of porous and heterogeneous rocks is of great importance in geophysics, e.g. earthquakes, and for various seismic engineering applications, e.g. hydrocarbon exploration. Within a heterogeneous medium oscillations with a characteristic resonance frequency, depending on the mass and internal length of the heterogeneity, can occur. When excited, heterogeneities can self-oscillate with their natural frequency. Another example of internal oscillations is the dynamical behavior of non-wetting fluid blobs or fluid patches in residually saturated pore spaces. Surface tension forces or capillary forces act as the restoring force that drives the oscillation. Whatever mechanism is involved, an oscillatory phenomena within a heterogeneous medium will have an effect on acoustic or seismic waves propagating through such a medium, i.e. wave velocity dispersion and frequency-dependent attenuation. We present two models for media exhibiting internal oscillations and discuss the frequency-dependent wave propagation mechanism. Both models give similar results: (1) The low-frequency (i.e. quasi-static) limit for the phase velocity is identical with the Gassmann-Wood limit and the high-frequency limit is larger than this value and (2) Around the resonance frequency a very strong phase velocity change and the largest attenuation occurs. (1) Model for a homogeneous medium exhibiting internal oscillations We present a continuum model for an acoustic medium exhibiting internal damped oscillations. The obvious application of this model is water containing oscillating gas bubbles, providing the material and model parameters for this study. Two physically based momentum interaction terms between the two inherent constituents are used: (1) A purely elastic term of oscillatory nature that scales with the volume of the bubbles and (2) A viscous term that scales with the specific surface of the bubble. The model is capable of taking into account an arbitrary number of oscillators with different resonance frequencies. Exemplarily, we show a log-normal distribution of resonance frequencies. Such a distribution changes the acoustic properties significantly compared to the case with only one resonance frequency. The dispersion and attenuation resulting from our model agree well with the dispersion and attenuation (1) derived with a more exact mathematical treatment and (2) measured in laboratory experiments. (2) Three-phase model for residually saturated porous media We present a three-phase model describing wave propagation phenomena in residually saturated porous media. The model consists of a continuous non-wetting phase and a discontinuous wetting phase and is an extension of classical biphasic (Biot-type) models. The model includes resonance effects of single liquid bridges or liquid clusters with miscellaneous eigenfrequencies taking into account a visco-elastic restoring force (pinned oscillations and/or sliding motion of the contact line). In the present investigation, our aim is to study attenuation due to fluid oscillations and due to wave-induced flow with a macroscopic three-phase continuum model, i.e. a mixture consisting of one solid constituent building the elastic skeleton and two immiscible fluid constituents. Furthermore, we study monochromatic waves in transversal and longitudinal direction and discuss the resulting dispersion relations for a typical reservoir sandstone equivalent (Berea sandstone).

Frehner, Marcel; Steeb, Holger; Schmalholz, Stefan M.

2010-05-01

153

Experimental velocities and accelerations in very steep wave events in deep water  

Microsoft Academic Search

The entire experimental velocity and acceleration fields in the six steepest cases of a campaign of totally 122 large wave events in deep water are documented. From observations in these six waves, totally 36000 experimental velocity vectors are put on non-dimensional form using a suitable reference velocity defined by ?g\\/k, where k and ? are obtained as follows: from the

John Grue; Atle Jensen

2006-01-01

154

Global Shear Wave Velocity Database for Probabilistic Assessment of the Initiation of Seismic-Soil Liquefaction  

Microsoft Academic Search

Engineering practitioners commonly use penetration-based methods (SPT & CPT) for assessment of seismic liquefaction triggering hazard. On the horizon, shear wave velocity (Vs) may offer engineers a third tool that is lower cost and provides more physically meaningful measurements. Development of the shear wave velocity liquefaction method has been hampered by a paucity of published velocity profiles; particularly in deeper

Robert Kayen; Raymond B. Seed; Robb E. S. Moss; Onder K. Cetin; Y. Tanaka; Kohji Tokimatsu

2004-01-01

155

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

156

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

157

New constraints on the arctic crust and uppermost mantle: surface wave group velocities, Pn, and Sn  

Microsoft Academic Search

We present the results of a study of surface wave dispersion across the Arctic region (>60°N) and compare the estimating group velocity maps with new maps of the body wave phases Pn and Sn. Data recorded at about 250 broadband digital stations from several global and regional networks were used to obtain Rayleigh and Love wave group velocity measurements following

A. L. Levshin; M. H. Ritzwoller; M. P. Barmin; A. Villaseñor; C. A. Padgett

2001-01-01

158

Mineralization, crystallography, and longitudinal seismic wave velocity of speleothems  

SciTech Connect

Speleothems, or cave deposits, of Fisher Cave in Sullivan, Missouri, display unique arrangements of overlapping and interwoven crystals that provide the foundation for intricate shapes and patterns. Research has found that the deposition of such forms are affected by many variables associated with carbonate petrology. An active cave system having a consistent yearly rate of water flow will deposit trace, non-carbonate mineralization at locations of initial contact with the cave environment. Deposit specimens were tested using X-ray diffraction methods resulting in the identification of a manganese oxide coating of a psilomelane. Speleothems, considered a natural resource, are protected along with the cave from industrial advancements outside the cave environment. When the detonation of explosives during the construction of a highway near Crystal Cave in Springfield, Missouri threatened the speleothegenic decoration, a study of the damaging resonant frequency of the speleothems was instigated. To calculate this frequency, the longitudinal and transverse seismic wave velocity was measured by geophysical techniques.

Williams, R.S.; Grant, S.K. (Univ. of Missouri, Rolla, MO (United States). Dept. of Geology and Geophysics); Haas, C.J. (Univ. of Missouri, Rolla, MO (United States). Dept. of Mining Engineering)

1993-03-01

159

Distinct Velocity variations around the Base of the Upper Mantle beneath Northeastern Asia  

Microsoft Academic Search

Both the global and regional P wave tomographic studies have revealed low-velocity anomalies beneath the high velocity descending slabs in the mantle transition zone and uppermost lower mantle under a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, makes it difficult to substantiate this feature and evaluate accurately

T. Wang; L. Chen

2007-01-01

160

Petrologic Constraints on Seismic Velocity Variations in the Upper Mantle Beneath Southern Africa  

Microsoft Academic Search

Three-dimensional tomographic images of seismic velocity perturbations obtained from analyses of P-wave and S-wave data from the southern Africa seismic experiment show that seismic velocities are about 0.5 to 1.0% higher in the upper mantle beneath Archean cratons relative to the upper mantle off-craton and in modified craton. We examine these seismic velocity perturbations quantitatively in terms of mineralogical and

D. E. James; R. W. Carlson; F. R. Boyd; P. E. Janney

2001-01-01

161

Instantaneous phase estimation to measure weak velocity variations: application to noise correlation on seismic data at the exploration scale  

NASA Astrophysics Data System (ADS)

Passive imaging from noise cross-correlation is a consolidated analysis applied at continental and regional scale whereas its use at local scale for seismic exploration purposes is still uncertain. The development of passive imaging by cross-correlation analysis is based on the extraction of the Green’s function from seismic noise data. In a completely random field in time and space, the cross-correlation permits to retrieve the complete Green’s function whatever the complexity of the medium. At the exploration scale and at frequency above 2 Hz, the noise sources are not ideally distributed around the stations which strongly affect the extraction of the direct arrivals from the noise cross-correlation process. In order to overcome this problem, the coda waves extracted from noise correlation could be useful. Coda waves describe long and scattered paths sampling the medium in different ways such that they become sensitive to weak velocity variations without being dependent on the noise source distribution. Indeed, scatters in the medium behave as a set of secondary noise sources which randomize the spatial distribution of noise sources contributing to the coda waves in the correlation process. We developed a new technique to measure weak velocity changes based on the computation of the local phase variations (instantaneous phase variation or IPV) of the cross-correlated signals. This newly-developed technique takes advantage from the doublet and stretching techniques classically used to monitor weak velocity variation from coda waves. We apply IPV to data acquired in Northern America (Canada) on a 1-km side square seismic network laid out by 397 stations. Data used to study temporal variations are cross-correlated signals computed on 10-minutes ambient noise in the frequency band 2-5 Hz. As the data set was acquired over five days, about 660 files are processed to perform a complete temporal analysis for each stations pair. The IPV permits to estimate the phase shift all over the signal length without any assumption on the medium velocity. The instantaneous phase is computed using the Hilbert transform of the signal. For each stations pair, we measure the phase difference between successive correlation functions calculated for 10 minutes of ambient noise. We then fit the instantaneous phase shift using a first-order polynomial function. The measure of the velocity variation corresponds to the slope of this fit. Compared to other techniques, the advantage of IPV is a very fast procedure which efficiently provides the measure of velocity variation on large data sets. Both experimental results and numerical tests on synthetic signals will be presented to assess the reliability of the IPV technique, with comparison to the doublet and stretching methods.

Corciulo, M.; Roux, P.; Campillo, M.; Dubucq, D.

2010-12-01

162

Monitoring stress related velocity variation in concrete with a 2.10-5 relative resolution using diffuse ultrasound  

NASA Astrophysics Data System (ADS)

Ultrasonic waves propagating in solids have stress-dependent velocities. The relation between stress (or strain) and velocity forms the basis of non-linear acoustics. In homogeneous solids, conventional time-of-flight techniques have measured this dependence with spectacular precision. In heterogeneous media like concrete, the direct (ballistic) wave around 500 kHz is strongly attenuated and conventional techniques are less efficient. In this manuscript, the effect of weak stress changes on the late arrivals constituting the acoustic diffuse coda is tracked. A resolution of 2.10-5 in relative velocity change is attained which corresponds to a sensitivity to stress change of better than 50 kPa. Therefore the technique described here provides an original way to measure the non-linear parameter with stress variations on the order of tens of kPa. From E. Larose and Stephen Hall, J. Acoust. Soc. Am. 125 1853-1857 (2009)

Larose, E. F.; Hall, S.

2009-12-01

163

A feasibility study to monitor variations of seismic velocities due to the Sequestration of CO2 with ambient seismic noise  

NASA Astrophysics Data System (ADS)

Monitoring of the subsurface becomes more and more a great interest. We work with data from a seismic network around the test-site for CO2 sequestration in Ketzin (Brandenburg, Germany), where CO2 is injected into a saline aquifer in about 650 m depths. Monitoring the expansion of the CO2-plume is essential not only for the detection of a possible leakage but also for a characterization of the reservoir. In contrast to expensive active seismics we investigate the potential of a continuous monitoring with the use of ambient seismic noise. With cross-correlating ambient seismic noise at two receivers, it is possible to reconstruct the Green's Function which describes the propagation of a seismic wave between the two receivers. Variations in the seismic velocities directly affect the shape of the cross-correlations. To detect small temporal changes in the seismic velocities, we use scattered waves, that can be identified at later parts in the coda. Because of their long propagation time, these waves are highly sensitive to small changes. Due to the almost continuous injection of CO2 we expect a monotonic decrease of the seismic velocities. By cross-correlating about 3 years of data from the beginning of the injection, we observed periodic velocity variations with a period of approximately one year that can't be caused by the CO2 injection. Based on an amplitude decrease of the observed velocity changes when using time-windows in the later part of the coda, we conclude the variations originate in the shallow subsurface and we suspect a meteorological effect. Annual variations are, however, observed in many environmental processes including the generation of seismic noise. To exclude the possibility that variations of the noise-field itself can cause the apparent velocity variations, we analyzed the prominent direction of the noise. The analysis showed a perfect stability, and therefor excludes the noise-field as a cause for the velocity variations. Though the observed periodic changes affect only the shallow subsurface, they mask potential signals of material changes from the reservoir depths.

Gassenmeier, M.; Sens-Schönfelder, C.; Korn, M.

2013-12-01

164

The core shadow zone boundary and lateral variations of the P velocity structure of the lowermost mantle  

Microsoft Academic Search

The recent determination of high-quality, short-period P-wave amplitude profiles near the core shadow zone for three source-receiver combinations allows an exploration of lateral variations in P velocity structure at the base of the mantle. Various radially symmetric models are tested by comparison of the data with amplitudes measured from generalized ray theory synthetics. The assumption necessary to justify one-dimensional modeling-that

Christopher J. Young; Thorne Lay

1989-01-01

165

Constraints on Crustal Shear Wave Velocity Structure beneath Central Tibet from 3-D Multi-scale Finite-frequency Rayleigh Wave Travel-time Tomography  

NASA Astrophysics Data System (ADS)

Surface wave travel-time tomography has been widely used as a powerful strategy to image shear wave velocity structure of the Earth's crust and upper mantle, providing comparable information other than body wave tomography. Traditionally, lateral variations of dispersive phase velocities are first obtained at multiple frequencies and then used to invert for shear wave velocity with 1-D depth-dependent sensitivity kernels. However, this approach runs short on considering the directional- and depth-dependence of scattering while surface wave propagating through laterally heterogeneous Earth. To refrain from these shortcomings, we here provide a fully 3-D finite-frequency method based on the Born scattering theory formulated with surface wave mode summation, and apply it to regional fundamental Rayleigh wave travel-time tomography in central Tibet. Our data were collected from Project Hi-CLIMB, which deployed an N-S trending linear array of over 100 broadband seismic stations with a large aperture of 800 km and very dense spacing of ~3-8 km across the Lhasa and Qiangtang terranes during 2004-2005. We follow a standard procedure of ambient noise cross correlation to extract empirical Green's functions of fundamental Rayleigh waves at 10-33 s between station pairs. A multi-taper method is employed to measure the phase differences as a function of period between observed and synthetic Rayleigh waves as well as the corresponding sensitivity kernels for the measured phase delays to 3-D shear wave velocity perturbations in a spherically-symmetric model suitable for central Tibet. A wavelet-based, multi-scale parameterization is invoked in the tomographic inversion to deal with the intrinsically multi-scale nature of unevenly distributed data and resolve the structure with data-adaptive spectral and spatial resolutions. The preliminary result shows that to the north of the Banggong-Nujiang suture (BNS), the crustal shear wave velocity beneath the Qiangtang terrane is generally very slow. The velocities beneath the Lhasa terrane appear to be relatively higher. The distinct velocity contrast across the BNS has also been observed previously in the tomographic models constrained by body-wave travel times. The absence of pervasive low velocity anomalies in the mid-to-lower crust revealed in our model indicates that the ductile channel flow of the lower crust is not active beneath southern Tibet. By integrating longer-period surface data from distant earthquakes, we will improve the model resolution in the lithospheric mantle and provide better constrains on the geodynamic process of the Himalayan-Tibetan orogeny involved in both the crust and lithospheric mantle.

Jheng, Y.; Hung, S.; Zhou, Y.; Chang, Y.

2012-12-01

166

A wave propagation model for the high velocity impact response of a composite sandwich panel  

Microsoft Academic Search

A solution methodology to predict the residual velocity of a hemispherical-nose cylindrical projectile impacting a composite sandwich panel at high velocity is presented. The term high velocity impact is used to describe impact scenarios where the projectile perforates the panel and exits with a residual velocity. The solution is derived from a wave propagation model involving deformation and failure of

Michelle S. Hoo Fatt; Dushyanth Sirivolu

2010-01-01

167

Feasibility of waveform inversion of Rayleigh waves for shallow shear-wave velocity using a genetic algorithm  

USGS Publications Warehouse

Conventional surface wave inversion for shallow shear (S)-wave velocity relies on the generation of dispersion curves of Rayleigh waves. This constrains the method to only laterally homogeneous (or very smooth laterally heterogeneous) earth models. Waveform inversion directly fits waveforms on seismograms, hence, does not have such a limitation. Waveforms of Rayleigh waves are highly related to S-wave velocities. By inverting the waveforms of Rayleigh waves on a near-surface seismogram, shallow S-wave velocities can be estimated for earth models with strong lateral heterogeneity. We employ genetic algorithm (GA) to perform waveform inversion of Rayleigh waves for S-wave velocities. The forward problem is solved by finite-difference modeling in the time domain. The model space is updated by generating offspring models using GA. Final solutions can be found through an iterative waveform-fitting scheme. Inversions based on synthetic records show that the S-wave velocities can be recovered successfully with errors no more than 10% for several typical near-surface earth models. For layered earth models, the proposed method can generate one-dimensional S-wave velocity profiles without the knowledge of initial models. For earth models containing lateral heterogeneity in which case conventional dispersion-curve-based inversion methods are challenging, it is feasible to produce high-resolution S-wave velocity sections by GA waveform inversion with appropriate priori information. The synthetic tests indicate that the GA waveform inversion of Rayleigh waves has the great potential for shallow S-wave velocity imaging with the existence of strong lateral heterogeneity. ?? 2011 Elsevier B.V.

Zeng, C.; Xia, J.; Miller, R. D.; Tsoflias, G. P.

2011-01-01

168

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

169

Determinants of brachial-ankle pulse wave velocity and carotid-femoral pulse wave velocity in healthy koreans.  

PubMed

The aim of this study was to determine the normal value of brachial-ankle pulse wave velocity (baPWV) and carotid-femoral pulse wave velocity (cfPWV) according to age group, gender, and the presence of cardiovascular risk factors in healthy Koreans, and to investigate the association between PWV and risk factors such as prehypertension, dyslipidemia, smoking, and obesity. We measured an arterial stiffness in 110 normal subjects who were 20 to 69 yr-old with no evidence of cardiovascular disease, cerebrovascular accident or diabetes mellitus. The mean values of baPWV and cfPWV were 12.6 (±2.27) m/sec (13.1±1.85 in men, 12.1±2.51 in women; P=0.019) and 8.70 (±1.99) m/sec (9.34±2.13 in men, 8.15±1.69 in women; P=0.001), respectively. The distribution of baPWV (P<0.001) and cfPWV (P=0.006) by age group and gender showed an increase in the mean value with age. Men had higher baPWV and cfPWV than women (P<0.001). There was a difference in baPWV and cfPWV by age group on prehypertension, dyslipidemia, current smoking, or obesity (P<0.001). In multiple linear regression, age and prehypertension were highly associated with baPWV and cfPWV after adjustment for confounding factors (P<0.001). The present study showed that baPWV and cfPWV are associated with age, gender, and prehypertension in healthy Koreans. PMID:24932081

Jang, Shin Yi; Ju, Eun Young; Huh, Eun Hee; Kim, Jung Hyun; Kim, Duk-Kyung

2014-06-01

170

Velocity of spherically-diverging detonation waves in RX26AF, LX17 and LX10  

Microsoft Academic Search

The velocity of spherically-diverging detonation waves was measured in RX-26-AF, LX-17 and LX-10 explosives at 20°C and -54°C for detonation wave radii from 13 to 50 mm. At the smaller radii the measured velocities were lower than published steady detonation velocities by as much as 4%. The detonation velocities measured at -54°C were usually higher than those measured at room

K. L. Bahl; R. S. Lee; R. C. Weingart

1983-01-01

171

Spatial correlation of shear-wave velocity within San Francisco Bay Sediments  

USGS Publications Warehouse

Sediment properties are spatially variable at all scales, and this variability at smaller scales influences high frequency ground motions. We show that surface shear-wave velocity is highly correlated within San Francisco Bay Area sediments using shear-wave velocity measurements from 210 seismic cone penetration tests. We use this correlation to estimate the surface sediment velocity structure using geostatistics. We find that the variance of the estimated shear-wave velocity is reduced using ordinary kriging, and that including this velocity structure in 2D ground motion simulations of a moderate sized earthquake improves the accuracy of the synthetics. Copyright ASCE 2006.

Thompson, E. M.; Baise, L. G.; Kayen, R. E.

2006-01-01

172

Variation of the upper mantle velocity structure along the central-south Andes  

NASA Astrophysics Data System (ADS)

Variations in the subduction angle of the Nazca plate beneath the South American plate has lead to different modes of deformation and volcanism along the Andean active margin. The volcanic gap between the central and southern Andean volcanic zones is correlated with the Pampean flat-slab subduction zone, where the subducting Nazca slab changes from a 30-degree dipping slab beneath the Puna plateau to a horizontal slab beneath the Sierras Pampeanas, and then to a 30-degree dipping slab beneath the south Andes from north to south. The Pampean flat-slab subduction correlates spatially with the track of the Juan Fernandez Ridge, and is associated with the inboard migration of crustal deformation. A major Pliocene delamination event beneath the southern Puna plateau has previously been inferred from geochemical and geological and preliminary geophysical data. The mechanisms for the transition between dipping- and flat-subduction slab and the mountain building process of the central Andean plateau are key issues to understanding the Andean-type orogenic process. We use a new frequency-time normalization approach with non-linear stacking to extract very-broadband (up to 300 second) empirical Green's functions (EGFs) from continuous seismic records. The long-period EGFs provide the deeper depth-sensitivity needed to constrain the mantle structure. The broadband waveform data are from 393 portable stations of four temporary networks: PUNA, SIEMBRA, CHARGE, RAMP, East Sierras Pampeanas, BANJO/SEDA, REFUCA, ANCORP, and 31 permanent stations accessed from both the IRIS DMC and GFZ GEOFON DMC. A finite difference waveform propagation method is used to generate synthetic seismograms from 3-D velocity model. We use 3-D traveltime sensitivity kernels, and traveltime residuals measurement by waveform cross-correlation to directly invert the upper mantle shear-wave velocity structure. The preliminary model shows strong along-strike velocity variations within in the mantle wedge and the subducting NAZCA slab. Low upper mantle velocities are north of 29°S and south of 35°S, corresponding to the low velocity mantle wedge of dipping-subduction. The upper mantle beneath the Sierras Pampeanas has a higher velocity than that beneath the central and south volcanic zones, which is consistent with the Pampeanas flat-slab. Though we observe substantial heterogenity within this flat-slab zone.

Liang, Xiaofeng; Sandvol, Eric; Shen, Yang; Gao, Haiying

2014-05-01

173

Interferometric methods for measurement of ultrasonic wave velocity: Classification, characteristics and basic defects  

NASA Technical Reports Server (NTRS)

Interferometric methods may be considered to include all methods in which the time necessary for a signal to pass through a medium is determined on the basis of the period of oscillation of which the signal consists, while the distance is determined by noting the coincidence between (1) interference between the measured signal and its reflection and ultrasonic interference, or between another signal serving as a reference and electrical interference in the electronic part of the device. In general, all interferometric methods are relatively simple and permit a high degree of accuracy. Theoretically 48 different variations may be distinguished, about 30 of which are capable of practical application. For liquids, where transverse waves do not exist, the number of possible variations is limited to 40, assuming that absolute velocity measurement is desired.

Kozlowski, Z.

1974-01-01

174

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

175

Shear Velocity Structure of the Tonga Arc and Lau Backarc Basin from Rayleigh Wave Tomography  

NASA Astrophysics Data System (ADS)

We analyze seismic data from the 2009 - 2010 Ridge2000 Lau Spreading Center project using the two-plane wave method of Rayleigh wave tomography to investigate the spatial distribution of melt. This project consists of 50 ocean bottom seismographs (OBSs) and 17 land-based seismic stations deployed in Fiji, Tonga, and the Lau basin for about one year. 1-D results show that a low-velocity zone exists beneath the Lau basin at the depths of 20 - 70 km, whereas the relict Lau ridge and the Fiji plateau are characterized by a high-velocity lid, implying a ~80 km-thick lithosphere of an extinct island arc. Furthermore, 3-D results indicate that the slowest velocities are located in a ~100 km wide region beneath the Central Lau Spreading Center to the north, where the spreading rate is fastest. The slow anomalies form an inclined zone, dipping to the west away from the arc. Although temperature can significantly influence seismic velocities, it cannot explain the extremely low mantle velocities (less than 3.5 km/s). Therefore, we interpret the slow anomalies in terms of partial melting, which commences at about 70 km depth in accord with petrological constraints. The asymmetric and broad distribution of inferred melt in the mantle across the Lau basin implies the passive decompression melting process governed by the mantle wedge flow pattern. The variations of velocity anomalies along the spreading centers indicate varying depths and in-situ melt contents from north to south. Slow anomalies deeper than 80 km beneath the Lau ridge suggest minor initial partial melting during asymmetric upwelling supplied by the Indian-Australian asthenosphere from west. These anomalies become shallower beneath the active volcano of Taveuni in Fiji. Smaller amplitude slow anomalies along the subduction-influenced Valu Fa ridge may indicate less in-situ melt content due to low melt viscosity and more efficient transport of the water-rich melt.aps of isotropic shear wave velocity at depths of 38, 58, 78, 98 km. The black dots and triangles represent the OBSs and the island-based seismic stations, respectively. The black curves indicate the spreading centers. The extensional transform zone (ETZ), the central Lau spreading center (CLSC), the eastern Lau spreading center (ELSC), and the Valu Fa ridge (VFR) are labeled in the map of 78 km.

Wei, S. S.; Wiens, D. A.; Webb, S. C.; Blackman, D. K.; Dunn, R.; Conder, J. A.

2012-12-01

176

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

177

Lithospheric Thickness Variations of India and the Himalaya from Seismic Surface Wave Tomography  

NASA Astrophysics Data System (ADS)

We use seismic surface wave data to determine the shear wave structure of the Indian and Tibetan lithosphere. Data come from permanent stations of the Global Seismic Network and Geoscope as well as from temporary field experiments and regional arrays. Data from the Himalayan Nepal Tibet PASSCAL Seismic Experiment (HIMNT) in particular greatly enhance the surface wave coverage across India. The shear velocity model is created from surface wave fundamental mode phase and group velocity measurements for Rayleigh and Love waves at periods of 15 to 150 seconds. The group velocity measurements are measured using the frequency- time analysis (Levshin et al., 1989). Phase velocities are from the data sets of Ekstrom et al. (1997) and Trampert and Woodhouse (1995). The measured dispersion curves are inverted to produce 2D maps of phase and group velocities for individual periods and wave types. At each geographical location, the dispersion curves are inverted to obtain a radially anisotropic 1D shear velocity model using a Monte-Carlo method (Shapiro and Ritzwoller, 2002). The resulting shear velocity model shows a fast velocity mantle lid that extends to 200 km depth beneath central India, but thins dramatically to just over 100 km thick beneath eastern India and Bangladesh. This trend continues to the north and may influence subduction style along the Himalaya. The thicker lithosphere does not appear to subduct as steeply as the thinner lithosphere to the east, suggesting that the thicker lithosphere is either more buoyant or more rigid than the thinner lithosphere. Variations in distance to the Indian flexural bulge support the hypothesis of the thicker Indian lithosphere having a greater flexural rigidity than the thin lithosphere. Deep continental earthquakes beneath southern Tibet are rare in the region of thick lithosphere, but are relatively common in the zone of thin lithosphere, perhaps due to increased bending stresses in the region of thin lithosphere.

Sheehan, A. F.; Ritzwoller, M. H.; Shapiro, N. M.

2006-12-01

178

On the Formation of Shock Waves in Subsonic Flows With Local Supersonic Velocities  

NASA Technical Reports Server (NTRS)

In the flow about a body with large subsonic velocity if the velocity of the approaching flow is sufficiently large, regions of local supersonic velocities are formed about the body. It is known from experiment that these regions downstream of the flow are always bounded by shock waves; a continuous transition of the supersonic velocity to the subsonic under the conditions indicated has never been observed. A similar phenomenon occurs in pipes. If at two cross sections of the pipe the velocity is subsonic and between these sections regions of local supersonic velocity are formed without completely occupying a single cross section, these regions are always bounded by shock waves.

Frankl, F. I.

1950-01-01

179

Shear wave velocity prediction using seismic attributes and well log data  

NASA Astrophysics Data System (ADS)

Formation's properties can be estimated indirectly using joint analysis of compressional and shear wave velocities. Shear wave data is not usually acquired during well logging, which is most likely for cost saving purposes. Even if shear data is available, the logging programs provide only sparsely sampled one-dimensional measurements: this information is inadequate to estimate reservoir rock properties. Thus, if the shear wave data can be obtained using seismic methods, the results can be used across the field to estimate reservoir properties. The aim of this paper is to use seismic attributes for prediction of shear wave velocity in a field located in southern part of Iran. Independent component analysis (ICA) was used to select the most relevant attributes to shear velocity data. Considering the nonlinear relationship between seismic attributes and shear wave velocity, multi-layer feed forward neural network was used for prediction of shear wave velocity and promising results were presented.

Gholami, Raoof; Moradzadeh, Ali; Rasouli, Vamegh; Hanachi, Javid

2014-03-01

180

ESTIMATING SUBSURFACE SHEAR WAVE VELOCITY STRUCTURE AND SITE AMPLIFICATIN CHARACTERISTICS OF PADANG, INDONESIA  

NASA Astrophysics Data System (ADS)

A shear wave velocity structure model of Padang, Indonesia was developed and the site amplification characteristics of earthquake ground motion were examined. The presented shear wave velocity structure model was obtained based on the microtremor survey. The inversion analysis of the dispersion curves of Rayleigh wave were performed for 11 sites where the array microtremor observations with 4 sensors were done and the shear wave velocity profiles were estimated for these sites. Furthermore the peak periods of HVSR for 130 sites were obtained and the distribution of the site dominant period was shown. The 3D shear wave velocity structure model was developed by considering the relation between the shear wave velocity profiles of 11 array sites and the site dominant period distribution. Finally the site amplification effect was investigated by conducting the finite element analysis.

Ono, Yusuke; Noguchi, Tatsuya; ??, ?; Rahmat Putra, Rusnardi; Uemura, Shuji; Ikeda, Tatsunori; Kiyono, Junji

181

Shear wave velocities of unconsolidated shallow sediments in the Gulf of Mexico  

USGS Publications Warehouse

Accurate shear-wave velocities for shallow sediments are important for a variety of seismic applications such as inver-sion and amplitude versus offset analysis. During the U.S. Department of Energy-sponsored Gas Hydrate Joint Industry Project Leg II, shear-wave velocities were measured at six wells in the Gulf of Mexico using the logging-while-drilling SonicScope acoustic tool. Because the tool measurement point was only 35 feet from the drill bit, the adverse effect of the borehole condition, which is severe for the shallow unconsolidated sediments in the Gulf of Mexico, was mini-mized and accurate shear-wave velocities of unconsolidated sediments were measured. Measured shear-wave velocities were compared with the shear-wave velocities predicted from the compressional-wave velocities using empirical formulas and the rock physics models based on the Biot-Gassmann theory, and the effectiveness of the two prediction methods was evaluated. Although the empirical equation derived from measured shear-wave data is accurate for predicting shear-wave velocities for depths greater than 500 feet in these wells, the three-phase Biot-Gassmann-theory -based theory appears to be optimum for predicting shear-wave velocities for shallow unconsolidated sediments in the Gulf of Mexico.

Lee, Myung W.

2013-01-01

182

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

183

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

184

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

185

Creep of porous rocks and measurements of elastic wave velocities under different hydrous conditions  

NASA Astrophysics Data System (ADS)

The long-term mechanical behavior of rocks is of prime importance for many geological hazards (e.g., landslides, rock falls, and volcanoes) as well as for the stability of man-made structures (underground mines, road cuts, and open pits). In some shallow environments, rocks exist in partially saturated conditions which can evolve with time according to variations in the relative humidity hr of the atmosphere (e.g., natural slopes, open cut excavations). In underground mines, rocks are also partially saturated because of artificial ventilation. These variations in liquid saturation may have a large impact on mechanical behavior since they imply variations in capillary pressure and, depending on the porosity and on the shape of the porous network, variations in the effective stresses. Therefore, knowledge of static fatigue under saturated and partially saturated conditions is important for estimating the long-term stability of such rock structures. Many studies have already shown that time-dependent weakening is much more important for a saturated rock than for a dry one and that the time to failure may decrease by several orders of magnitude for saturated rocks as compared to dry rocks. In addition, the weakening effect of water is more significant in long-term experiments than in short-term ones (instantaneous loading). A physical explanation for these results may be the enhancement of subcritical crack growth by stress corrosion at crack tips which is often considered to be the main cause of time-dependent behavior of rocks. The failure of brittle rocks during compression tests is preceded by the formation, growth, and coalescence of microcracks. Elastic wave velocities are reduced due to the presence of open microcraks and fractures and may be used to monitor the progressive damage of rocks. The specific experimental setup available in our lab allows the simultaneous measurement of five velocities (with different polarizations and directions) and two directions of strains on a same sample, under uniaxial compression. Different hydrous conditions (saturated or partially saturated) are tested in all creep tests. In this paper, we focus on the time-dependent behavior and short-term mechanical behavior of iron ore and limestone in saturated and partially saturated domains. The main outcomes of these experiments are: (i) identification of the apparent dynamic stiffness tensor from elastic wave velocity measurements; (ii) assessment of velocity anisotropy, and its evolution under uniaxial loading. This last step allows for the quantification of the intrinsic and stress-induced anisotropies, leading eventually to an estimation of the microcracks density and distribution evolutions in the rock sample under loading.

Eslami, J.; Grgic, D.; Hoxha, D.

2009-04-01

186

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

187

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography  

NASA Astrophysics Data System (ADS)

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ˜600 stations and ˜150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ˜5000 and ˜45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ˜1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods (<20 s), very low phase velocities are imaged in the major basins, including the Tarim, Qaidam, Junggar, and Sichuan basins, and in the Ordos Block. At intermediate and long periods (>20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3-D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

Yang, Yingjie; Zheng, Yong; Chen, John; Zhou, Shiyong; Celyan, Savas; Sandvol, Eric; Tilmann, Frederik; Priestley, Keith; Hearn, Thomas M.; Ni, James F.; Brown, Larry D.; Ritzwoller, Michael H.

2010-08-01

188

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

189

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.

Xu, Zhen J.; Song, Xiaodong

2009-01-01

190

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

191

Velocity of Spherically-Diverging Detonation Waves in RX-26-AF, LX-17 and LX-10.  

National Technical Information Service (NTIS)

The velocity of spherically-diverging detonation waves was measured in RX-26-AF, LX-17 and LX-10 explosives at 20 exp 0 C and -54 exp 0 C for detonation wave radii from 13 to 50 mm. At the smaller radii the measured velocities were lower than published st...

K. L. Bahl R. C. Weingart R. S. Lee

1983-01-01

192

Laboratory velocities and attenuation of P-waves in limestones during freeze-thaw cycles  

Microsoft Academic Search

The velocity and the attenuation of compressional P-waves, measured in the laboratory at ultrasonic frequencies during a series of freezing and thawing cycles, are used as a method for predicting frost damage in a bedded limestone. Pulse transmission and spectral ratio techniques are used to determine the P-wave velocities and the attenuation values relative to an aluminum reference samples with

Jean-Michel Remy; M. Bellanger; F. Homand-Etienne

1994-01-01

193

Correlation of liquefaction resistance with shear wave velocity based on laboratory study using bender element  

Microsoft Academic Search

Recent studies using field case history data yielded new criteria for evaluating liquefaction potential in saturated granular deposits based on in situ, stress-corrected shear wave velocity. However, the conditions of relatively insufficient case histories and limited site conditions in this approach call for additional data to more reliably define liquefaction resistance as a function of shear wave velocity. In this

ZHOU Yan-guo; CHEN Yun-min; KE Han

2005-01-01

194

Comparision between crustal density and velocity variations in Southern California  

USGS Publications Warehouse

We predict gravity from a three-dimensional Vp model of the upper crust and compare it to the observed isostatic residual gravity field. In general this comparison shows that the isostatic residual gravity field reflects the density variations in the upper to middle crust. Both data sets show similar density variations for the upper crust in areas such as the Peninsular Ranges and the Los Angeles basin. Both show similar variations across major faults, such as the San Andreas and Garlock faults in the Mojave Desert. The difference between the two data sets in regions such as the Salton Trough, the Eastern California Shear Zone, and the eastern Ventura basin (where depth to Moho is <30 km), however, suggests high-density middle to lower crust beneath these regions. Hence the joint interpretation of these data sets improves the depth constraints of crustal density variations.

Langenheim, V. E.; Hauksson, E.

2001-01-01

195

Elastic wave velocities and permeability evolution during compaction of Bleuswiller sandstone.  

NASA Astrophysics Data System (ADS)

Field observations and laboratory experiments have recently documented the formation of compaction bands in porous sandstones ([Mollema and Antonellini, 1996],[Olsson and Holcomb, 2000], [Besuelle, 2001a], [Klein et al., 2001]). It has been observed experimentally ([Wong et al., 2001], [Baud et al., 2003],[Fortin et al., 2003] that under axisymmetric compression, compaction bands develop sub-perpendicular to the main compressive stress which is predicted theoretically in the framework of strain localization theory ([Bésuelle, 2001a],[Issen and Rudnicki, 2000]). Volumetric strain, fluid transport and elastic properties are intimately coupled to one another, for they all depend on few intrinsic parameters such as the porosity, the crack density, and the matrix and fluid elastic properties. On one hand, [Scott et al., 1993] showed that elastic wave velocities were clearly affected during the deformation of porous sandstones. On the other hand, [Zhu and Wong, 1997] showed that the relation between the evolution of permeability and volumetric strain during compaction of sandstones was not straightforward. In this tudy, we present for the first time the simultaneous evolution of volumetric strain, elastic wave velocities and permeability for a set of deformation experiments of Bleuswiller sandstone. We show that, although very coherent to one another, those three sets are not systematically correlated. Indeed, inelastic compaction, whether it is distributed or localized, is accompanied by a drastic decrease of elastic wave velocities due to grain crushing, a decrease of permeability and porosity due to pore collapse. Using simple statistical physics concepts based on [Kachanov, 1993] and [Guéguen and Dienes, 1989], we try to understand and address the issue of coupling/decoupling between volumetric strain (mainly sensitive to equant porosity variations), elastic properties (mainly sensitive to crack density) and permeability (theoretically sensitive to both) during the formation of compaction bands. Finally, we show that the mineral composition of a sandstone is a key parameter controlling the effective pressure at which the onset of pore collapse P* takes place.

Fortin, J.; Schubnel, A.; Gueguen, Y.

2004-05-01

196

Upper mantle structure of South America from joint inversion of waveforms and fundamental mode group velocities of Rayleigh waves  

Microsoft Academic Search

A new tomographic S wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S and Rayleigh waveforms and fundamental mode Rayleigh wave group velocities, to better constrain upper mantle S velocity structure and Moho depth. We used ~5700 Rayleigh wave group velocity dispersion curves and 1537

Mei Feng; Suzan van der Lee; Marcelo Assumpção

2007-01-01

197

Upper mantle structure of South America from joint inversion of waveforms and fundamental mode group velocities of Rayleigh waves  

Microsoft Academic Search

A new tomographic S wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S and Rayleigh waveforms and fundamental mode Rayleigh wave group velocities, to better constrain upper mantle S velocity structure and Moho depth. We used ?5700 Rayleigh wave group velocity dispersion curves and 1537

Mei Feng; Suzan van der Lee; Marcelo Assumpção

2007-01-01

198

Resonant three-wave interaction of Holmboe waves in a sharply stratified shear flow with an inflection-free velocity profile  

NASA Astrophysics Data System (ADS)

Within the context of the well-known interpretation in terms of the wave interaction [P. G. Baines and H. Mitsudera, J. Fluid Mech. 276, 327 (1994); J. R. Carpenter et al., Phys. Fluids 22, 054104 (2010)], instability of sharply stratified (so that the vertical scale l of density variation is much smaller than the scale ? of velocity shear) flows with inflection-free velocity profiles should be treated as Holmboe's instability. In such flows with a relatively weak stratification (when the bulk Richardson number J < (l/?)3/2), eigenoscillations (i.e., Holmboe waves) have much the same phase velocities in a broad spectral range. This creates favorable conditions for a wide variety of three-wave interactions, in contrast to the homogeneous boundary layers where subharmonic resonance is the only effective three-wave process. In the paper, evolution equations are derived which describe three-wave interactions of Holmboe waves and have the form of nonlinear integral equations. Analytical and numerical methods are both used to find their solutions in different cases, and it is shown that at the nonlinear stage disturbances increase, as a rule, explosively. Some possible relations of the results obtained with those of numerical simulations and laboratory experiments are briefly discussed.

Churilov, S. M.

2011-11-01

199

Positive phase-velocity tapering of broadband helix traveling-wave tubes for efficiency enhancement  

NASA Astrophysics Data System (ADS)

A positive phase-velocity tapering of 1.5 octave broadband helix traveling-wave tubes for efficiency enhancement, where the phase velocity is linearly increased in the output section, was studied by using the one-dimensional nonlinear theory. At high frequencies, the electromagnetic wave in the positively tapered section traps the fastest electrons in the decelerating electric field, extracting more energy from the electron beam. At low frequencies, a decreased velocity difference between the electron beam and the electromagnetic wave destroys the phase condition for second-harmonic generation, retaining fundamental wave efficiency as well as reducing its second-harmonic power.

Jung, Sun-Shin; Soukhov, Andrei V.; Jia, Baofu; Park, Gun-Sik

2002-04-01

200

Velocity of spherically-diverging detonation waves in RX-26-AF, LX-17 and LX-10  

SciTech Connect

The velocity of spherically-diverging detonation waves was measured in RX-26-AF, LX-17 and LX-10 explosives at 20/sup 0/C and -54/sup 0/C for detonation wave radii from 13 to 50 mm. At the smaller radii the measured velocities were lower than published steady detonation velocities by as much as 4%. The detonation velocities measured at -54/sup 0/C were usually higher than those measured at room temperature. These results indicate that a significant part of the excess transit time observed when an explosive is initiated by a point or hemispherical detonator may be due to a low detonation velocity.

Bahl, K.L.; Lee, R.S.; Weingart, R.C.

1983-07-15

201

Upper mantle P wave velocity structure of the northern part of China and Mongolia  

Microsoft Academic Search

The average upper mantle P wave velocity structure and lateral heterogeneity in the northern part of China and Mongolia are\\u000a investigated by waveform inversion of broadband body waveform data recorded by CDSN and digital stations around China. The\\u000a average model has a low P wave velocity lid (about 7.8–8.0 km·s?1) with thickness about 60 km, and two discontinuities with velocity

Jian-Ping Wu; Rong-Sheng Zeng; Yue-Hong Ming

1998-01-01

202

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

203

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

204

Time varying velocity structures in Earth's outer core: Constraints from exotic P-waves  

NASA Astrophysics Data System (ADS)

The outer core is one of the most dynamic divisions of our planet. However, despite undergoing vigorous convection, the outer core is not necessarily a uniform, homogeneous layer of the Earth. Accumulation of light element enriched iron at the top of the outer core, below the core-mantle boundary, may lead to the formation of a stably stratified layer, corresponding to the E' layer as defined by Bullen. The E' layer would have different properties to the rest of the outer core and may be a source of scattering. The lowermost outer core, the F layer, may also have different physical properties than the rest of the outer core, either due to the crystallisation of iron or the release of light elements as the inner core grows. Time varying structure in the Earth's core has been observed in some previous studies, particularly using earthquake doublets. The vigorous convection in the outer core may lead to small-scale lateral variations in its velocity structure over time, due to the movement of fluids and slurry near to the core-mantle and inner core boundaries. We investigate the velocity and attenuation structure of the upper 1500 km of the outer core using high frequency PmKP seismic phases. PmKP waves travel as P-waves throughout the Earth, bouncing m-1 times on the underside of the core-mantle boundary. By analysing the relative arrival times and amplitudes of the PmKP waves and other seismic phases, and comparing these to synthetic waveforms, it is possible to constrain the velocity and attenuation characteristics of the upper 1500 km of the outer core. We correct for known mantle structure and explore the effects of core-mantle boundary topography. To investigate the scattering characteristics of the uppermost outer core and the sharpness of any stratified layers we search for precursors to PmKP phases, which are elusive. P4KP-PcP differential travel times suggest that the uppermost 1300 km of the outer core is up to 0.4% slower than PREM. There is some evidence for time variations in the velocity structure of Earth's outer core from P4KP-PcP differential travel times, indicating that there may be transient features in the uppermost outer core.

Day, E. A.; Irving, J. C.; Deuss, A. F.; Cormier, V. F.

2011-12-01

205

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

206

Diabetic retinopathy is associated with pulse wave velocity, not with the augmentation index of pulse waveform  

Microsoft Academic Search

BACKGROUND: To investigate the clinical differences between pulse wave velocity and augmentation index in diabetic retinopathy. METHODS: The subjects were 201 patients with type 2 diabetes. These subjects were measured for both augmentation index (AI) and brachial-ankle pulse wave velocity (baPWV) by a pulse wave analyzer. The relationships between AI, baPWV, and diabetic retinopathy were examined. RESULTS: BaPWV was significantly

Osamu Ogawa; Kiyoko Hiraoka; Takahiro Watanabe; Junichiro Kinoshita; Masahiko Kawasumi; Hidenori Yoshii; Ryuzo Kawamori

2008-01-01

207

Tidally induced velocity variations of the Beardmore Glacier, Antarctica, and their representation in satellite measurements of ice velocity  

NASA Astrophysics Data System (ADS)

Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, both linearly and non-linearly. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge, which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, the importance of which is highlighted here in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of >50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This variability decreases rapidly to <5% only 15 km upstream of the grounding line. Daily fluctuations are smoothed to <1% in the 11-day repeat pass TerraSAR-X imagery, but fortnightly variations over this period are still visible and show that satellite-velocity measurements can be affected by tides over longer periods. The measured tidal displacement observed in radar look direction over floating ice also allows the grounding line to be identified, using differential speckle tracking where phase information cannot be easily unwrapped.

Marsh, O. J.; Rack, W.; Floricioiu, D.; Golledge, N. R.; Lawson, W.

2013-09-01

208

Velocity selective trapping of atoms in a frequency-modulated standing laser wave: wave function and stochastic trajectory approaches  

NASA Astrophysics Data System (ADS)

The wave function of a moderately cold atom in a stationary near-resonant standing light wave delocalizes very fast due to wave packet splitting. However, frequency modulation of the field may suppress packet splitting for some atoms having specific velocities in a narrow range. These atoms remain localized in a small space for a long time. We show that modulated field can not only trap, but also cool the atoms. We perform a numerical experiment with a large atomic ensebmble having wide initial velocity and energy distribution. During the experiment, most of atoms leave the wave while trapped atoms have narrow energy distribution

Argonov, Victor

2014-03-01

209

Left ventricular ejection time, not heart rate, is an independent correlate of aortic pulse wave velocity.  

PubMed

Several studies showed a positive association between heart rate and pulse wave velocity, a sensitive marker of arterial stiffness. However, no study involving a large population has specifically addressed the dependence of pulse wave velocity on different components of the cardiac cycle. The aim of this study was to explore in subjects of different age the link between pulse wave velocity with heart period (the reciprocal of heart rate) and the temporal components of the cardiac cycle such as left ventricular ejection time and diastolic time. Carotid-femoral pulse wave velocity was assessed in 3,020 untreated subjects (1,107 men). Heart period, left ventricular ejection time, diastolic time, and early-systolic dP/dt were determined by carotid pulse wave analysis with high-fidelity applanation tonometry. An inverse association was found between pulse wave velocity and left ventricular ejection time at all ages (<25 years, r(2) = 0.043; 25-44 years, r(2) = 0.103; 45-64 years, r(2) = 0.079; 65-84 years, r(2) = 0.044; ? 85 years, r(2) = 0.022; P < 0.0001 for all). A significant (P < 0.0001) negative but always weaker correlation between pulse wave velocity and heart period was also found, with the exception of the youngest subjects (P = 0.20). A significant positive correlation was also found between pulse wave velocity and dP/dt (P < 0.0001). With multiple stepwise regression analysis, left ventricular ejection time and dP/dt remained the only determinant of pulse wave velocity at all ages, whereas the contribution of heart period no longer became significant. Our data demonstrate that pulse wave velocity is more closely related to left ventricular systolic function than to heart period. This may have methodological and pathophysiological implications. PMID:24052034

Salvi, Paolo; Palombo, Carlo; Salvi, Giovanni Matteo; Labat, Carlos; Parati, Gianfranco; Benetos, Athanase

2013-12-01

210

Radial anatomic variation of ultrasonic velocity in human cortical bone.  

PubMed

Quantitative ultrasound techniques can be used to retrieve cortical bone quality. The aim of this study was to investigate the anatomic variations in speed of sound (SOS) in the radial direction of cortical bone tissue. SOS measurements were realized in 17 human cortical bone samples with a 3.5-MHz transverse transmission device. The radial dependence of SOS was investigated in a direction perpendicular to the periosteum. For each sample, bone porosity was measured using an X-ray micro-computed tomography device. The mean SOS was 3586 ± 255 m/s. For 16 of 17 specimens, similar radial variations in SOS were observed. In the periosteal region, SOS first decreased in the direction of the endosteum and reached a minimum value approximately in the middle of the cortical bone. SOS then increased, moving to the endosteal region. A significant negative correlation was obtained between SOS and porosity (R = -0.54, p = 0.02). PMID:23969161

Mathieu, Vincent; Chappard, Christine; Vayron, Romain; Michel, Adrien; Haïat, Guillaume

2013-11-01

211

Using CAD variation goemetric approach solving velocity\\/acceleration of a 4SPS &UPU parallel manipulator  

Microsoft Academic Search

A novel 4SPS&UPU parallel manipulator is proposed. Its pose, velocity, and acceleration are solved by using a CAD variation geometric approach. First, a novel 4SPS&UPU simulation mechanism and its four isomeric simulation mechanisms are created by using CAD variation geometric techniques, and their kinematic characteristics are analyzed. Second, a velocity\\/acceleration simulation mechanism of the 4SPS&UPU parallel manipulator is created. Third,

Yi Lu; Tatu Leinonen

2007-01-01

212

Model for the relation between shock velocity and particle velocity in weak shock waves in metals  

SciTech Connect

The weak-shock theory of D. C. Wallace (Phys. Rev. B {bold 22}, 1487 (1980)) is used to calculate shock velocities as a function of particle velocity. The calculated shock velocities are based on thermoelastic behavior, plasticity behavior, and precursor behavior. The results are calculated and compared with data for 6061T6 Al, Be, Cu, Fe, 21-6-9 stainless steel, and U. Except for Fe, the agreement is reasonable. The contributions to the shock velocity from material strength and precursor behavior are rather small but, nevertheless, are calculated fairly well with the present theory.

Tonks, D.L. (Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico (USA))

1991-10-15

213

Lithostratigraphy and shear-wave velocity in the crystallized Topopah Spring Tuff, Yucca Mountain, Nevada  

USGS Publications Warehouse

Evaluation of the potential future response to seismic events 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 (Vs), 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 Vs values compared to samples from lithophysal zones. Some samples have Vs values that are outside 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 Vs data from small-scale samples (typical and "flawed" core) to larger scale transects 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.

Buesch, D. C.; Stokoe, II, K. H.; Won, K. C.; Seong, Y. J.; Jung, J. L.; Schuhen, M. D.

2006-01-01

214

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

215

Lithospheric structure of the Chinese mainland determined from joint inversion of regional and teleseismic Rayleigh-wave group velocities  

NASA Astrophysics Data System (ADS)

We processed a large number of vertical-component seismograms recorded by broadband seismic stations in and around China and ultimately retrieved ˜33 140 regional and ˜10 360 teleseismic fundamental-mode group-velocity measurements at 40 s periods, with fewer measurements for shorter and longer periods. We directly inverted the processed group-velocity measurements for a three-dimensional lithospheric S-velocity model of the Chinese mainland. Synthetic test results and data fit or misfit analysis demonstrated the reliability of our surface-wave tomographic inversion. The imaged upper-crustal low velocities are consistent with variations in sediment thickness; for example, the Tarim Basin, which contains a great thickness of sediments, is characterized by a strong, shallow, low-velocity anomaly. High lithospheric velocities are observed to varying depths in the North China Craton, Yangtze Craton, and Tarim Craton, indicating varying thicknesses of the lithosphere beneath these cratonic areas. Low asthenospheric velocities are widely imaged in eastern and southern China, around the Tarim Basin, and along a roughly north-south belt in central China. The low asthenospheric velocities in eastern and southern China are attributed to partial melting of a subducted slab associated with the westward subduction of an oceanic plate, while those in central China and around the Tarim Basin are attributed to Indian-Eurasian collision.

Feng, Mei; An, Meijian

2010-06-01

216

Kinetic Theory for Electrostatic Waves Due to Transverse Velocity Shears.  

National Technical Information Service (NTIS)

Shear in the flow velocity of a fluid leads to the low frequency and long wavelength Kelvin-Helmholtz (K-H) instability. The velocity shear can be generated in a number of ways. In a plasma the existence of an inhomogeneous electric field component transv...

G. Ganguli Y. C. Lee P. J. Palmadesso

1988-01-01

217

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

218

Solitary Waves of the MRLW Equation by Variational Iteration Method  

SciTech Connect

In a recent publication, Soliman solved numerically the modified regularized long wave (MRLW) equation by using the variational iteration method (VIM). In this paper, corrected numerical results have been computed, plotted, tabulated, and compared with not only the exact analytical solutions but also the Adomian decomposition method results. Solitary wave solutions of the MRLW equation are exactly obtained as a convergent series with easily computable components. Propagation of single solitary wave, interaction of two and three waves, and also birth of solitons have been discussed. Three invariants of motion have been evaluated to determine the conservation properties of the problem.

Hassan, Saleh M. [Department of Mathematics, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451 (Saudi Arabia); Department of Mathematics, College of Science, Ain Shams University, Abbassia 11566, Cairo (Egypt); Alamery, D. G. [Department of Mathematics, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451 (Saudi Arabia)

2009-09-09

219

S-Wave Velocity Structure of the High Lava Plains, Oregon, from Rayleigh-Wave Inversion  

NASA Astrophysics Data System (ADS)

The High Lava Plains (HLP) "hotspot" track is a prominent volcanic lineament that trends SE-NW from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. The HLP volcanism is a rough mirror image both temporally and geographically of the Snake River Plain (SRP) Yellowstone track to the east. Unlike the SRP, however, HLP volcanism remains an enigma, apparently unrelated to motion of the North American plate. Moreover, many of the tectonic or physiographic features that characterize the SRP volcanic province are not mimicked in HLP terrane. While the recently initiated HLP Seismic Experiment will ultimately consist of a dense array of about 170 broadband stations for high-resolution imaging of the mantle beneath the HLP and adjacent terranes, the preliminary analysis reported here is based on a relatively sparse array (the 16 stations of the current HLP deployment, as well as nearby stations from USArray, the USNSN, and the Berkeley network) and is intended to highlight regional contrasts in uppermost mantle structure. We image the upper-mantle velocity structure beneath the High Lava Plains and adjacent tectonic provinces by analyzing fundamental-mode Rayleigh-wave phase velocities regionalized along a series of two-station propagation paths that lie within and adjacent to the HLP. The resulting dispersion curves, which typically give robust results over the period range 40--90 seconds (and in some cases to as high as 140 seconds), cross the region along several different azimuths and allow relatively localized velocity anomalies to be isolated and quantified. Directly beneath the HLP, we find very low velocities with no significant mantle lid. Uppermost mantle velocities in the HLP region appear to average about 4.2 km/s, indicating high mantle temperatures and perhaps at least a small degree of partial melting. The region to the southwest of the HLP exhibits an upper-mantle lid nearly 100 km thick that is underlain by a low-velocity zone (v_s ~ 4.2 km/s) extending to at least 150--200 km depth.

Snoke, J. A.; Warren, L. M.; James, D. E.

2006-12-01

220

Effect of cracks on the pressure dependence of P wave velocities in crystalline rocks  

NASA Astrophysics Data System (ADS)

To test the "bed of nails" model, we have made detailed measurements of P wave velocities in five low-porosity, crystalline rocks at effective pressures to 500 MPa and fit two equations based on the model to the laboratory data. The first equation, V(P) = V0(1 + P/ Pi)(1 - m)/2, applies at relatively low pressures because it assumes that the grain modulus is very much larger than the crack modulus. It can be fit to four of the five data sets. The fit to the data for a monomineralic epidote yields values for V0, Piand m of 8.02±0.02 km/s, 1.2±0.5 MPa, and 0.9845±0.0004, respectively, with a rms error of 6.28 m/s. The second equation, 1/V2 (P) = (1/Vc2 - Vg2)/(1 + P/Pi)1 - m + 1/Vg2 assigns a constant velocity to the grains and applies when the modulus of the cracks is of the order of the grain modulus at high pressures. This equation can be fit to three of the data sets; the fit to data for a diopside pyroxenite yields values of Vc, Vg, Pi, and m of 6.20±0.04 km/s, 8.28±0.02 km/s, 7±1 MPa, and 0.20±0.05, with a rms error of 17.9 m/s. For all seven fits to the laboratory data the rms errors range from 0.1 to 0.3% and are of the order of the limits of precision of the measurements. The "bed of nails" model explains the pressure dependence of P wave velocities in the samples remarkably well, as evidenced by the small rms errors. The variation with pressure of P wave velocities in these rocks clearly reflects the increasing stiffness of cracks. The fact that the first equation fits four of five data sets is one of several indications that cracks significantly affect the mechanical properties of the rocks even at 500 MPa. Finally, we note that different kinds of cracks have markedly different mechanical properties; the best fitting model parameters reflect the nature of the cracks which populate the samples.

Carlson, Richard L.; Gangi, Anthony F.

1985-09-01

221

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

222

Estimation of shear wave velocity profiles by the inversion of spatial autocorrelation coefficients  

NASA Astrophysics Data System (ADS)

The subsurface shear-wave velocity (Vs) is considered to be a key parameter for site characterization and assessment of earthquake hazard because of its great influence on local ground-motion amplification. Array microtremor measurements are widely used for the estimation of shear-wave velocities. Compared to other methods such as frequency-wavenumber (f-k) methods, the spatial autocorrelation (SPAC) method requires fewer sensors and thus is relatively easier to implement and gives robust estimations of shear-wave velocity profiles for depths down to a few hundred meters. The quantity derived from observed data is the SPAC coefficient, which is a function of correlation distance, frequency and phase velocity. Generally, estimation of Vs profiles is a two stage process: Estimation of the dispersion data from the SPAC coefficients and inversion of the dispersion data for shear-wave velocity structure. In this study, instead of inverting dispersion curves, a more practical approach is used; that is, observed SPAC coefficients are directly inverted for the S-wave velocities. A synthetic case and a field data application are presented to test the potential of the inversion algorithm. We obtain an iterative damped least-squares solution with differential smoothing. The differential smoothing approach constrains the change in shear-wave velocities of the adjacent layers and thus stabilizes the inversion.

Kocao?lu, Argun H.; F?rtana, Karolin

2011-10-01

223

Estimation of S-Wave Velocity Structure at Taichung Area, Taiwan, Using Array Records of Microtremors  

NASA Astrophysics Data System (ADS)

S-wave velocities have widely been used for earthquake ground-motion site characterization. Thus, here, the S-wave velocity structures of Taichung city, Taiwan are investigated using the array records of microtremors at 21 sites. The dispersion curves at these sites are calculated using the F-K method (Capon, 1969); then, the S-wave velocity structures in Chia-Yi city are estimated by employing the surface wave inversion technique (Herrmann, 1991). At most sites, observed phase velocities are almost flat with the phase velocity of about 1000 m/sec in the frequency range from 0.5 to 2Hz. This suggests that a thickness layer with an S-wave velocity of about 1km/sec was deposited. If the S-wave velocity of bedrock is assumed to be 1500m/sec, the depth of the alluvium at the Taichung area is about 600m~1300m. The depth of the alluvium gradually increases from east to west and from north to south.

Huang, H.; Hsu, C.

2009-12-01

224

Velocity of Sound Behind Strong Shock Waves in 2024 Al.  

National Technical Information Service (NTIS)

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...

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

1983-01-01

225

PARTICLE ACCELERATION BY ELECTROSTATIC WAVES WITH SPATIALLY VARYING PHASE VELOCITIES  

Microsoft Academic Search

We present here the results of numerical calculations connected with a mechanism recently proposed [Swift, 1968] for the acceleration of charged particles in a strong dc magnetic field. An electrostatic plasma wave propagates nearly perpendicularly to the magnetic field, and a weak density gradient exists parallel to the magnetic field. The wave propagates into the region of increasing plasma density

S. Peter Gary; David Montgomery; Daniel W. Swift

1968-01-01

226

Spatial correlation of shear-wave velocity in the San Francisco Bay Area sediments  

USGS Publications Warehouse

Ground motions recorded within sedimentary basins are variable over short distances. One important cause of the variability is that local soil properties are variable at all scales. Regional hazard maps developed for predicting site effects are generally derived from maps of surficial geology; however, recent studies have shown that mapped geologic units do not correlate well with the average shear-wave velocity of the upper 30 m, Vs(30). We model the horizontal variability of near-surface soil shear-wave velocity in the San Francisco Bay Area to estimate values in unsampled locations in order to account for site effects in a continuous manner. Previous geostatistical studies of soil properties have shown horizontal correlations at the scale of meters to tens of meters while the vertical correlations are on the order of centimeters. In this paper we analyze shear-wave velocity data over regional distances and find that surface shear-wave velocity is correlated at horizontal distances up to 4 km based on data from seismic cone penetration tests and the spectral analysis of surface waves. We propose a method to map site effects by using geostatistical methods based on the shear-wave velocity correlation structure within a sedimentary basin. If used in conjunction with densely spaced shear-wave velocity profiles in regions of high seismic risk, geostatistical methods can produce reliable continuous maps of site effects. ?? 2006 Elsevier Ltd. All rights reserved.

Thompson, E. M.; Baise, L. G.; Kayen, R. E.

2007-01-01

227

Thermal dehydration reactions characterized by combined measurements of electrical conductivity and elastic wave velocities  

NASA Astrophysics Data System (ADS)

Combined laboratory measurements of seismic velocities and electrical conductivity as a function of PT and drainage conditions have been performed on various rocks containing hydrous minerals. This paper presents experimental results for evaporite rocks containing gypsum (CaSO4 x 2H2O) and carnallite (KMgCl3 x 6H2O) and for serpentinite. The experiments on the evaporite rocks were carried out in a triaxial cell in the range 20-130 C and 5.0-24.0 MPa confining pressure. The measurements on serpentinite covered the range 20-750 C at pressures of up to 200 MPa and were performed in a cubic pressure apparatus. The thermally induced onset of dehydration was indicated by the pronounced discontinuous behaviour of conductivity, corresponding to a marked drop in the elastic wave velocities. The respective Poisson ratios were inversely correlated with permeability and seem to be a sensitive parameter to describe the reaction-related variations of pore space. The seismic properties can be satisfactory modelled by using the self-consistent approximation of O'Connell and Budiansky, illustrating the effects of variations in pore space and saturation. The dehydration behavior of the various hydrous rock types was found to be different because the changes in the rock physical properties are closely linked to the internally created pore fluid, to the changes in the porosity and pore geometry, and to the resulting pore pressure. Progressive thermal dehydration reactions induce an opening of pore space accompanied by lowered saturation conditions whereby the rate of fluid release depends mainly on temperature and also on the drainage conditions of the system. Our findings, as reflected by the variations in the geophysical parameters, may be of importance for the interpretation of natural dehydration processes in the crust caused by prograde metamorphism.

Popp, T.; Kern, H.

1993-11-01

228

New Insights on Lithospheric Structure beneath Isparta Angle and the Surroundings from Rayleigh Wave Phase Velocity Inversions  

NASA Astrophysics Data System (ADS)

The primary objective of this study is to obtain the lithospheric structure of Anatolian-African Subduction zone including the Isparta Angle (IA) from Phase velocity inversion of Rayleigh waves and subsequent Shear wave velocity inversions. The ongoing subduction of the African Plate under Anatolian Plateau results in a highly complex tectonic structure beneath Southwestern Turkey and the surroundings. IA is seismically quite active and formed by the intersection of two very different subduction zones: The Hellenic arc to the west and the Cyprian arc to the east. The geometric difference between Hellenic Arc (relatively steep retreating subduction) and Cyprus Arc (shallower subduction) might be indicating a tear or gap in the subducting African Lithosphere beneath the Anatolian Plate responsible for the active deformation. A temporary seismic network consisting of 10 3-component BB stations were installed in August 2006 with the support from University of Missouri and 9 more stations in March 2007 in addition to the 21 existing permanent stations of Kandilli Observatory and Earthquake Research Institute (KOERI) and two from Süleyman Demirel University (SDU). 8 stations from Geofon Network were also included to extend the station coverage. We used earthquakes in a distance range of 30-120 degrees with body wave magnitude larger than 5.5. Depending on the signal to noise ratio, azimuthal coverage of events, and coherence from station, 68 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 the pattern of amplitude variations effectively in many cases. To begin with, an average phase velocity dispersion curve (35 -140 seconds) was obtained and used as an input for 2-D inversions. Updated 2-D tomographic maps of phase velocities were constructed along with the resolution tests. Phase velocities can only tell us integrated information about the upper mantle. Furthermore, we inverted phase velocities for shear wave velocities in order to obtain direct information at various depths (35- ~300 km) that can be interpreted in terms of temperature anomalies, the presence of melt or dissolved water, etc.,

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

2010-12-01

229

Estimating shear wave velocity of soil deposits using polynomial neural networks: Application to liquefaction  

NASA Astrophysics Data System (ADS)

Geophysical and geotechnical field investigations have introduced several techniques to measure in-situ shear wave velocity of soils. However, there are some difficulties for the easy and economical use of these techniques in the routine geotechnical engineering works. For the soil deposits, researchers have developed correlations between shear wave velocity and SPT-N values. In the present study, a new database containing the measured shear wave velocity of soil deposits have been compiled from the previously published studies. Using polynomial neural network (PNN), a new correlation has been subsequently developed for the prediction of shear wave velocity. The developed relationship shows an acceptable performance compared with the available relationships. Three examples are then presented to confirm accuracy and applicability of the proposed equation in the field of liquefaction potential assessment.

Ghorbani, Ali; Jafarian, Yaser; Maghsoudi, Mohammad S.

2012-07-01

230

Energy velocity and quality factor of poroelastic waves in isotropic media.  

PubMed

The energy velocity and Q factor of poroelastic acoustic waves in the context of classical isotropic Biot's theory are revisited. Special attention is paid to the high frequency regime when interphase interaction is viscoelastic. The analogy with viscoelastic behavior is emphasized in derivation of the energy balance equations which relate kinetic energy, potential energy, viscous power dissipation, and elastic energy stored associated with each wave. These lead to exact closed form expressions for the energy velocity and Q factor for both longitudinal and shear waves from energy principles. Most notably, the analysis of the resulting expressions reveals that the energy velocity of both longitudinal and shear waves equals (exceeds) the corresponding phase velocity in the case of the low (full) frequency range theory, and that the exact expression for the Q factor contains an additive correction due to viscoelastic interphase interaction. PMID:21568384

Gerasik, Vladimir; Stastna, Marek

2011-05-01

231

Partial Derivatives of Surface Wave Phase Velocity with Respect to Physical Parameter Changes within the Earth.  

National Technical Information Service (NTIS)

By using energy equations, which are equivalent to fundamental equations and boundary conditions in surface wave problems, we can get analytical expressions for partial derivatives of phase velocity with respect to changes of physical parameters within th...

H. Takevchi J. Dorman M. Saito

1964-01-01

232

Savani: A variable resolution whole-mantle model of anisotropic shear velocity variations based on multiple data sets  

NASA Astrophysics Data System (ADS)

We present a tomographic model of radially anisotropic shear velocity variations in the Earth's mantle based on a new compilation of previously published data sets and a variable block parameterization, adapted to local raypath density. We employ ray-theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface wave phase delays from fundamental modes up to the sixth overtone, measured at periods between 25 and 350 s, as well as cross-correlation traveltimes of major body wave phases. Before inversion, we apply crustal corrections using the crustal model CRUST2.0, and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani, additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25° in well-sampled and 5° in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S wave tomography. Our model highlights (i) a distinct ocean-continent anisotropic signature in the uppermost mantle, (ii) an oceanic peak in above average ?<1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness, and (iii) a long-wavelength pattern of ?<1 associated with the large low-shear velocity provinces in the lowermost mantle.

Auer, L.; Boschi, L.; Becker, T. W.; Nissen-Meyer, T.; Giardini, D.

2014-04-01

233

Precise continuous monitoring of seismic velocity variations and their possible connection to solid earth tides  

Microsoft Academic Search

Seismic velocities in the siting area of the Norwegian Seismic Array (Norsar) have been monitored over a time period of 1 week by using a hydroelectric power plant as a continuous wave generator. Propagational phase angle differences have been measured over travel distances ranging from 4.7 to 13.7 km, and group velocities of the order of 3.5 km\\/s are derived.

Hilmar Bungum; Torben Risbo; Erik Hjortenberg

1977-01-01

234

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

235

Velocity and current distributions in the spent beam of the backward-wave oscillator  

Microsoft Academic Search

Results are presented of an experimental study of the spent beam of a backward-wave oscillator. The instantaneous velocity and current of the spent beam are measured using a velocity analyzer built onto the collector of a scaled 80-mc backward-wave oscillator. The tube employs a sheet beam and interdigital line, 12 feet long. It is designed to be representative of large-space-charge

J. W. Gewartowski

1958-01-01

236

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

237

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

238

Comparison of the Effects of Applied Loads and Temperature Variations on Guided Wave Propagation  

NASA Astrophysics Data System (ADS)

Most guided wave-based damage detection methods for structural health monitoring rely upon detecting small damage-induced changes in ultrasonic guided wave signals. However, the structure of interest is generally exposed to variable loads and temperatures during normal usage, and received signals can be significantly affected. These signal changes are of concern because of their potential to cause false alarms during in situ monitoring. Load-induced signal changes are similar in some respects to those caused by temperature variations because both cause changes in bulk wave speeds and specimen dimensions. However, load-induced changes, unlike temperature changes, can produce a slight anisotropy of the structure, causing the changes in velocities to depend on the direction of propagation. Here we present experimental results that show the anisotropic effect of applied loads on the direct arrivals of various Lamb wave modes and compare that to both theory and to the isotropic effect of temperature.

Lee, Sang Jun; Gandhi, Navneet; Michaels, Jennifer E.; Michaels, Thomas E.

2011-06-01

239

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

240

The Shear Wave Velocity Structure of the Marmara Region by Using Receiver Function Analysis  

NASA Astrophysics Data System (ADS)

In this study, we investigated crustal structure of the Marmara Region, Turkey by receiver function analysis. The study area comprises between 40-41.5 N and 26-29.5 E geographical coordinates including the Marmara Sea. Aim of this study was to reveal the crustal velocity structure beneath the land and sea stations in the Marmara region. We analysed receiver function data recorded by 38 land stations between January of 2008 and April of 2012, and 5 cabled Sea Bottom Observatories which were deployed at the end of 2010 by KOERI. Receiver functions were computed using approximately 250 teleseismic events from different azimuthal coverage within a 30°-90° epicentral distance with magnitudes greater than Mw 5.5. We determined receiver functions in the frequency domain using the water-level deconvolution technique of Langston (1979). Receiver functions with similar back-azimuths were stacked in order to improve their signal to noise ratios. In the next step, we will compute the receiver functions in the time domain using the iterative deconvolution technique suggested by Ligorria and Ammon (1999), and compare the results in each domain. Finally, The shear-wave velocity models will be obtained beneath each station and Moho depth variation will be mapped.

Buyukakpinar, P.; Gürbüz, C.; Zor, E.

2012-12-01

241

Vertical Energy Propagation of Inertial Waves: A Vector Spectral Analysis of Velocity Profiles  

Microsoft Academic Search

Vertical propagation of near-inertial period waves has been detected in a series of recent velocity profiles by a technique of vector spectral analysis. This method, previously applied to vector series in time, has been used to study the vertical spatial structure of velocity profiles obtained in the Mid-Ocean Dynamics Experiment (MODE). Prior to the use of spectral analysis, however, it

KEVIN D. LEAMANAND; Thomas B. Sanford

1975-01-01

242

Electromagnetic plane waves with negative phase velocity in charged black strings  

SciTech Connect

We investigate the propagation regions of electromagnetic plane waves with negative phase velocity in the ergosphere of static charged black strings. For such a propagation, some conditions for negative phase velocity are established that depend on the metric components and the choice of the octant. We conclude that these conditions remain unaffected by the negative values of the cosmological constant.

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

2013-02-15

243

P wave velocity structure in the Yucca Mountain, Nevada, region  

NASA Astrophysics Data System (ADS)

We have performed a crustal tomographic inversion using over 250,000 P arrival times from local earthquake sources and surface explosions in the Yucca Mountain, Nevada, region. Within the shallowest 2-3 km, topographic features tend to dominate the structure with high velocities imaged under Bare Mountain, the Funeral Mountains, and higher terrain to the east of Yucca Mountain and low velocities imaged under Crater Flat, Jackass Flat, the Amargosa Desert, and the caldera complexes. Imaged shallow velocities also show correlation with several known gravity and aeromagnetic anomalies. Below the basins (˜2-3 km depth), velocities vary between 5.5 and 6.5 km/s and lose many of the correlations seen in the shallowest layers; however, a few major structures, such as the Bare Mountain block, can be traced to at least 10 km depth. Additionally, we image structures that may be associated with the Wahmonie intrusion and pre-Tertiary structural trends. Yucca Mountain itself is underlain by a high-velocity upper crustal-scale structure similar to other structures in the region such as Bare Mountain and may represent a Basin and Range style back-tilted block, which may provide a structural explanation for Yucca Mountain's topographic expression. Additionally, the imaged, relatively low velocity basement under Crater Flat may provide a preferred conduit for magma intrusion into Crater Flat compared to Yucca Mountain, accounting for the lack of post-Miocene volcanism observed at the mountain proper. We explore our tomographic results in the context of four major tectonic models that have been proposed for the Yucca Mountain region.

Preston, Leiph; Smith, Ken; von Seggern, David

2007-11-01

244

Simultaneous structure and elastic wave velocity measurement of SiO[subscript 2] glass at high pressures and high temperatures in a Paris-Edinburgh cell  

SciTech Connect

An integration of multi-angle energy-dispersive x-ray diffraction and ultrasonic elastic wave velocity measurements in a Paris-Edinburgh cell enabled us to simultaneously investigate the structures and elastic wave velocities of amorphous materials at high pressure and high temperature conditions. We report the first simultaneous structure and elastic wave velocity measurement for SiO{sub 2} glass at pressures up to 6.8 GPa at around 500 C. The first sharp diffraction peak (FSDP) in the structure factor S(Q) evidently shifted to higher Q with increasing pressure, reflecting the shrinking of intermediate-range order, while the Si-O bond distance was almost unchanged up to 6.8 GPa. In correlation with the shift of FSDP position, compressional wave velocity (Vp) and Poisson's ratio increased markedly with increasing pressure. In contrast, shear wave velocity (Vs) changed only at pressures below 4 GPa, and then remained unchanged at {approx}4.0-6.8 GPa. These observations indicate a strong correlation between the intermediate range order variations and Vp or Poisson's ratio, but a complicated behavior for Vs. The result demonstrates a new capability of simultaneous measurement of structures and elastic wave velocities at high pressure and high temperature conditions to provide direct link between microscopic structure and macroscopic elastic properties of amorphous materials.

Kono, Yoshio; Park, Changyong; Sakamaki, Tatsuya; Kenny-Benson, Curtis; Shen, Guoyin; Wang, Yanbin (CIW) [CIW; (UC)

2012-05-09

245

The relationship between shear wave velocity, temperature, attenuation and viscosity in the shallow part of the mantle  

NASA Astrophysics Data System (ADS)

Surface wave tomography, using the fundamental Rayleigh wave velocities and those of higher modes between 1 and 4 and periods between 50 and 160 s, is used to image structures with a horizontal resolution of ˜250 km and a vertical resolution of ˜50 km to depths of ˜300 km in the mantle. A new model, PM_v2_2012, obtained from 3×106 seismograms, agrees well with earlier lower resolution models. It is combined with temperature estimates from oceanic plate models and with pressure and temperature estimates from the mineral compositions of garnet peridotite nodules to generate a number of estimates of SV(P,T) based on geophysical and petrological observations alone. These are then used to estimate the unrelaxed shear modulus and its derivatives with respect to pressure and temperature, which agree reasonably with values from laboratory experiments. At high temperatures relaxation occurs, causing the shear wave velocity to depend on frequency. This behaviour is parameterised using a viscosity to obtain a Maxwell relaxation time. The relaxation behaviour is described using a dimensionless frequency, which depends on an activation energy E and volume Va. The values of E and Va obtained from the geophysical models agree with those from laboratory experiments on high temperature creep. The resulting expressions are then used to determine the lithospheric thickness from the shear wave velocity variations. The resolution is improved by about a factor of two with respect to earlier models, and clearly resolves the thick lithosphere beneath active intracontinental belts that are now being shortened. The same expressions allow the three dimensional variations of the shear wave attenuation and viscosity to be estimated.

Priestley, Keith; McKenzie, Dan

2013-11-01

246

Estimation of skin elasticity by measuring surface wave velocity under impulse stimulus using compact optical sensors  

Microsoft Academic Search

Tissue elastography characterizes tissue mechanical properties, which can provide important information for diagnosis. While popular methods track shear wave propagation inside tissue, this paper proposes a method to estimate elasticity by measuring group velocity of the surface Raleigh wave. This method features noncontact, noninvasive and low-cost and have a great potential for clinic applications. By measuring the impulse response of

Bo Qiang; Xiaoming Zhang; James Greenleaf

2009-01-01

247

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

National Technical Information Service (NTIS)

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 vel...

D. E. McNamara W. R. Walter

1997-01-01

248

Velocity measurements at high temperatures by ultrasound Doppler velocimetry using an acoustic wave guide  

Microsoft Academic Search

Ultrasound Doppler velocimetry (UDV) was used to measure flow velocities at temperatures up to 620 °C. To overcome the thermal restriction of the ultrasonic transducers an acoustic wave guide was used. The acoustic wave guide and the piezoelectric element are combined in the form of an integrated sensor. This approach allowed the first successful application of the ultrasound Doppler technique

S. Eckert; G. Gerbeth; V. I. Melnikov

2003-01-01

249

Laser light scattering diagnostic for measurement of flow velocity in vicinity of propagating shock waves  

Microsoft Academic Search

A laser light scattering diagnostic for measurement of dynamic flow velocity at a point is described. The instrument is being developed for use in the study of propagating shock waves and detonation waves in pulse detonation engines under development at the NASA Glenn Research Center (GRC). The approach uses a Fabry-Perot interferometer to measure the Doppler shift of laser light

Richard G. Seasholtz; Alvin E. Buggele

2001-01-01

250

PS-wave processing and S-wave velocity inversion of OBS data from Northern South China Sea  

NASA Astrophysics Data System (ADS)

It is very important to convert seismic data from the time domain to the depth domain so we can explain the geological information more obviously. This paper provides a method for time to depth conversion. PS-wave data from an Ocean bottom seismograph (OBS) survey of the Northern South China Sea was processed through the separation of PP and PS wavefields and the rotation of horizontal components. An inverse modeling of travel times was performed for the determination of the S-wave velocity (Vs). The migration section of the single channel seismic data was used to define the model horizons and to help control their geometry. Wide angle hydrophone data of OBS were used to determine the P-wave travel times and wide angle horizontal component data of OBS were used to determine the PS-wave travel times. The chosen travel times from various shots were inverted for P- and S-wave interval velocities using RayInvr, which calculates theoretical travel times via ray tracing. Damped least squares optimization is performed to fine-tune the fits between observed and calculated travel times. The achieved trends of the P-wave velocity (Vp) and Vs curves are similar and the velocity increases in the layer where gas hydrates are present.

Wang, Xiangchun; Qian, Rongyi; Xia, Changliang

2014-01-01

251

Measurements of velocity and trajectory of water particle for internal waves in two density layers  

Microsoft Academic Search

This article discusses the kinetics of internal waves, which propagate in a two-layer fluid system having a constant water depth, using particle image velocimetry (PIV). The experimental vector field of velocity and vertical distributions of its components were estimated at several phases in one wave cycle and compared with the corresponding predictions on the basis of third-order Stokes internal-wave theory.

Motohiko Umeyama; Shogo Matsuki

2011-01-01

252

Runup and green water velocities due to breaking wave impinging and overtopping  

Microsoft Academic Search

The present study investigates, through measurements in a 2D wave tank, the velocity fields of a plunging breaking wave impinging\\u000a on a structure. As the wave breaks and overtops the structure, so-called green water is generated. The flow becomes multi-phased\\u000a and chaotic as a large aerated region is formed in the flow in the vicinity of the structure while water

Yonguk Ryu; Kuang-An Chang; Richard Mercier

2007-01-01

253

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

254

Velocity-matched slow-wave electrodes for integrated electro-optic modulators  

NASA Astrophysics Data System (ADS)

Coplanar slow-wave electrode structures capable of matching the velocities of microwaves to those of optical waves in compound semiconductor based electro-optic modulators are described. In such an electrode microwaves are slowed by periodically adding pairs of capacitive loading fins to the electrode to increase its capacitance per unit length, without obtaining a corresponding decrease in its inductance per unit length. Electro-optic modulators having wide bandwidths and requiring small amounts of modulating power may be realized by using slow-wave electrodes to achieve the velocity-match condition. The theory of operation of, and the results of some measurements on, electrodes of this type are presented.

Jaeger, Nicolas A.; Lee, Zachary K.

1993-01-01

255

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

256

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

257

Estimation of global aortic pulse wave velocity by flow-sensitive 4D MRI.  

PubMed

The aim of this study was to determine the value of flow-sensitive four-dimensional MRI for the assessment of pulse wave velocity as a measure of vessel compliance in the thoracic aorta. Findings in 12 young healthy volunteers were compared with those in 25 stroke patients with aortic atherosclerosis and an age-matched normal control group (n = 9). Results from pulse wave velocity calculations incorporated velocity data from the entire aorta and were compared to those of standard methods based on flow waveforms at only two specific anatomic landmarks. Global aortic pulse wave velocity was higher in patients with atherosclerosis (7.03 +/- 0.24 m/sec) compared to age-matched controls (6.40 +/- 0.32 m/sec). Both were significantly (P < 0.001) increased compared to younger volunteers (4.39 +/- 0.32 m/sec). Global aortic pulse wave velocity in young volunteers was in good agreement with previously reported MRI studies and catheter measurements. Estimation of measurement inaccuracies and error propagation analysis demonstrated only minor uncertainties in measured flow waveforms and moderate relative errors below 16% for aortic compliance in all 46 subjects. These results demonstrate the feasibility of pulse wave velocity calculation based on four-dimensional MRI data by exploiting its full volumetric coverage, which may also be an advantage over standard two-dimensional techniques in the often-distorted route of the aorta in patients with atherosclerosis. PMID:20512861

Markl, Michael; Wallis, Wolf; Brendecke, Stefanie; Simon, Jan; Frydrychowicz, Alex; Harloff, Andreas

2010-06-01

258

Velocity ratio measurement using the frequency of gyro backward wave.  

National Technical Information Service (NTIS)

The operating diagram of a low quality factor, 8GHz TE(sub 01)(sup 0) gyrotron exhibits oscillations between 6.8 and 7.3GHz. These oscillations are identified as the backward wave component of the TE(sub 21)(sup 0) traveling mode. As the resonance conditi...

P. Muggli M. Q. Tran T. M. Tran

1990-01-01

259

Imaging small velocity variations trough improved noise correlation approach: application to northern Fennoscandia  

NASA Astrophysics Data System (ADS)

Ambient noise tomography is now widely used method to imaging of the Earth crust and uppermost mantle, and it can in many cases yield higher resolution than tomographies that use seismic sources. Many of these applications are carried out in settings with strong velocity contrasts, for example between sediment and bedrock. Small velocity variations (around 1%) are potentially within the limit of resolution of the method, because non-isotropic noise sources, or small time averaging can provide an estimated Green's function affected by errors larger than the velocity variations. Here we present the result of a noise correlation tomography using one year of data (2008) continuously recorded by the LAPNET seismological array in the northern part of the Baltic shield. This zone is characterized by the absence of undeformed sediments and the lateral velocity variations are the order of 1-2 %. To extract such small velocity variations we use a novel correlation approach, able to provide high quality estimated Green's function. The noise correlation functions calculated between each station pair were then used to produce a high-resolution 3D Vs model of the area from to a depth of 20 km. The model is in good agreement with previous 2-D active seismic profiles and tectonic models of the area. With our tomography, we obtain 3-D constraints on the crustal structure, and more particularly on the Archean-Proterozoic transition.

Poli, P.; Pedersen, H. A.; Campillo, M.; Polenet/Lapnet Worging Group

2012-04-01

260

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

261

Variation of Seismic Velocity Structure around the Mantle Transition Zone and Conjecture of Deep Water Transport by Subducted Slabs  

NASA Astrophysics Data System (ADS)

Seismic tomography models published in the past two decades determined common long-wavelength features of subducting plates as high velocity anomalies and upwelling plumes as low velocity anomalies, and have led to a new class of high-resolution three-dimensional (3D) modeling of global mantle convection with a link to tomography models [e.g., Becker and Boschi, 2002; Ritsema et al., 2007; Schuberth et al., 2009a,b]. However, even such high resolution numerical models do not account for the variation associated with different behaviors of subducting plates as they enter the mantle transition zone (MTZ), i.e., some flatten to form stagnant slabs with a large lateral extent and others descend further into the lower mantle. There are conventional interpretations applied for the cause of variation of the subducted slab behaviors, i.e., temperature difference due to different plate age, different geochemical compositions, different water content and subsequent possible reduction of viscosity etc., which could be taken as non-unique and somewhat equivocal. These parameters and conditions have been tested in two-dimensional numerical simulations, while the water content in the MTZ or the mechanisms of hydration and dehydration through subduction process are still in the realm of conjecture. Recent models of seismic P- and SH-wave velocities derived for the mantle structure beneath northeast China [Wang and Niu, 2010; Ye et al., 2011] using reflectivity synthetics with data from the dense Chinese networks of broadband seismic instruments, show a broader 660 km discontinuity (by about 30 to 70 km) and slower shear velocities above the MTZ than a global standard model iasp91 (Kennett and Engdahl, 1991). These features were interpreted with a mixture of different chemical properties which show delayed phase transformation, and effects of water above the flattened slab. Nonetheless, the SH-wave model has a structure similar to model TNA above the MTZ, which was derived for the structure beneath tectonic North America [Grand and Helmberger, 1984] and has a sharp discontinuity. Tajima and Nakagawa [2006] have already shown that SH waveforms which sampled the stagnant slab in the northwestern Pacific subduction zone could be modeled with TNA well. The reflectivity synthetic approaches attempted to model a large number of waveform data obtained from an array with a layered structure, and tend to average the discontinuity depth variation and other features. Our 3D finite difference waveform modeling clearly suggests variation of relatively sharp phase transformation depths beneath stagnant slabs. The results were interpreted with variation of geochemical properties under wet condition at the base of MTZ. Accordingly we explore how well the hydrous and dry conditions can be delineated for varying chemical properties from seismic waveform analysis.

Tajima, F. C.; Stahler, S. C.; Ohtani, E.; Yoshida, M.; Sigloch, K.

2011-12-01

262

Wave equation tomographic velocity inversion method based on the Born/Rytov approximation  

NASA Astrophysics Data System (ADS)

This paper discusses Born/Rytov approximation tomographic velocity inversion methods constrained by the Fresnel zone. Calculations of the sensitivity kernel function and traveltime residuals are critical in tomographic velocity inversion. Based on the Born/Rytov approximation of the frequency-domain wave equation, we derive the traveltime sensitivity kernels of the wave equation on the band-limited wave field and simultaneously obtain the traveltime residuals based on the Rytov approximation. In contrast to single-ray tomography, the modified velocity inversion method improves the inversion stability. Tests of the near-surface velocity model and field data prove that the proposed method has higher accuracy and Computational efficiency than ray theory tomography and full waveform inversion methods.

Zhang, Kai; Yin, Zheng; Li, Zhen-Chun; Chen, Yong-Rui

2013-06-01

263

Kinetic Alfvén wave and ion velocity distribution functions in the solar wind  

NASA Astrophysics Data System (ADS)

Using 1D test particle simulations, the effect of a kinetic Alfvén wave on the velocity distribution function of protons in the collisionless solar wind is investigated. We first use linear Vlasov theory to obtain the property of a kinetic Alfvén wave numerically (the wave propagates in the direction almost perpendicular to the background magnetic field). We then numerically simulate how the wave will shape the proton velocity distribution function. It is found that Landau resonance may be able to generate two components in the initially Maxwellian proton velocity distribution function: a tenuous beam component along the direction of the background magnetic field and a core component. The streaming speed of the beam relative to the core proton component is about 1.2 -- 1.3 Alfvén speed. However, no perpendicular ion heating is observed from the simulation. Reference: Li, X., Lu, Q.M., Chen, Y., Li, B., Xia, L.D., ApJ, 719, L190, 2010.

Li, X.; Lu, Q.; Chen, Y.; Li, B.; Xia, L.

2010-12-01

264

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

265

Measurement of local pulse wave velocity: effects of signal processing on precision.  

PubMed

Pulse wave velocity (PWV) provides information about the mechanical properties of the vessel: the stiffer the artery is, the higher the PWV will be. PWV measured over a short arterial segment facilitates direct characterization of local wall properties corrected for prevailing pressure without the necessity of measuring pulse pressure locally. Current methods for local PWV assessment have a poor precision, but it can be improved by applying linear regression to a characteristic time-point in distension waveforms as recorded simultaneously by multiple M-line ultrasounds. We investigated the precision of this method in a phantom scaled according to realistic in vivo conditions. Special attention was paid to the identification of the foot of the wave, using the maximum of the second derivative, the intersecting tangent and the 20% threshold method. Before foot detection, the distension waveforms were subjected to preprocessing with various filters. The precision of the maximum of the second derivative had a coefficient of variation (CV) of 0.45% and 10.45% for an eighth and second order low pass filter, respectively. The intersecting tangent and the threshold method were less sensitive to filtering; the CVs were 0.66% and 0.68% for the high order filter and 2.36% and 1.43% for the low order filter, respectively. We conclude that foot detection by a threshold of 20% or by the tangent method are more suitable to identify the foot of the wave to measure local PWV. Both methods are less sensitive to (phase) noise than the maximum of the second derivative method and exhibit good precision with a CV of less than 1%. PMID:17383803

Hermeling, Evelien; Reesink, Koen D; Reneman, Robert S; Hoeks, Arnold P G

2007-05-01

266

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

267

A three-dimensional P wave velocity model for the Charlevoix seismic zone, Quebec, Canada  

NASA Astrophysics Data System (ADS)

A three-dimensional P wave velocity model has been developed for the Charlevoix seismic zone (CSZ). The CSZ is located along the St. Lawrence River ˜100 km northeast of Quebec City, Canada, and is one of the most active seismic zones in eastern North America. Five earthquakes with magnitudes equal to or exceeding 6.0 have occurred in the CSZ in historic time, and around 200 earthquakes occur annually. Hypocenters are located in Precambrian basement rocks. Basement rocks have been affected by numerous tectonic events including Grenvillian collision, Iapetan rifting, and meteor impact. We performed a sequential, tomographic inversion for P wave velocity structure based upon 3093 P wave arrivals from 489 earthquakes recorded by 12 stations. High velocity is associated with the center of the impact crater. The region of high velocity is surrounded by low velocities interpreted to be highly disrupted rocks. An elongated, high-velocity region is present at midcrustal depths that trends parallel to the St. Lawrence River. Earthquakes avoid the high-velocity body and separate into two bands, one on either side of the feature. Larger earthquakes (magnitude ? 4) have occurred along the northern edges of the high-velocity region.

Vlahovic, Gordana; Powell, Christine; Lamontagne, Maurice

2003-09-01

268

The variation in strength of transverse shocks in detonation waves  

Microsoft Academic Search

The variation of strength along a transverse shock associated with a Mach stem interaction at the detonation front in 2CO + O2 at 50 torr was measured by analysis of interferograms. An approximate solution to the flow behind the nonsteady reactive wave is obtained on the assumption of a power-law form for the density profile which is a reasonable representation

D. H. Edwards; A. T. Jones

1978-01-01

269

Rayleigh wave group velocity tomography in the Aegean area  

Microsoft Academic Search

Data from a large-scale experiment which took place in Greece during the period January–July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996–2000 from other temporary networks have been included in the dataset. One

E. E. Karagianni; D. G. Panagiotopoulos; G. F. Panza; P. Suhadolc; C. B. Papazachos; B. C. Papazachos; A. Kiratzi; D. Hatzfeld; K. Makropoulos; K. Priestley; A. Vuan

2002-01-01

270

Estimating near-surface shear wave velocities in Japan by applying seismic interferometry to KiK-net data  

NASA Astrophysics Data System (ADS)

We estimate shear wave velocities in the shallow subsurface throughout Japan by applying seismic interferometry to the data recorded with KiK-net, a strong motion network in Japan. Each KiK-net station has two receivers; one receiver on the surface and the other in a borehole. By using seismic interferometry, we extract the shear wave that propagates between these two receivers. Applying this method to earthquake-recorded data at all KiK-net stations from 2000 to 2010 and measuring the arrival time of these shear waves, we analyze monthly and annual averages of the near-surface shear wave velocity all over Japan. Shear wave velocities estimated by seismic interferometry agree well with the velocities obtained from logging data. The estimated shear wave velocities of each year are stable. For the Niigata region, we observe a velocity reduction caused by major earthquakes. For stations on soft rock, the measured shear wave velocity varies with the seasons, and we show negative correlation between the shear wave velocities and precipitation. We also analyze shear wave splitting by rotating the horizontal components of the surface sensors and borehole sensors and measuring the dependence on the shear wave polarization. This allows us to estimate the polarization with the fast shear wave velocity throughout Japan. For the data recorded at the stations built on hard rock sites, the fast shear wave polarization directions correlate with the direction of the plate motion.

Nakata, N.; Snieder, R.

2012-01-01

271

Characteristics of Wave Packets in the Upper Troposphere. Part II: Seasonal and Hemispheric Variations.  

NASA Astrophysics Data System (ADS)

Gridded 300-hPa meridional wind data produced by the ECMWF reanalysis project were analyzed to document the seasonal and hemispheric variations in the properties of upper-tropospheric wave packets. The properties of the wave packets are mainly illustrated using time-lagged one-point correlation maps performed on . Based on indices that show the coherence of wave propagation, as well as examination of correlation maps, schematic waveguides were constructed for the summer and winter seasons of both hemispheres along which waves preferentially propagate with greatest coherence. In the summers, the waveguides basically follow the position of the midlatitude jets. In the Northern Hemisphere winter, the primary waveguide follows the subtropical jet over southern Asia into the Pacific, but there is a secondary branch running across Russia, joining the primary waveguide near the entrance to the Pacific storm track. Over the Atlantic, the waveguide passes east-southeastward toward North Africa, then back to southern Asia. During the Southern Hemisphere winter, the primary waveguide splits in two around 70°E, with the primary (more coherent) branch deviating equatorward to join up with the subtropical waveguide, and a secondary branch spiraling poleward along with the subpolar jet and storm track maxima. Wave packet envelopes were also defined and group velocities of wave packets were computed based on correlations performed on packet envelopes. These group velocities were found to agree qualitatively with those defined previously based on wave activity fluxes.By examining the wave coherence indices, as well as individual correlation maps and Hovmöller diagrams of correlations computed along the primary waveguides, it was concluded that wave propagation is least coherent in Northern Hemisphere summer, and that waves in Southern Hemisphere summer are not necessarily more coherent than those in Southern Hemisphere winter. Data from a GCM experiment were also analyzed and showed that wave packets in the GCM also display such a seasonal variation in coherence. Results from experiments using an idealized model suggest that coherence of wave packets depends not only on the baroclinicity of the large-scale flow, but also on the intensity of the Hadley circulation, which acts to tighten the upper-tropospheric potential vorticity gradient.

Chang, Edmund K. M.

1999-06-01

272

Low-velocity fault-zone guided waves: Numerical investigations of trapping efficiency  

USGS Publications Warehouse

Recent observations have shown that shear waves trapped within low-velocity fault zones may be the most sensitive measure of fault-zone structure (Li et al., 1994a, 1994b). Finite-difference simulations demonstrate the effects of several types of complexity on observations of fault-zone trapped waves. Overlying sediments with a thickness more than one or two fault-zone widths and fault-zone step-overs more than one or two fault widths disrupt the wave guide. Fault kinks and changes in fault-zone width with depth leave readily observable trapped waves. We also demonstrate the effects of decreased trapped wave excitation with increasing hypocentral offset from the fault and the effects of varying the contrast between the velocity in the fault zone and surrounding hard rock. Careful field studies may provide dramatic improvements in our knowledge of fault-zone structure.

Li, Y. -G.; Vidale, J. E.

1996-01-01

273

Extensional wave attenuation and velocity in partially-saturated sand in the sonic frequency range  

SciTech Connect

Extensional wave attenuation and velocity measurements on a high permeability Monterey sand were performed over a range of gas saturations for imbibition and degassing conditions. These measurements were conducted using extensional wave pulse propagation and resonance over a 1 - 9 kHz frequency range for a hydrostatic confining pressure of 8.3 MPa. Analysis of the extensional wave data and the corresponding X-ray CT images of the gas saturation show strong attenuation resulting from the presence of the gas (QE dropped from 300 for the dry sand to 30 for the partially-saturated sand), with larger attenuation at a given saturation resulting from heterogeneous gas distributions. The extensional wave velocities are in agreement with Gassmann theory for the test with near-homogeneous gas saturation and with a patchy saturation model for the test with heterogeneous gas saturation. These results show that partially-saturated sands under moderate confining pressure can produce strong intrinsic attenuation for extensional waves.

Liu, Z.; Rector, J.W.; Nihei, K.T.; Tomutsa, L.; Myer, L.R.; Nakagawa, S.

2002-06-17

274

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

275

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

276

Relationship between Elastic wave Velocity and Permeability of Rock Model with penny-shaped cracks  

NASA Astrophysics Data System (ADS)

Estimating underground fluid-flow is of great importance in petroleum engineering and carbon capture and storage (CCS). Permeability is one of the most important parameters which show how easily fluid passes through rock mass. It could be acquired just by measuring rock samples near borehole in lab-experiments. It means that permeability except near borehole should be estimated, considering other information. In this research, elastic wave velocity is focused as a tool of estimating permeability, because it is one of the most popular parameter which has underground information. The relationship between permeability and elastic wave velocities should be revealed, in order to establish a methodology to estimate rock permeability from elastic wave velocity. These two parameters are controlled by pore geometry of rock. Therefore, we focused on pore geometry as connecting bridge between the two parameters: permeability, elastic wave velocity. We modeled the considering rock as a solid mass containing a lot of same-sized penny-shaped cracks randomly. LBM (Lattice Boltzmann Method), which is one of the computational fluid dynamics methods, is adopted for calculating permeability in our study. This method has a storing point especially under complicated fluid-solid boundary condition. Elastic wave velocities are derived from effective elastic moduli (i.e., bulk modulus, stiffness). They are estimated by self-consistent approximation, which needs porosity of rock model, aspect ratio of penny-shaped cracks and volume fraction of each phase. In this research, we assume that solid phase is composed only by quartz and rock's pore space is filled with water. The simulated results demonstrate that aspect ratio of crack can be estimated by P- and S-wave velocity, and aspect ratio and P-wave velocity can determine porosity. Whereas, the relationship between porosity and permeability is dependent on aspect ratio, which means permeability can be estimated by aspect ratio and porosity. Therefore, this research reveals that permeability can be estimated by P-wave velocity and S-wave velocity if the rock is composed by same-sized penny cracks.

Yamabe, H.; Tsuji, T.; Matsuoka, T.

2011-12-01

277

Time-lapse Measurements of Scholte Wave Velocity Over a Compacting Oil Field  

NASA Astrophysics Data System (ADS)

Acquisition of time-lapse seismic data over producing oil and gas fields is a proven method for optimizing hydrocarbon production. Most current data have been acquired using towed-streamer seismic vessels but new systems incorporating permanent Ocean Bottom Cable (OBC) systems are gaining in popularity, both as a way to achieve better repeatability and also to reduce the cost of acquiring many time-lapse repeats of the baseline survey. Over the last three years, more than seven repeat data sets have been acquired at the permanent OBC system installed (by the operator, BP) over the Valhall oil field located offshore Norway. This system contains ~2400 four-component receiver stations that are recorded using a dense areal shot grid ("carpet" shoot) that provides high fold and has delivered excellent time-lapse signals starting from the first repeat occurring just three months after the baseline. Time-lapse OBC data are conventionally used to measure amplitude and velocity changes of body wave reflections (PP and PS) but other measurements are also possible. In particular, Scholte waves are strongly visible on records acquired everywhere in the field on appropriately processed data and, given the high fold (because of the dense shots), Scholte wave velocity and anisotropy time-lapse changes obtained with both hydrophone and geophone sensors are accurately and robustly estimated. The resulting shallow velocity maps are very sensitive to the seabed strains and show large velocity changes overlying deep production. Also, reconstruction of compressional "head wave" velocity difference measurements and vertically propagating shear wave shallow time-lapse statics produce maps that resemble the Scholte wave maps, with differences that reflect the physics of the propagation modes and effective fold. A reservoir model that includes deep reservoir volume changes together with appropriate geomechanical properties in the overburden and a shallow conversion of strain to velocity is used to successfully predict the measured velocity changes. The strain/velocity conversion requires asymmetry between crack opening and closing as well as velocity hysteresis and, in fact, the measurements provide an excellent laboratory for testing fracture-model/velocity conversion on in-situ rocks. After calibration, the model together with the data can constrain both volume changes in the reservoir, for making drilling decisions as well as the overburden geomechanical rock properties model, which itself is used for well- path selection and facilities decisions. Scholte wave velocity measurements can also be made using an oil platform as a "passive" source, removing the need for a conventional source near the seafloor. Finally, these measurements might be applicable on time- lapse controlled source measurements of greater generality in a wider geophysical context wherever an accurate measurement of a time-varying surface strain is desired.

Wills, P. B.; Hatchell, P. J.

2007-12-01

278

3-D Seismic Velocity Structure of the Hawaii Hotspot from Joint Inversion of Body Wave and Surface Wave data  

NASA Astrophysics Data System (ADS)

The Hawaii hotspot and the associated chain of islands have been long regarded as the case example of a deep-rooted mantle plume. However the efforts to detect a thermal plume seismically have been inconclusive. In this study we combine the complementary sensitivities of body- and surface-waves in order to improve resolution of mantle structure beneath Hawaii. Adding surface-wave constraints to the body wave inversion improves the resolution of the crustal and upper mantle structure. We used data from the deployment of temporary broadband ocean-bottom seismometers (OBS) of the Hawaiian Plume-Lithosphere Undersea Melt Experiment (PLUME) together with data from the on-shore stations in order to make the most complete dataset available. In a first step, we obtained stable and reliable OBS orientations over a range of earthquake back-azimuths using teleseismic P-wave particle motions. Due to the high noise of the OBS data in some frequency bands, we began by filtering in the period band of 0.04-1Hz. Using the proper channel orientations, we measured ~800 S-wave relative arrival times (direct S and SKS phases) on the SV component using muti-channel cross correlation. We applied the two-plane wave tomography method to generate surface wave phase velocity information. We use surface waves from 71 events with magnitude greater than 5.8 to generate phase velocity maps from 25 sec to 100 sec. These maps clearly show the low velocities beneath the islands surrounded by relatively high phase velocity. The pure S wave inversion result shows the 3-D structure beneath the PLUME array to a depth of 1000km and reveals a several-hundred-kilometer-wide region of low velocities beneath Hawaii that dips to the southeast. The low velocities continue downward through the mantle transition zone and extend into the uppermost lower-mantle where our resolution begins to degrade. These images are consistent with the interpretation that the Hawaiian hotspot is the result of an upwelling high-temperature plume from the lower mantle. The broader upper-mantle low-velocity region immediately beneath the Hawaiian Islands likely reflects the horizontal spreading of the plume material beneath the lithosphere. We also obtain result from the joint body-wave and surface-wave inversion. The same shallow low velocity zone is imaged along the island chain and the deeper part is identical to the body-wave inversion image. The low velocity to the southwest of the island is also clear to a depth of 600km. However, this low velocity limb is relatively small compared to the one from the southeast. This suggests this southwest limb is the result of spreading and interaction of the plume with the lithosphere. If the observation here is true, it will provide a hot environment for the 660km discontinuity to the west of the Hawaii and may give a new perspective to the plume origin debate.

Cheng, C.; Allen, R. M.; Porritt, R. W.

2012-12-01

279

Impact of desiccation on compressional and shear-wave velocities in clay-rocks: a laboratory study  

NASA Astrophysics Data System (ADS)

The study of the impact of desiccation on the mechanical parameters of clay-rocks is of crucial importance to characterize the desaturated zone close to the walls of a deep underground repository, excavated into clay-rocks. Three core samples were taken from the Callovo-Oxfordian argillite formation located at the MHM-URL laboratory in Eastern France (40% of clay minerals in average); rich of illites/smectites. In order to desaturate the core samples, we followed two desiccation paths. In a first step (desaturation phase); each sample was dried at ambient air (relative humidity in the range 32 to 42 % and at average room temperature 20 ? C). In a second step (heating phase), the same samples were heated by four temperature levels from 65? C to 105? C. Velocity measurements were carried out on the core samples using an ultrasonic (1 MHz) pulse transmission technique to obtain P and perpendicularly polarized shear (Sv, Sh) wave velocities during both of desaturation and heating phases. The results show that when the degree of saturation decreased, both P and S wave velocities increased. During these desiccation paths, the initial anisotropy was not significantly enhanced. The increase in S wave velocity, by as much as 10% and the associated increase in dynamic shear modulus following desiccation, suggests the presence of desiccation-driven hardening, which is commonly observed in clay soils. The existence of a such a phenomenon proves that the classical models (e.g., the Gassmann equation) used to study the effect of saturating fluids, fail to correctly assess the influence of variations in water content on seismic velocities measured in clay-rocks. In the case of clay-rocks subjected to very low confining pressures and high desiccation conditions (high ventilation rates), new models which explicitly account for textural changes in clay will need to be developed.

Ghorbani, A.; Zamora, M.; Cosenza, Ph.

2009-04-01

280

Ultrasonic elastic wave velocity measurements of polycrystalline MgAl2O4 spinel at high pressure and high temperature  

NASA Astrophysics Data System (ADS)

Magnesium aluminate (MgAl2O4) spinel is considered an important functional/structural material widely used at extreme conditions, due to its good mechanical strength, high resistance to chemical attack, good stability, and excellent optical and dielectric properties. Moreover, MgAl2O4 spinel also plays a significant role in geophysics as an important rock-forming mineral, which constitutes peridotites from the uppermost of the Earth's mantle. Therefore, understanding the elasticity and sound velocities of MgAl2O4 spinel are of great interest in the fields ranging from materials physics to geophysics by various experimental techniques. Previous experimental studies on MgAl2O4 spinel were carried out either at high temperature or high temperature. To date, there are no direct measurements of the elastic wave velocities of MgAl2O4 spinel at simultaneous high-pressure and high-temperature conditions. Recently, elasticity and sound velocities of polycrystalline MgAl2O4 spinel have been firstly measured up to 14 GPa and 900 K using ultrasonic interferometry in conjunction with energy-dispersive synchrotron X-ray diffraction. It is found that compressional wave velocity (VP) increases with pressure and decreases with temperature, whereas the shear wave velocity (VS) decreases with both pressure and temperature. Two-dimensional linear fittings of the present data give: KS0 =195 (1) GPa, dKs/dP = 4.4(1), dKs/dT = -0.023(1) GPa/K, G0 = 108(1) GPa, dG/dP = 0.38(1), dG/dT = -0.014(1) GPa/K. Moreover, the bulk modulus (KS) exhibits obvious increase with pressure, while the shear modulus (G) shows a weak variation with pressure, which may be due to the shear deformation relaxation by the coupling between atomic displacements and shear strains.

Irifune, T.; Zou, Y.; Greaux, S.; Zhou, C.; Whitaker, M. L.; Higo, Y.; Li, B.

2012-12-01

281

Coseismic velocity variation of Wenchuan aftershocks measured from active source monitoring  

Microsoft Academic Search

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

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

2008-01-01

282

Characteristics of group velocities of backward waves in a hollow cylinder.  

PubMed

It is known that modes in axially uniform waveguides exhibit backward-propagation characteristics for which group and phase velocities have opposite signs. For elastic plates, group velocities of backward Lamb waves depend only on Poisson's ratio. This paper explores ways to achieve a large group velocity of a backward mode in hollow cylinders by changing the outer to inner radius ratio, in order that such a mode with strong backward-propagation characteristics may be used in acoustic logging tools. Dispersion spectra of guided waves in hollow cylinders of varying radii are numerically simulated to explore the existence of backward modes and to choose the clearly visible backward modes with high group velocities. Analyses of group velocity characteristics show that only a small number of low order backward modes are suitable for practical use, and the radius ratio to reach the highest group velocity corresponds to the accidental degeneracy of neighboring pure transverse and compressional modes at the wavenumber k?=?0. It is also shown that large group velocities of backward waves are achievable in hollow cylinders made of commonly encountered materials, which may bring cost benefits when using acoustic devices which take advantage of backward-propagation effects. PMID:24907803

Cui, Hanyin; Lin, Weijun; Zhang, Hailan; Wang, Xiuming; Trevelyan, Jon

2014-06-01

283

Shear Velocity Structure of the Indian Shield From Surface Wave Tomography and Receiver Function Inversion  

NASA Astrophysics Data System (ADS)

We present an improved shear velocity model for the crust and upper mantle of the Indian shield, an assembly of Precambrian cratons bounded by mobile belts and paleo-rifts, obtained by inverting receiver function data and a set of recently constructed Rayleigh wave group velocity maps of India and the surrounding regions. The latter provide a higher resolution dataset than previous global and regional studies and the inclusion of shorter paths (<500km) increases the high frequency content, providing better constraints on upper crustal structure. Dispersion curves were extracted from the group velocity maps for points on a 0.5° by 0.5° grid and inverted for 1D shear velocity structure to 120km depth using a linearised least squares inversion method. Group velocity dispersion of surface waves is sensitive to shear velocity structure but does not contain information on discrete velocity boundaries; synthetic tests show that such boundaries are blurred to the order of 20km. The availability of receiver functions which are more sensitive to impedance contrasts, allows a more accurate determination of crustal structure below seismic stations and data for more than 40 stations across India have been re-examined and jointly inverted with the dispersion data from this study. The resulting shear wave velocity structure is not only better constrained for the areas where both data sets are available, but equally importantly, provides a means of using the more extensive surface wave data to calibrate crustal thickness elsewhere in the region. Our results show that thicker crust in the region of 40-45km is present beneath the Delhi-Aravalli fold belt and an area of central India straddling the Satpura Mobile Belt, with crust as thick as 54km observed beneath the Western Dharwar Craton. We also observe the Indian crust flexed beneath the Himalayan foreland basin and high velocity anomalies in the upper mantle beneath the Dharwar Craton in the south and the Singbhum and Bundelkhand Cratons in the north.

Acton, C.; Priestley, K.; Gaur, V. K.; Rai, S. S.

2008-12-01

284

An estimation technique of Rayleigh wave phase velocities using arrays with arbitrary geometry  

NASA Astrophysics Data System (ADS)

The mictotremor survey method (MSM) is one of the most practical techniques to estimate velocity structure of shear waves in sedimentary layers. In the MSM, the velocity models are determined by inversion analysis of the Rayleigh wave phase velocity dispersion curve observed from microtremors. In most of the cases, the phase velocity dispersion curve is obtained by either the spatial autocorrelation (SPAC) technique or the frequency-wavenumber (F-K) technique applied to array measurements of microtremors. These techniques place significant restrictions on the array geometry and number of stations required, which limits the applicability of MSM, especially in urban areas. We have derived a new technique for estimating phase velocities of Rayleigh waves. This new technique (the direct estimation method: DEM) enables to the use of flexible array configurations and a minimal number of stations. Moreover, the DEM can be applied to records from existing station arrays, such as those in an earthquake monitoring network. In the DEM, microtremors detected by arrays with arbitrary geometry can be represented by complex coherence functions (CCFs: Shiraishi et. al. 2006) of the Rayleigh wave. The CCF is derived from analytic solution of Lamb's problem, and it consists of the Bessel function of the first kind J0(?r/c) (?: angular frequency, r: distance between the stations, c: phase velocity), which is well-known function and is used in the SPAC technique to estimate phase velocity. The phase velocities can be estimated by solving the equations with the least squares approach to minimize the residual error between the observed and the theoretical values. A field experiment has been carried out to verify the effectiveness of the DEM, and the phase velocities obtained by the DEM with an array of arbitrary geometry are in excellent agreement with those obtained using the SPAC technique.

Shiraishi, H.; Asanuma, H.

2008-12-01

285

High-resolution Shear Wave Velocity Structure beneath the Western Sichuan from Ambient Noise Array Tomography  

NASA Astrophysics Data System (ADS)

Since 2006, the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, has deployed a dense seismograph array in SW China in order to understand the earthquake dynamics in this region. Using about one year continuous recordings at 156 stations over western Sichuan(100°~105°E, 29°~32°N), we obtained the surface wave empirical Green’s functions from ambient noise cross-correlation, and then measured about 10,000 Rayleigh wave phase velocity dispersion curves. Rayleigh wave phase velocity maps (2-35s) are constructed using the generalized linear inversion method with lateral resolution about 20-30 kilometers. Finally, we invert for the 3-D shear wave velocity structure in the crust using the Neighborhood Algorithm with constraints (e.g., Moho depth) from the receiver function inversions (Liu et al., 2008). The results show significant differences of the crustal structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. The phase velocity maps at short periods also show clear boundaries around the Longmenshan faults and Xianshuihe faults. The Sichuan basin shows significant higher velocity feature than the Chuandian and Songpan-Garze block at the middle and lower crustal depth, and appears much slower in the upper crust, suggesting the existence of a thick sediment layer. And the crust thickness of Sichuan basin becomes thinner from south to north. In the Songpan-Garze block, a low-velocity layer exists in the middle and lower crust. In the Chuandian block exists a high-velocity layer in the middle crust near the Xianshuihe faults, but a low-velocity layer in the lower crust. Our study manifests that Sichuan basin vertically contacts with the Songpan-Garze block without obvious westward subduction, although the crust of the Songpan-Garze block seems mechanically much weaker as inferred from the widespread crustal low-velocity zone (Liu et al, 2008).

Li, Y.; Yao, H.; Chen, J.; Huang, H.; Liu, Q.; van der Hilst, R. D.

2009-12-01

286

Deep crustal structure of the Indian shield from joint inversion of P wave receiver functions and Rayleigh wave group velocities: Implications for Precambrian crustal evolution  

Microsoft Academic Search

The S wave velocity structure of the crust and uppermost mantle of the Indian shield has been investigated by jointly inverting P wave receiver functions and Rayleigh wave group velocities at 38 broadband stations in the subcontinent. The Indian shield is an amalgamation of several terranes of Archean and Proterozoic age that were partly flooded by Deccan Trap volcanism during

S. Jagadeesh; S. S. Rai; T. J. Owens

2009-01-01

287

Along-strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound  

NASA Astrophysics Data System (ADS)

Around 1100 years ago, the Seattle fault, which trends east-west beneath Puget Sound and the greater Seattle metropolitan area, experienced a M > 7 earthquake. We present high-resolution images of the shallow P wave velocity variation across the fault zone. These images were obtained by tomographic inversion of the first arrivals recorded along two north-south oriented seismic reflection lines shot within Puget Sound near Seattle. Just beneath the seafloor, the fault zone includes uplifted Tertiary rocks with seismic velocities in the range of 2300 to 2600 m s-1. These velocities contrast markedly with values of ˜1600 m s-1 in shallow Holocene sediments. South of the Seattle fault zone volcanic rocks of the Crescent Formation, which exhibit velocities >3700 m s-1, are identified at depths of only 900 m. Seismic velocities of around 2600 m s-1, which represent Oligocene rocks, are found in the hanging wall of the Seattle fault beneath eastern Puget Sound. In the west, lower, 2300 m s-1 seismic velocities occur, probably due to the presence of Miocene rocks, which are not found in the east. Along-strike velocity variations arise from the folding of Tertiary rocks and the presence of distinct fault splays, including a north striking tear fault characterized by depressed seismic velocities that was intersected by the eastern seismic line. Along-strike differences in the uplift of Tertiary rocks beneath Puget Sound are likely associated with the existence of a segment boundary of the Seattle fault system.

Calvert, Andrew J.; Fisher, Michael A.; Johnson, Samuel Y.

2003-01-01

288

New constraints on the arctic crust and uppermost mantle: surface wave group velocities, P n , and S n  

Microsoft Academic Search

We present the results of a study of surface wave dispersion across the Arctic region (>60°N) and compare the estimating group velocity maps with new maps of the body wave phases Pn and Sn. Data recorded at about 250 broadband digital stations from several global and regional networks were used to obtain Rayleigh and Love wave group velocity measurements following

A. L. Levshin; M. H. Ritzwoller; M. P. Barmin; A. Villaseñor; C. A. Padgett

2001-01-01

289

Validity and reproducibility of arterial pulse wave velocity measurement using new device with oscillometric technique: A pilot study  

Microsoft Academic Search

BACKGROUND: Availability of a range of techniques and devices allow measurement of many variables related to the stiffness of large or medium sized arteries. There is good evidence that, pulse wave velocity is a relatively simple measurement and is a good indicator of changes in arterial properties. The pulse wave velocity calculated from pulse wave recording by other methods like

Madireddy Umamaheshwar Rao Naidu; Budda Reddy; Sridhar Yashmaina; Amar Narayana Patnaik; Pingali Usha Rani

2005-01-01

290

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

291

Crust and upper mantle P wave velocity structure beneath Valles caldera, New Mexico: Results from the Jemez teleseismic tomography experiment  

SciTech Connect

New results are presented from the teleseismic component of the Jemez Tomography Experiment conducted across Valles caldera in northern New Mexico. We invert 4872 relative {ital P} wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km. The three principle features of our model for Valles caldera are: (1) near-surface low velocities of {minus}17{percent} beneath the Toledo embayment and the Valle Grande, (2) midcrustal low velocities of {minus}23{percent} in an ellipsoidal volume underneath the northwest quadrant of the caldera, and (3) a broad zone of low velocities ({minus}15{percent}) in the lower crust or upper mantle. Crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast. Near-surface low velocities are interpreted as thick deposits of Bandelier tuff and postcaldera volcaniclastic rocks. Lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast, beneath the Valle Grande. We interpret the midcrustal low-velocity zone to contain a minimum melt fraction of 10{percent}. While we cannot rule out the possibility that this zone is the remnant 1.2 Ma Bandelier magma chamber, the eruption history and geochemistry of the volcanic rocks erupted in Valles caldera following the Bandelier tuff make it more likely that magma results from a new pulse of intrusion, indicating that melt flux into the upper crust beneath Valles caldera continues. The low-velocity zone near the crust-mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle. In either case, this low-velocity anomaly indicates that underplating by mantle-derived melts has occurred. {copyright} 1998 American Geophysical Union

Steck, Lee K.; Fehler, Michael C.; Roberts, Peter M.; Baldridge, W. Scott; Stafford, Darrik G. [Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico (United States)] [Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico (United States); Lutter, William J.; Sessions, Robert [Department of Geology and Geophysics, University of Wisconsin-Madison (United States)] [Department of Geology and Geophysics, University of Wisconsin-Madison (United States)

1998-10-01

292

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

293

Velocity-Space Diffusion Coefficients Due to Full-Wave ICRF Fields in Toroidal Geometry  

SciTech Connect

Jaeger et al. have calculated bounce-averaged QL diffusion coefficients from AORSA full-wave fields, based on non-Maxwellian distributions from CQL3D Fokker-Planck code. A zero banana-width approximation is employed. Complementing this calculation, a fully numerical calculation of ion velocity diffusion coefficients using the full-wave fields in numerical tokamak equilibria has been implemented to determine the finite orbit width effects. The un-approximated Lorentz equation of motion is integrated to obtain the change in velocity after one complete poloidal transit of the tokamak. Averaging velocity changes over initial starting gyro-phase and toroidal angle gives bounce-averaged diffusion coefficients. The coefficients from the full-wave and Lorentz orbit methods are compared for an ITER DT second harmonic tritium ICRF heating case: the diffusion coefficients are similar in magnitude but reveal substantial finite orbit effects.

Harvey, R.W. [CompX, P.O. Box 2672, Del Mar, CA 92014-5672 (United States); Jaeger, F.; Berry, L.A.; Batchelor, D.B.; D'Azevedo, E.; Carter, M.D. [ORNL, Oak Ridge, TN (United States); Ershov, N.M.; Smirnov, A.P. [Moscow State Univ. (Russian Federation); Bonoli, P.; Wright, J.C. [PSFC, MIT, Boston, MA (United States); Smithe, D.N. [ATK-Mission Research (United States)

2005-09-26

294

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

295

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.

296

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

297

Effect of slope variation and skating technique on velocity in cross-country skiing.  

PubMed

The purpose of the present study was to investigate the effect of slope variations upon the maximal velocity attainable by cross-country ski racers using three skating techniques (V-1, V-2, and Gunde). Nine (2 females, 7 males) junior ski racers (16.0 +/- 0.4 yr of age, mean +/- SEM) exhibiting strong technical skills participated in the study. The subjects were required to skate at maximal velocity over five different courses (length 120-200 m) with mean slopes of -1, 0, 6, 9, and 12%. Video analysis was used to determine skiing velocity, cycle length, and cycle rate. Heart rate was monitored to verify intensity of exertion. Results indicated that intensities were similar to those observed while racing. Velocities for the three techniques were not different for -1, 0, and 6% slopes. However V-1 was significantly faster (P < 0.01) at 9% and 12% inclines. Velocity was highly correlated to cycle length (P < 0.01) but unrelated to cycle rate. Cycle length was significantly different between techniques (P < 0.01) and varied across slopes. Cycle rate was significantly different between techniques (P < 0.01) but did not vary across slopes. Thus, our results indicate that any of the three techniques is adequate on flat and rolling terrain, but V-1 should be used at slopes of 9% and above. It is also concluded that during short trials skied at maximal velocity, skiing velocity is highly dependent on cycle length and independent of cycle rate. PMID:7723654

Boulay, M R; Rundell, K W; King, D L

1995-02-01

298

Phase velocity and azimuthal anisotropy variations beneath the central Tien Shan  

NASA Astrophysics Data System (ADS)

The goal of this study is to construct 3-D shear-wave structure from surface waves beneath the central part of the Tien Shan, the world’s largest and most active intracontinental orogen. We have analyzed fundamental-mode Rayleigh wave data recorded at the CHENGIS and KNET local seismic networks, which consist of 41 broadband seismic stations. Two different methods, the two-plane-wave inversion technique and the cross-correlation of ambient seismic noise, have been adopted to solve for phase velocities. We have applied the first method on fundamental mode Rayleigh wave trains that are extracted from 52 teleseismic events at central frequencies from 7.5 mHz to 50 mHz with a 10 mHz frequency interval. Rayleigh waves at short periods from 10 to 30 s have been determined by stacking two years ambient seismic noise at pairs of seismic stations. Combining the two techniques we are able to generate phase velocity maps at the periods of 10-133 s, which reflect structure from middle crust to about 200 km depth. A clear low velocity zone is imaged beneath the western part of the central Tien Shan range at short periods, indicating a thick and /or slow crust. The slow anomaly region shifts to north at intermediate periods of 50 to 100 s, which are most sensitive to the shallow upper mantle. On the other hand, fast anomaly is observed in the eastern part of the central Tien Shan at the same depth range. At long periods of 100 to 133 s, a slow anomaly is imaged in southwest of the central Tien Shan and a fast anomaly is present beneath the Tarim Basin. We have also estimated the azimuthal anisotropy from Rayleigh wave data and found an ENE-WSW fast direction at 20-50 s and WNW-ESE direction at long periods of 60 to 110 s. We will solve for 3-D shear wave structure from the phase velocity maps and discuss the important features of the results and their implications to the formation and evolution of the Tien Shan.

Lisi, A.; Li, A.

2009-12-01

299

Surface acoustic wave velocity in single-crystal AlN substrates  

Microsoft Academic Search

The surface acoustic wave velocity has been measured on a-plane (c-propagation) and c-plane oriented bulk aluminum nitride (AlN) single crystals using the S11-parameter method in the frequency range 160-360 MHz. The SAW velocity is 5760 m\\/s for both orientations. From comparison of this value with the simulations using various elastic constants of AlN available in literature, we estimated the elastic

Gang Bu; D. Ciplys; M. Shur; L. J. Schowalter; S. Schujman; R. Gaska

2006-01-01

300

Determination of S-wave velocity structure using microtremors and spac method applied in Thessaloniki (Greece)  

Microsoft Academic Search

Array measurements of microtremors at 16 sites in the city of Thessaloniki were performed to estimate the Vs velocity of soil formations for site effect analysis. The spatial autocorrelation method was used to determine phase velocity dispersion curves in the frequency range from 0.8–1.5 to 6–7 Hz. A Rayleigh wave inversion technique (stochastic method) was subsequently applied to determine the

Paschalis Apostolidis; Dimitrios Raptakis; Zafeiria Roumelioti; Kyriazis Pitilakis

2004-01-01

301

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

302

Single Tracking Location Methods Suppress Speckle Noise in Shear Wave Velocity Estimation  

PubMed Central

In ultrasound-based elastography methods, the estimation of shear wave velocity typically involves the tracking of speckle motion due to an applied force. The errors in the estimates of tissue displacement, and thus shear wave velocity, are generally attributed to electronic noise and decorrelation due to physical processes. We present our preliminary findings on another source of error, namely, speckle-induced bias in phase estimation. We find that methods that involve tracking in a single location, as opposed to multiple locations, are less sensitive to this source of error since the measurement is differential in nature and cancels out speckle-induced phase errors.

Elegbe, Etana C.; McAleavey, Stephen A.

2014-01-01

303

ESTIMATION OF S-WAVE VELOCITY STRUCTURE OF FUKUI PLAIN BASED ON MICROTREMOR ARRAY OBSERVATION  

NASA Astrophysics Data System (ADS)

The precise evaluations of Quaternary structure of the region are indispensable in order to accurately predict the seismic damage. However, deep borehole, PS-logging and elastic wave exploration have been executed only on limited points around the Fukui Plain. The problem analyzed in this study is statistical estimation of the 3D S-wave velocity structure down to the Tertiary bedrock of the Fukui Plain based on the data from 75 microtremor array observation sites. The Rayleigh wave phase velocities at each array site were calculated by the spatial autocorrelation method. The phase velocities at each site were inverted to a 1D S-wave profile using a genetic inversion. The 3-components single-site microtremor observations were carried out to compensate the array observations. The 3D S-wave velocity structure around the Fukui plain have been interpolated by using Kriging and Co-Kriging techniques. In the Co-Kriging procedure, the correlations between the estimated depths of Quaternary and the observed predominant periods of the sites were taken into account. The validity of the estimated structure from the microtremor observation was confirmed by comparing with the density structure and with the existing PS-logging data.

Kojima, Keisuke; Moto, Koudai

304

Along-strike variations in the shallow seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound  

Microsoft Academic Search

Around 1100 years ago, the Seattle fault, which trends east-west beneath Puget Sound and the greater Seattle metropolitan area, experienced a M > 7 earthquake. We present high-resolution images of the shallow P wave velocity variation across the fault zone. These images were obtained by tomographic inversion of the first arrivals recorded along two north-south oriented seismic reflection lines shot

Andrew J. Calvert; Michael A. Fisher; Samuel Y. Johnson

2003-01-01

305

Crustal shear-wave velocity and radial anisotropy models of North China Craton revealed by ambient noise tomography  

NASA Astrophysics Data System (ADS)

The North China Craton (NCC) is of the Archean craton on the Earth preserving crust remnants as old as 3.8 Ga. The craton experienced a sequence of major tectonic events from cratonization to reactivation, and thus, it shows complex lithotectonic assemblages, structural-styles and tectonic evolution. In the last decade, intense geological and geochemical investigations to the outcrops of the old crustal rocks have greatly improved the understanding of craton formation and evolution in the Precambrian. Simultaneously, geophysical imaging of the present crust places key constrains for selecting and justifying the candidate tectonic model. The crust structure of the NCC revealed by the previous geophysical studies is of high resolution but fragmented. Therefore, the integrated and high-resolution image of the crust will provide some new insights into the tectonic assembly and structure of the NCC. We combined the seismic data from both the permanent network of the China National Seismic Network (CNSN) and the temporary seismic arrays of the North China Interior Structure Project (NCISP) in the central and western NCC. The study region covers the Ordos Block (WNCC), the Khondalite Belt, the Yinshan Block and the Trans North China Orogen (TNCO). The ambient noise tomography was applied to obtain the 3-D models of shear-wave velocity and radial anisotropy in the crust. The empirical Green's functions (EGFs) of Rayleigh and Love waves were derived from the cross-correlation functions of Vertical and Transverse components using the more than one-year-long continuous raw data at 208 broadband seismic stations. We measured the Rayleigh and Love wave dispersion curves at the periods of 8 - 40s from the EGFs and inverted for the 3-D crustal shear wave velocity (Vsv and Vsh) models. The radial anisotropy was carried out by 2(VSH-VSV)/(VSH+VSV). The shear-wave velocity models show complex variations in all directions, which reflect the remnants in the crust caused by polyphase deformation and metamorphism. The Fenwei rift and the Yinchuan-Hetao rift are observed as concordant low velocity zones from the upper to the lower crust. We suggest that the two rifts may be caused by the rifting involving the whole crust. The crust of the Ordos Block primarily displays weak radial anisotropy, while the crust of the TNCO displays several patterns of positive anisotropy. We interpret the variations of anisotropy may reflect the tectonic transition from the extension in the TNCO to stability in the Ordos Block.

Jiang, M.; Ai, Y.

2011-12-01

306

Tomography and velocity structure of the crust and uppermost mantle in southeastern Europe obtained from surface wave analysis  

Microsoft Academic Search

A set of two hundred shear-wave velocity models of the crust and uppermost mantle in southeast Europe is determined by application\\u000a of a sequence of methods for surface-waves analysis. Group velocities for about 350 paths have been obtained after analysis\\u000a of more than 600 broadband waveform records. Two-dimensional surface-wave tomography is applied to the group-velocity measurements\\u000a at selected periods and

R. Raykova; S. Nikolova

2007-01-01

307

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

308

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

309

Shear-wave velocity structure of the western part of the Mediterranean Sea from Rayleigh-wave analysis  

NASA Astrophysics Data System (ADS)

The lithospheric structure of the western part of the Mediterranean Sea is shown by means of S-velocity maps, for depths ranging from 0 to 35 km, determined from Rayleigh-wave analysis. The traces of 55 earthquakes, which occurred from 2001 to 2003 in and around the study area have been used to obtain Rayleigh-wave dispersion. These earthquakes were registered by 10 broadband stations located on Iberia and the Balearic Islands. The dispersion curves were obtained for periods between 1 and 45 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source zones to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and after inverted according to the generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks with a size of 1° × 1°. The shear velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several depths. These maps show the existence of lateral and vertical heterogeneity. In these maps is possible to distinguish several types of crust with an average S-wave velocity ranging from 2.6 to 3.9 km/s. The South Balearic Basin (SBB) is more characteristic of oceanic crust than the rest of the western Mediterranean region, as it is demonstrated by the crustal thickness. We also find a similar S-wave velocity (ranging from 2.6 km/s at the surface to 3.2 km/s at 10 km depth) for the Iberian Peninsula coast to Ibiza Island, the North Balearic Basin (NBB) and Mallorca Island. In the lower crust, the shear velocity reaches a value of 3.9 km/s. The base of the Moho is estimated from 15 to 20 km under Iberian Peninsula coast to Ibiza Island, continues towards NBB and increases to 20-25 km beneath Mallorca Island. While, the SBB is characterized by a thinner crust that ranges from 10 to 15 km, and a faster velocity. A gradual increase in velocity from the north to the south (especially in the upper 25 km) is obtained for the western part of the Mediterranean Sea. The base of the crust has a shear-wave velocity value around of 3.9 km/s for the western Mediterranean Sea area. This area is characterized by a thin crust in comparison with the crustal thickness of the eastern Mediterranean Sea area. This thin crust is related with the distensive tectonics that exists in this area. The low S-wave velocities obtained in the upper mantle might be an indication of a serpentinized mantle. The obtained results agree well with the geology and other geophysical results previously obtained. The shear velocity generally increases with depth for all paths analyzed in the study area.

Corchete, Victor; Chourak, M.

2010-07-01

310

Estimation of S-wave Velocity Profile by Inversion of Coda H/V Spectrum  

NASA Astrophysics Data System (ADS)

A new method using inversion of coda H/V spectrum is examined to estimate the S-wave velocity profile down to seismic bedrock. The inverse analysis is based on the theoretical H/V spectrum of surface wave proposed for microtremor H/V spectra analyses. In order to confirm the applicability of this estimating method, the coda H/V spectra analysis are conducted at KiK-net Narita and Miyakoji observatory sites using three-component seismometers. The estimated S-wave velocity profiles are consistent with the available profiles derived from the PS loggings, vertical seismic array analyses and other explorations. It becomes clear that the inversion of coda H/V spectrum can almost estimate the S-wave velocity profile down to seismic bedrock. To conduct the inverse analyses sufficiently well, it is necessary to consider the effects of the higher modes of Rayleigh and Love waves, especially Rayleigh-to-Love-wave amplitude ratio for horizontal motions.

Kobayashi, Genyuu; Mamada, Yutaka

311

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.

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

2013-01-01

312

Length and activation dependent variations in muscle shear wave speed.  

PubMed

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 to 5.6 m s(-1) 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(-1) 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

Chernak, L A; DeWall, R J; Lee, K S; Thelen, D G

2013-06-01

313

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

314

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

315

Detailed three-dimensional shear wave velocity structure of the northwestern United States from Rayleigh wave tomography  

NASA Astrophysics Data System (ADS)

Since the mid-Miocene, the northwestern United States has experienced extensive flood basalt volcanism, followed by the formation of two time-progressive tracks of silicic volcanism: the Yellowstone/Snake River Plains (YSRP) and the High Lava Plains (HLP). The YSRP track progresses towards the northeast, parallel to North American plate motion, and has therefore often been attributed to a deep mantle plume source. However, the HLP track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion. The causes of the mid-Miocene flood basalts and the tracks of the YSRP and HLP are a matter of ongoing debate. We present results of Rayleigh wave phase velocity inversions and inversions for 3-D shear wave velocity structure of the northwestern United States using data collected from the High Lava Plains seismic experiment and the EarthScope USArray Transportable Array (TA). The large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area. Our velocity images indicate that low S-wave velocities in the uppermost mantle do not well match the track of HLP volcanism. While at the surface the Newberry caldera appears to anchor the NW end of the HLP hotspot track, the seismic results show that it lies in a separate, north-south trending low velocity band just east of the Cascades that is distinct from the main HLP trace. The ultra-low S-wave velocities beneath the YSRP track extend locally to at least 175 km depth and are by far the most prominent seismic anomalies in the region. Along axis, the YSRP hotspot track is characterized by a discrete low velocity channel in the upper mantle that shallows, narrows and intensifies to the northeast, but then deepens rapidly to the north beneath Yellowstone. The shallowing of the low velocity anomaly to the northeast is consistent with a stationary heat source beneath a moving plate, coupled with Basin and Range extension and heating.

Wagner, Lara; Forsyth, Donald W.; Fouch, Matthew J.; James, David E.

2010-11-01

316

Tracing causes for the stress sensitivity of elastic wave velocities in dry Castlegate sandstone  

NASA Astrophysics Data System (ADS)

The stress sensitivity of elastic wave velocities in dry rock is a resultant of two types of processes-elastic and non-elastic processes. Both processes are activated under stress and both are associated with stress-induced changes in the rock structure. Although they are of the same origin, their stress-dependency may differ. In this study, a set of tests that separate the elastic and non-elastic processes is used to evaluate the impact of each process on the stress sensitivity of the elastic wave velocities. The tests rely on comparing the stress sensitivity of wave velocities during uniform loading/unloading over a long stress interval (involving both elastic and non-elastic processes) with the stress sensitivity seen in low-amplitude stress oscillations (assumed to be affected mainly by elastic processes). Our study on dry, weak sandstone shows that the impact of elastic and non-elastic processes on the stress sensitivity of the elastic wave velocities is significantly different. This implies that the processes ought to be described separately in order to provide a better foundation for predictive rock physics models. Observations on artificially fractured samples indicate that large, horizontal fractures reduce the axial wave velocities, whereas they have no notable impact on the stress sensitivity of the velocities. This suggests that the closed macro-fractures contain damaged areas with reduced stiffness which are apparently insensitive to stress changes. A few basic processes-elastic opening and closure of cracks, friction-controlled shear sliding of closed cracks associated with opening or closure of wing cracks, and crushing of asperities in fractures or grain contacts-may be used to intuitively explain the observations.

Stroisz, Anna Magdalena; Fjær, Erling

2013-01-01

317

Agradient velocity, vortical motion and gravity waves in a rotating shallow-water model  

NASA Astrophysics Data System (ADS)

A new approach to modelling slow vortical motion and fast inertia-gravity waves is suggested within the rotating shallow-water primitive equations with arbitrary topography. The velocity is exactly expressed as a sum of the gradient wind, described by the Bernoulli function,B, and the remaining agradient part, proportional to the velocity tendency. Then the equation for inverse potential vorticity,Q, as well as momentum equations for agradient velocity include the same source of intrinsic flow evolution expressed as a single term J (B, Q), where J is the Jacobian operator (for any steady state J (B, Q) = 0). Two components of agradient velocity are responsible for the fast inertia-gravity wave propagation similar to the traditionally used divergence and ageostrophic vorticity. This approach allows for the construction of balance relations for vortical dynamics and potential vorticity inversion schemes even for moderate Rossby and Froude numbers assuming the characteristic value of |J(B, Q)| = to be small. The components of agradient velocity are used as the fast variables slaved to potential vorticity that allows for diagnostic estimates of the velocity tendency, the direct potential vorticity inversion with the accuracy of 2 and the corresponding potential vorticity-conserving agradient velocity balance model (AVBM). The ultimate limitations of constructing the balance are revealed in the form of the ellipticity condition for balanced tendency of the Bernoulli function which incorporates both known criteria of the formal stability: the gradient wind modified by the characteristic vortical Rossby wave phase speed should be subcritical. The accuracy of the AVBM is illustrated by considering the linear normal modes and coastal Kelvin waves in the f-plane channel with topography.

Sutyrin Georgi, G.

2004-07-01

318

Regional P wave velocity structure of the Northern Cascadia Subduction Zone  

USGS Publications Warehouse

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 the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25-35 km depth) having relatively low velocities of 6.4-6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes withi the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2-7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M>4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate. Copyright 2006 by the American Geophysical Union.

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

2006-01-01

319

Three dimensional shear wave velocity structure of the crust and upper mantle beneath China from ambient noise surface wave tomography  

NASA Astrophysics Data System (ADS)

We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green’s functions obtained from seismic ambient noise cross-correlation. The data we use are from the China National Seismic Network, global and regional networks and PASSCAL stations in the region. We first acquire cross-correlation seismograms between all possible station pairs. We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis 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 velocity structure of the crust and upper mantle beneath China at each grid node. The inversion results show large-scale structures that correlate well with surface geology. Near the surface, velocities in major basins are anomalously slow, consistent with the thick sediments. East-west contrasts are striking in Moho depth. There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau (TP) and Tianshan (Junggar, Tarim, Ordos, and Sichuan). These strong blocks, therefore, appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape. In northwest TP in Qiangtang, slow anomalies extend from the crust to the mantle lithosphere. Meanwhile, widespread, a prominent low-velocity zone is observed in the middle crust beneath most of the central, eastern and southeastern Tibetan plateau, consistent with a weak (and perhaps mobile) middle crust.

Sun, Xinlei; Song, Xiaodong; Zheng, Sihua; Yang, Yingjie; Ritzwoller, Michael H.

2010-10-01

320

Eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures  

NASA Astrophysics Data System (ADS)

We automatically analyzed 32,000 fundamental mode Love and Rayleigh wave signals with earthquake-station paths traversing Eurasia and Indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 Love and 4020 Rayleigh paths. These were inverted to give phase velocity maps at 14 fixed periods. Resolution tests suggest that features with diameter >750 km and >500 km are resolved over most of Eurasia and central/SE Asia respectively. Low-period Love waves image areas with thick sedimentary cover as low-velocity zones, and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity. At long periods, both Love and Rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots. We observe significant along-strike heterogeneity in the Zagros fold belt and Tien Shan-Altai system. Taking sections across Eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly. For example, the Red River and East Vietnam Boundary faults are traced on their eastern side by low velocities which extend at depth into Indonesia. We relate this to mantle upwelling associated with early Eocene rotation of Indochina and reversal of the sense of shear across the Red River fault post-20 Ma. We observe dipping subduction of the Mediterranean beneath the Aegean, of the Philippine Sea beneath Indonesia, and of the Indian shield beneath Tibet. We also image a fossil subducted plate beneath NE Borneo which we associate with subduction of the proto-South China Sea between 50 Ma and 15 Ma.

Curtis, Andrew; Trampert, Jeannot; Snieder, Roel; Dost, Bernard

1998-11-01

321

Shallow shear wave velocity and Q structures at the El Centro strong motion accelerograph array  

NASA Astrophysics Data System (ADS)

At three sites in the U.S. Geological Survey El Centro Strong Motion Accelerograph Array (EO5, EO6 and EO7), we have used the dispersive and attenuative properties of artificially generated Rayleigh waves to infer the depth dependence of shear wave velocity (?) and quality factor (Q) and a site dependent equivalent elastic source spectrum. This approach provides an inexpensive means for determining two important quantities (? and Q) required to estimate the response of sites to earthquakes. We find that the shear velocities and the gradients in shear velocities down to depths of 100 meters at station EO6 (in the wedge between the Imperial and Brawley faults) are nearly twice those at either station EO5 and EO7 (east and west, respectively, of the wedge). Also, the seismogram character, surface wave dispersion and shear wave velocity structure at stations EO5 and EO7 are similar to each other while they are different from EO6. These results show significant structural differences between the wedge beneath EO6 and surrounding areas. The anomalously high accelerations recorded at EO6 during the 1979 Imperial Valley earthquake may be related to these differences.

Barker, Terrance G.; Stevens, Jeffry L.

1983-09-01

322

Nonlinear effects in velocity space and drift wave transport in tokamaks.  

National Technical Information Service (NTIS)

It is pointed out that the use of kinetic models for drift wave transport in general makes it necessary to treat consistently the transport in both real space and velocity space. For a broad class of collisionless or marginally collisionless plasmas, nonl...

J. Weiland

1991-01-01

323

Erratum: pulse wave velocity testing in the Baltimore longitudinal study of aging.  

PubMed

A correction was made to Pulse Wave Velocity Testing in the Baltimore Longitudinal Study of Aging. There was an error with the author, Majd AlGhatrif's, name. The author's name has been corrected to: Majd AlGhatrif instead of: Majd al Ghatrif. PMID:24809417

2014-01-01

324

Reciprocal Influence of Slow Waves Extracted in Intracranial Pressure, Arterial Pressure and Cerebral Blood Velocity Signals.  

National Technical Information Service (NTIS)

Slow waves in intracranial pressure (ICP) and related signals seems of interest to evaluate the dynamic autoregulation, i.e. by comparison of the frequency links between an intracranial signal e.g. ICP, cerebral blood velocity (CBV), and arterial blood pr...

F. Cervenansky J. J. Lemaire J. Y. Boire

2001-01-01

325

Assessment of relationships between blood pressure, pulse wave velocity and digital volume pulse  

Microsoft Academic Search

Aortic arterial stiffness is an independent predictor of cardiovascular risk. Pulse wave velocity (PWV) is the most validated and universally accepted measure of arterial stiffness. The Digital Volume Pulse (DVP) is an accurate and non invasive method to obtain information on the pressure pulse waveform, and provides two indexes: stiffness index (SIDVP) which relates with large artery stiffness, and the

J. M. Padilla; E. J. Berjano; J. Saiz; L. Facila; P. Diaz; S. Merce

2006-01-01

326

Assessment of Relationships be tween Blood Pressure, Pulse Wave Velocity and Digital Volume Pulse  

Microsoft Academic Search

Aortic arterial stiffness is an independent predictor of cardiovascular risk. Pulse wave velocity (PWV) is the most validated and universally accepted measure of arterial stiffness. The Digital Volume Pulse (DVP) is an accurate and non invasive method to obtain information on the pressure pulse waveform, and provides two indexes: stiffness index (SIDVP) which relates with large artery stiffness, and the

JM Padilla; EJ Berjano; J Sáiz; L Fácila; P Díaz

327

Relationship between hyperinsulinemia and pulse wave velocity in moderately hyperglycemic patients  

Microsoft Academic Search

Aim: Arterial stiffness assessed by pulse wave velocity (PWV) reflects early stage arteriosclerosis. The influence of hyperinsulinemia on peripheral vascular disease (PVD) is still unknown. We determined the influences of hyperinsulinemia on PVD assessed by PWV in moderately hyperglycemic patients. Methods: Thirty-six moderately hyperglycemic, outcoming patients were recruited in this study. All subjects were divided into two groups by fasting

Miki Koizumi; Hiroyuki Shimizu; Kenju Shimomura; Shinsuke Oh-I; Yasuyo Tomita; Tomohiro Kudo; Ken-Ichi Iizuka; Masatomo Mori

2003-01-01

328

Evaluation of shear wave velocity profile using SPT based uphole method  

Microsoft Academic Search

The uphole method is a field seismic test which uses receivers on the ground surface and an underground source. A modified form of the uphole method is introduced in order to obtain efficiently the shear wave velocity (VS) profile of a site. This method is called the standard penetration test (SPT)-uphole method because it uses the impact energy of the

Eun-Seok Bang; Dong-Soo Kim

2007-01-01

329

Relationship between Pulse Wave Velocity and Carotid Atherosclerosis in Geriatric People  

Microsoft Academic Search

Objective: To investigate the relationship between the pulse wave velocity (PWV) and angiographic carotid atherosclerosis in elderly patients. Method: 103 consecutive elderly patients were divided into two groups according to the results of cerebral angiography: carotid atherosclerosis group and normal carotid angiogram group. Basic clinical information was required by a standardized questionnaire. Carotid-femoral PWV (cfPWV) as a marker of Stiffness

L. Shen; W. Wu; B. You; H. Gao; C. Wang; Y. Liu

2011-01-01

330

Brachio-ankle pulse wave velocity and cardio-ankle vascular index (CAVI)  

Microsoft Academic Search

In order to diagnose arteriosclerosis in any part of the body, pulse wave velocity (PWV) measurement is a useful approach. However, it is considered that the technique of PWV measurement should be simplified. A new method for measuring PWV has therefore been proposed in Japan. The PWV of the brachial artery (ba) and the ankle was measured by applying air

Tomoyuki Yambe; Makoto Yoshizawa; Yoshifumi Saijo; Tasuku Yamaguchi; Muneichi Shibata; Satoshi Konno; Shinichi Nitta; Takashi Kuwayama

2004-01-01

331

Profiles of stationary shock waves for hexagonal discrete velocity model with all triple collisions  

Microsoft Academic Search

Stationary shock wave profiles are investigated for the hexagonal discrete velocity model with binary and all triple collisions. The singular points corresponding to equilibrium limit states are studied analytically in the phase space of the relevant dynamical systems. The influence of triple collisions on various macroscopic quantities which characterize the shock profiles is investigated. The infinite Mach number case is

T. Platkowski

1992-01-01

332

3D shear-wave velocity structure of the Eifel plume, Germany  

Microsoft Academic Search

The Quaternary Eifel volcanic fields, situated on the Rhenish Massif in Germany, are the focus of a major interdisciplinary project. The aim is a detailed study of the crustal and mantle structure of the intraplate volcanic fields and their deep origin. Recent results from a teleseismic P-wave tomography study reveal a deep low-velocity structure which we infer to be a

Matthias Keyser; Joachim R. R. Ritter; Michael Jordan

2002-01-01

333

Relations between dairy food intake and arterial stiffness: pulse wave velocity and pulse pressure.  

PubMed

Modifiable risk factors, such as diet, are becomingly increasingly important in the management of cardiovascular disease, one of the greatest major causes of death and disease burden. Few studies have examined the role of diet as a possible means of reducing arterial stiffness, as measured by pulse wave velocity, an independent predictor of cardiovascular events and all-cause mortality. The aim of this study was to investigate whether dairy food intake is associated with measures of arterial stiffness, including carotid-femoral pulse wave velocity and pulse pressure. A cross-sectional analysis of a subset of the Maine-Syracuse Longitudinal Study sample was performed. A linear decrease in pulse wave velocity was observed across increasing intakes of dairy food consumption (ranging from never/rarely to daily dairy food intake). The negative linear relationship between pulse wave velocity and intake of dairy food was independent of demographic variables, other cardiovascular disease risk factors, and nutrition variables. The pattern of results was very similar for pulse pressure, whereas no association between dairy food intake and lipid levels was found. Further intervention studies are needed to ascertain whether dairy food intake may be an appropriate dietary intervention for the attenuation of age-related arterial stiffening and reduction of cardiovascular disease risk. PMID:22431583

Crichton, Georgina E; Elias, Merrrill F; Dore, Gregory A; Abhayaratna, Walter P; Robbins, Michael A

2012-05-01

334

An experimental study on the correlation between the elastic wave velocity and microfractures in coal rock from the Qingshui basin  

NASA Astrophysics Data System (ADS)

In order to study the impact of microfractures on the elastic wave velocity, multi-azimuth elastic wave velocity experiments were performed with coal rock samples under a confining pressure similar to the one in the buried coal strata of the Qinshui basin. The results indicate that (1) the microfractures in coal rock reduce the elastic wave velocity. In the experiments, the velocity was increased with a confining pressure. However, when the confining pressure was increased to 10 MPa, the velocity became stable because most of the microfractures in the coal rock samples were closed. (2) The elastic wave velocity is sensitive to the microfracture orientation in coal rock. The elastic wave propagates rapidly along the microfracture direction, while it propagates slowly perpendicular to the microfracture direction. This research provides the basic data to predict microfractures in coal rock.

Feng, Zhou; Mingjie, Xu; Zhonggao, Ma; Liang, Cai; Zhu, Zhu; Juan, Li

2012-12-01

335

3-D P-wave Velocity Structure in Western Greece Determined from Tomography Using Earthquake Data Recorded at the University of Patras Seismic Network (PATNET)  

NASA Astrophysics Data System (ADS)

The 3-D P-wave velocity structure of the upper crust in the region of western Greece is investigated by inversion of about 1500 residuals of P-wave arrival times from local earthquake data recorded in the year 1996 by the newly established University of Patras Seismic Network (PATNET). The resulting velocity structure shows strong horizontal variations due to the complicated structure and the variation of crustal thickness. Relatively low-velocity contours are observed in the area defined by Cephallonia-Zakynthos Islands and northwestern Peloponnesos. This is in addition to some well localized peaks of relatively higher values of P-wave velocity may be related to the zone of Triassic evaporites in the region and correspond to diapirism that breaks through to the uppermost layer. Finally, a low P-velocity `deeping' zone extending from Zakynthos to the Gulf of Patras is correlated with Bouguer anomaly map and onshore and offshore borehole drillings which indicate that thick sediments overly the evaporites which exist there at depth greater than 2 km.

Melis, N. S.; Tselentis, G.-A.

336

Atmospheric parameters of a sample of giant stars with accurate radial velocity variations  

NASA Astrophysics Data System (ADS)

In da Silva et al. (2005) is presented the detailed spectroscopic analysis of 60 evolved stars, which were previously studied for accurate radial velocity variations (Setiawan et al. 2003). The observations were made with FEROS and the 1.52m ESO telescope of La Silla, partially in the ESO-Observatório Nacional (Brazil) agreement. The main goal of this study is to determine precise atmospheric fundamental data (Teff, log g, [Fe/H] and micro-turbulence velocity) to derive ages and masses of the stars, trying to link them to the detected radial velocity variabilities. Here, due the short space at our disposal, we discuss only the effective temperature determination, comparing the values obtained from the excitation equilibrium of FeI and from (V-K) index with those from (B-V).

da Silva, L.; Girardi, L.; Pasquini, L.; Döllinger, M.; de Medeiros, R.; Setiawan, J.; Hatzes, A.; Weiss, A.; di Mauro, M. Pia

337

Relative array analysis of upper mantle lateral velocity variations in Southern California  

NASA Technical Reports Server (NTRS)

The many densely spaced stations of the Southern California Seismic Network, SCARLET, provide a unique opportunity to examine local variations in upper mantle velocities. By averaging the incoming teleseismic wavefronts over 100-km aperture subarrays, ray parameter dT/dDelta, azimuth, and average arrival time estimates were obtained for each event. These values are compared to whole array plane fit parameters with relative array diagrams and representations of net subarray delays. The conducted study confirms the existence of a high-velocity body at depth beneath the Transverse Ranges and a low-velocity region under the Salton Trough. An uppermost mantle anomaly beneath the San Bernardino Mountains is well established both by this study and from local data. The relation of this body to the deeper structure is investigated.

Walck, M. C.; Minster, J. B.

1982-01-01

338

P-wave velocity structure of the uppermost mantle beneath Hawaii from traveltime tomography  

USGS Publications Warehouse

We examine the P-wave velocity structure beneath the island of Hawaii using P-wave residuals from teleseismic earthquakes recorded by the Hawaiian Volcano Observatory seismic network. The station geometry and distribution of events makes it possible to image the velocity structure between ~ 40 and 100 km depth with a lateral resolution of ~ 15 km and a vertical resolution of ~ 30 km. For depths between 40 and 80 km, P-wave velocities are up to 5 per cent slower in a broad elongated region trending SE-NW that underlies the island between the two lines defined by the volcanic loci. No direct correlation between the magnitude of the lithospheric anomaly and the current level of volcanic activity is apparent, but the slow region is broadened at ~ 19.8??N and narrow beneath Kilauea. In the case of the occanic lithosphere beneath Hawaii, slow seismic velocities are likely to be related to magma transport from the top of the melting zone at the base of the lithosphere to the surface. Thermal modelling shows that the broad elongated low-velocity zone cannot be explained in terms of conductive heating by one primary conduit per volcano but that more complicated melt pathways must exist.

Tilmann, F. J.; Benz, H. M.; Priestley, K. F.; Okubo, P. G.

2001-01-01

339

Seismicity and 3-D Velocity Structure of the Himalayan Collision Zone: Lateral Variations in Lithospheric Structure  

NASA Astrophysics Data System (ADS)

Using P and S arrival time data from the 29-station Himalayan Nepal Tibet Seismic Experiment (HIMNT), we performed local earthquake tomography for eastern Nepal and southern Tibet. Our results show that local earthquakes beneath the network reach depths up to 100 km below sea level, and over a hundred seismic events recorded during the one year deployment locate in the upper mantle. These subcrustal earthquakes suggest that in some areas beneath the Himalayan collision zone, the upper mantle deforms by brittle processes. There are no clear gaps in the vertical distribution of the seismicity. Our tomographic images show dramatic crustal thickening from an average of 45 km beneath southern Nepal to 75 km beneath southern Tibet. The presence of earthquakes around Moho depths and high P wave velocities (up to 8.7 km/s) in the upper mantle beneath the High Himalayas and southern Tibet suggest a relatively cold upper mantle. Most of the lower crustal and subcrustal earthquakes beneath the High Himalaya and southern Tibet are concentrated east of longitude 86.5E, where the lower crust has relatively low P wave velocities (between 6.2 and 6.5 km/s). West of that longitude, earthquakes deeper than 50 km are rare, and P wave velocities in the lower crust are as high as 7.5 km/s. We interpret these high velocities in the Tibetan lower crust as the result of metamorphic reactions causing partial transformation into eclogite of materials in the lower Indian crust. The Vp/Vs ratio is anomalously low (an average of 1.66) in the upper crust of southern Tibet (Tethyan Himalaya), which may correspond to a very silicic basement of the sedimentary sequence seen at the surface.

Monsalve, G.; Sheehan, A.; Rowe, C.

2005-12-01

340

Shear velocity variation within the D? region beneath the central Pacific  

Microsoft Academic Search

Small-scale variability of shear velocity (Vs) structure in the D? region beneath the central Pacific is imaged using 442 broadband tangential component S waveforms recorded in western North America for 37 intermediate and deep focus Tonga-Fiji earthquakes. Double-array stacking of spatially binned subsets of data reveals lateral variations in the relative timing and amplitude of deep mantle discontinuity reflections on

Megan Avants; Thorne Lay; Sara A. Russell; Edward J. Garnero

2006-01-01

341

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

342

Evidence for P wave velocity discontinuities at depths greater than 650 km in the mantle  

NASA Astrophysics Data System (ADS)

The arrival times of seismic P waves recorded at long lines of portable seismographs deployed on the shield region of central Australia show evidence of breaks in the travel-time curve at epicentral distances near 30, 39 and 43°. These breaks are additional to those at about 20 and 24° (associated with the 400- and 650-km discontinuities) and imply that the P wave velocity structure of the mantle does not increase smoothly in the depth range 650-1100 km, but rather consists of regions of nearly constant velocity separated by small but significant velocity increases at depths of approximately 770, 980 and 1080 km. These conclusions are in agreement with those previously inferred from first and later arrivals at the Warramunga Seismic Array.

Muirhead, K. J.; Hales, A. L.

1980-12-01

343

Evolution of microstructure and elastic wave velocities in dehydrated gypsum samples  

NASA Astrophysics Data System (ADS)

This study aims at contributing to the experimental database of changes in rock physical properties, particularly elastic wave velocities, induced by devolatilization reactions. Cylindrical samples of natural gypsum were dehydrated in air for up to 800 h at ambient pressure and temperatures between 378 and 423 K. Subsequently, the transformation kinetics, reaction induced changes in microstructure and porosity and the concurrent evolution of the sample P and S-wave velocities were constrained. Weighing the heated samples in predefined time intervals yielded the reaction progress where the stoichiometric mass balance indicated an ultimate dehydration to anhydrite regardless of temperature. Porosity was observed to continuously increase with reaction progress from approximately 2 % for fully hydrated samples to 30 % for completely dehydrated ones, whilst the initial bulk volume was preserved. In a first set, P-wave velocity was measured at ambient conditions with ultrasonic transducers indicating a linear decrease with porosity from 5.2 km/s at 2 % to 1.0 km/s at 30 %. Results of a second set of ultrasonic measurements for both P and S-waves will be presented as well aiming at a spatially resolved wave velocity dependence on microstructure. For P-waves three different effective medium models - Voigt, Wyllie (Reuss), and Nur - were compared to the data. The linear dependence of P-wave velocity on porosity observed is best represented by the Voigt bound. The Voigt bound, however, overestimates the measured values significantly. The Wyllie-Equation (the Reuss bound) does not replicate the linear decrease in P-wave velocity with porosity and generally underestimates the data. However, at porosities above approximately 25 % the agreement with measured values is excellent. The Nur-Model yields a nonlinear dependence but replicates the data best for model-inherent critical porosities between 0.25 and 0.3. Thin section micrographs taken on selected samples reveal a sharp reaction front progressively migrating sample inwards. SEM imaging confirmed this observation, additionally showing (1) that the cylindrical outer rim consists of a highly porous network within an anhydrite matrix and (2) that the remaining inner cylinder appears unaltered at 388 K whereas bassanite needles progressively turning into anhydrite can be found at 398 K.

Milsch, H.; Priegnitz, M.

2012-04-01

344

Investigation of the phase velocities of guided acoustic waves in soft porous layers  

NASA Astrophysics Data System (ADS)

A new experimental method for measuring the phase velocities of guided acoustic waves in soft poroelastic or poroviscoelastic plates is proposed. The method is based on the generation of standing waves in the material and on the spatial Fourier transform of the displacement profile of the upper surface. The plate is glued on a rigid substrate so that it has a free upper surface and a nonmoving lower surface. The displacement is measured with a laser Doppler vibrometer along a line corresponding to the direction of propagation of plane surface waves. A continuous sine with varying frequencies was chosen as excitation signal to maximize the precision of the measurements. The spatial Fourier transform provides the wave numbers, and the phase velocities are obtained from the relationship between wave number and frequency. The phase velocities of several guided modes could be measured in a highly porous foam saturated by air. The modes were also studied theoretically and, from the theoretical results, the experimental results, and a fitting procedure, it was possible to determine the frequency behavior of the complex shear modulus and of the complex Poisson ratio from 200 Hz to 1.4 kHz, in a frequency range higher than the traditional methods. .

Boeckx, L.; Leclaire, P.; Khurana, P.; Glorieux, C.; Lauriks, W.; Allard, J. F.

2005-02-01

345

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

346

Parallel-velocity-shear-modified drift wave in negative ion plasmas  

NASA Astrophysics Data System (ADS)

A systematic investigation of the effects of a parallel velocity shear and negative ions on the collisionless drift wave instability has for the first time been realized by simultaneously using a segmented tungsten hot plate of a Q-machine and sulfur hexafluoride (SF6) gas in a magnetized potassium plasma. The parallel velocity shear of the positive ion flow tends to decrease the fluctuation level of the drift wave. The introduction of negative ions first increases the fluctuation level and then starts to decrease it at the negative ion exchange fraction of around 10%, while keeping the above-mentioned shear effect qualitatively. In addition, a simple dispersion relation based on the local model has been calculated to show that it can predict wave characteristics similar to the experimental results. Our findings provide a potential for gaining a more profound insight into the physics of space/circumterrestrial plasmas.

Ichiki, R.; Kaneko, T.; Hayashi, K.; Tamura, S.; Hatakeyama, R.

2009-03-01

347

Tomographic imaging of the P-wave velocity structure beneath the Kamchatka peninsula  

NASA Astrophysics Data System (ADS)

A total of 5270 shallow and intermediate-depth earthquakes recorded by the 32 stations of the regional seismic network of the Geophysical Service of Russia are used to assess the P-wave velocity structure beneath the Kamchatka peninsula in the Western Pacific. The tomographic inversion is carried out in three steps. First, a 1-D tomographic problem is solved in order to obtain an initial velocity model. Based on the 1-D velocity model, 3-D tomographic inversions with homogeneous and heterogeneous starting models are obtained. The Conrad (15 km depth) and Moho (35 km depth) discontinuities determined from the 1-D tomographic inversion, and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray-path determinations. Both velocity structure and hypocentral locations are determined simultaneously in the inversion. The spacing of the grid nodes is a half-degree in the horizontal direction and 20-50 km in the vertical direction. A detailed P-wave tomographic image is determined down to a depth of 200 km. The resulting tomographic image has a prominent low-velocity anomaly that shows a maximum decrease in P-wave velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes. At depth, low-velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km. These anomalies are apparently associated with the volcanic activity. The sedimentary basin of the Central Kamchatsky graben, to the west of the volcanic front, and the accretionary prism at the trench correlate with shallow low-velocity anomalies. High-velocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the Ganalsky-Valaginskoe uplift and upper crust of Shipunsky cape. The results also suggest that the subducted Pacific plate has P-wave velocities approximately 2-7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km.

Gorbatov, A.; Domínguez, J.; Suárez, G.; Kostoglodov, V.; Zhao, D.; Gordeev, E.

1999-05-01

348

Hybrid Control of an Euler-Bernoulli Beam Using Direct Velocity Feedback and Wave-Filter-Based Active Wave Control  

NASA Astrophysics Data System (ADS)

Active wave control strategy enables the inactivation of vibration mode, which is valid for suppressing the vibration of a distributed parameter structure. However, when active wave control is applied, new vibration modes are produced in the uncontrolled region. To overcome this problem, this paper proposes a novel control strategy based on a hybrid combination of direct velocity feedback (DVFB) and active wave control. The two control methods have complementary qualities; DVFB is for improving the stability, and active wave control is for its unique control effects. First, a transfer matrix method in the Laplace domain is introduced to describe wave propagation phenomena of an Euler-Bernoulli beam. Then the wave filtering method which uses point sensors is presented. Based on the filtering method, the characteristic equation and control laws of the reflected wave absorbing control are derived. Next, the independence of the two control methods in the proposed hybrid control system is investigated by a numerical simulation. This is followed by the discussion of the stability problem of the hybrid control system via a Nyquist diagram method and three types of root loci. Finally, the control effects of the proposed control system are presented, demonstrating the validity of the proposed method.

Iwamoto, Hiroyuki; Tanaka, Nobuo; Hill, Simon G.

349

Inversion of Surface Wave Phase Velocities for Radial Anisotropy to an Depth of 1200 km  

NASA Astrophysics Data System (ADS)

This study aims to evaluate three dimensional radial anisotropy to an depth of 1200 km. Radial anisotropy describes the difference in velocity between horizontally polarized Rayleigh waves and vertically polarized Love waves. Its presence in the uppermost 200 km mantle has well been documented by different groups, and has been regarded as an indicator of mantle convection which aligns the intrinsically anisotropic minerals, largely olivine, to form large scale anisotropy. However, there is no global agreement on whether anisotropy exists in the region below 200 km. Recent models also associate a fast vertically polarized shear wave with vertical upwelling mantle flow. The data used in this study is the globally isotropic phase velocity models of fundamental and higher mode Love and Rayleigh waves (Visser, 2008). The inclusion of higher mode surface wave phase velocity provides sensitivities to structure at depth that extends to below the transition zone. While the data is the same as used by Visser (2008), a quite different parameterization is applied. All the six parameters - five elastic parameters A, C, F, L, N and density - are now regarded as independent, which rules out possible biased conclusions induced by scaling relation method used in several previous studies to reduce the number of parameters partly due to limited computing resources. The data need to be modified by crustal corrections (Crust2.0) as we want to look at the mantle structure only. We do this by eliminating the perturbation in surface wave phase velocity caused by the difference in crustal structure with respect to the referent model PREM. Sambridge's Neighborhood Algorithm is used to search the parameter space. The introduction of such a direct search technique pales the traditional inversion method, which requires regularization or some unnecessary priori restriction on the model space. On the contrary, the new method will search the full model space, providing probability density function of each anisotropic parameter and the corresponding resolution.

Xing, Z.; Beghein, C.; Yuan, K.

2012-12-01

350

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

351

Rail movement and ground waves caused by high-speed trains approaching track-soil critical velocities  

Microsoft Academic Search

The increased speeds of modern trains are normally accompanied with increased transient movements of the rail and ground, which are especially high when train speeds approach some critical wave velocities in the track-ground system. These transient movements may cause large rail deflections, as well as structural vibrations and associated noise in nearby buildings. There are two main critical wave velocities

V V Krylov; A R Dawson; M E Heelis; A C Collop

2000-01-01

352

Variational Iteration Method and Homotopy-Perturbation Method for Solving Different Types of Wave Equations  

Microsoft Academic Search

Due to wide range of interest in use of wave equations to gain insight in to vibrating problems, Homotopy Perturbation Method (HPM) and Variational Iteration Method (VIM) are employed to approximate the solution of the three types of wave equations including one-dimensional wave equation, kinematic wave equation and non-linear homogeneous wave equation. The final results obtained by HPM and VIM

A. Barari; Abdoul R. Ghotbi; F. Farrokhzad; D. D. Ganji

2008-01-01

353

Velocity model of the Hronov-Po?í?í Fault Zone from Rayleigh wave dispersion  

NASA Astrophysics Data System (ADS)

The Hronov-Po?í?í Fault Zone (HPFZ) is an active tectonic area with regularly occurring shallow earthquakes up to magnitude 5. For their exact locations, at least an average velocity model of the area is needed. A method of measuring local phase velocities of surface waves using the array of stations deployed permanently in the HPFZ is introduced. Seven regional and teleseismic events are selected to represent different backazimuths of propagation. Applicable range of periods is estimated for each event. The coherency of the waves reaching the array is constraining the short period range. The dimension of the array is a limiting factor for the long-periods. A dispersion curve of Rayleigh wave phase velocity measured at the vertical component and characterizing 1D properties of the target area is determined using the seven measurements for the interval from 1 to 40 s. An isometric method is used to invert the determined dispersion curve for shear and longitudinal velocity distribution from the surface to the depth of 65 km.

Kolínský, Petr; Valenta, Jan; Málek, Ji?í

2014-07-01

354

Interferometric laser technique for accurate velocity measurement in shock wave physics  

Microsoft Academic Search

An interferometric system for continuously measuring specimen velocity is described. The Döppler shift of a laser beam reflected from the specimen's free surface is transformed into a variation of the diameter of Fabry-Perot interference rings, and the evolution of the diameter versus time is continuously recorded by a streak camera. Two examples of the use of this system for study

M. Durand; P. Laharrague; P. Lalle; A. Le Bihan; J. Morvan; H. Pujols

1977-01-01

355

PICASSO: Shear velocities in the Western Mediterranean from Rayleigh Wave tomography  

NASA Astrophysics Data System (ADS)

The Western Mediterranean has been affected by complex subduction and slab rollback, simultaneously with compression due to African-European convergence. The deformed region occupies a wide area from the intra-continental Atlas mountain belt in Morocco to the southern Iberian Massif in Spain. Evolutionary models of the Western Mediterranean invoke extensive slab rollback and compression in the Cenozoic, as well as likely upper mantle delamination scenarios during formation of the Alboran domain, the Betics, Rif, and Atlas Mountains. PICASSO (Program to Investigate Convective Alboran Sea System Overturn) is a multidisciplinary, international investigation of the Alboran System and surrounding areas. In this study we have analyzed data from the 95 PICASSO broadband stations with data from the Spanish IberArray and Siberia Array in Spain and Morocco, the University of Muenster array in the Atlas Mountains and the permanent Spanish and Portuguese networks. We present Rayleigh wave tomography results made from 168 teleseimic events recorded by 237 stations from April 2009 to April 2011. We measured Rayleigh phase velocities using the two-plane-wave method to remove complications due to multi-pathing, and finite-frequency kernels to improve lateral resolution. Phase velocities were then inverted for shear velocity structure on a grid of 0.5 by 0.5 degree to form a well-resolved 3D shear velocity model to 230 km depth. Our results show low S-velocities (2.9 km/s) in the crust beneath the Gibraltar Strait. Low upper mantle S-velocities are mapped beneath the Middle and High Atlas at ~60 km depth suggesting an elevated asthenosphere beneath these young mountain belts, in agreement with receiver functions analysis (Thurner et al, this session). Beneath the Western Alboran Sea, upper-mantle velocities change laterally from high velocities (>4.5 km/s) in the east to lower velocities to the west (~4.3 km/s). The Rayleigh wave tomography is consistent with P-tomography that shows a high velocity slab sinking vertically beneath the western Alboran Sea (Bezada et al., this session).

Palomeras, I.; Thurner, S.; Levander, A.

2012-12-01

356

Redetermination of galactic spiral density wave parameters based on spectral analysis of masers radial velocities  

NASA Astrophysics Data System (ADS)

To redetermine the Galactic spiral density wave parameters, we have performed a spectral (Fourier) analysis of the radial velocities for 44 masers with known trigonometric parallaxes, proper motions, and line-of-sight velocities. The masers are distributed in awide range of Galactocentric distances (3.5 kpc < R < 13.2 kpc) and are characterized by a wide scatter of position angles ? in the Galactic XY plane. This has required an accurate allowance for the dependence of the perturbation phase both on the logarithm of the Galactocentric distances and on the position angles of the objects. To increase the significance of the extraction of periodicities from data series with large gaps, we have proposed and implemented a spectrum reconstruction method based on a generalized maximum entropy method. As a result, we have extracted a periodicity describing a spiral density wave with the following parameters from the maser radial velocities: the perturbation amplitude f R = 7.7{-1.5/+1.7} km s-1, the perturbation wavelength ? = 2.2{-0.1/+0.4} kpc, the pitch angle of the spiral density wave i = -5{-0.9°/+0.2°}, and the phase of the Sun in the spiral density wave ? ? = -147{-17°/+3°}.

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

2012-09-01

357

An improved approximation of Bergmann's form for the Rayleigh wave velocity.  

PubMed

In the present paper an improved approximation for the Rayleigh wave velocity in isotropic elastic solids is obtained using the method of least squares. It is of Bergmann's form, i.e. the form of the ratio of two binomials. It is shown that this approximation is the best one of the Rayleigh wave velocity, in the sense of least squares, with respect to the class of functions whose elements are the ratio of two binomials. This approximation is much more accurate than Bergmann's one. Its maximum percentage error is 10 times smaller than that of Bergmann's. It is 7.6 times better than the one obtained recently by Royer and Clorennec [D. Royer, D. Clorennec, An improved approximation for the Rayleigh wave equation, Ultrasonics 46 (2007) 23-24]. An approximation of Bergmann's form for the squared Rayleigh wave velocity is also derived and its maximum percentage error is 5 times smaller than that of Royer and Clorennec's approximation. Some polynomial approximations with very high accuracy are also obtained. PMID:17825868

Vinh, Pham Chi; Malischewsky, Peter G

2007-12-01

358

Phase velocities and attentuations of shear, Lamb, and Rayleigh waves in plate-like tissues submerged in a fluid (L)  

PubMed Central

In the past several decades, the fields of ultrasound and magnetic resonance elastography have shown promising results in noninvasive estimates of mechanical properties of soft tissues. These techniques often rely on measuring shear wave velocity due to an external or internal source of force and relating the velocity to viscoelasticity of the tissue. The mathematical relationship between the measured velocity and material properties of the myocardial wall, arteries, and other organs with non-negligible boundary conditions is often complicated and computationally expensive. A simple relationship between the Lamb–Rayleigh dispersion and the shear wave dispersion is derived for both the velocity and attenuation. The relationship shows that the shear wave velocity is around 20% higher than the Lamb–Rayleigh velocity and that the shear wave attenuation is about 20% lower than the Lamb–Rayleigh attenuation. Results of numerical simulations in the frequency range 0–500?Hz are presented.

Nenadic, Ivan Z.; Urban, Matthew W.; Bernal, Miguel; Greenleaf, James F.

2011-01-01

359

Phase velocities and attenuations of shear, Lamb, and Rayleigh waves in plate-like tissues submerged in a fluid (L).  

PubMed

In the past several decades, the fields of ultrasound and magnetic resonance elastography have shown promising results in noninvasive estimates of mechanical properties of soft tissues. These techniques often rely on measuring shear wave velocity due to an external or internal source of force and relating the velocity to viscoelasticity of the tissue. The mathematical relationship between the measured velocity and material properties of the myocardial wall, arteries, and other organs with non-negligible boundary conditions is often complicated and computationally expensive. A simple relationship between the Lamb-Rayleigh dispersion and the shear wave dispersion is derived for both the velocity and attenuation. The relationship shows that the shear wave velocity is around 20% higher than the Lamb-Rayleigh velocity and that the shear wave attenuation is about 20% lower than the Lamb-Rayleigh attenuation. Results of numerical simulations in the frequency range 0-500 Hz are presented. PMID:22225009

Nenadic, Ivan Z; Urban, Matthew W; Bernal, Miguel; Greenleaf, James F

2011-12-01

360

Shear wave anisotropy imaging  

Microsoft Academic Search

Shear wave anisotropy imaging (SWAI) is a novel method that images local variations in tissue shear wave velocity. A commercial ultrasound scanner is used to generate and track propagating shear waves. Radiation force from a brief high-energy pulse generates the shear waves. The pulse and resultant shear waves are steered in order to launch the waves at oblique angles. The

Stephen J. Hsu; Mark L. Palermi; Kathryn R. Nightingale; Stephen A. McAleavey; Jeremy D. Dahl; Gregg E. Trahey

2003-01-01

361

Carotid-Femoral Pulse Wave Velocity: Impact of Different Arterial Path Length Measurements  

PubMed Central

Background Carotid-femoral pulse wave velocity (PWV) is the most established index of arterial stiffness. Yet there is no consensus on the methodology in regard to the arterial path length measurements conducted on the body surface. Currently, it is not known to what extent the differences in the arterial path length measurements affect absolute PWV values. Methods Two hundred fifty apparently healthy adults (127 men and 123 women, 19-79 years) were studied. Carotid-femoral PWV was calculated using (1) the straight distance between carotid and femoral sites (PWVcar–fem), (2) the straight distance between suprasternal notch and femoral site minus carotid arterial length (PWV(ssn–fem)-(ssn–car)), (3) the straight distance between carotid and femoral sites minus carotid arterial length (PWV(car–fem)-(ssn–car)), and (4) the combined distance from carotid site to the umbilicus and from the umbilicus to femoral site minus carotid arterial length (PWV(ssn–umb–fem)-(ssn–car)). Results All the calculated PWV were significantly correlated with each other (r=0.966-0.995). PWV accounting for carotid arterial length were 16-31% lower than PWVcar–fem. PWVcar–fem value of 12 m/sec corresponded to 8.3 m/sec for PWV(ssn–fem)-(ssn–car), 10.0 m/sec for PWV(car–fem)-(ssn–car), and 8.9 m/sec for PWV(ssn–umb–fem)-(ssn–car). Conclusion Different body surface measurements used to estimate arterial path length would produce substantial variations in absolute PWV values.

Sugawara, Jun; Hayashi, Koichiro; Yokoi, Takashi; Tanaka, Hirofumi

2009-01-01

362

Bifurcations of dust ion acoustic travelling waves in a magnetized dusty plasma with a q-nonextensive electron velocity distribution  

SciTech Connect

Nonlinear ion acoustic waves in the magnetized dusty plasma in the presence of superthermal electron have been studied. We have used the reductive perturbation method to derive a Kadomtsev-Petviashili equation for dust ion acoustic waves in a magnetized dusty plasma with q-nonextensive velocity distributed electrons. By applying the bifurcation theory of planar dynamical systems to this equation, the existence of solitary wave solutions and periodic travelling wave solutions is proved. Two exact solutions of the above waves are obtained.

Kumar Samanta, Utpal [Department of Mathematics, Bankura Christian College, Bankura, West Bengal (India); Department of Mathematics, Siksha Bhavana, Visva Bharati University, Santiniketan (India); Saha, Asit [Department of Mathematics, Sikkim Manipal Institute of Technology, Majitar, Rangpo, East-Sikkim (India); Chatterjee, Prasanta [Department of Mathematics, Siksha Bhavana, Visva Bharati University, Santiniketan (India)

2013-02-15

363

Compressional and EM wave velocity anisotropy in a temperate glacier due to basal crevasses and implications for water content estimation  

NASA Astrophysics Data System (ADS)

Radar velocity measurements have long been used to infer englacial water content via isotropic mixing rules. However, when the water is present in a fracture system with preferred orientation, failure to account for the resultant anisotropy can result in serious over- or underestimation of the volumetric water content. We have conducted a series of experiments designed to investigate elastic and electromagnetic (EM) velocity anisotropy associated with a preferentially aligned fracture system on a temperate valley glacier in south-central Alaska. Video in 28 boreholes coupled with a variety of geophysical observations, provide evidence that the extensive fracture system provides a substantial reservoir for englacial water storage. Basal crevassing produces the fracture system and borehole measurements have shown that the fractures are water-filled, subvertical, and have a preferred orientation distributed over an azimuthal aperture of approximately 30 degrees. We conducted a 3D compressional wave (P-wave) seismic reflection survey with the primary objective of identifying azimuthal anisotropy in the velocity structure which would then determine the preferred orientation of the fracture system. The 3D survey patch was 300 x 300 m with a 214 channel recording system arranged in a 100 m x 100 m checkerboard pattern. Shooting geometry was designed to ensure 90 degrees of azimuthal coverage over the majority of the CDP grid. Results of multi-azimuthal velocity analysis show azimuthal anisotropy of over 3% with a velocity uncertainty of 0.8% or less along three primary azimuthal components. To investigate anisotropy of the dielectric permittivity we conducted a multi-azimuth, multi-offset, polarimetric radar reflection experiment. Data were acquired with a 25 MHz pulsed radar system along 5 azimuths ranging from 0 to 94 degrees relative to the direction of glacier flow. We repeated the measurements with TE, TM, and cross-polarized antenna configurations. A range of radar velocities were recorded that varied systematically with azimuth and antenna polarization. Anisotropic analysis of these variations shows the mean orientation of the fractures and these results are consistent with seismic and borehole measurements. Isotropic analysis would incorrectly yield two widely varying estimates; polarization parallel to the fracture system would result in a water content estimate of about 3.8% whereas polarization orthogonal to the fracture system gives 0.3%. Anisotropic analysis of the polarimetric data gives a single volumetric water content estimate of 1.2%. We conclude that meaningful estimates of water content in glaciers based on EM velocity measurements require collecting data such that the presence of anisotropy can be evaluated and an anisotropic analysis employed when necessary.

Bradford, J. H.; Nichols, J.; Harper, J. T.

2011-12-01

364

Composite materials characterization using phase velocities measured with laser generated and detected ultrasonic waves  

NASA Astrophysics Data System (ADS)

When a laser beam is focused on the surface of a composite material, anisotropy gives rise to folded ray curves in which the acoustic rays are more concentrated in some directions than in others. In particular, the energy density can be very high at the cuspidal edge. The propagation in such media gives rise to internal diffraction by which waves are observed in directions that are not included in the cusp sector. These waves are not explained by ray theory. The measurement of the stiffness tensor of an anisotropic material by means of laser generated ultrasound is a nontrivial matter for essentially two reasons. First, the recovering of the coefficients from the group velocities is a double iterative numerical process that requires a high accuracy in the velocity measurement. Second, internal diffraction is not taken into account by such an algorithm and it provides undesired velocity data which induces a shift of the identified stiffness coefficients. In this paper, it is shown both numerically and experimentally that phase velocities can be measured using signals generated by a line source. The measurement of the stiffness coefficients from such velocities avoids the aforementioned difficulties. .

Reverdy, F.; Audoin, B.

2000-05-01

365

Pn Wave velocities beneath the Ethiopian Plateau and Main Ethiopian Rift.  

NASA Astrophysics Data System (ADS)

In this study, uppermost P mantle wave velocities beneath Ethiopia have been modeled using P-wave travel time data from the Ethiopia Broadband Seismic Experiment. The study region covers the rift faults of the Cenozoic East African rift system in Ethiopia, which have developed mainly within the Proterozoic Mozambique belt. Over 3000 travel times from some 250 local and regional earthquakes have been used in a Pn tomography to image uppermost mantle structure. The earthquakes are located mainly within the rift system. The highest density of crossing ray path exist within and surrounding the Main Ethiopian Rift (MER) and the western side of the Ethiopian Plateau. In the tomography, Pn travel times were inverted using a damped generalized inverse algorithm. Results show that Pn velocities are between 7.9-8.1 km/s beneath the eastern and western sides of the Ethiopian Plateau, as well as beneath the southern end of the MER. This is in contrast to Pn velocities of about 7.6 km/s found beneath the central and northern parts of the MER. The change in Pn velocity along the MER from about 7.9 km in the south to about 7.6 km in the north coincides with a change in the volcanic activity in the MER, with more recent volcanism found in the central and northern MER. These results are consistent with Pn velocities obtained from refraction profiles in a number of locations.

Brazier, R. A.; Nyblade, A. A.

2006-12-01

366

Rayleigh Wave Phase Velocity Structure of the Cameroon Volcanic Line Region  

NASA Astrophysics Data System (ADS)

The Cameroon Volcanic Line (CVL) is an 1800 km lineament of Cenozoic volcanism that extends from the Gulf of Guinea into Central Africa. With most of the line having experienced recent holocene volcanism, the CVL is one of the more prominent volcanic tracks exhibiting little to no discernible age progression. Previous interpretations of the CVL have been diverse and included volcanic activity associated with laterally transported material from a plume, multiple plumes, rift-flank processes, plate-driven Richter rolls, plume-fed small-scale convection cells, or propagating lithosphere cracks in order to reconcile the lack of age progression with the strong linearity and position of the CVL. Discriminating between these models can be significantly aided by the determination of the extent and depth of the source of the volcanism. In early 2005, a dense broadband seismometer array located over the continental section of the CVL was deployed to image the underlying mantle structure and resolve the source of the volcanism. This two-year PASSCAL deployment consisted of 8 trial stations for the first year and the following year was upgraded to 32 stations with a station spacing of 50 to 200 km. We present preliminary results of surface wave phase velocity maps from this new dataset that successfully image a low-velocity anomaly beneath the CVL as well as high velocities associated with the lithosphere of the Congo Craton. Phase velocity maps were determined by an inversion of teleseismic Rayleigh waves with Forsyth and Lee's two-plane wave method to reduce bias from multipathing. The low-velocity anomaly is observed over a period range of 20 to 140 s suggesting that the CVL anomaly extends across a substantial depth range of the lithosphere and asthenosphere. Furthermore, the lowest velocities of the anomaly are centered at the junction of the 'Y' in the volcanic track, 400 km northeast (inland) of the CVL's largest active volcano Mt. Cameroon.

Shore, P.; Euler, G. G.; Tibi, R.; Wiens, D. A.; Larson, A. M.; Nyblade, A.; Tokam, A. P.; Tabod, C.; Nnange, J. M.; Ateba, B.

2007-12-01

367

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.