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1

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

Microsoft Academic Search

Intraseasonal vertical velocity variation investigated using observation dataUpwelling event in the central equatorial Indian OceanStructure of the quasi-biweekly mixed Rossby-gravity wave in the ocean

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

2011-01-01

2

Performance testing of lead free primers: blast waves, velocity variations, and environmental testing  

E-print Network

Results are presented for lead free primers based on diazodinitrophenol (DDNP)compared with tests on lead styphnate based primers. First, barrel friction measurements in 5.56 mm NATO are presented. Second, shot to shot variations in blast waves are presented as determined by detonating primers in a 7.62x51mm rifle chamber with a firing pin, but without any powder or bullet loaded and measuring the blast wave at the muzzle with a high speed pressure transducer. Third, variations in primer blast waves, muzzle velocities, and ignition delay are presented after environmental conditioning (150 days) for two lead based and two DDNP based primers under cold and dry (-25 deg C,0% relative humidity), ambient (20 deg C, 50% relative humidity), and hot & humid (50 deg C, 100% relative humidity) conditions in 5.56 mm NATO. Taken together, these results indicate that DDNP based primers are not sufficiently reliable for service use.

Courtney, Elya; Summer, Peter David; Courtney, Michael

2014-01-01

3

A preliminary investigation of the effect of compositional variation on compressional-wave velocity in transversely isotropic media  

E-print Network

A PRELIMINARY INVESTIGATION OF THE EFFECT OF COMPOSITIONAL VARIATION ON COMPRESSIONAL-WAVE VELOCITY IN TRANSVERSELY ISOTROPIC MEDIA A Thesis by PATRICR JOHN MELIA Submitted to the Graduate College of Texas A&M University in partial... fulfillment of the requirement for the degree of MASTER OF SCIENCE August 1983 Ma)or Sub)ect: Geophysics A PRELIMINARY INVESTIGATION OF THE EFFECT ON COMPOSITIONAL VARIATION ON COMPRESSIONAL-WAVE VELOCITY IN TRANSVERSELY ISOTROPIC MEDIA A Thesis...

Melia, Patrick John

2012-06-07

4

Variation of seismic-wave velocities in westerly granite under stress  

E-print Network

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

Al-Shaibani, Abdulaziz Muhareb

2012-06-07

5

Energy momentum, wave velocities and characteristic shocks in Euler's variational equations with application to the Born-Infeld theory  

NASA Astrophysics Data System (ADS)

We consider the Euler's variational equations deriving from a general Lagrangian L(??qr,qs). Under the assumption of convexity of energy, we write down some inequalities for the energy-momentum tensor including Hawking-Ellis energy conditions. We show that there exists the same number of positive and negative wave velocities and no velocity can change sign. Finally, we study the structure of the characteristic shocks with particular attention to the generalized Born-Infeld Lagrangian describing the electron with spin.

Boillat, Guy; Ruggeri, Tommaso

2004-09-01

6

Variation of P-Wave Velocity before the Bear Valley, California, Earthquake of 24 February 1972.  

PubMed

Residuals for P-wave traveltimes at a seismnograph station near Bear Valley, California, for small, precisely located local earthquakes at distances of 20 to 70 kilometers show a sharp increase of nearly 0.3 second about 2 months before a magnitude 5.0 earthquake that occurred within a few kilometers of the station. This indicates that velocity changes observed elsewhere premonitory to earthquakes, possibly related to dilatancy, occur along the central section of the San Andreas fault system. PMID:17784227

Robinson, R; Wesson, R L; Ellsworth, W L

1974-06-21

7

Velocities under Water Waves  

NSDL National Science Digital Library

This site from the Center of Applied Coastal Research at the University of Delaware features an interactive animation of wave form that users can modify then watch the results of their input. Parameters include wave height, wave period, and local depth. The resulting animation shows the associated water particle motion, plus the velocity vector of the user's chosen wave.

Dalrymple, Robert A.; Delaware, University O.

8

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. B1, PAGES 393-405, JANUARY 10, 1987 P Wave Velocity Variations in the Coso Region, California,  

E-print Network

the Long Valley caldera of eastern California. They identify areas of low P wave velocity with dimensionsof of southeasternCalifornia in order to spatially locate any upper crustal velocity anomalies.Like Long Valley velocity variations in the Indian Wells Valley-Coso region of southeasternCalifornia. The residuals

Clayton, Robert W.

9

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

10

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

NASA Astrophysics Data System (ADS)

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

Tsuji, Sugane; Hiramatsu, Yoshihiro

2014-12-01

11

LOGARITHMIC VELOCITY VARIATIONS FOR LONGITUDINAL ULTRASONIC WAVES IN CUBIC BaTiO3 NEAR  

E-print Network

defined values for the critical exponents of ultra- sonic velocity and attenuation; these last two quan typical ferroelectric crystals, has many properties which are not quite elucidated at this time. As it is possible at present to obtain large ( ~ 1 cm3) single crystals of good crystal- lographic quality, we have

Paris-Sud XI, Université de

12

Physics of the Earth and Planetary Interiors 154 (2006) 180195 Constraining P-wave velocity variations in the upper  

E-print Network

Physics of the Earth and Planetary Interiors 154 (2006) 180­195 Constraining P-wave velocity¨oz Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 sensitivity kernels to combine the datasets, and mantle structure was parameterized with an irregular grid

van der Hilst, Robert Dirk

13

S velocity variations beneath North America  

Microsoft Academic Search

We investigate S velocity variation in the upper mantle beneath North American to better understand the effects of data heterogeneity, model parameterization, and regularization. To this end, we analyzed and fit regional S and Rayleigh wave trains generated by earthquakes around North America that occurred between the years 2000 through 2006, including waveforms from the Transportable Array stations of EarthScope's

Heather Bedle; Suzan van der Lee

2009-01-01

14

Temporal velocity variations beneath the Coso geothermal field observed using seismic double difference tomography of compressional and shear wave arrival times  

NASA Astrophysics Data System (ADS)

Microseismic imaging can be an important tool for characterizing geothermal reservoirs. Since microseismic sources occur more or less continuously due to the operations of a geothermal plant and the naturally occurring background seismicity, passive seismic monitoring is well suited for quantifying the temporal variations in reservoir properties that occur within the geothermal reservoir during production. In this study we will use microseismic data recorded between 1996 and 2008 to investigate the temporal variations in seismic velocity below the Coso geothermal field in California. In this study we will apply the double difference tomography method to simultaneously locate a suite of microseismic events and determine the compressional and shear wave velocity as well as their ratio. The double-difference method uses both absolute and relative arrival times of earthquakes measured at the same station, which allows a more precise determination of the relative locations of earthquakes. In particular, we apply a cross-correlation technique to improve the measurement of relative traveltimes. The large number of microearthquakes observed between 1996 and 2008 allows us to characterize subsurface velocity and to investigate changes in velocity that accompany production from the geothermal reservoir.

Seher, T.; Zhang, H.; Fehler, M. C.; Newman, G. A.

2011-12-01

15

Shear wave splitting and velocity variations measured from noise auto-correlation reveal crack healing after the 2007 Chuetsu-Oki earthquake in Japan  

NASA Astrophysics Data System (ADS)

The 17 July 2007 Mw=6.7 Chuetsu-Oki earthquake occurred within a dense network of permanent seismometers, making it an excellent place to examine time variations of seismic properties. We use seismic noise analysis of auto-correlated waveforms (ACF) for stations in the region to determine isotropic velocity variations. ACFs are computed from the band-pass filtered seismic noise portion recorded with each short-period seismometer for each day. A new multiple filter automatic shear wave splitting measurement technique is applied to S phases from aftershocks to determine fast polarization (?) of the initial wave and delay time (dt) between the two split shear waves. We have examined two stations, IZUMOZ and N.KZKF, whose S arrival times were determined by the JMA network operators, and which are both at the edge of the rupture area. Autocorrelation functions show a sharp jump in the phase shift (change of lag time) of about 0.05 s immediately after the mainshock, followed by a decay to background levels over a period of about one month. These changes are seen on several phases at different lag times after the main shock. The phases are interpreted as reflections from interfaces at 8 km - 10 km depth. Before the mainshock, few earthquakes occurred in the region with which to measure shear wave splitting, but afterwards many aftershocks allowed us to determine a decay of dt over time. Average time delays between the two split shear waves decrease by about 0.1 s after the mainshock, following a decay curve that is similar to that of the phase shifts in the ACFs interpreted as isotropic velocity changes. Both stations exhibit similar dt decays, and such decays are seen in earthquakes from several different sections of the aftershock zone. ? also shows a change in rotation that correlates with the time delay changes. We suggest that after the earthquake, cracks opened up below the stations to decrease isotropic velocities and increase anisotropy. Crack healing over a period of a month returned both the isotropic and anisotropic velocities to their background values. (Left scale) Individual splitting delay time measurements (green dots) and moving averages over window of 10 and 20 points, plotted as a function of time. (Right scale) 5-day moving average of autocorrelation function phase shifts for phases A, B, and C, of which lag times are 2.8 s - 3.5 s. All for station IZUMOZ.

Savage, M. K.; Ohmi, S.

2010-12-01

16

Gravity Water Wave Refractions Using Group Velocity.  

National Technical Information Service (NTIS)

The report documents the incorporation of group velocity into an existing digital computer program which uses phase velocity for calculating gravity water wave rays. Multiperiod rays are used instead of single-period rays. The original gravity water wave ...

K. C. Matson

1970-01-01

17

1 Problem Galilean Transformation of Wave Velocity  

E-print Network

In Galilean relativity an object with velocity u in one inertial frame appears to have velocity u ? = u ? v (1) to an observer moving with velocity v with respect to the original frame. What is the Galilean transformation for the phase velocity up = ? ˆ k/k of a wave of angular frequency ?>0 and wave vector k, for waves where the phase velocity is much less than the speed of light? Show that the component of phase velocity perpendicular to velocity v is not invariant under the Galilean transformation, so that the sign of the transverse velocity can be opposite in different frames of reference. Discuss also the Galilean transformation of group velocity, and of the wave equation. Restrict your discussion to classical physics. The possibly paradoxical behavior of Galilean transformations of quantum theoretic wave functions is reviewed in [1]. For completeness, consider also the Lorentz transformations of phase and group velocity.

José Luis; Junquera Fernandez-diez; Kirk T. Mcdonald

18

Compressional and Shear Wave Velocities in Meteorites  

NASA Astrophysics Data System (ADS)

Elastic properties of asteroids govern much of their response to impact events, and are germane to impact mitigation and exploitation strategies. However, elastic wave velocities have been one of the least explored physical quantities of meteorites. Velocities measured to date (roughly 30 determinations) show a wide range for both compression and shear wave velocities and a strong dependence upon porosity. We have measured compressional and shear wave velocities in 72 meteorites. Meteorites were measured in the form of slabs as commonly found in collections. Pieces with two roughly parallel flat sides are required to accommodate the velocity transducers' shape. Each measurement was made three times at different positions to improve statistics and to test for sample variation. Bulk densities were measured with a modified Archimedean method using ~1 mm-diameter glass beads. Porosities were measured for 9 specimens in a commercial He pycnometer. Porosities for other meteorites measured were taken from literature sources. Bulk and grain densities and porosities were generally found to be consistent with previous work, although rare large discrepancies were found. Seismic velocities were also found to be generally similar to previously obtained values. Various relationships have been quantified; velocity vs. fall date, class, petrologic type, porosity, bulk density, and darkness. Derived relationships were generally restricted to only the data obtained for ordinary chondrites, as they constituted the largest sampled population (and presumably represent the bulk of the undifferentiated asteroids). Falls from the last 50 years exhibit a smaller range of velocities indicating that older samples have experienced variable subtle weathering effects (probably crack propagation vs. filling pores by secondary minerals) from exposure to the Earth's atmosphere. This indicates that meteorite physical property studies as proxies for asteroidal properties must be restricted to the freshest possible material, and that recently fallen meteorites must be curated in inert atmospheres. Velocities inversely correlate to porosities as found in previous work, but were also found to correlate (inversely) to petrologic type in contrast to previous work. Possible darkening due to weathering complicates interpreting the observed correlation between velocity and the darkness of a meteorite (presumed to be related to shock history).

Hons, M. S.; Hildebrand, A. R.

2004-05-01

19

Seasonal cycles of seismic velocity variations detected using coda wave interferometry at Fogo volcano, São Miguel, Azores, during 2003-2004  

NASA Astrophysics Data System (ADS)

Fogo volcano is an active central volcano, with a lake filled caldera, in the central part of São Miguel Island, Azores, whose current activity is limited to hydrothermal manifestations such as active fumarolic fields, thermal and CO 2 cold springs and soil diffuse degassing areas. It is affected by important active tectonic structures, with high seismic activity and practically continuous micro-seismicity. A recurrent feature from the seismicity observed in volcanic regions is the occurrence of clusters of similar earthquakes, whose origin can be attributed to the repeated action of a similar source mechanism at the same focal area. Doublets/multiplets were identified in this study within a catalogue of small magnitude (usually < 3) volcano tectonic events recorded in 2003-2004 by a selection of stations around Fogo volcano. All events have been cross-correlated and pairs whose waveforms exhibited a cross-correlation coefficient equal to or higher than 0.9 were analysed using the coda-wave interferometry technique. Subtle velocity variations found between events highlight a seasonal cycle of the velocity patterns, with lower velocity in winter time and higher velocity during summer months. Those results, together with quantitative differences between the same doublets at different stations, exhibit an excellent correlation with rainfall. A seasonal effect can also be broadly seen in the seismicity occurrence, and some of the swarms recorded over the two year period occur during the wettest season or close to episodes of abundant (above average) rainfall. Moreover, temporal and spatial analysis of several swarms highlighted the lack of any mainshock-aftershock sequence and organized migration of the hypocenters. This is suggestive of a very heterogeneous stress field. Vp/Vs is found to be lower than usually observed in volcanic areas, an occurrence likely related to the presence of steamy fluid associated with the geothermal system. Taken together, these observations suggest that pore pressurisation plays a major role in controlling a considerable part of the recorded seismicity. The geothermal fluids around Fogo massif have been identified as derived from meteoric water, which infiltrates through Fogo Lake and the volcano flanks and flows from south to north on the northern flank. All those elements seem to point to a role played by rainfall in triggering seismicity at São Miguel, possibly through pressure changes at depth in response to surface rain and/or an interaction with the geothermal system.

Martini, Francesca; Bean, Christopher J.; Saccorotti, Gilberto; Viveiros, Fatima; Wallenstein, Nicolau

2009-04-01

20

Joint inversion of surface wave velocity and gravity observations and its application to central Asian basins shear velocity structure  

Microsoft Academic Search

We implement and apply a method to the jointly inverted of surface wave group velocities and gravity anomalies observations. Surface wave dispersion measurements are sensitive to seismic shear wave velocities, and the gravity measurements supply constraints on rock density variations. Our goal is to obtain a self-consistent three-dimensional shear velocity–density model with increased resolution of shallow geologic structures. We apply

Monica Maceira; Charles J. Ammon

2009-01-01

21

Shear wave velocities in the earth's mantle.  

NASA Technical Reports Server (NTRS)

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

Robinson, R.; Kovach, R. L.

1972-01-01

22

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

23

Periodic radial velocity variations in RU Lupi  

NASA Astrophysics Data System (ADS)

Context: RU Lup is a Classical T Tauri star with unusually strong emission lines, which has been interpreted as manifestations of accretion. Recently, evidence has accumulated that this star might have a variable radial velocity. Aims: We intended to investigate in more detail the possible variability in radial velocity using a set of 68 high-resolution spectra taken at the VLT (UVES), the AAT (UCLES) and the CTIO (echelle). Methods: Using standard cross-correlation techniques, we determined the radial velocity of RU Lup. We analysed these results with Phase-dispersion minimization and the Lomb-Scargle periodogram and searched for possible periodicities in the obtained radial velocities. We also analysed changes in the absorption line shapes and the photometric variability of RU Lup. Results: Our analysis indicated that RU Lup exhibits variations in radial velocity with a periodicity of 3.71 days and an amplitude of 2.17 km s-1. These variations can be explained by the presence of large spots, or groups of spots, on the surface of RU Lup. We also considered a low-mass companion and stellar pulsations as alternative sources for these variations but found these to be unlikely.

Stempels, H. C.; Gahm, G. F.; Petrov, P. P.

2007-01-01

24

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

25

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

E-print Network

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

Paris-Sud XI, Université de

26

Lateral variations in lower mantle seismic velocity  

NASA Technical Reports Server (NTRS)

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

Duffy, Thomas S.; Ahrens, Thomas J.

1992-01-01

27

Copernicus observations of Iota Herculis velocity variations  

NASA Technical Reports Server (NTRS)

Observations of Iota Her at 109.61-109.67 nm obtained with the U1 channel of the Copernicus spectrophotometer at resolution 5 pm during 3.6 days in May, 1979, are reported. Radial-velocity variations are detected and analyzed as the sum of two sinusoids with frequencies 0.660 and 0.618 cycles/day and amplitudes 9.18 and 8.11 km/s, respectively. Weak evidence supporting the 13.9-h periodicity seen in line-profile variations by Smith (1978) is found.

Rogerson, J. B., Jr.

1984-01-01

28

Eurasian surface wave tomography: Group velocities  

Microsoft Academic Search

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

Michael H. Ritzwoller; Anatoli L. Levshin

1998-01-01

29

Variational Principles for Water Waves  

E-print Network

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

Boris Kolev; David H. Sattinger

2007-12-01

30

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

31

Compressional and shear wave velocities in granular materials to 2.5 kilobars  

NASA Technical Reports Server (NTRS)

The velocities of seismic compressional waves and, for the first time, shear wave velocities in silica sand, volcanic ash, and basalt powder were determined under hydrostatic confining pressures to 2.5 kb. Simultaneously, the porosity of these materials was obtained as a function of confining pressure. The presented results have important implications for the self-compaction hypothesis that has been postulated to explain the lunar near-surface seismic velocity variation.

Talwani, P.; Nur, A.; Kovach, R. L.

1973-01-01

32

Tomographic imaging of P and S wave velocity structure beneath northeastern Japan  

Microsoft Academic Search

The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexity shaped seismic velocity discontinuities and with three-dimensional variations in the velocities in the modeling space. This method is applied to 18,679 arrival times from 470 shallow and intermediate-depth earthquakes in order to study P and S wave

Dapeng Zhao; Akira Hasegawa; Shigeki Horiuchi

1992-01-01

33

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

34

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

35

Anisotropic Rayleigh wave phase velocity maps of eastern China  

NASA Astrophysics Data System (ADS)

We explore the variations of Rayleigh wave phase velocity beneath eastern China in a broad period range (20-200 s). Rayleigh wave dispersion curves are measured by the two-station technique for a total of 734 interstation paths using vertical component broadband waveforms at 39 seismic stations in eastern China from 466 global earthquakes. In addition, 599 waveform inversion interstation measurements were added to this data set. The interstation dispersion curves are then inverted for high-resolution isotropic and azimuthally anisotropic phase velocity maps at periods between 20 and 200 s. At shorter periods sampling the crustal depth range, phase velocities are higher in the southeastern part of the region, reflecting the thinner crust there. The Jiangnan Belt separates Cathaysia from the Yangtze Craton, the latter with thicker crust and a deep, high-velocity cratonic root. The eastern part of Yangtze Craton, however, east of 115-116°E, does not display a deep root and has a thin lithosphere. Azimuthal anisotropy at long periods (>120 s) shows fast propagation directions broadly similar to that of the absolute plate motion. Beneath Cathaysia and eastern Yangtze Craton, anisotropy in the asthenosphere is strong and suggests coast-perpendicular flow. Asthenospheric flow from beneath China's thick continental lithosphere toward the thinner lithosphere of the margin and the resulting decompression melting may be the fundamental causes of the intraplate basaltic volcanism along the eastern coast of China.

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

2014-06-01

36

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

NASA Technical Reports Server (NTRS)

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

Nakanishi, I.; Anderson, D. L.

1983-01-01

37

Rayleigh-wave Phase-velocity Maps beneath Eastern China  

NASA Astrophysics Data System (ADS)

Eastern China is a geologically complex region with strong lateral changes in Moho depth. It is also a tectonically active region with active faults and protocratonic units. We investigated the variations of isotropic and anisotropic Rayleigh-wave phase velocity beneath eastern China using broadband records at 38 stations with roughly even distribution from the China National Seismic Network. Rayleigh-wave dispersion curves are manually measured by the two-station technique for a total of 741 inter-station paths from the vertical-component waveforms. We complemented this dataset with 599 automated inter-station measurements. When selecting the data, we imposed an upper bound of 10° for the angle between the great circle connecting a pair of stations and the great circle connecting the stations and the event. The inter-station distances are in the range 250-2500 km, enabling phase-velocity measurements over a broad period range, 8-200 s. We extracted 59306 records from 438 events with epicentral distances between 10° and 170°. These dispersion curves are then inverted using the LSQR algorithm for the high-resolution isotropic and azimuthally anisotropic phase-velocity maps at selected periods between 16 and 200 s. The isotropic as well as anisotropic models of Rayleigh-wave phase velocities we obtain are consistent with the tectonic features observed in this region. Furthermore, the anisotropic anomalies we observe are compatible with previous SKS splitting measurements. Interestingly, we observe different azimuthal anisotropy patterns in several distinct period ranges, suggesting both lateral and depth variations of azimuthal anisotropy in this region. At crustal depths, the isotropic structure exhibits a clear contrast between the Yangtze Craton in the southeast, which appears faster than regional average by up to 5%, and the northwest region, which is slower than average by about 3-4%. The Jiangnan Belt separates regions with different velocity expressions, indicating a possible suture between the Yangtze Craton and Cathaysia Unit. Geodynamical interpretations for azimuthal anisotropy patterns are proposed in specific regions, in particular an abrupt change in Moho depth in the eastern border of the Tibetan Plateau, and a lithospheric thinning beneath the Cathaysia Unit. The anisotropic structures are in agreement with absolute plate motion and the overall upper-mantle flow. However, more local variations are related to the tectonics of the region, such as the border between the Ordos and Sichuan Basins.

Legendre, C. P.; Deschamps, F.; Zhao, L.; Lebedev, S.; Chen, Q.

2013-12-01

38

A simple method of predicting S-wave velocity  

USGS Publications Warehouse

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

Lee, M. W.

2006-01-01

39

Variation of Flow Propagation Velocity with Age  

Microsoft Academic Search

Flow propagation velocity is a new color Doppler M-mode measurement of left ventricular filling characteristics. This study was designed to establish normal values for this measurement in healthy individuals and to compare these findings with pulsed Doppler transmitral velocities. Complete M-mode, two-dimensional, and Doppler echocardiographic studies were performed on 64 volunteers between 21 and 79 years of age. Significant negative

David M. Mego; Vincent S. DeGeare; Sheri Y. Nottestad; Vernadette P. Lamanna; Lori C. Oneschuk; Bernard J. Rubal; Miguel Zabalgoitia

1998-01-01

40

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

41

P/n/ velocity and cooling of the continental lithosphere. [upper mantle compression waves in North America  

NASA Technical Reports Server (NTRS)

The average upper mantle compressional wave velocity and heat flow figures presently computed for continental physiographic provinces in North America exhibit an inverse relationship, and possess a statistically significant correlation coefficient. A correlation is also demonstrated between compressional wave velocity and material temperature by estimating crust-mantle boundary temperatures from heat flow values. The dependency of compressional wave velocity on temperature implies that the observed geographical distribution in upper mantle seismic velocity may be due to the temperature effect character of upper mantle compressional wave velocity variation.

Black, P. R.; Braile, L. W.

1982-01-01

42

Velocity diffusion in plasma waves excited by electron beams  

NASA Astrophysics Data System (ADS)

New results provided by numerical simulations of the weak instability of a warm electron beam in a collisionless plasma are presented. The theoretical model considers the self-consistent resonant interactions of beam particles with wave packets of broad spectra; it is derived using some of the initial approximations of the standard derivation of the quasilinear diffusion equation in the weak turbulence approach, without, however, the assumption of randomly phased waves. A huge number of particle trajectories calculated over long times by a symplectic code are analyzed using various statistical algorithms. The dynamics of the beam relaxation and the saturation of the wave spectrum are studied and compared with the analytical solutions provided by the quasilinear theory of weak turbulence. The most interesting results concern the presence of strong and persistent irregularities in the wave energy spectrum at saturation, which are linked to large velocity variations observed in the particles' dynamics and to non-Gaussian local diffusion. Quantitative estimates of the diffusion coefficients are given and compared with predictions of the weak turbulence theory.

Volokitin, A.; Krafft, C.

2012-08-01

43

P And S Wave Velocity Determination  

E-print Network

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

Willis, M. E.

1983-01-01

44

Radial Velocity Variations in K Giants: Planets or Pulsations?  

E-print Network

Abstract: Radial velocity observations of K giants have revealed periodic variations for some of the stars. The periods range from approximately 180 to 1000 or more days with velocity amplitudes ranging from approximately 50 m/s to 500 m/s. Except for Iota Draconis (Frink et al. 2002), all variations are nearly sinusoidal. Companions with zero orbital eccentricity as well as pulsations may cause these radial velocity variations. Additional high resolution spectra were taken in order to reveal which of these mechanisms is at work. No conclusions can be drawn from this work yet, although it seems likely that at least for some stars pulsations are the best explanation. A theoretical study is performed to investigate what kind of pulsations could cause these long period radial velocity variations. 1

Saskia Hekker; Sabine Reffert; Andreas Quirrenbach

2005-01-01

45

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

46

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

Microsoft Academic Search

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver

Jianghai Xia; Richard D. Miller; Choon B. Park; James A. Hunter; James B. Harris; Julian Ivanov

2002-01-01

47

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

48

Shear velocity structure of the Mariana mantle wedge from Rayleigh wave phase velocities  

Microsoft Academic Search

We examine the seismic structure of the Mariana mantle wedge using data from a combined deployment of ocean bottom seismographs and land stations in 2003–2004. We measure Rayleigh wave phase velocities and invert these results for the shear velocity structure and azimuthal anisotropy in the region. In the back-arc region low phase velocities indicate shear velocities as low as 3.9

Moira L. Pyle; Douglas A. Wiens; Dayanthie S. Weeraratne; Patrick J. Shore; Hajime Shiobara; Hiroko Sugioka

2010-01-01

49

Measurement of velocity field in parametrically excited solitary waves  

E-print Network

Paramerically excited solitary waves emerge as localized structures in high-aspect-ratio free surfaces subject to vertical vibrations. Herein, we provide the first experimental characterization of the hydrodynamics of thess waves using Particle Image Velocimetry. We show that the underlying velocity field of parametrically excited solitary waves is mainly composed by an oscillatory velocity field. Our results confirm the accuracy of Hamiltonian models with added dissipation in describing this field. Remarkably, our measurements also uncover the onset of a streaming velocity field which is shown to be as important as other crucial nonlinear terms in the current theory. The observed streaming pattern is particularly interesting due to the presence of oscillatory meniscii.

Gordillo, Leonardo

2014-01-01

50

Implications of elastic wave velocities for Apollo 17 rock powders  

NASA Technical Reports Server (NTRS)

Ultrasonic P- and S-wave velocities of lunar rock powders 172701, 172161, 170051, and 175081 were measured at room temperature and to 2.5 kb confining pressure. The results compare well with those of terrestrial volcanic ash and powdered basalt. P-wave velocity values up to pressures corresponding to a lunar depth of 1.4 km preclude cold compaction alone as an explanation for the observed seismic velocity structure at the Apollo 17 site. Application of small amounts of heat with simultaneous application of pressure causes rock powders to achieve equivalence of seismic velocities for competent rocks.

Talwani, P.; Nur, A.; Kovach, R. L.

1974-01-01

51

Calculating wave-generated bottom orbital velocities from surface-wave parameters  

NASA Astrophysics Data System (ADS)

Near-bed wave orbital velocities and shear stresses are important parameters in many sediment-transport and hydrodynamic models of the coastal ocean, estuaries, and lakes. Simple methods for estimating bottom orbital velocities from surface-wave statistics such as significant wave height and peak period often are inaccurate except in very shallow water. This paper briefly reviews approaches for estimating wave-generated bottom orbital velocities from near-bed velocity data, surface-wave spectra, and surface-wave parameters; MATLAB code for each approach is provided. Aspects of this problem have been discussed elsewhere. We add to this work by providing a method for using a general form of the parametric surface-wave spectrum to estimate bottom orbital velocity from significant wave height and peak period, investigating effects of spectral shape on bottom orbital velocity, comparing methods for calculating bottom orbital velocity against values determined from near-bed velocity measurements at two sites on the US east and west coasts, and considering the optimal representation of bottom orbital velocity for calculations of near-bed processes. Bottom orbital velocities calculated using near-bed velocity data, measured wave spectra, and parametric spectra for a site on the northern California shelf and one in the mid-Atlantic Bight compare quite well and are relatively insensitive to spectral shape except when bimodal waves are present with maximum energy at the higher-frequency peak. These conditions, which are most likely to occur at times when bottom orbital velocities are small, can be identified with our method as cases where the measured wave statistics are inconsistent with Donelan's modified form of the Joint North Sea Wave Project (JONSWAP) spectrum. We define the "effective" forcing for wave-driven, near-bed processes as the product of the magnitude of forcing times its probability of occurrence, and conclude that different bottom orbital velocity statistics may be appropriate for different problems.

Wiberg, Patricia L.; Sherwood, Christopher R.

2008-10-01

52

Calculating wave-generated bottom orbital velocities from surface-wave parameters  

USGS Publications Warehouse

Near-bed wave orbital velocities and shear stresses are important parameters in many sediment-transport and hydrodynamic models of the coastal ocean, estuaries, and lakes. Simple methods for estimating bottom orbital velocities from surface-wave statistics such as significant wave height and peak period often are inaccurate except in very shallow water. This paper briefly reviews approaches for estimating wave-generated bottom orbital velocities from near-bed velocity data, surface-wave spectra, and surface-wave parameters; MATLAB code for each approach is provided. Aspects of this problem have been discussed elsewhere. We add to this work by providing a method for using a general form of the parametric surface-wave spectrum to estimate bottom orbital velocity from significant wave height and peak period, investigating effects of spectral shape on bottom orbital velocity, comparing methods for calculating bottom orbital velocity against values determined from near-bed velocity measurements at two sites on the US east and west coasts, and considering the optimal representation of bottom orbital velocity for calculations of near-bed processes. Bottom orbital velocities calculated using near-bed velocity data, measured wave spectra, and parametric spectra for a site on the northern California shelf and one in the mid-Atlantic Bight compare quite well and are relatively insensitive to spectral shape except when bimodal waves are present with maximum energy at the higher-frequency peak. These conditions, which are most likely to occur at times when bottom orbital velocities are small, can be identified with our method as cases where the measured wave statistics are inconsistent with Donelan's modified form of the Joint North Sea Wave Project (JONSWAP) spectrum. We define the 'effective' forcing for wave-driven, near-bed processes as the product of the magnitude of forcing times its probability of occurrence, and conclude that different bottom orbital velocity statistics may be appropriate for different problems. ?? 2008 Elsevier Ltd.

Wiberg, P. L.; Sherwood, C. R.

2008-01-01

53

Regional wave propagation characteristics and velocity structure in Asia  

Microsoft Academic Search

One of the most tectonically active continental regions in the world is in east Asia where continental collision occurs. In this dissertation an extensive study of seismic wave propagation recorded from earthquakes and nuclear explosions in China and its surrounding regions is presented. High-frequency Sn and Lg wave amplitude ratios are used to map lateral variations of shear wave attenuation.

Richard Ronald Rapine

2000-01-01

54

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

55

Variational wave functions for homogenous Bose systems  

Microsoft Academic Search

We study variational wave functions of the product form, factorizing according to the wave vectors k, for the ground state of a system of bosons interacting via positive pair interactions with a positive Fourier transform. Our trial functions are members of different orthonormal bases in Fock space. Each basis contains a quasiparticle vacuum state and states with an arbitrary finite

Andras Sueto; Peter Szepfalusy

2008-01-01

56

Variational wave functions for homogenous Bose systems  

Microsoft Academic Search

We study variational wave functions of the product form, factorizing according to the wave vectors k , for the ground state of a system of bosons interacting via positive pair interactions with a positive Fourier transform. Our trial functions are members of different orthonormal bases in Fock space. Each basis contains a quasiparticle vacuum state and states with an arbitrary

András Süto; Péter Szépfalusy

2008-01-01

57

Tracer vs. pressure wave velocities through unsaturated saprolite  

SciTech Connect

Saprolite is a form of weathered bedrock that is commonly used as the host material at waste disposal sites in the Southeastern Piedmont. However, estimating the unsaturated hydraulic and transport properties of saprolite is difficult due to saprolite's low permeability. The authors demonstrate the use of short-duration fluid irrigation pulses for maintaining unsaturated conditions in intact saprolite columns. Concomitant Cl{sup {minus}} tracer experiments demonstrate that irrigated waters moved through an effective volumetric porosity substantially less than the ambient water-filled porosity. The authors observed the unexpected result that irrigation-induced pressure wave velocities were {approx}1,000 times faster than tracer velocities. The relationship between pressure wave velocities and fluid velocities is described using kinematic wave theory, presented for four parametric representations (Brooks-Corey, van Genuchten-Mualen, Broadbridge-White, and the Galileo Number), that predicts fluid pressure velocities to be from approximately two to fifteen times faster than saprolite tracer velocities. None of the kinematic models was able to reproduce observed rapid pressure wave velocities. A hydraulic form of the advection-diffusion equation based on Richards' equation is presented that favorably predicts the shape of pressure response curves only when the kinematic velocity is ignored and the hydraulic diffusivity of the unsaturated saprolite is considered. Based on the advection-diffusion equation, diffusion-dominated soil water pressure wave velocities should decrease with depth, eventually conforming with kinematic wave theory. Pressure pulse velocity monitoring may be an additional tool for estimating unsaturated hydraulic properties in low permeability media.

Rasmussen, T.C.; Baldwin, R.H. Jr.; Dowd, J.F.; Williams, A.G.

2000-02-01

58

Nonlinear shock acceleration. III - Finite wave velocity, wave pressure, and entropy generation via wave damping  

NASA Technical Reports Server (NTRS)

The nonlinear theory of shock acceleration developed in earlier papers, which treated the waves as being completely frozen into the fluid, is generalized to include wave dynamics. In the limit where damping keeps the wave amplitude small, it is found that a finite phase velocity (V sub ph) of the scattering waves through the background fluid, tempers the acceleration generated by high Mach number shocks. Asymptotic spectra proportional to 1/E sq are possible only when the ratio of wave velocity to shock velocity is less than 0.13. For a given asymptotic spectrum, the efficiency of relativistic particle production is found to be practically independent of the value of V sub ph, so that earlier results concerning its value remain valid for finite V sub ph. In the limit where there is no wave damping, it is shown that for modest Alfven Mach numbers, approximately greater than 4 and less than 6, the magnetic field is amplified by the energetic particles to the point of being in rough equipartition with them, as models of synchrotron emission frequently take the field to be. In this case, the disordering and amplification of field energy may play a major role in the shock transition.

Eichler, D.

1985-01-01

59

Upper mantle phase velocity and shear wave velocity structure beneath the Cameroon Volcanic Line region  

NASA Astrophysics Data System (ADS)

The Cameroon Volcanic Line (CVL) extends 1800km SW-NE from the Gulf of Guinea into Central Africa. Cenozoic volcanism associated with the CVL lacks the typical age progression along the line exhibited by hotspot-related volcanic tracks, calling into question the geodynamic source of this linear volcanic feature. Numerous models have been proposed to explain the linear nature of the CVL in the absence of age progression, including laterally transported material from a single or multiple plumes, rift-flank processes, reactivation of the Central African Shear Zone, and edge-flow convection associated with the neighboring Congo Craton. To characterize the upper mantle velocity structure beneath the CVL and to address the uncertain geodynamic source, we investigate surface wave and upper mantle shear wave velocity structure beneath the continental portion of the CVL. Rayleigh wave phase velocities are measured at periods from 18 to 182 seconds following the two-plane wave methodology developed by Forsyth and Li (2005) using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007. These phase velocities are then inverted to build a model of shear wave velocities in the upper mantle beneath the CVL. Preliminary results indicate a linear zone of reduced velocities beneath the CVL at periods up to 124s. A zone of elevated velocities is found to the south beneath the Archean-PaleoProterozoic Congo Craton. We present a series of velocity maps demonstrating the change in phase velocities at different periods and across geologic terrains, as well as cross-sections through the shear wave velocity model, to evaluate potential geodynamic sources of the CVL.

Adams, A. N.; Euler, G. G.; Wiens, D. A.; Nyblade, A.; Shore, P.; Tibi, R.; Tokam, A.; Tabod, C.; Bekoa, A.

2012-12-01

60

High precision measurement of stellar radial velocity variations  

NASA Technical Reports Server (NTRS)

New techniques for the high precision measurement of variations in stellar radial velocities now allow a precision of better than 5 m/s to be obtained in routine telescopic observations. Some of the factors limiting velocity precision and how they can be overcome are discussed. Also presented are applications of these new techniques to the problems of detection of planetary systems around other stars and the use of stellar oscillations to determine interior properties of the stars.

Cochran, William D.; Hatzes, Artie P.

1990-01-01

61

Local variations of seismic velocity in the Imperial Valley, California  

SciTech Connect

The authors inverted local earthquake arrival times to estimate spatial variations of seismic velocity. Their model consisted of near-surface station corrections and local perturbations to a standard crustal velocity model. The authors found a zone of relatively high-velocity trending southeast from the Salton Sea. This zone corresponds to the region of thickest sediments. The authors compared results with those of teleseismic studies by Savino et al (1977). The agreement was excellent, suggesting that the teleseismic delays are caused primarily by crustal velocity variations. Residual delays between the teleseismic observations and predicted crustal delays imply crustal thinning of 3 or 4 km along the axis of the valley. Known geothermal resource areas at Salton Sea (or Obsidian Buttes), Brawley, and East Mesa, lie on the axis of a zone of thin crust, and they may be intimately related to the Brawley fault. Neither local earthquake nor teleseismic arrival times can discriminate between these hypotheses, but the issue might be resolved by combining both types of data. Known geothermal resource areas at Heber, Dunes, and Glamis, lie away from the projected trace of the Brawley fault. These areas are nearly aseismic, and overlie crust with apparent seismic velocities only mildly higher than the regional average. These apparent velocity anomalies could be related to crustal thinning, but because of the very mild Bouguer gravity anomalies in these areas, it seems more likely that the velocity anomalies occur entirely within the crust. 7 references, 6 figures, 3 tables.

Jackson, D.D.; Lee, W.B.

1981-12-01

62

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

Microsoft Academic Search

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

K. L. Prindle-Sheldrake; T. Tanimoto

2003-01-01

63

Lithology and shear-wave velocity in Memphis, Tennessee  

USGS Publications Warehouse

We have derived a new three-dimensional model of the lithologic structure beneath the city of Memphis, Tennessee, and examined its correlation with measured shear-wave velocity profiles. The correlation is sufficiently high that the better-constrained lithologic model may be used as a proxy for shear-wave velocities, which are required to calculate site-amplification for new seismic hazard maps for Memphis. The lithologic model and its uncertainties are derived from over 1200 newly compiled well and boring logs, some sampling to 500 m depth, and a moving-least-squares algorithm. Seventy-six new shear-wave velocity profiles have been measured and used for this study, most sampling to 30 m depth or less. All log and velocity observations are publicly available via new web sites.

Gomberg, J.; Waldron, B.; Schweig, E.; Hwang, H.; Webbers, A.; VanArsdale, R.; Tucker, K.; Williams, R.; Street, R.; Mayne, P.; Stephenson, W.; Odum, J.; Cramer, C.; Updike, R.; Hutson, S.; Bradley, M.

2003-01-01

64

Group velocity of gravitational waves in an expanding universe  

E-print Network

The group velocity of gravitational waves in a flat Friedman-Robertson-Walker universe is investigated. For plane waves with wavelength well inside the horizon, and a universe filled with an ideal fluid with the pressure to density ratio less than 1/3, the group velocity is greater than the velocity of light. As a result, a planar pulse of gravitational waves propagating through the universe during the matter/dark energy dominated era arrives to the observer with the peak shifted towards the forefront. For gravitational waves emitted by inspiralling supermassive black holes at the edge of the observable universe, the typical shift that remains after the effects of nonplanarity are suppressed is of order of ten picoseconds.

Vladimír Balek; Vratko Polák

2007-07-10

65

Shear-wave velocity estimation from different type of sediments.  

PubMed

This paper presents estimates of shear-wave velocity profiles for different types of sediments by inverting the dispersion curves of interface waves. Data used in this study were collected at different locations with different recording devices in different experiments. Four data sets were analyzed. Data from one set were recorded by hydrophone array, and data from other three sets were collected by multi-component ocean bottom seismic cables. The configuration of the experimental setup and the environmental conditions were different for the different experiments. Water depth varied from 2 m to 365 m. The frequency range of the interface-wave data was from 1.5 Hz to around 20 Hz. The phase-velocity dispersion curves of the interface waves were extracted using different time-frequency analysis methods. The maximum penetration depth of the interface waves to the sediments varied from 15m to 180 m. A Bayesian nonlinear inversion approach was used for estimating shear-wave velocity profiles as a function of depth in the sediments and the uncertainties. The estimated shear-wave velocity profiles from different experiment and different sediment type were discussed and compared. PMID:25235100

Dong, Hefeng

2014-04-01

66

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

67

Irregular wave induced velocities in shallow water  

E-print Network

, 2x Usual W ?. . 114 Figure Page 68 Vertical Velocity Peaks Probability Distribution, 0. 5x Usual W, . 115 69 Parameterization of Beta-Rayleigh Distribution, U, 116 70 Parameterization of Beta-Rayleigh Distribution, U, 117 71 Parameterization.... 58 Page 134 88 Time Series No. 59 135 89 Time Series No. 65 136 90 Time Series No. 68 137 91 Time Series No. 69 138 92 Time Series No. 75 139 93 Time Series No. 78 140 94 Time Series No 79 141 95 Time Series No. 85 142 96 Time Series No. 88...

Sultan, Nels John

2012-06-07

68

A Local Concept of Wave Velocities  

E-print Network

The classical characterization of \\wp, as a typical concept for far field phenomena, has been successfully applied to many \\w phenomena in past decades. The recent reports of superluminal tunnelling times and negative group velocities challenged this concept. A new local approach for the definition of \\wvs avoiding these difficulties while including the classical definitions as particular cases is proposed here. This generalisation of the conventional non-local approach can be applied to arbitrary \\w forms and propagation media. Some applications of the formalism are presented and basic properties of the concept are summarized.

I. V. Drozdov; A. A. Stahlhofen

2007-04-11

69

Surface wave velocity structure of the western Himalayan syntaxis  

NASA Astrophysics Data System (ADS)

The Nanga Parbat Haramosh massif (NPHM) is located in the western syntaxis of the India-Eurasia collision zone and is subject to erosion rates that are so extreme as to impact the isostatic equilibrium of the massif. In order to investigate the interaction between large scale tectonic forces and local isostatic processes, we employ a Rayleigh wave tomography method to measure phase velocities within the massif and surrounding region at crust and mantle depths. Our inversion solves for phase velocity anomalies by representing perturbations in the wavefield as the interference of two plane waves. Our data set was obtained from a temporary seismic array deployed in 1996 and includes 53 teleseismic events with Mw ? 5.0, at periods from 20 to 79 s. Phase velocities at short periods are low, ranging from 3.2 km s-1 at 20 s, and increasing gradually to 3.5 km s-1 at 40 s. These velocities are 11 per cent lower than velocities observed in the Indian continental Plate at periods below 45 s. Above 50 s, phase velocities in the Nanga Parbat region are significantly higher, ranging from 3.7 km s-1 at 45 s to 4.0 km s-1 at 79 s. These high phase velocities above 60 s are consistent with average velocities measured within the Indian Plate. Comparison of these results with surface wave studies in other regions of the Tibetan plateau including the eastern syntaxis and central Tibet show a similar low velocity anomaly below 45 s. Phase velocities above 55 s, however, are significantly higher in the Nanga Parbat region compared to velocities reported for all other regions of the plateau. Shear wave inversions produce significantly low velocities in the upper crust of the NPHM but exceed average lithospheric velocities below the Moho. We suggest the combination of anomalously low velocities in the upper crust and high velocities at lithospheric depths is due to rapid exhumation of deep crustal material causing elevated geothermal gradients. Azimuthal anisotropy shows a NNW-SSE fast direction at 1.5 per cent peak to peak for periods from 22 to 80 s. This suggests deformation occurs at lithospheric depths, that is driven by large scale stresses of the India-Eurasia collision zone, along an axis parallel to the least principal stress direction.

Hanna, A. C.; Weeraratne, D. S.

2013-09-01

70

S wave velocity structure below central Mexico using highresolution surface wave tomography  

E-print Network

Click Here for Full Article S wave velocity structure below central Mexico using highresolution 2010; published 15 June 2010. [1] Shear wave velocity of the crust below central Mexico is estimated. The results show large differences, especially in the lower crust, among backarc, volcanic arc, and forearc

Clayton, Robert W.

71

Group-Velocity-Matched Three Wave Mixing in Birefringent Crystals  

SciTech Connect

We show that the combination of pulse-front slant, k-vector tilt, and crystal birefringence often permits exact matching of both phase and group velocities in three wave mixing in birefringent crystals. This makes possible more efficient mixing of short light pulses, and it permits efficient mixing of chirped or broad bandwidth light. We analyze this process and present examples. Differences in the group velocities of the three interacting waves in a nonlinear crystal often limits the effective interaction length. For example, in mixing very short pulses, temporal walk off can stretch the pulses in time unless the crystal is very short. Efficient mixing with such short crystals requires high irradiances, but the irradiances are limited by higher order nonlinear effects such as intensity-dependent refractive index and two-photon absorption. Improved matching of the group velocities can alleviate this problem, allowing longer crystal and lower irradiances. Similarly, for high energy pulses, practical limits on crystal apertures mandate temporally stretching the pulses to reduce irradiances. For the resulting chirped pulses, temporal walk off restricts the chirp range unless the group velocities are well matched. In addition to perfectly matching the group velocities of all three waves, it is sometimes useful to match two velocities, such as the signal and idler in parametric amplification, permitting broadband parametric amplification, or to arrange the velocities of two inputs to bracket the generated sum frequency pulse, giving pulse compression under suitable circumstances.

SMITH,ARLEE V.

2000-12-12

72

Compressional and shear wave velocities in serpentinized peridotites  

NASA Astrophysics Data System (ADS)

Serpentinized peridotites in the wedge mantle play key roles in the transport of water and the slab-mantle coupling. Geophysical mapping of serpentinized regions is essential for good understanding of subduction zone processes. Tomographic studies have related low-velocity and high Poisson's ratio (high Vp/Vs) to serpentinized peridotites [e.g., Kamiya and Kobayashi (2000)]. Their interpretations are based on velocity measurements mostly on Low-T type (containing lizardite and/or chrysotile) serpentinized peridotites [e.g., Christensen (1996)]. However, it is questionable whether velocities of Low-T type are applicable to warm subduction zones like Costa Rica, where the other serpentine mineral antigorite is expected. From crystallographic studies, higher elastic stiffness is expected in antigorite. Watanabe et al. (2007) have shown that antigorite-bearing serpentinized peridotites have higher velocity and lower Poisson's ratio than Low-T type serpentinized peridotites. However, their argument was based on the arithmetic mean of velocities in three mutually orthogonal directions. Anisotropic nature of rock samples should be properly taken into account in calculating the average velocity. The temperature dependence of velocities should also be investigated. In order to solve these problems, we are now conducting (1) determination of all elastic moduli from velocity measurements in various directions and (2) velocity measurements under high pressure and temperature conditions. Compressional and shear wave velocities are measured on octahedral specimens by the pulse transmission technique in various directions of propagation and polarization. Specimens were prepared from antigorite- bearing serpentinized peridotites collected in Hida outer belt. Measurements are conducted at the room temperature and under the confining pressure up to 180 MPa. All elastic moduli with orthorhombic symmetry are determined, and isotropic velocities and Poisson's ratio are calculated assuming that similar rocks are randomly packed. The higher velocity and lower Poisson's ratio of antigorite-bearing serpentinized peridotites has been confirmed. Velocities of an antigorite rock are measured up to 600°C at 1 GPa. The rock sample shows numerous parallel lines, which are interpreted to be parallel to b-axis of antigorite grains. The compressional wave velocity was 8.4 km/s and 6.6 km/s in the direction parallel and perpendicular to the lines, respectively. No significant temperature dependence of the velocity was observed in the direction parallel to the lines, while the compressional wave velocity decreases by 4% from the room temperature to 600°C in the direction perpendicular to the lines. Observed anisotropic behaviors are consistent with crystallographic natures of antigorite.

Watanabe, T.; Oguri, H.; Yano, H.; Yoneda, A.

2007-12-01

73

Acoustic Interferometer for Localized Rayleigh Wave Velocity Measurements  

NASA Astrophysics Data System (ADS)

Two instrumentation systems for measurement of Rayleigh surface wave (RSW) velocity are described. The first system consists of a more conventional methodology using matched RF amplifiers and phase detector/mixer circuits. In the second system, a lock-in amplifier, operating at high frequency, replaces the matched RF amplifiers and phase detector/mixer circuit, therefore simplifying the instrumentation. Both systems have been used to measure relative Rayleigh wave velocity using a cylindrically focused acoustic transducer consisting of three elements. A high-precision relative velocity measurement of Rayleigh surface waves is performed by exciting the central element and one of the outer elements with a tone burst signal and measuring the phase difference between the two received signals.

Martin, Richard W.; Sathish, Shamachary; Reibel, Richard; Moran, Thomas J.; Blodgett, Mark P.

2003-03-01

74

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

USGS Publications Warehouse

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

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

2003-01-01

75

The Effect of Area Variation on Wave Rotor Elements  

NASA Technical Reports Server (NTRS)

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

Wilson, Jack

1997-01-01

76

One-dimensional Shear Velocity Structure of Northern Africa from Rayleigh Wave Group Velocity Dispersion  

NASA Astrophysics Data System (ADS)

- Rayleigh wave group velocity dispersion measurements from 10s to 160s periods have been made for paths traversing Northern Africa. Data were accumulated from the IRIS DMC, GEOSCOPE, and MEDNET seismic networks covering the years 1991-1997. The group velocity measurements are made including the effects of debiasing for instantaneous period and a single-iteration, mode-isolation (phase match) filter. The curves are grouped by tectonic province and compared to tomographic model-based curves in an effort to test and validate the tomographic models. Within each tectonic category (rift, orogenic zone, or craton) group velocity curves from various provinces are similar. Between tectonic categories, however, there are marked differences. The rift related paths exhibit the lowest group velocities observed, and cratonic paths the fastest. One-dimensional shear velocity inversions are performed, and while highly nonunique, the ranges of models show significant differences in upper mantle velocities between the tectonic provinces.This work is part of a larger project to determine group velocity maps for North Africa and the Middle East. The work presented here provides important tools for the validation of tomographic group velocity models. This is accomplished by comparing group velocity curves calculated from the tomographic models with carefully selected high-quality group velocity measurements. The final group velocity models will be used in Ms measurements, which will contribute to the mb:Ms discriminant important to the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The improved shear wave velocity models provided by this study also contribute to the detection, location, and identification of seismic sources.

Hazler, S. E.; Sheehan, A. F.; McNamara, D. E.; Walter, W. R.

77

S wave velocity structure of the Northern Cascadia subduction zone  

Microsoft Academic Search

Teleseismic receiver functions from an array of portable broadband seismograph stations located in southwestern British Columbia are interpreted to estimate the S wave velocity structure to upper mantle depths across the northern Cascadia subduction zone. At our westernmost station on central Vancouver Island, a prominent low-velocity zone (DeltaVs=-1 km s-1) with a high Poisson's ratio is estimated at 36-41 km

John F. Cassidy; Robert M. Ellis

1993-01-01

78

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

79

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

80

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

81

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations  

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

82

An inexpensive instrument for measuring wave exposure and water velocity  

USGS Publications Warehouse

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

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

2011-01-01

83

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

84

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

NASA Astrophysics Data System (ADS)

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

Wang, Tao; Chen, Ling

2009-02-01

85

Formula for the phase velocity of electromagnetic waves.  

PubMed

A new formula for the phase velocity of electromagnetic waves presented by Chen [Appl. Phys. Lett. 88, 121125 (2006)] is investigated and discussed here. The difference between the result obtained with the new formula and that obtained directly using the phase term is small for a fundament-mode Gaussian laser beam. However, this difference is qualitative in some high-order Gaussian-mode laser beams. Using the new formula for such beams, discontinuities arise in the distribution of the phase velocity. This distribution is not rotationally symmetric with respect to the optical axis, and an imaginary phase velocity may appear near these discontinuities. PMID:19518391

Huang, Shihua; Wu, Fengmin; Hu, Bo

2009-04-01

86

Surface Wave Velocity Structure of the Western Himalayan Syntaxis  

NASA Astrophysics Data System (ADS)

The Nanga Parbat Haramosh massif (NPHM) is located within the western Himalayan syntaxis, and is characterized by extreme topography and very high erosion rates. The NPHM has undergone rapid uplift and exhumation at a rate of 5-10 mm/yr over the last 3 Ma. This uplift rate may be a simple lithostatic response to the high rate of erosion, or may include uplift due to tectonic compression. In this project, we seek to constrain the mantle velocity structure of the NPHM by using Rayleigh wave phases. We apply a surface wave inversion to a data set of ~90 teleseismic events collected from an array of 10 broadband stations of the Nanga Parbat experiment in 1996. This inversion consists of a two-plane wave approximation which represents perturbations to the wave field by the interference of two plane waves, allowing us to solve for phase velocities at various periods. We find that phase velocities increase steadily from 3.2 km/s to 4.0 km/s over the period range 20-70 seconds, without demonstrating a significant Moho discontinuity. Phase velocities are somewhat low for typical continental crust, possibly indicating higher temperatures. Rays passing through the Indian plate show higher velocities than the surrounding terrane. Azimuthal anisotropy is ~1.5% for periods below 40 seconds, and is mostly limited to the crust. This level of anisotropy predicts an SKS splitting time of ~0.4 seconds, which agrees with previous SKS studies in the area. The anisotropic orientation is N 60 W at periods below 40 seconds, consistent with the least principal stress direction associated with NE-SW convergence of India and Eurasia. Vertical constraints of crust and mantle velocities in the NPHM compared to its surrounding lithospheric blocks will help differentiate between lithostatic and compressional tectonic models for uplift.

Hanna, A. C.; Weeraratne, D. S.; Meltzer, A. S.

2008-12-01

87

Linking sperm length and velocity: the importance of intramale variation  

PubMed Central

Selection imposed through sperm competition is commonly thought to promote the evolution of longer sperm, since sperm length is assumed to be positively associated with sperm swimming velocity. Yet, the basis for this assumption remains controversial, and there is surprisingly little intraspecific evidence demonstrating such a link between sperm form and function. Here, we show that sperm length and velocity are highly correlated in the sea urchin Heliocidaris erythrogramma, but importantly we report that failure to account for within-male variation in these sperm traits can obscure this relationship. These findings, in conjunction with the mounting evidence for extremely high levels of intra-specific variance in sperm traits, suggest that a functional link between sperm morphology and velocity may be more prevalent than what current evidence suggests. Our findings also suggest that selection for faster swimming sperm may promote the evolution of longer sperm, thereby supporting recent findings from macroevolutionary studies. PMID:20484233

Fitzpatrick, John L.; Garcia-Gonzalez, Francisco; Evans, Jonathan P.

2010-01-01

88

Arterial Pulse Wave Velocity as a Psychophysiological Measure  

Microsoft Academic Search

It has been predicted theoretically and shown experimentally that the arterial pulse wave velocity will depend on the state of the arterial wall, this varying between individuals as a function of age and within the individual according to site and, because the arterial wall does not obey Hooke's law, according to the internal pressure at any given moment. In the

JAMES G. L. WILLIAMS; BARBARA WILLIAMS

89

Tomographic P-wave velocity images of the Loma Prieta earthquake asperity  

SciTech Connect

Tomographic inversion is applied to delay times from local earthquakes to image 3-D velocity variations surrounding the main rupture of the 1989 Loma Prieta earthquake. The 55{times}45 square km region is represented by blocks of 1 km per side laterally and by 8 layers of varying thickness to 18 km depth. High quality P-wave arrival times recorded on the USGS CALNET array from 549 crustal earthquakes with depths of 0 to 25 km were used as sources. Preliminary results note several velocity variations (5-12%) that correlate with specific characteristics of the 1989 rupture. These include prominent high-velocity anomalies near the mainshock hypocenter and prominent low-velocity anomalies where the dip of the San Andreas fault appears to change significantly. The termination of prominent low velocity features existing primarily in the hanging wall to depths of 7-9 km, correlates with the top of the rupture zone. High-velocity variations along the fault dominate where aftershock activity is high. The high velocity anomaly located at depth along the fault is interpreted as imaging the asperity on which the Loma Prieta earthquake occurred.

Lees, J.M. (Yale Univ., New Haven, CT (USA))

1990-08-01

90

Detonation wave velocity and curvature of brass encased PBXN-111  

SciTech Connect

Detonation velocities and wave front curvatures were measured for PBXN-111 charges encased in 5 mm thick brass tubes. In all the experiments (charge diameters from 19 to 47 mm) the brass case affected the detonation properties of PBXN-111. Steady detonation waves propagated in brass encased charges with diameters as small as 19 mm, which is about half of the unconfined failure diameter. The radii of curvature of the detonation waves at the center of the wave fronts ranged from 52 to 141 mm for charge diameters of 25 to 47 mm. The angles between the detonation wave fronts and the brass/charge interfaces were between 72 and 74 degrees. {copyright} {ital 1996 American Institute of Physics.}

Forbes, J.W.; Lemar, E.R. [Naval Surface Warfare Center, Indian Head Division, Silver Spring, Maryland 20903-5640 (United States)

1996-05-01

91

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

92

Surface Wave Group Velocity Tomography and Lithospheric S-velocity Structure of South American Continent  

Microsoft Academic Search

A new group velocity tomography is presented for the South American continent including additional stations deployed in the northern and northeastern part of the continent, which had not been covered by previous works. More than 10,000 seismograms were examined with paths principally propagating across the continent of South America. 4000 Rayleigh- and 2270 Love-wave dispersion curves with good quality were

M. Feng; M. Assumpcao; S. van der Lee

2003-01-01

93

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

USGS Publications Warehouse

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

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

2002-01-01

94

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

PubMed Central

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

Varghese, Tomy; Madsen, Ernest L.

2011-01-01

95

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

96

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

97

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

98

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

99

Estimation of seabed shear-wave velocity profiles using shear-wave source data.  

PubMed

This paper estimates seabed shear-wave velocity profiles and their uncertainties using interface-wave dispersion curves extracted from data generated by a shear-wave source. The shear-wave source generated a seismic signature over a frequency range between 2 and 60 Hz and was polarized in both in-line and cross-line orientations. Low-frequency Scholte- and Love-waves were recorded. Dispersion curves of the Scholte- and Love-waves for the fundamental mode and higher-order modes are extracted by three time-frequency analysis methods. Both the vertically and horizontally polarized shear-wave velocity profiles in the sediment are estimated by the Scholte- and Love-wave dispersion curves, respectively. A Bayesian approach is utilized for the inversion. Differential evolution, a global search algorithm is applied to estimate the most-probable shear-velocity models. Marginal posterior probability profiles are computed by Metropolis-Hastings sampling. The estimated vertically and horizontally polarized shear-wave velocity profiles fit well with the core and in situ measurements. PMID:23862796

Dong, Hefeng; Nguyen, Thanh-Duong; Duffaut, Kenneth

2013-07-01

100

P-wave and S-wave velocity structure of northern Cascadia margin gas hydrates  

NASA Astrophysics Data System (ADS)

Single-channel seismic and wide-angle reflection data collected in September 2005 were analysed along a 2-D profile of 10 ocean bottom seismometers (OBSs) on the continental slope region off Vancouver Island, near ODP Site 889 and IODP Site U1327. The objectives were to determine the shallow P-wave and S-wave velocity structure associated with marine gas hydrates and to estimate the hydrate concentration and distribution in the sediment pore space. Combined traveltime inversion of single-channel and OBS data produced a P-wave velocity model down to the depth of the bottom-simulating reflector (BSR) at 230(±5) m below the seafloor (mbsf). Mean velocities, which increased from 1.50 km s-1 at the seafloor to 1.88 km s-1 at the BSR, are in good agreement with the sonic log data from Sites 889 and U1327. The increase in P-wave velocity of the hydrate-bearing sediments relative to a background no-hydrate velocity was utilized to estimate the hydrate concentration by using effective medium theory. An average concentration of 13 per cent in the interval from 120-230 mbsf was estimated from the P-wave velocity model. Lateral continuity of the model data confirms that these average hydrate concentrations are also found around the drillsites out to distances of a few kilometres. Forward modelling of S-waves was carried out using the data from the OBS horizontal components. Above the BSR, S-wave velocities are higher than a background velocity profile based on a rock physics model and on global averages for unconsolidated sediments. This increase in velocity suggests that the hydrate is distributed as part of the load-bearing matrix to increase the rigidity of the sediment.

Dash, Ranjan; Spence, George

2011-12-01

101

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

102

Estimation of shear velocity contrast from transmitted Ps amplitude variation with ray-parameter  

NASA Astrophysics Data System (ADS)

Amplitude versus offset of P to P reflection is commonly used by the exploration seismology community for hydrocarbon exploration. In this paper, we investigate the feasibility to estimate crustal velocity structure from transmitted P- to S-wave amplitude variation with ray-parameter. First the transmission coefficient for the plane P wave converting to S wave (P- to -S) is approximated and expressed as a function of slowness. The resulting linear relation involves two coefficients (intercept, X and gradient, Y), which are functions of velocities and densities. Due to the stable nature of X and the fact that P- to -S amplitudes are weakly dependent on the density contrast, we use this parameter next to estimate the shear wave velocity contrast across an interface using the forward scattered P- to -S amplitude versus slowness data. We report on the effectiveness of the approach using various synthetics data sets. The present methodology is also tested on real data sets from two broad-band seismic stations from HYB and COR.

Kumar, Prakash; Sen, Mrinal K.; Haldar, Chinmay

2014-09-01

103

Rayleigh Wave Phase Velocity Maps in South-central Mongolia  

NASA Astrophysics Data System (ADS)

Mongolia orogenic is located between the Siberian platform, Sino-Korean platform and Tarim platform, and thousands of kilometers away from the Indian-Asian continental collision front. Geological structure in this area is influenced by the complicated orogeny. Active tectonics is marked by the progressive transition from south to north. We collected the vertical component data recorded by 69 broadband seismic stations deployed in the south-central Mongolia from August 2011 to July 2013. By using the frequency-time analysis of wavelet transformation method, we extracted the inter-station phase velocity dispersion curves of fundamental Rayleigh wave and then constructed the phase velocity maps at periods from 12s to 70s. At shorter periods (e.g. 12s), the results show that the phase velocity distributions are well correlated with the large crustal structural domains, with low phase velocities in the gobi basin, while high phase velocities in the mountainous areas. The distributions at middle periods (e.g. 30s) show that low phase velocities are located in the central region and high phase velocities in the southeast and north, which corresponds well with the crustal thickness. The maps at long periods (e.g. 50s, 60s), reveal that a obvious low velocity zone at 106°N, which may indicate the high temperatures and be related with mantle upwelling. In contrast, the southeast region still shows high velocity. This suggests that the shear velocity structure in our study area may be not as simple as previously thought. This study was supported by the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120).

Yu, D.; Wu, Q.; Gao, M.; Munkhuu, U.; Demberel, S. G.

2013-12-01

104

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 3D 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 six months in the Aegean area within the framework of a large-scale experiment. For each epicenter-station ray path an averaged group velocity has been determined using the method of frequency time analysis (FTAN) and the data are used in order to determine the local group velocities for different periods over the area covered by the seismic ray paths by means of Yanovskaya's formulation for laterally heterogeneous media. Taking into account the resolution of the local group velocities, a grid of 0.5 degree was adopted for the Aegean area and a local dispersion curve was defined for each geographical point. More than 80 local dispersion curves were finally inverted using a non-linear inversion approach, deriving the corresponding 1D shear-velocity models. The interpolation of these models results in a 3D S-wave tomographic image of the crust and uppermost mantle in the broader Aegean area. As expected, due to 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 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 28-30km. 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 about 30-34 km. Strong lateral variations of the shear-wave velocities are found throughout the Aegean area. For shallow depths (<10 km) low S-wave velocities are observed in the sedimentary basins of the North Aegean Sea, of the gulf of Thermaikos 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/sec) are observed at depths of about 30-40 km just below the Moho discontinuity, while in the rest of the inner Aegean Sea and the continental Greece the uppermost mantle is characterized by velocities around 4.3-4.4 km/sec. This low-velocity zone in the Southern Aegean Sea can be associated with the presence of a partially melt mantle wedge in the Southern Aegean subduction zone in agreement with previous studies.

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

2003-04-01

105

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

SciTech Connect

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. Ranier, high velocity anomalies may reflect buried plutons.

Lees, J.M.; Crosson, R.S. (Univ. of Washington, Seattle (United States))

1990-04-10

106

Tomographic Images of The Upper Mantle From Observations of Surface Wave Phase and Group Velocity  

NASA Astrophysics Data System (ADS)

Lateral variations in the global properties of the Earth's upper mantle are best con- strained on the basis of measurements of surface wave propagation, and subsequent to- mographic images. In recent years, the Harvard group has developed a new technique to measure anomalies in phase velocity of Love and Rayleigh waves at intermediate- long period (35-300 s). These data have then been used to generate new 3-D tomo- graphic images; a new inversion algorithm has been designed to account for lateral variations in the surface wave sensitivity functions: strong crustal heterogeneities af- fect the sensitivity of surface waves to the underlying structure. To further improve our model's resolution of the shallow mantle, we have modified our software to fit, in a simultaneous least-squares inversion, observations of group and phase velocities of surface waves. Our new, radially anisotropic global model of shear velocities in the upper mantle is based on the Harvard phase velocities data set, and on a set of regional observations of group velocities (from the Lawrence Livermore Laboratory), restricted to Europe, Northern Africa and the Mediterranean Basin. As more data are now available in this region, we have parameterized it more finely than the rest of the globe. We describe our method, analyze the new data, and discuss our new images of the upper mantle, focusing on the lateral structure of the Mediterranean Basin. We con- clude that our (relatively) long-wavelength image of the Mediterranean upper mantle represents a good background map for finer resolution models of the area.

Boschi, L.; Kustowski, B.; Ekström, G.

107

Measurements of parallel electron velocity distributions using whistler wave absorption  

NASA Astrophysics Data System (ADS)

We describe a diagnostic to measure the parallel electron velocity distribution in a magnetized plasma that is overdense (?pe > ?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 ?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 ? - k?v? = ?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.

2012-08-01

108

A 1D P wave velocity model under the pacific region using multiply reflected P waves  

NASA Astrophysics Data System (ADS)

In order to constrain the shallow structure of the Earth in global tomography, Love and Rayleigh waves are often used. However these waves are mostly sensitive to the S wave velocity structure. P-wave energy is either evanescent, or leaking away at every surface reflection that generates an S wave which travels much deeper into the mantle. For that reason, to study the shallow P velocity structure of the Earth, we need to study P-waves at regional distances if a good seismic station coverage is available. Otherwise we can use multiply reflected P waves at teleseismic distance when regional data are not available (as in the oceans for instance). We used 203 events of magnitude Mw > 6.0 recorded from the dense network of US ARRAY, which allows us to make a very large number of group arrival and slowness measurements of multiply reflected P waves . Our study shows that two times reflected PPP and three times reflected PPPP waves are very well observed despite the ray- theoretical prediction that at certain distances almost all of their compressional energy is converted to shear waves. We also observed Four times reflected 5P and five times reflected 6P which show a strong interference for epicentral distances larger than 80 degree. These observations of multiply reflected P waves allow us to inferred a 1D P wave model for the shallow structure under the pacific region.

Foundotos, M.; Nolet, G.

2012-12-01

109

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

110

Negative phase velocity of electromagnetic waves and the cosmological constant  

E-print Network

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

Tom G. Mackay; Sandi Setiawan; Akhlesh Lakhtakia

2004-11-29

111

The measurements of the compressional wave velocity of soils during unconsolidated-undrained triaxial testing  

NASA Astrophysics Data System (ADS)

In this study, a conventional triaxial cell was modified to measure the compressional wave velocity during a triaxial test. Two air-dry remolded soils taken from the counties in Sharkey and Neshoba, MS, were chosen for the study. Unconsolidated-undrained triaxial tests with pore pressure measurement were carried out. Soil samples were isotropically consolidated up to three different cell pressures, axially compressed with the axial strain up to 22%, and subjected to the unload-reload stress path cycles before and after soil failures. The velocity of the compressional wave in the axial direction as a function of the axial strain was measured along with the measurement of the stress-strain response. A comparison of the load-deformation behavior with the load-acoustic velocity behavior was made. The variation of the acoustic velocity with the effective stresses during the isotropic loading, normally consolidated compression and unload-reload stress path cycles were examined. Several empirical expressions of the compressional wave velocity in terms of the effective stresses and the over-consolidation ratio were proposed and examined with the measured data.

Lu, Zhiqu; Hickey, C. J.; Sabatier, J. M.

2003-04-01

112

Dispersion and Anisotropy of Elastic Wave Velocities In Cracked Rocks  

NASA Astrophysics Data System (ADS)

It is well known that the measured high frequency velocities are generally faster than those predicted by the equations of Poroelasticity. The low frequency limit corre- sponds to a situation where fluid pressure is equilibrated in pores and cracks. This is no longer true at high frequencies because the fluid has no time to move and equili- brate. In other words, a fluid pressure gradient is induced when a seismic wave passes through a rock, because the fluid pressure change is low in stiff porous voids (equant pores) and high in compliant ones (microcracks). Depending on the frequency, the fluid pressure has enough time or not to equilibriate. This velocity dispersion experi- mentally observed in isotropic media and generally called "squirt flow" mechanism is of primary importance to interpret seismic data and/or to extrapolate laboratory elastic wave velocities measurements. We propose here a method, based on Kachanov's mod- els (Kachanov, M., [1993], Adv. Appl. Mech., vol. 30, pp. 259-445) and poroelasticity to calculate the low frequency compliances from the compliances of the solid grain rock, porosity, crack density tensor and the fluid bulk modulus. Results show that dis- persion increases with increasing crack density. Moreover, for high crack density, low frequency P wave anisotropy is much larger than expected from laboratory.

Schubnel, Alexandre; Guéguen, Yves

113

Advantages of Using Multichannel Analysis of Love Waves (MALW) to Estimate Near-Surface Shear-Wave Velocity  

NASA Astrophysics Data System (ADS)

As theory dictates, for a series of horizontal layers, a pure, plane, horizontally polarized shear (SH) wave refracts and reflects only SH waves and does not undergo wave-type conversion as do incident P or Sv waves. This is one reason the shallow SH-wave refraction method is popular. SH-wave refraction method usually works well defining near-surface shear-wave velocities. Only first arrival information is used in the SH-wave refraction method. Most SH-wave data contain a strong component of Love-wave energy. Love waves are surface waves that are formed from the constructive interference of multiple reflections of SH waves in the shallow subsurface. Unlike Rayleigh waves, the dispersive nature of Love waves is independent of P-wave velocity. Love-wave phase velocities of a layered earth model are a function of frequency and three groups of earth properties: SH-wave velocity, density, and thickness of layers. In theory, a fewer parameters make the inversion of Love waves more stable and reduce the degree of nonuniqueness. Approximating SH-wave velocity using Love-wave inversion for near-surface applications may become more appealing than Rayleigh-wave inversion because it possesses the following three advantages. (1) Numerical modeling results suggest the independence of P-wave velocity makes Love-wave dispersion curves simpler than Rayleigh waves. A complication of "Mode kissing" is an undesired and frequently occurring phenomenon in Rayleigh-wave analysis that causes mode misidentification. This phenomenon is less common in dispersion images of Love-wave energy. (2) Real-world examples demonstrated that dispersion images of Love-wave energy have a higher signal-to-noise ratio and more focus than those generated from Rayleigh waves. This advantage is related to the long geophone spreads commonly used for SH-wave refraction surveys, images of Love-wave energy from longer offsets are much cleaner and sharper than for closer offsets, which makes picking phase velocities of Love waves easier and more accurate. (3) Real-world examples demonstrated that inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves. This is due to Love-wave's independence of P-wave velocity, which results in fewer unknowns in the MALW method compared to inversion methods of Rayleigh waves. This characteristic not only makes Love-wave dispersion curves simpler but also reduces the degree of nonuniqueness leading to more stable inversion of Love-wave dispersion curves.

Xia, Jianghai; Xu, Yixian; Luo, Yinhe; Miller, Richard D.; Cakir, Recep; Zeng, Chong

2012-09-01

114

Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments  

NASA Astrophysics Data System (ADS)

The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850 ?m aluminum (6061-T6) flyer plates to peak velocities of 34 km/s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during acceleration to peak velocity. In addition, we present results pertaining to plate impact, shock wave experiments in which the aluminum flyer plates were magnetically accelerated across a vacuum gap and impacted z-cut, ?-quartz targets. Accurate measurements of resulting quartz shock velocities are presented and analyzed through high-fidelity MHD simulations enhanced using optimization techniques. Results show that a fraction of the flyer remains at solid density at impact, that the fraction of material at solid density decreases with increasing magnetic pressure, and that the observed abrupt decrease in the quartz shock velocity is well correlated with the melt transition in the aluminum flyer.

Lemke, R. W.; Knudson, M. D.; Bliss, D. E.; Cochrane, K.; Davis, J.-P.; Giunta, A. A.; Harjes, H. C.; Slutz, S. A.

2005-10-01

115

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

116

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

117

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

E-print Network

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

118

Adhesion Molecule Polymorphisms and Pulse Wave Velocity in American Youth  

PubMed Central

Background Our research group recently reported that aorto-radial (radial) and aorto-dorsalis-pedis (foot) pulse wave velocity (PWV) as proxies of arterial stiffness are substantially heritable in healthy youth. This paper aimed at uncovering the genetic contributions of adhesion molecules, key members in the inflammatory process, to PWV in these young individuals. Methods Radial and foot PWV were non-invasively measured with applanation tonometry in 702 black and white subjects (42% blacks, mean age 17.7 ±3.3 years) from the Georgia Cardiovascular Twin Study. Eight functional polymorphisms from genes for E-selectin (SELE), P-selectin (SELP), intercellular adhesion molecules-1 (ICAM1), and vascular cell adhesion molecules-1 (VCAM1) were genotyped. Results Youth with Ser290Asn or Asn290Asn genotype (SELP) compared to those with Ser290Ser had an increase in both radial and foot PWV (6.61±0.07 vs. 6.41±0.05 m/s, p=0.026; 7.22±0.05 vs. 7.04±0.04 m/s, p=0.007). TT homozygotes of rs2244529 (SELP) had higher foot PWV (7.28±0.07 vs. 7.06±0.03 m/s, p=0.002) than CT heterozygotes and CC homozygotes. There appeared to be a decrease in foot PWV in youth with the 241Arg allele (ICAM1) as compared to those without (6.96±0.08 vs. 7.14±0.03 m/s, p=0.005). For the Asp693Asp (C to T) polymorphism (VCAM1), CC genotype had higher foot PWV than CT and TT genotypes (7.18±0.04 vs. 6.95±0.06 m/s, p<0.0001). There was an epistatic interaction between Ser290Asn, Gly241Arg, and Asp693Asp on foot PWV (p=0.017), explaining 3.6% variance of the foot PWV. Conclusion Genetic variation of adhesion molecules may be implicated in the development of arterial stiffness. Screening for adhesion molecule polymorphisms may help identify high-risk youth. PMID:18828734

Zhu, Haidong; Yan, Weili; Tan, Yuande; Li, Ke; Kapuku, Gaston; Treiber, Frank A.; Su, Shaoyong; Harshfield, Gregory A.; Snieder, Harold; Dong, Yanbin

2013-01-01

119

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

120

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

121

Surface wave and shear wave velocity structure beneath Kekexili area in northern Tibet  

NASA Astrophysics Data System (ADS)

While several portable seismic arrays had been deployed in Tibet during the last decade most of these arrays covered the southern and eastern region of the plateau, leaving the central northern part of the plateau, the Kekexili area, out of the inversion range of most of these tomographic studies. In 2008 following the 2008 Ms 7.3Yutian earthquake Peking University deployed a temporal seismic array along the southern boundary of the Tarim basin. Here we report preliminary results of a surface wave study utilizing the ray paths between this PKU Yutian array and the on-going Indepth IV passive array (also known as the ASCENT array) to invert for the average shear-wave velocity structure of this little studied Kekexili area. We also show the results of the average shear-wave velocity structure of the Tarim basin using the surface waves pasting through the PKU Yutian array and the CEA provincial permanent stations in the Tianshan region. We divide the ray paths into two groups that have sampled the northern and southern Kekexili region separated by the Kunlun fault zone. Phase- and group-velocity curves of the Rayleigh waves were measured at the periods of 15 to 125 s using conventional two-station method. Further inversion of phase velocities yields one-dimensional model of the average shear wave velocity structure for the northern and southern Kekexili areas. Both of the phase- and group-velocities measured for the ray paths in the northern Kekexili area are higher than that in the southern Kekexili area for periods shorter than 75 s and however, the difference is insignificant at periods longer than 75 s. The inverted Moho depth is about 50 km for the northern Kekexili area, and about 65 km for the southern Kekexili area. There exists a high velocity zone at depths of 50-70 km near the Moho in the northern Kekexili area. Finally, the measured phase velocities for the ray paths across the Tarim basin are significantly lower than the AK135 global average model at periods less than 75 seconds. The inverted Moho depth is about 50 km and the shear wave velocities in the crust is lower than the AK135 model for the Tarim basin.

He, W.; Chen, Y. J.; Fan, W.; Tang, Y.; Seismotectonics at Peking University

2011-12-01

122

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

123

Latitudinal Variations Observed in Gravity Waves with Short Vertical Wavelengths  

Microsoft Academic Search

Knowledge of the latitudinal variations in the occurrence of gravity waves is important for their parameter- ization in global models. Observations of gravity waves with short vertical scales have shown a pronounced peak in wave activity at tropical latitudes. In this paper, it is shown that such a peak may be a natural consequence of the latitudinal variation in the

M. Joan Alexander; Toshitaka Tsuda; Robert A. Vincent

2002-01-01

124

Three dimensional P-wave velocity structure at Popocatépetl Volcano, Mexico  

NASA Astrophysics Data System (ADS)

Popocatépetl Volcano is an active andesitic stratovolcano (5460m) of the Trans Mexican Volcanic Belt that has been erupting since December 1994. We used three-dimensional seismic tomography to detect variations in the P-wave velocity structure during three different volcanic cycles, which are separated by the two largest eruptions that occurred at Popocatépetl on June 30th, 1997 and January 22nd, 2001. The start of the first dataset is defined by the eruption in June 1996; the last dataset ends with the eruption in September 2003. The P-wave velocity structure of the volcano has been determined to 10 km depth below the summit using nearly 4000 P-arrival times from about 900 volcano tectonic events recorded on a local 10-station network. The Root Mean Square (RMS) of the arrival time residuals was reduced by 22% - 30% from the initial RMS of about 0.30 s, after running seven iterations of our tomography code. In the three eruption cycles, we observe velocity structures which complement the results of former studies, as well as image previously unrecognized velocity anomalies. We observe velocity changes of ~1km/s in certain areas before and after large eruptions. We compare the tomographic results with geologic, geophysical and geochemical investigations and interpret low velocity zones as fractured rock (SE-zone) or as thermally disturbed zones including hot rock and melt (below the crater region and north flank), from which unexpected future eruptions or flank collapses may occur. High velocity zones are interpreted as roofs of magma reservoirs, old dike systems or relicts of the ancient volcano. The observed changes in velocity anomalies before and after large eruptions, determined by four-dimensional tomography, indicate a change in the internal volcanic fluid-filled structures during volcanic eruptions and show the need for time-resolved geophysical techniques when investigating volcanic structures.

Berger, P.; Valdes-Gonzales, C. M.

2009-12-01

125

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

126

Microwave acoustics handbook. Volume 4: Bulk wave velocities: Numerical data  

NASA Astrophysics Data System (ADS)

Information useful for the design of acoustic delay lines, resonators, and other miniature, low-cost devices for use in communications and electronic sensing is given. Numerical data on bulk acoustic wave velocities and power flow angles are given for longitudinal, and two shear waves for various orientations of the following single crystalline materials: Ba2NaNb5O15, Bi12/geO20, CdS, Diamond, Eu3Fe5O15, GaAs, Gadolinium Gallium Garnet, Germanium, InSb, InAs, Lead Molybdate, PbS, LiNbO3, LiTaO3, MgO, Quartz, Rutile, Sapphire, Silicon, Spinel, TeO2, YAG, YGaG, YIG, and ZnO. This present volume is intended to be used as a supplement to Volume 3 whenever accurate numerical data is required rather than the more convenient graphical information.

Slobodnik, A. J., Jr.; Delmonico, R. T.; Conway, E. D.

1980-06-01

127

A model of elastically and anelastically produced temperature derivatives of acoustic wave velocities in inorganic oxide glasses  

Microsoft Academic Search

The temperature dependence of 15 MHz ultrasonic bulk wave velocity in the range 4 to 600 K has been measured in the entire range of glasses that can be prepared by mixing MoO3 with P2O5 in open crucibles. In all cases the temperature gradients are negative. However, when the contribution to the velocity variation caused by (anelastic) two-well relaxational effects

B. Bridge; N. D. Patel

1987-01-01

128

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

129

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

130

Coseismic velocity variations caused by static stress changes associated with the 2001 Mw = 4.3 Agios Ioanis earthquake in the Gulf of Corinth, Greece  

Microsoft Academic Search

The analysis of temporal variations in the seismic velocity across faults can be used to estimate in situ stress changes. Seismic velocity of propagation depends on the fault stiffness, which is a function of stress. The coda wave interferometry technique is applied to seven families of repeating earthquakes (multiplets) recorded on the southern shore of the Gulf of Corinth, Greece,

L. Cociani; C. J. Bean; H. Lyon-Caen; F. Pacchiani; A. Deschamps

2010-01-01

131

The Generation and Propagation of Ocean Waves and Swell. I. Wave Periods and Velocities  

Microsoft Academic Search

A method is described for measuring and examining ocean waves in a way which allows their amplitude and period to be determined with some precision. Data obtained in this way are compared with meteorological charts of the ocean in an attempt to assess the velocity of propagation of swell over long distances. A critical estimate is only possible when the

N. F. Barber; F. Ursell

1948-01-01

132

Variational wave functions for homogenous Bose systems  

SciTech Connect

We study variational wave functions of the product form, factorizing according to the wave vectors k, for the ground state of a system of bosons interacting via positive pair interactions with a positive Fourier transform. Our trial functions are members of different orthonormal bases in Fock space. Each basis contains a quasiparticle vacuum state and states with an arbitrary finite number of quasiparticles. One of the bases is that of Valatin and Butler (VB), introduced fifty years ago and parametrized by an infinite set of variables determining Bogoliubov's canonical transformation for each k. In another case, inspired by Nozieres and Saint James the canonical transformation for k=0 is replaced by a shift in the creation/annihilation operators. For the VB basis we prove that the lowest energy is obtained in a state with {approx}{radical}(volume) quasiparticles in the zero mode. The number of k=0 physical particles is of the order of the volume and its fluctuation is anomalously large, resulting in an excess energy. The same fluctuation is normal in the second type of optimized bases, the minimum energy is smaller and is attained in a vacuum state. Associated quasiparticle theories and questions about the gap in their spectrum are also discussed.

Sueto, Andras [Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P. O. Box 49, H-1525 Budapest (Hungary); Szepfalusy, Peter [Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P. O. Box 49, H-1525 Budapest (Hungary); Department of Physics of Complex Systems, Eoetvoes University, H-1117 Budapest (Hungary)

2008-02-15

133

Shear wave velocity structure in North America from large-scale waveform inversions of surface waves  

USGS Publications Warehouse

A two-step nonlinear and linear inversion is carried out to map the lateral heterogeneity beneath North America using surface wave data. The lateral resolution for most areas of the model is of the order of several hundred kilometers. The most obvious feature in the tomographic images is the rapid transition between low velocities in the technically active region west of the Rocky Mountains and high velocities in the stable central and eastern shield of North America. The model also reveals smaller-scale heterogeneous velocity structures. A high-velocity anomaly is imaged beneath the state of Washington that could be explained as the subducting Juan de Fuca plate beneath the Cascades. A large low-velocity structure extends along the coast from the Mendocino to the Rivera triple junction and to the continental interior across the southwestern United States and northwestern Mexico. Its shape changes notably with depth. This anomaly largely coincides with the part of the margin where no lithosphere is consumed since the subduction has been replaced by a transform fault. Evidence for a discontinuous subduction of the Cocos plate along the Middle American Trench is found. In central Mexico a transition is visible from low velocities across the Trans-Mexican Volcanic Belt (TMVB) to high velocities beneath the Yucatan Peninsula. Two elongated low-velocity anomalies beneath the Yellowstone Plateau and the eastern Snake River Plain volcanic system and beneath central Mexico and the TMVB seem to be associated with magmatism and partial melting. Another low-velocity feature is seen at depths of approximately 200 km beneath Florida and the Atlantic Coastal Plain. The inversion technique used is based on a linear surface wave scattering theory, which gives tomographic images of the relative phase velocity perturbations in four period bands ranging from 40 to 150 s. In order to find a smooth reference model a nonlinear inversion based on ray theory is first performed. After correcting for the crustal thickness the phase velocity perturbations obtained from the subsequent linear waveform inversion for the different period bands are converted to a three-layer model of S velocity perturbations (layer 1, 25-100 km; layer 2, 100-200 km) layer 3, 200-300 km). We have applied this method on 275 high-quality Rayleigh waves recorded by a variety of instruments in North America (IRIS/USGS, IRIS/IDA, TERRAscope, RSTN). Sensitivity tests indicate that the lateral resolution is especially good in the densely sampled western continental United States, Mexico, and the Gulf of Mexico.

Alsina, D.; Woodward, R. L.; Snieder, R. K.

1996-01-01

134

Determining uncertainties in Rayleigh wave group velocity dispersion curve measurements from the ambient noise  

NASA Astrophysics Data System (ADS)

Ambient noise tomography has emerged as a powerful tool used to characterize the Earth's crust. Typically, cross correlations of ambient seismic noise are performed to visualize the propagation of surface waves between any two stations. Surface wave group velocity dispersion curves are then measured and inverted, following a standard procedure, to produce tomographic models of the crust (Bensen et al, 2007). However, the uncertainties in noise dispersion curve measurements are not well understood, and these are translated to unknown uncertainties in the tomographic models. In this study, we analyze 12 months of 24-hour vertical-component data from the KMA, KIGAM, and F-Net accelerometer and broadband seismometer networks in Korea and Japan. Noise correlations are performed in period bands of 2.2-10 s, 10-20 s, 2.2-20 s, and 15-50 s. The correlations are then stacked from 1 month to 12 months, and Rayleigh wave group velocity dispersion curves are measured. Preliminary results show monthly variations in the noise correlations. We define the signal to noise ratio as the ratio of the maximum amplitude to the standard deviation of the entire noise correlation. By measuring the signal to noise ratios, we derive relationships between measurement uncertainties and seasonal variations. These signal to noise ratios can then be used to approximate measurement uncertainties in future studies. We also explore the relationship between uncertainties in the noise correlations and the dispersion curve measurements.

Witek, M.; Kang, T.; van der Lee, S.; Chang, S.

2012-12-01

135

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

NASA Astrophysics Data System (ADS)

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

Foster, Anna E.

136

Accuracy of Velocity Estimation Using Global Variational Methods  

NASA Astrophysics Data System (ADS)

In this work a method of processing digital images, such as those from PIV, MTV, or LIF, for flow velocities using Global Variational Method (GVM) is investigated. This technique is based on principles of Frobenius-Perron (FP) operator theory in which image sequences can be related to the infinitesimal generator of the FP operator to motivate a flow-recovery constraint. A regularization method is then used to minimize this constraint along with an additional constraint required to stabilize a solution. Synthetic images, with typical MTV tagging patterns, and variable noise levels were first created and then displaced using analytically derived flow fields. Displacements were calculated from pairs of images and the error was determined by comparing the measured displacements to those of the analytical flow field. A direct correlation technique (DCT) was also used to process the synthetically derived images for comparison. Results show that the GVM error levels are nominally 5-10 times higher than for the DCT. While the error is higher for GVM compared to DCT the results show potential for using this technique to provide quantitative flow measurements in cases where DCT cannot be applied.

Bohl, Doug; Santitissadeekorn, Naratip; Bollt, Erik

2008-11-01

137

Searching for Radial Velocity Variations in eta Carinae  

NASA Technical Reports Server (NTRS)

A hot companion of eta Carinae has been detected using high resolution spectra (905 - 1180 A) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite (see poster by Sonneborn et al.). Analysis of the far-UV spectrum shows that eta Car B is a luminous hot star. The N II 1084-86 emission feature indicates that the star may be nitrogen rich. The FUV continuum and the S IV 1073 P-Cygni wind line suggest that the effective temperature of eta Car B is at least 25,000 K. FUV spectra of eta Carinae were obtained with the FUSE satellite at 9 epochs between 2000 February and 2005 July. The data consists of 12 observations taken with the LWRS aperture (30x30 arcsec), three with the HIRS aperture (1.25x20 arcsec), and one MRDS aperture (4x20 arcsec). In this paper we discuss the analysis of these spectra to search for radial velocity variations associated with the 5.54-year binary orbit of Eta Car AB.

Iping, R. C.; Sonneborn, G.; Gull, T. R.; Ivarsson, S.; Nielsen, K.

2006-01-01

138

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

NASA Astrophysics Data System (ADS)

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 petrological constraints, including depth and temperature of origin, obtained from more than 100 well-characterized mantle samples in kimberlite pipes both on and off craton. We use laboratory-determined bulk and shear moduli, as well as their temperature and pressure derivatives, to compute the seismic properties of the mineral aggregates at the estimated depths and temperatures in the mantle from which the samples were derived. Xenolith samples from the Kaapvaal craton are organized by petrologic subgroups: (1) low temperature garnet lherzolites and harzburgites that originate predominantly from depths in the range 100-150 km; (2) high temperature garnet lherzolites originating at depths in excess of 170 km; and (3) spinel lherzolites and harzburgites that are thought to comprise a continuous upper layer (depth < 80 km) of the cratonic lithosphere. Off-craton peridotites of Proterozoic age (ca 1-2 Ga) are typically more fertile (enriched in Al, Ca, and Fe) than on-craton samples, and they tend to lie on a mantle geotherm displaced toward higher temperatures. All of the mantle samples used in the present study consist dominantly of olivine and orthopyroxene, with or without lesser amounts of garnet, clinopyroxene, and spinel. P and S wave velocities are computed for each xenolith sample based on the modal proportions and chemical composition of individual mineral phases. Where possible, each velocity determination is corrected for the pressure and temperature at which the sample was in equilibrium in the mantle. A significant uncertainty in the velocity determinations results from an incomplete, and in some cases inconsistent, elasticity database. Notably absent from the database are a number of temperature derivatives, many compositional and second-order derivatives, and virtually all P-T cross-derivatives. Within the limits of these uncertainties, however, we have selected a representative set of elastic parameters that we apply uniformly to all specimens of the xenolith suite for the computation of seismic velocities. The contributions of varying mineral modes, phase compositions, and geothermal gradient to seismic velocity variations are examined in the terms of observed velocity perturbations across southern Africa. We evaluate the relative importance of the geothermal gradient vs. average rock composition of the upper mantle as factors in producing the seismic velocity variations observed between regions that are on-craton, off-craton, and in disturbed craton.

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

2001-05-01

139

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

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

140

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

141

Surface wave tomography with USArray: Rayleigh wave phase velocity, ellipticity, and local amplification  

NASA Astrophysics Data System (ADS)

The deployment of the EarthScope/USArray Transportable Array allows detailed empirical study of the surface-wave wavefield on a large scale. In this presentation, we show that three local properties of Rayleigh waves, i.e. phase velocity, ellipticity (or H/V ratio), and local amplification, can be determined across the array in the western US between 24 and 100 sec period based on teleseismic measurements. More than 900 earthquakes are analyzed where phase velocity and local amplification are determined based on empirical phase travel time and amplitude mapping. The three Rayleigh wave properties, which are all sensitive to the 1D structure beneath each location, have very distinct depth sensitivity to Vs, Vp/Vs ratio, and density. Joint inversion of these quantities therefore reduces the trade-off between the three different parameters at different depths. Including the H/V ratio, in particular, allows the uppermost (0-3 km) crustal velocity and density structure to be constrained, and our new results are in excellent agreement with known surface features. Pronounced low Vs, low density, and high Vp/Vs anomalies are imaged in the locations of several major sedimentary basins including the Williston, Powder River, Green River, Denver, and San Juan basins. Preliminary results on the inverted 3D Vs, Vp/Vs ratio, and density structure in the crust and upper mantle will also be discussed. (a)-(c) 30-sec Rayleigh-wave phase velocity, local amplification, and H/V ratio observed across USArray in the western US. The red lines denote the tectonic boundaries and the triangles in (b)-(c) shown the stations used. The thick black lines indicate 3-km sediment contours for several major sedimentary basins (WB: Williston Basin; PR: Powder River Basin; GR: Green River Basin; DB: Denver Basin). (d)-(f) The Vs, density, and Vp/Vs ratio in the uppermost crust (0-3 km) inverted by phase velocity and H/V ratio measurements.

Lin, F.; Schmandt, B.; Tsai, V. C.

2012-12-01

142

Compositional causes of velocity variations in garnet peridotite xenoliths from the Kaapvaal Craton, South Africa  

NASA Astrophysics Data System (ADS)

To investigate the effects of compositional variations on the velocity and density of the upper mantle, we perform a principle component analysis on the modal variability of > 200 garnet peridotite xenoliths from the Archean Kaapvaal craton. Since peridotite melting relations change with depth, we consider xenolith compositional variations as a function of equilibration pressure, by examining all xenoliths that lie within a 1 GPa wide pressure window. The center of the window is moved in 0.05 GPa increments from 3-5.5 GPa. For each pressure window, the xenoliths that have equilibration pressures within this pressure range are identified, and their P-wave and S-wave velocities, and densities, calculated using the technique of Schutt and Lesher [2006; JGR, v. 111]. To remove the effects of pressure and temperature on the scatter in velocities, we calculate all velocities using the pressure and temperature at the center of the window, assuming a 41 mW/m2 geotherm. In general, V_P and V_S vary by about 1-1.5% due to compositional variations, so compositional effects are potentially resolvable with seismic tomography. Compositional variations within the examined suite of xenoliths are calculated using principle component analysis on the mode variation of the xenoliths. Statistical robustness of the principle components is calculated by bootstrapping analysis. We restrict the first principle component to explaining the effects of Mg# (and thus melt depletion) on mode. Interestingly, this Mg# component shows an increase in orthopyroxene stability between ~4.1-5.2 GPa, consistent with experimental melting studies. Only one other significant component is found, and this second component shows a strong trade-off between olivine and orthopyroxene mode, a trend we associate with silica enrichment [Kelemen et al., 1998; EPSL, v. 164], probably arising from infiltration of metasomatic fluids associated with Archean subduction. To assess the effect of the principle components on xenolith velocity and density, we calculate each xenolith's mode in terms of the two significant principle components and perform a bivariate regression of xenolith V_P, V_S, and density against these principle components. In general, ~50-90% of the variance in velocity and density can be explained by the effects of melt depletion and silica enrichment. For example, a 1% increase in Mg# increases V_P and V_S by as much as 0.5% and 0.2% respectively, and decreases density by ~1%. An increase in orthopyroxene mode by a weight fraction of 0.1 affects V_P and V_S by ~-0.7% and 0.2%, respectively, and increases density by as much as 2%. This suggests not only that depleted mantle may be imaged through seismic means, but also that low velocity upper mantle may indicate silica enrichment and perhaps even zones of ancient subduction.

Schutt, D. L.; Lesher, C. E.

2006-12-01

143

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

144

Microwave acoustics handbook. Volume 3: Bulk wave velocities  

NASA Astrophysics Data System (ADS)

Information useful for the design of acoustic delay lines, resonators, and other miniature, low cost, reliable devices for use in communications and electronic sensing is given in this report. Computations of bulk acoustic wave velocities, power flow angles, and coupling to electric fields are plotted for various orientations of the following single crystalline materials: Ba2NaNb5O15, Bi12GeO20, CdS, Diamond, Eu3Fe5O15, Gadolinium Gallium Garnet, GaAs, Germanium, InSb, InAs, Lead Molybdate, PbS, LiNbO3, LiTaO3, MgO, Quartz, Rutile, Sapphire, Silicon, Spinel, TeO2, YAG, YGaG, YIG, and ZnO. Particular cuts of interest, including cases for common metals, are then chosen for more detailed numerical calculations of mechanical and electrical parameters governing acoustic wave propagation in these media. A list of material constants is also included.

Slobodnik, A. J., Jr.; Delmonico, R. T.; Conway, E. D.

1980-05-01

145

Evidence for anisotropy in north east Africa, from geographical and azimuthal distribution of Rayleigh wave velocities, and average upper mantle structure  

NASA Astrophysics Data System (ADS)

Long period surface waves from WWSSN, GDSN, SRO and GEOSCOPE stations are analysed to obtain the dispersion of surface waves in the North-East of Africa. Lateral variations of Rayleigh wave velocities are retrieved simultaneously with the average azimuthal anisotropy. As a first result, the low velocity anomaly under the Red Sea is apparent at all periods. A second result is a nearly North-South direction of maximum group and phase velocities. This azimuthal anisotropy direction is in agreement with the direction of absolute plate motion in this region. Anisotropy is also evidenced by simultaneous inversion of Love and Rayleigh wave dispersion along the path TAM-AGD. In the resulting models, isotropic and anisotropic, very low mantle SV-wave velocities extend up to the Moho. For SH waves, we find a strong discrepancy with SV waves, SH-wave velocities being higher by 5% down to 200 km under the crust. This is characteristic of lithospheric polarization anisotropy. The S-wave anisotropy (polarization and azimuthal) found in this region, including the Red Sea, is compatible with the absolute motion of the African plate: it can be interpreted, considering the low lithospheric S-velocities, as a consequence of plastic flow in the weak lithosphere of an anomalous mantle. This anisotropy, evidenced here on S waves, may be due, as Pn and lithospheric S-wave anisotropy found in other continental areas, to dynamical processes related to plate motion.

Hadiouche, Ouiza; Jobert, Nelly

1988-04-01

146

Distribution of Longitudinal Wave Velocities in Bovine Cortical Bone in vitro  

Microsoft Academic Search

The distribution of longitudinal wave velocities and longitudinal moduli in a bovine femoral cortical bone was experimentally investigated. In all parts of the long cylindrical bone, the velocities and longitudinal moduli in the axial direction were the highest. In the anterior (A) part, the velocities in the axial direction were high and almost constant, whereas the velocities in the proximal

Yu Yamato; Hideo Kataoka; Mami Matsukawa; Kaoru Yamazaki; Takahiko Otani; Akira Nagano

2005-01-01

147

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

148

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

149

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

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

150

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

NASA Technical Reports Server (NTRS)

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

Chung, D. H.

1973-01-01

151

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

152

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

NASA Astrophysics Data System (ADS)

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

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

2012-05-01

153

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

154

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

NASA Astrophysics Data System (ADS)

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

Xia, Jianghai

2014-04-01

155

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

156

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

157

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

PubMed

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

Ricci, Stefano; Bassi, Luca; Tortoli, Piero

2014-02-01

158

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

159

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

160

One-dimensional models of shear wave velocity for the eastern Mediterranean obtained from the inversion of Rayleigh wave phase velocities and tectonic implications  

Microsoft Academic Search

On a SW-NE profile from the Libyan coast towards central Turkey phase velocity curves of the fundamental Rayleigh mode were measured using a two-station method. The inversion of phase velocity curves yields 1-D models of shear wave velocity down to approximately 200 km depths that may be interpreted as estimates of average models between neighbouring stations on the profile. Strong

T. Meier; K. Dietrich; B. Stöckhert; H.-P. Harjes

2004-01-01

161

Pulse wave velocity and cognitive function in older adults.  

PubMed

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

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

2014-01-01

162

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

163

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

164

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

PubMed Central

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

Xu, Zhen J.; Song, Xiaodong

2009-01-01

165

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

166

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

167

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

168

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

Microsoft Academic Search

Evaluation of rock velocities and comparison with velocity profiles defined by seismic refraction experiments are a crucial approach for understanding the petrological structure of the crust. In this study, we calculated the seismic wave velocities of various types of rocks from the Oman ophiolite in order to constrain a petrological structure of the oceanic crust. Christensen & Smewing (1981, JGR)

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

2010-01-01

169

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

170

Determination of relationship between Rayleigh wave velocity and stress with laser Doppler velocimeter  

NASA Astrophysics Data System (ADS)

A non-contact measurement technique of Rayleigh wave velocity is proposed. In the non-contact measurement system, a laser Doppler velocimeter is used to determine wave motions. With above technique, the relationship between Rayleigh wave velocity and stress for an aluminum alloy 5052 and steel SS400 is determined, and the results are in good agreement with that obtained by contact measurement method.

He, Lingfeng; Kobayashi, Shoichi

2002-05-01

171

Velocity and attenuation of seismic waves in random media: A spectral function approach  

NASA Astrophysics Data System (ADS)

This contribution investigates the scattering of scalar and elastic waves in two-phase materials and single-mineral-cubic, hexagonal, orthorhombic-polycrystalline aggregates with randomly oriented grains. Based on the Dyson equation for the mean field, explicit expressions for the imaginary part of Green's function in the frequency-wavenumber domain (?,p), also known as the spectral function, are derived. This approach allows the identification of propagating modes with their relative contribution, and the computation of both attenuation and phase velocity for each mode. The results should be valid from the Rayleigh (low-frequency) to the geometrical optics (high-frequency) regime. Applications of the proposed theory to the structure of the inner core of the Earth will be presented. In particular, it will be shown that our scattering theory can explain the striking correlation between velocity and attenuation and the associated hemispherical variations revealed by PKP waves propagating through the inner core of the Earth. The implications for inner core dynamics will be summarized.

Margerin, Ludovic; Calvet, Marie; Monnereau, Marc; Souriau, Annie

2013-04-01

172

Spatial and seasonal variation in wave attenuation over Zostera noltii  

NASA Astrophysics Data System (ADS)

Wave attenuation is a recognized function of sea grass ecosystems which is believed to depend on plant characteristics. This paper presents field data on wave attenuance collected over a 13 month period in a Zostera noltii meadow. The meadow showed a strong seasonality with high shoot densities in summer (approximately 4,600 shoots/m2) and low densities in winter (approximately 600 shoots/m2). Wave heights and flow velocities were measured along a transect at regular intervals during which the site was exposed to wind waves and boat wakes that differ in wave period and steepness. This difference was used to investigate whether wave attenuation by sea grass changes with hydrodynamic conditions. A seasonal change in wave attenuation was observed from the data. Results suggest that a minimum shoot density is necessary to initiate wave attenuation by sea grass. Additionally, a dependence of wave attenuation on hydrodynamics was found. Results suggest that the threshold shoot density varies with wave period and a change in energy dissipation toward the shore was observed once this threshold was exceeded. An attempt was made to quantify the bed roughness of the meadow; the applicability of this roughness value in swaying vegetation is discussed. Finally, the drag coefficient for the meadow was computed: A relationship between wave attenuance and vegetation Reynolds number was found which allows comparing the wave attenuating effect of Zostera noltii to other plant species.

Paul, M.; Amos, C. L.

2011-08-01

173

Variational Formulation of Eikonal Theory for Vector Waves.  

National Technical Information Service (NTIS)

The eikonal theory of wave propagation is developed by means of a Lorentz-covariant variational principle, involving functions defined on the natural eight-dimensional phase space of rays. The wave field is a four-vector representing the electromagnetic p...

A. N. Kaufman, H. Ye Y. Hui

1986-01-01

174

Variational Formulation of Covariant Eikonal Theory for Vector Waves.  

National Technical Information Service (NTIS)

The eikonal theory of wave propagation is developed by means of a Lorentz-covariant variational principle, involving functions defined on the natural eight-dimensional phase space of rays. The wave field is a four-vector representing the electromagnetic p...

A. N. Kaufman, H. Ye Y. Hui

1986-01-01

175

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

E-print Network

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

Denny, Mark

176

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

USGS Publications Warehouse

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

Brocher, T. M.

2008-01-01

177

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

PubMed

Wave intensity (WI) is a hemodynamics index, which is the product of changes in pressure and velocity across the wave-front. Wave Intensity Analysis, which is a time domain technique allows for the separation of running waves into their forward and backward directions and traditionally uses the measured pressure and velocity waveforms. However, due to the possible difficulty in obtaining reliable pressure waveforms non-invasively, investigating the use of wall displacement instead of pressure signals in calculating WI may have clinical merits. In this paper, we developed an algorithm in which we use the measured diameter of flexible tube's wall and flow velocity to separate the velocity waveform into its forward and backward directions. The new algorithm is also used to separate wave intensity into its forward and backward directions. In vitro experiments were carried out in two sized flexible tubes, 12mm and 16mm in diameters, each is of 2 m in length. Pressure, velocity and diameter were taken at three measuring sites. A semi-sinusoidal wave was generated using a piston pump, which ejected 40cc water into each tube. The results show that separated wave intensity into the forward and backward directions of the new algorithm using the measured diameter and velocity are almost identical in shape to those traditionally using the measured pressure and velocity. We conclude that the new algorithm presented in this work, could have clinical advantages since the required information can be obtained non-invasively. PMID:18002125

Feng, J; Khir, A W

2007-01-01

178

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

179

Depth Extent of the Fault Zone Wave Guide: Effects of Fault Variation With Depth  

NASA Astrophysics Data System (ADS)

Deformation in major fault zones creates a low-velocity zone that can trap or guide seismic waves. Previous computer modeling has indicated that sources off the fault should not generate fault zone guided waves unless they are beyond the end of the wave guide. However, guided waves have recently been observed from off-fault earthquakes. In addition, guided wave delay times have not always systematically increased with distance between source and seismometer. These observations have been cited as evidence that the fault zone waveguide does not extend beyond a few kilometers in depth. Previous computer modeling has usually assumed a homogenous fault. However, stress-induced closure of porosity causes seismic velocity both outside and inside the fault to increase substantially with depth. This velocity increase will cause the dominant frequency of the waveguide to increase. Using reasonable geological models for major faults in California, the dominant frequency might change from ~5 Hz to perhaps greater than 20 Hz. Few studies have investigated guided waves with a broad range of frequencies on a single fault. The evidence for lack of a deep waveguide at low frequency may not indicate that the fault zone waveguide disappears at depth, but rather that its dominant frequency changes with depth. We have computed for sources in and out of the fault finite-difference synthetic seismic guided waves for a shallow fault waveguide and for a deep fault waveguide whose properties change realistically with depth. The results show a frequency dependence of guided waves caused by fault zone variation with depth. This may shed light on the current controversy over the depth extent of the low velocity fault zone.

Wu, J.; Hole, J. A.; Snoke, J. A.; Imhof, M. G.

2006-12-01

180

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

181

A Variational Property of the Velocity Distribution in a System of Material Particles  

ERIC Educational Resources Information Center

A simple variational property concerning the velocity distribution of a set of point particles is illustrated. This property provides a full characterization of the velocity distribution which minimizes the kinetic energy of the system for prescribed values of linear and angular momentum. Such a characterization is applied to discuss the kinetic…

Siboni, S.

2009-01-01

182

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

183

Shear wave velocities from noise correlation at local scale  

Microsoft Academic Search

Cross correlations of ambient seismic noise recordings have been studied to infer shear seismic velocities with depth. Experiments have been done in the crowded and noisy historical centre of Napoli over inter-station distances from 50 m to about 400 m, whereas active seismic spreadings are prohibitive, even for just one receiver. Group velocity dispersion curves have been extracted with FTAN

G. de Nisco; C. Nunziata; F. Vaccari; G. F. Panza

2008-01-01

184

Rayleigh wave phase velocity maps from the ambient noise tomography in central Mongolia  

NASA Astrophysics Data System (ADS)

The study area (103°E-111°E, 44°N-49°N) located in the Mongolian fold belts and situated at the southeast of Baikal rift zone which is known as one of the most active regions on the Earth due to integrated influence of the India-Asia collision and compression and the subduction of the Pacific Plate. Additionally, it also located in the north of South-North earthquake belts of China. So, it is believed to be an ideal site for understanding intraplate dynamics. Seismic ambient noise tomography has been performed all over the world these years, and it has been proved it's a powerful way to image and study the structure of crust and uppermost mantle due to its exclusive capability to extract estimated Green's functions for short period surface waves. Compared with traditional earthquake tomography methods of surface waves, ambient noise tomography hasn't limitations related to the distribution of earthquakes as well as errors in earthquake locations and source mechanisms. A new scientific project was carried out in 2011 by Institute of Geophysics of China Earthquake Administration (IGP-CEA) and Research center of Astronomy and Geophysics of Mongolian Academy of Science (RCAG-MAS). In the seismic sub-project 60 portable seismic stations were deployed in central Mongolia in August 2011. Continuous time-series of vertical component between August 2011 and July 2012 have been collected and cross-correlated to obtain estimated Green's functions (EGF) of Rayleigh wave. Using the frequency and time analysis technique based on continuous wavelet transformation, 1258 of phase velocity dispersion curves of Rayleigh wave were extracted from EGFs. High resolution phase velocity maps at periods of 5, 10, 20 and 30 s were reconstructed with grid size 0.5°x0.5° by utilizing a generalized 2-D-linear inversion method developed by Ditmar & Yanovskaya. The tomography results reveal lateral heterogeneity of shear wave structure in the crust and upper mantle in the study region. For periods shorter than 10 s, the phase velocity variations are well correlated with the principal geological units, with low-speed anomalies corresponding to the sedimentary basins and high-speed anomalies coinciding with the main mountain ranges. Within the period range from 20 s to 30 s, phase velocity distribution is correlated to the crust thickness. However, the value of phase velocities have little lateral changes with ~0.15km/s on each map for the whole period band ranging from 5 s to 30 s, indicating that it doesn't have big lateral heterogeneity for shear wave structure in the crust and upper mantle in the study region.This study was supported by the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120) and NSFC (41104029)

Pan, J.; Wu, Q.; Gao, M.; Li, Y.; Demberel, S. G.; Munkhuu, U.

2013-12-01

185

The primitive nature of large low shear-wave velocity provinces  

NASA Astrophysics Data System (ADS)

The lowermost (>2400 km) Earth's mantle mapped by seismic tomography is strongly heterogeneous, the most striking feature being two large regions where shear-wave velocity drops by a few percent compared to averaged mantle. Additional seismic observations indicate that these structures cannot result from purely thermal effects. Compositional anomalies are required to fully explain seismic observations, but their exact nature is still debated. Here, we show that low shear-wave velocity provinces unlikely consist of recycled oceanic crust (MORB). We calculated seismic sensitivity to high-pressure MORB, and found that in the lowermost mantle shear-wave velocity increases with increasing fraction of MORB. Therefore, unless they are heated up to unrealistic temperatures, high-pressure MORB would induce high shear-wave velocity, in contradiction with the observations. Instead, material enriched in iron by ˜3.0% and in (Mg,Fe)-perovskite by ˜20% compared to regular mantle provides a good explanation for the low shear-wave velocity provinces and for the high bulk-sound velocities observed in the same areas. In addition, several geochemical and geodynamical arguments support a primitive origin for this material. Low shear-wave velocity provinces may thus consist of reservoirs of primitive material that have differentiated early in the Earth's history.

Deschamps, Frédéric; Cobden, Laura; Tackley, Paul J.

2012-10-01

186

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

187

Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan  

NASA Astrophysics Data System (ADS)

At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.

Shiina, Takahiro; Nakajima, Junichi; Toyokuni, Genti; Matsuzawa, Toru

2014-12-01

188

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

189

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

NASA Technical Reports Server (NTRS)

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

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

2012-01-01

190

Application of a simple relation for describing wave velocity as a function of pressure in rocks containing microcracks  

Microsoft Academic Search

The occurrence of microcracks and pore space reduces the P and S wave velocities in rocks from those expected for the intrinsic mineral matrix. In the low-pressure regime, before microcracks close, the elastic properties and wave velocities are characteristically nonlinear. This nonlinear behavior reflects the effects of the crack closure spectra on the P and S wave velocities. Several efforts

Roy J. Greenfield; E. K. Graham

1996-01-01

191

Application of a simple relation for describing wave velocity as a function of pressure in rocks containing microcracks  

Microsoft Academic Search

The occurrence of microcracks and pore space reduces the P and S wave velocities in rocks from those expected for the intrinsic mineral matrix. In the low-pressure regime, before microcracks close, the elastic properties and wave velocities are, characteristically nonlinear. This nonlinear behavior reflects the effects of the crack closure spectra on the P and S wave velocities. Several efforts

Roy J. Greenfield; E. K. Graham

1996-01-01

192

IPS measurements and azimuthal variation of solar-wind velocity on the heliospheric current sheet  

NASA Astrophysics Data System (ADS)

The solar-wind velocity tends to be a minimum along the heliospheric current sheet and increases with heliomagnetic latitude. In addition, an azimuthal variation of solar-wind velocity is found to exist on the current sheet. In this study of the solar-wind velocity variations on the current sheet, we have made use of interplanetary scintillation (IPS) measurements. This method provides solar-wind data over a larger range of heliolatitudes: +/- 80 deg or greater. We found that there exists a clear longitudinal variation of solar-wind velocity on the current sheet during 1973-1985. It is also noticeable that the pattern of variation changes with the phase of the solar cycle. On average, the solar-wind velocity varied from 300 to 550 km/s 1 on the current sheet during the period of study. It is also found that the longitudinal variation of the solar-wind velocity on the current sheet obtained using near-earth satellite data agrees well with that obtained using IPS data.

Bala, B.; Prabhakaran Nayar, S. R.

1993-02-01

193

Variational theory for thermodynamics of thermal waves  

NASA Astrophysics Data System (ADS)

We discuss description of macroscopic representations of thermal fields with finite signal speed by composite variational principles involving suitably constructed potentials along with original physical variables. A variational formulation for a given vector field treats all field equations as constraints that are linked by Lagrange multipliers to the given kinetic potential. We focus on the example of simple hyperbolic heat transfer, but also stress that the approach can be easily extended to the coupled transfer of heat, mass, and electric charge. With our approach, various representations may be obtained for physical fields in terms of potentials (gradient or nongradient representations). Corresponding Lagrangian and Hamiltonian formalism can be developed. Symmetry principles yield components of the energy-momentum tensor for the given kinetic potential. The limiting reversible case appears as a special yet suitable reference frame to describe irreversible phenomena. With the conservation laws resulting from the least action principle and the Gibbs equation, the variational scheme of nonequilibrium thermodynamics follows. Its main property is abandoning the assumption of local thermal equilibrium.

Sieniutycz, Stanislaw; Berry, R. Stephen

2002-04-01

194

Compressional and shear wave velocities in serpentinized peridotites  

Microsoft Academic Search

Serpentinized peridotites in the wedge mantle play key roles in the transport of water and the slab-mantle coupling. Geophysical mapping of serpentinized regions is essential for good understanding of subduction zone processes. Tomographic studies have related low-velocity and high Poisson's ratio (high Vp\\/Vs) to serpentinized peridotites [e.g., Kamiya and Kobayashi (2000)]. Their interpretations are based on velocity measurements mostly on

T. Watanabe; H. Oguri; H. Yano; A. Yoneda

2007-01-01

195

S wave velocity structure of the northern Cascadia subduction zone  

Microsoft Academic Search

The shear velocity structure across the northern Cascadia subduction zone is examined using three-component broadband digital seismographs. Locally generated P-to-S conversions are analyzed to estimate the S velocity structure to upper mantle depths and to constrain the subduction geometry of the Juan de Fuca plate. The oceanic crust is at 47-53 km beneath central Vancouver Island, 60-65 km beneath Georgia

John F. Cassidy; Robert M. Ellis

1993-01-01

196

The upper crustal P-wave velocity structure of Newberry volcano, Central Oregon.  

E-print Network

?? The upper-crustal seismic-velocity structure of Newberry volcano, central Oregon, is imaged using P-wave travel time tomography. The inversion combines a densely-spaced seismic line collected… (more)

Beachly, Matthew William

2011-01-01

197

The Upper Crustal P-wave Velocity Structure of Newberry Volcano, Central Oregon .  

E-print Network

??The upper-crustal seismic-velocity structure of Newberry volcano, central Oregon, is imaged using P-wave travel time tomography. The inversion combines a densely-spaced seismic line collected in… (more)

Beachly, Matthew William, 1986-

2011-01-01

198

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

199

Influence of small temperature variations on the ultrasonic velocity in concrete  

NASA Astrophysics Data System (ADS)

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

Niederleithinger, E.; Wunderlich, C.

2013-01-01

200

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

NASA Astrophysics Data System (ADS)

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

Garth, Tom; Rietbrock, Andreas

2014-09-01

201

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

202

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

203

Near-Surface Shear-Wave Velocity Measurements in Unlithified Sediment  

NASA Astrophysics Data System (ADS)

Shear-wave (S-wave) velocity can be directly correlated to material stiffness making it a valuable physical property that has found uses in construction, engineering, and environmental projects. This study compares three different methods, Multichannel Analysis of Surface Waves (MASW), S-wave tomography, and downhole seismic for measuring S-wave velocities, investigates and identifies the differences among the methods' results, and prioritizes the different methods for S-wave use at the U. S. Army's Yuma Proving Grounds (YPG) north of Yuma, AZ. A large signal-to-noise ratio and a layered depositional architecture at the study site gives the MASW method much potential, but higher-mode energy resulting from velocity discontinuities reduces the effectiveness of the method shallower than 20 ft. First arrival analysis provides evidence of a velocity discontinuity within the first 10 feet of unconsolidated sediment. S-wave first arrivals were picked using impulsive sledgehammer data which were then used for both tomographic inversion and refraction analysis. Three-component downhole seismic data were collected by using a locking geophone coupled with the borehole casing to estimate seismic velocities directly. This study helps to identify the strengths and weaknesses of each of these methods at sites similar to YPG. MASW results show a low-velocity layer at a depth of about 50 feet that is verified by downhole seismic data and is undetectable through traditional refraction tomography. However S-wave refraction tomography provides more convincing results at shallow depths where the MASW method fails. Using both methods in an integrated fashion provide the most accurate depiction of S-wave velocity characteristics in the shallow unconsolidated sediments at YPG.

Rickards, Benjamin Thomas

204

Wave-induced velocities inside a model seagrass bed  

E-print Network

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

Luhar, Mitul

205

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

206

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

USGS Publications Warehouse

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

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

2011-01-01

207

Hurricane Directional Wave Spectrum Spatial Variation at Landfall  

NASA Technical Reports Server (NTRS)

On 26 August 1998, hurricane Bonnie was making landfall near Wilmington, NC. The NASA airborne scanning radar altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane hunter aircraft at 2.2 km height documented the sea surface directional wave spectrum in the region between Charleston, SC and Cape Hatteras, NC. The aircraft ground track included both segments along the shoreline and Pamlico Sound as well as far offshore. An animation of the directional wave spectrum spatial variation at landfall will be presented and contrasted with the spatial variation when Bonnie was in the open ocean on 24 August 1998.

Walsh, Edward J.; Wright, C. Wayne; Vandemark, Douglas C.; Krabill, William B.; Garcia, Andrew W.; Houston, Samuel H.; Powell, Mark D.; Black, Peter G.; Marke, Frank D.; Busalacchi, Antonio J. (Technical Monitor)

2000-01-01

208

Low Velocity Detonation of Nitromethane Affected by Precursor Shock Waves Propagating in Various Container Materials  

Microsoft Academic Search

It is well known that some liquid explosives have two detonation behaviors, high velocity detonation (HVD) or low velocity detonation (LVD) can propagate. A physical model to describe the propagation mechanism of LVD in liquid explosives was proposed that LVD is not a self-reactive detonation, but rather a supported-reactive detonation from the cavitation field generated by precursor shock waves. However,

Hideki Hamashima; Akinori Osada; Yukio Kato; Shigeru Itoh

2007-01-01

209

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

Microsoft Academic Search

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

Éric Beucler; Éléonore Stutzmann; Jean-Paul Montagner

2003-01-01

210

Regional P wave velocity structure of the Northern Cascadia Subduction Zone  

Microsoft Academic Search

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

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

2006-01-01

211

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

212

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

Microsoft Academic Search

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

Francis Birch

1960-01-01

213

Investigation of local rayleigh wave velocity dispersion due to surface residual stress  

Microsoft Academic Search

Residual stress induced in materials by surface modification techniques creates a stress gradient very near the top surface. The modified layer can lead to dispersion of Rayleigh surface wave (RSW) velocity. An analysis of the dispersion of RSW is expected to provide a means for nondestructive evaluation of the stress gradient. Small changes in RSW velocity has been measured using

Richard W. Martin; Richard S. Reibel; Shamachary Sathish; Mark P. Blodgett

2004-01-01

214

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

215

Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle  

USGS Publications Warehouse

Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, QS-1, and seismic velocity, VS, we compare global maps of the thermal structure of the continental upper mantle with global QS-1 and Vs maps as determined from Rayleigh waves at periods between 40 and 150 S. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of <0.56 between VS and T and of <0.47 between QS and T at any depth. Such low correlation coefficients can partially be attributed to modelling arrefacts; however, they also suggest that not all of the VS and QS anomalies in the continental upper mantle can be explained by T variations. Global maps show that, by the sign of the anomaly, VS and QS usually inversely correlate with lithospheric temperatures: most cratonic regions show high VS and QS and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero QS anomalies approximately correspond to the 1000 ??C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000-1100 ??C. East-west profiles of VS, QS and T where continental data coverage is best (50??N latitude for North America and 60??N latitude for Eurasia) further demonstrate that temperature plays a dominant, but non-unique, role in determining the value of lithospheric VS and QS. At 100 km depth, where the resolution of seismic models is the highest, we compare observed seismic VS and QS with theoretical VST and QST values, respectively, that are calculated solely from temperature anomalies and constrained by experimental data on temperature dependencies of velocity and attenuation. This comparison shows that temperature variations alone are sufficient to explain seismic VS and QS in ca 50 per cent of continental regions. We hypothesize that compositional anomalies resulting from Fe depletion can explain the misfit between seismic and theoretical VS in cratonic lithosphere. In regions of active tectonics, temperature effects alone cannot explain seismic VS and QS in the lithosphere. It is likely that partial melts and/or fluids may affect seismic parameters in these regions. This study demonstrates that lithospheric temperature plays the dominant role in controlling VS and QS anomalies, but other physical parameters, such as compositional variations, fluids, partial melting and scattering, may also play a significant role in determining VS and QS variations in the continental mantle. ?? 2004 RAS.

Artemieva, I. M.; Billien, M.; Leveque, J. -J.; Mooney, W. D.

2004-01-01

216

Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle  

NASA Astrophysics Data System (ADS)

Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, Q-1s, and seismic velocity, Vs, we compare global maps of the thermal structure of the continental upper mantle with global Q-1s and Vs maps as determined from Rayleigh waves at periods between 40 and 150 s. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high. The available data set does not indicate that, at a global scale, seismic anomalies in the upper mantle are controlled solely by temperature variations. Continental maps have correlation coefficients of <0.56 between Vs and T and of <0.47 between Qs and T at any depth. Such low correlation coefficients can partially be attributed to modelling artefacts; however, they also suggest that not all of the Vs and Qs anomalies in the continental upper mantle can be explained by T variations. Global maps show that, by the sign of the anomaly, Vs and Qs usually inversely correlate with lithospheric temperatures: most cratonic regions show high Vs and Qs and low T, while most active regions have seismic and thermal anomalies of the opposite sign. The strongest inverse correlation is found at a depth of 100 km, where the attenuation model is best resolved. Significantly, at this depth, the contours of near-zero Qs anomalies approximately correspond to the 1000 °C isotherm, in agreement with laboratory measurements that show a pronounced increase in seismic attenuation in upper mantle rocks at 1000-1100 °C. East-west profiles of Vs, Qs and T where continental data coverage is best (50°N latitude for North America and 60°N latitude for Eurasia) further demonstrate that temperature plays a dominant, but non-unique, role in determining the value of lithospheric Vs and Qs. At 100 km depth, where the resolution of seismic models is the highest, we compare observed seismic Vs and Qs with theoretical VTs and QTs values, respectively, that are calculated solely from temperature anomalies and constrained by experimental data on temperature dependencies of velocity and attenuation. This comparison shows that temperature variations alone are sufficient to explain seismic Vs and Qs in ca 50 per cent of continental regions. We hypothesize that compositional anomalies resulting from Fe depletion can explain the misfit between seismic and theoretical Vs in cratonic lithosphere. In regions of active tectonics, temperature effects alone cannot explain seismic Vs and Qs in the lithosphere. It is likely that partial melts and/or fluids may affect seismic parameters in these regions. This study demonstrates that lithospheric temperature plays the dominant role in controlling Vs and Qs anomalies, but other physical parameters, such as compositional variations, fluids, partial melting and scattering, may also play a significant role in determining Vs and Qs variations in the continental mantle.

Artemieva, Irina M.; Billien, Magali; Lévêque, Jean-Jacques; Mooney, Walter D.

2004-05-01

217

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

218

Average shear wave velocity models of the crustal structure at Mt. Vesuvius  

Microsoft Academic Search

Some Mt. Vesuvius events recorded in 1989–1999 period at Osservatorio Vesuviano (OSVE)-INGV stations have been processed by means of frequency time analysis (FTAN) method. The group velocities of the fundamental mode of Rayleigh-wave have been extracted in the period range 0.3–2s. Group velocity data, in the period range 10–35s, and phase velocity data in the period range 25–100s, obtained in

M. Natale; C. Nunziata; G. F. Panza

2005-01-01

219

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

Microsoft Academic Search

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. Villaseñor; H. M. Benz; L. Filippi; G. De Luca; R. Scarpa; G. Patanè; S. Vinciguerra

1998-01-01

220

Phase reconstruction from velocity-encoded MRI measurements - A survey of sparsity-promoting variational approaches  

NASA Astrophysics Data System (ADS)

In recent years there has been significant developments in the reconstruction of magnetic resonance velocity images from sub-sampled k-space data. While showing a strong improvement in reconstruction quality compared to classical approaches, the vast number of different methods, and the challenges in setting them up, often leaves the user with the difficult task of choosing the correct approach, or more importantly, not selecting a poor approach. In this paper, we survey variational approaches for the reconstruction of phase-encoded magnetic resonance velocity images from sub-sampled k-space data. We are particularly interested in regularisers that correctly treat both smooth and geometric features of the image. These features are common to velocity imaging, where the flow field will be smooth but interfaces between the fluid and surrounding material will be sharp, but are challenging to represent sparsely. As an example we demonstrate the variational approaches on velocity imaging of water flowing through a packed bed of solid particles. We evaluate Wavelet regularisation against Total Variation and the relatively recent second order Total Generalised Variation regularisation. We combine these regularisation schemes with a contrast enhancement approach called Bregman iteration. We verify for a variety of sampling patterns that Morozov's discrepancy principle provides a good criterion for stopping the iterations. Therefore, given only the noise level, we present a robust guideline for setting up a variational reconstruction scheme for MR velocity imaging.

Benning, Martin; Gladden, Lynn; Holland, Daniel; Schönlieb, Carola-Bibiane; Valkonen, Tuomo

2014-01-01

221

Effect of a Shallow Water Obstruction on Long Wave Runup and Overland Flow Velocity  

E-print Network

, as noted in a handful of post-tsunami surveys, appears to be as an energy reducer: areas protected:6 455 CE Database subject headings: Tsunamis; Wave runup; Reefs; Numerical models; Velocity; Overland flow. Introduction As a nonlinear long wave, such as a tsunami, approaches the shoreline, it inevitably

Lynett, Patrick

222

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

Microsoft Academic Search

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

Terrance G. Barker; Jeffry L. Stevens

1983-01-01

223

Asymptotic solutions of 2D wave equations with variable velocity and localized right-hand side  

Microsoft Academic Search

In the paper, we consider the Cauchy problem for the inhomogeneous wave equation with variable velocity and with a perturbation in the form of a right-hand side localized in space (near the origin) and in time. In particular, this problem is connected with the question about the creation of tsunami and Rayleigh waves. Using abstract operator theory and in particular

S. Yu. Dobrokhotov; V. E. Nazaikinskii; B. Tirozzi

2010-01-01

224

Multiparameter full waveform inversion of multicomponent ocean-bottom-cable data from the Valhall field. Part 2: imaging compressive-wave and shear-wave velocities  

NASA Astrophysics Data System (ADS)

Multiparameter elastic full waveform inversion (FWI) is a promising technology that allows inferences to be made on rock and fluid properties, which thus narrows the gap between seismic imaging and reservoir characterization. Here, we assess the feasibility of 2-D vertical transverse isotropic visco-elastic FWI of wide-aperture multicomponent ocean-bottom-cable data from the Valhall oil field. A key issue is to design a suitable hierarchical data-driven and model-driven FWI workflow, the aim of which is to reduce the nonlinearity of the FWI. This nonlinearity partly arises because the shear (S) wavespeed can have a limited influence on seismic data in marine environments. In a preliminary stage, visco-acoustic FWI of the hydrophone component is performed to build a compressional (P)-wave velocity model, a density model and a quality-factor model, which provide the necessary background models for the subsequent elastic inversion. During the elastic FWI, the P and S wavespeeds are jointly updated in two steps. First, the hydrophone data are inverted to mainly update the long-to-intermediate wavelengths of the S wavespeeds from the amplitude-versus-offset variations of the P-P reflections. This S-wave velocity model is used as an improved starting model for the subsequent inversion of the better-resolving data recorded by the geophones. During these two steps, the P-wave velocity model is marginally updated, which supports the relevance of the visco-acoustic FWI results. Through seismic modelling, we show that the resulting visco-elastic model allows several P-to-S converted phases recorded on the horizontal-geophone component to be matched. Several elastic quantities, such as the Poisson ratio, and the ratio and product between the P and S wavespeeds, are inferred from the P-wave and S-wave velocity models. These attributes provide hints for the interpretation of an accumulation of gas below lithological barriers.

Prieux, Vincent; Brossier, Romain; Operto, Stéphane; Virieux, Jean

2013-09-01

225

Wave Packet for Massless Fermions and its Implication to the Superluminal Velocity Statistics of Neutrino  

E-print Network

Non-dispersive wave packet for massless fermions is formulated on the basis of squeezed coherent states that are put in a form of common eigenfunction for the Hamiltonian and the helicity operator, starting from the Dirac equation. The wave packet thus constructed is demonstrated to propagate at a constant velocity as that of light. This explicit expression of wave packet for the massless fermions can facilitate theoretical analysis of problems where a wave packet is of formal significance. Furthermore, extensive wave packet may result in a superluminal velocity statistics if determined from the time-of-flight measurement, as recently done on muon neutrinos, when a threshold particle flux or energy transfer, which is eventually referred to the propagation of wave packet, to invoke a detection event is assumed.

Kelin Wang; Zexian Cao

2012-01-06

226

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

NASA Technical Reports Server (NTRS)

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

Nakanishi, I.; Anderson, D. L.

1984-01-01

227

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

228

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

229

Shear-Wave Velocity Structure in the Northern Basin and Range Province from the Combined Analysis of Receiver Functions and Surface Waves  

Microsoft Academic Search

A new method based on the joint inversion of receiver functions and surface-wave phase velocities results in well-determined shear-velocity structures that are consistent with the compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. This new inversion method takes advantage of average-velocity information present in the surface-wave method and differential velocity information contained in the

Serdar Ozalaybey; Martha K. Savage; Anne F. Sheehan; John N. Louie; James N. Brune

1997-01-01

230

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

231

Study on Correlation Between Shear Wave Velocity and Ground Properties for Ground Liquefaction Investigation of Silts  

NASA Astrophysics Data System (ADS)

Shear wave velocity (Vs) of soil is one of the key parameters used in assessment of liquefaction potential of saturated soils in the base with leveled ground surface; determination of shear module of soils used in seismic response analyses. Such parameter can be experimentally obtained from laboratory soil tests and field measurements. Statistical relation of shear wave velocity with soil properties based on the surface wave survey investigation, and resonant column triaxial tests, which are taken from more than 14 sites within the depth of 10 m under ground surface, is obtained in Tianjin (China) area. The relationship between shear wave velocity and the standard penetration test N value (SPT-N value) of silt and clay in the quaternary formation are summarized. It is an important problem to research the effect of shear wave velocity on liquefaction resistance of saturated silts (sandy loams) for evaluating liquefaction resistance. According the results of cyclic triaxial tests, a correlation between liquefaction resistance and shear wave velocity is presented. The results are useful for ground liquefaction investigation and the evaluation of liquefaction resistance.

Che, Ailan; Luo, Xianqi; Qi, Jinghua; Wang, Deyong

232

Velocity-Space Diffusion in a Perpendicularly Propagating Electrostatic Wave  

E-print Network

propagation in outlined. Link: http://charles.karney.info/biblio/karney79b.html #12;VELOCITY-SPACE DIFFUSION I at those points where the phase u h l y varying, i.e., at =v. The t r j a t o r y of the ion is given. 1.) The multing #12;diRerence quationr are u-9-p, v ==B+P, where Here r is the normalized

Karney, Charles

233

Temporal variations in flow velocity at Finsterwalderbreen, a Svalbard surge-type glacier  

NASA Astrophysics Data System (ADS)

Inter- and intra-annual velocity variations are well known on alpine glaciers, but their importance for Arctic glaciers has only been recognized more recently. This paper presents flow velocity data from Finsterwalderbreen, a 35 km2 polythermal surge-type glacier in southern Svalbard that is presently ˜100 years into its quiescent phase. Field measurements of glacier surface velocities are available from 1950-52 and 1994-97, and mean velocities for the last decade are estimated for the lower glacier using cables drilled to the glacier bed. These velocities show substantial seasonal variations indicating that basal sliding is an important component of surface velocities and interannual fluctuations of up to 75%, possibly indicating variations in subglacial water storage. Several lines of evidence indicate that this glacier has an extensive subglacial hydrological system, generally considered to be a prerequisite for surge-type glaciers, which is at least partly pressurized. Information on surface morphology from 1898 onwards shows that the glacier has experienced continuous retreat since the last surge in about 1910, and has now retreated ˜1.5 km further back than its previous pre-surge position in 1898. Tracking of moraine loops on terrestrial and aerial photographs acquired over a 100 year period indicates that the surge period of Finsterwalderbreen may be lengthening in response to climate changes.

Nuttall, Anne-Marie; Hodgkins, Richard

234

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

235

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

236

Reflection and transmission of acoustical waves from a layer with space-dependent velocity.  

NASA Technical Reports Server (NTRS)

The refraction of acoustical waves by a moving medium layer is theoretically treated and the reflection and transmission coefficients are determined. The moving-medium-layer velocity is uniform but with a space dependence in one direction. A partitioning of the moving medium layer into constant-velocity sublayers is introduced and numerical results for a three-sublayer approximation of Poiseuille flow are presented. The degenerate case of a single constant-velocity layer is also treated theoretically and numerically. The numerical results show the reflection and transmission coefficients as functions of the peak moving-medium-layer normalized velocity for several angles of incidence.

Steinmetz, G. G.; Singh, J. J.

1972-01-01

237

Total-variation-based methods for gravitational wave denoising  

E-print Network

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

Torres, Alejandro; Font, José A; Ibáñez, José M

2014-01-01

238

Three-dimensional variations in the Tibetan mantle lid velocity from Pn tomography  

NASA Astrophysics Data System (ADS)

We have used Pn arrivals from seismic experiments and networks in Tibet and surrounding regions to image the structure of the mantle lid. Previous Pn tomography studies have remarked on the contrast between high velocities beneath southern Tibet and low velocities beneath northern Tibet. We find that this model is oversimplified. Longer raypaths show a vastly different velocity structure than do the shorter raypaths, thus there is significant vertical variation in the velocity structure of the mantle lid. Only longer raypaths show low velocity beneath northern Tibet. The lowest velocities occur beneath the central Qiangtang and Songpan-Ganzi terranes, but are not coincident with the Bangong-Nujiang suture that separates northern and southern Tibet. These deep low velocity zones suggest a thin lithosphere there. There is also low velocity mantle beneath the northeast Songpan-Ganzi Terrane (in Gansu province). Shorter raypaths image the top of the mantle lid and show two distinct high velocity zones beneath the Himalayas. Several similar high velocity regions occur north of the eastern Himalayan syntaxis beneath the Lhasa, Qiangtang, and Songpan-Ganzi terranes. We interpret these as underthusted Indian continental lithosphere segments. The east-west variability of Pn velocity beneath the Himalayas and southern Tibet indicates that the underthrusted Indian continental lithosphere is not a homogeneous body. Qaidam Basin along with the Gonghe and Xining Basins east of it show an east-west high velocity anomaly. We interpret this as a preexisting cratonic core that deformed little while the Tibetan Plateau to the south and the Qilian Shan to the north did.

Hearn, T. M.; Wang, H.; Chen, Y. J.; Sandvol, E. A.; Ni, J. F.

2011-12-01

239

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

240

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

241

Seasonal and interannual variations in the velocity field of the South China Sea  

Microsoft Academic Search

A three-dimensional numerical model is used to simulate sea level and velocity variations in the South China Sea for 1992–1995.\\u000a The model is driven by daily wind and daily sea surface temperature fields derived from the NCEP\\/NCAR 40-year reanalysis project.\\u000a The four-year model outputs are analyzed using time-domain Empirical Orthogonal Functions (EOF). Spatial and temporal variations\\u000a of the first two

Chau-Ron Wu; Ping-Tung Shaw; Shenn-Yu Chao

1998-01-01

242

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

243

RESOLUTION OF SHALLOW SHEAR STRUCTURE USING REGIONAL SURFACE WAVES FROM MINING EXPLOSIONS AND GENETIC ALGORITHMS Fundamental Rayleigh wave group velocity dispersion curves  

E-print Network

AND GENETIC ALGORITHMS Test Fundamental Rayleigh wave group velocity dispersion curves from MFA (Multiple/sec. A genetic algorithm (GA) was used for crustal shear-wave velocity inversion. As a global optimization method and observed group-velocity. This fitness function in combination with the genetic algorithm provided improved

Stump, Brian W.

244

The change in Cosmic rays intensity variation with the Solar wind velocity variation  

Microsoft Academic Search

GRAPES-3 experiment is situated at Ooty in South India 76.7 East 11.4 North. Effective ob- servation area of our muon telescopes is 560 m 2 . They are the largest detector in the world of its kind. There were several reports that increase of the solar wind velocity suppresses the intensity of cosmic rays. But there are few which studied

Y. HAYASHI; K. HAYASHI; S. KAWAKAMI; T. MATSUYAMA; M. MINAMINO; T. OKUDA; S. OGIO; A. OSHIMA; N. SHIMIZU; S. K GUPTA; A. IYER; P. JANGADEESAN; S. KARTHIKEYAN; P. K MOHANTY; S. D MORRIS; P. K NAYAK; B. S RAO; K. C RAVINDRAN; H. TANAKA; S. C TONWAR; S. SHIBATA; I. MORISHITA

245

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

246

Gradient index lenses for flexural waves based on thickness variations  

NASA Astrophysics Data System (ADS)

This work presents a method for the realization of gradient index devices for flexural waves in thin plates. Unlike recent approaches based on phononic crystals, the present approach is based on the thickness-dependence of the dispersion relation of flexural waves, which is used to create gradient index devices by means of local variations of the plate's thickness. Numerical simulations of known circularly symmetrical gradient index lenses have been performed. These simulations have been done using the multilayer multiple scattering method and the results prove their broadband efficiency and omnidirectional properties. Finally, finite element simulations employing the full three-dimensional elasticity equations also support the validity of the designed approach.

Climente, Alfonso; Torrent, Daniel; Sánchez-Dehesa, José

2014-08-01

247

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

248

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

Microsoft Academic Search

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

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

1984-01-01

249

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

250

Velocity Variation Assessment of Red Blood Cell Aggregation with Spectral Domain Doppler Optical Coherence Tomography  

E-print Network

Velocity Variation Assessment of Red Blood Cell Aggregation with Spectral Domain Doppler Optical coherence tomography (SD-D-OCT) to qualitatively mea- sure red blood cell aggregation. Variance/standard deviation (SD) of the Doppler frequency spectrum in Doppler variance imaging of flowing blood under shearing

Chen, Zhongping

251

Modeling and Simulation of Fixed bed Adsorption column: Effect of Velocity Variation  

Microsoft Academic Search

The kinetic behavior of a fixed-bed adsorber can be explained and the characteristic breakthrough curve of the adsorption phenomena can be obtained through mathematical models. In the earlier models, the kinetics is explained using a mathematical model that takes into account of external and internal mass- transfer resistances with a nonideal plug flow behavior. The variation of fluid velocity along

B. V. Babu; Suresh Gupta

252

S velocity variations beneath North America Heather Bedle1,2  

E-print Network

, mineralogical, and thermal states of the man- tle. It is crucial to quantify even minor velocity variations when feature of the North American continent is the Rocky Mountain Cordillera, which extends from the far north in Alaska, through Canada, the United States and into Mexico. This mountain belt extends to and abuts

van der Lee, Suzan

253

Variational Partitioned Runge-Kutta methods for Lagrangians linear in velocities  

E-print Network

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

Tomasz M. Tyranowski; Mathieu Desbrun

2014-01-30

254

Abnormalities of T waves in effort angina pectoris patients at rest evaluated by spatial velocity electrocardiogram.  

PubMed

The normal shape of the T wave on the standard 12-lead electrocardiogram (ECG) is asymmetrical. The spatial velocity electrocardiogram (SVECG) is a method of recording the slope of the wave of the ECG and makes it possible to quantify the symmetry of the ECG-T wave. The aim of this study was to diagnose resting effort angina pectoris not detectable by conventional resting ECG using SVECG. We studied SVECG-T waves in effort angina pectoris patients with significant coronary artery stenosis but with normal resting ECG (AP group: 50 subjects) and in an age- and sex-matched normal controls (N group: 30 subjects). The SVECG is depicted as a simple differential wave, and all SVECG-T waves had two peaks. The first peak of the SVECG-T wave was designated as the 'a' wave and the second peak as the 'c' wave. The heights (a wave-mag, c wave-mag) and c wave-mag/a wave-mag ratio (c/a ratio) were calculated. The c/a ratio was significantly decreased in the AP group (1.52 +/- 0.29 vs 1.87 +/- 0.27, p < 0.0001). In cases below the cut off c/a ratio of 1.7, the diagnostic sensitivity, specificity and accuracy for effort angina pectoris were retrospectively evaluated to be 82%, 70% and 78%, respectively. The heights of the 'a' and 'c' waves correspond to the maximum slope of the ascending and descending limbs of the T wave on ECG. The symmetrization of the T wave on ECG is considered to occur when the c/a ratio decreases and approaches 1.0. In conclusion, the c/a ratio of the SVECG-T wave is a useful index to diagnose effort angina pectoris at rest when the resting ECG is normal. PMID:9057682

Sasaki, A; Takimiya, A; Arai, T; Song, Y; Nakajima, S; Muto, K; Ibukiyama, C

1996-11-01

255

Intracyclic velocity variation and arm coordination assessment in swimmers with Down syndrome.  

PubMed

This study examined the differences in intracycle velocity variation and arm coordination in front crawl in swimmers with Down syndrome in three breathing conditions. International swimmers with Down syndrome (N = 16) performed 3 × 20 m front crawl at 50 m race speed: without breathing, breathing to the preferred side, and breathing to the nonpreferred side. A two dimensional video movement analysis was performed using the APASystem. Breathing conditions were compared using Repeated Measures ANOVA. Swimming velocity was higher without breathing and intracyclic velocity variation was higher while breathing. Swimmers tended to a catch up arm coordination mode for both breathing conditions and a superposition mode when not breathing. These data reflect arm coordination compromising swimming performance, particularly when comparing with non disabled swimmers in literature. The physical and perhaps cognitive impairment associated with Down syndrome may result in a disadvantage in both propulsion and drag, more evident when breathing. PMID:23283027

Marques-Aleixo, Inês; Querido, Ana; Figueiredo, Pedro; Vilas-Boas, João Paulo; Corredeira, Rui; Daly, Daniel; Fernandes, Ricardo J

2013-01-01

256

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

E-print Network

known as the Blue Mountain Province (BMP) collided with Nevada during the mid to late JurassicDetailed three-dimensional shear wave velocity structure of the northwestern United States from a Department of Geological Sciences, University of North Carolina-Chapel Hill, Chapel Hill, NC, United States b

Fouch, Matthew J.

257

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

258

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

259

Joint Inversion of Multimode Love Wave Dispersion Curves for Accurately Appraising of Shear Wave Velocity Reversal Profile  

NASA Astrophysics Data System (ADS)

The use of multimode dispersion curves (DCs) in surface wave inversion to derive shear wave velocity (VS) profiles has increased in the past decade as the inclusion of higher mode data can improve the accuracy of the inversion results. This paper presents a joint inversion codes to invert multimode Love wave (fundamental plus higher modes) DCs and focuses on the attempt to improve the accurate appraisal of the VS reversal profile. The multimode Love wave DCs were synthesized using full SH (Shear Horizontal) waveform reflectivity as the true model. In this study, we used a frequency-slowness (f-p) transform to extract DC from full SH waveform reflectivity. DCs overlain in the dispersion images were picked manually. The DCs were then inverted using modified genetic algorithm (GA). Our numerical modeling showed that, the joint inversion of multimode Love wave DCs can significantly improve the accurate assessment of VS reversal profiles.

Hamimu, La; Nordin, M. Nawawi M.; Safani, Jamhir

2010-07-01

260

Kinetic theory for electrostatic waves due to transverse velocity shears  

NASA Technical Reports Server (NTRS)

A kinetic theory in the form of an integral equation is provided to study the electrostatic oscillations in a collisionless plasma immersed in a uniform magnetic field and a nonuniform transverse electric field. In the low temperature limit the dispersion differential equation is recovered for the transverse Kelvin-Helmholtz modes for arbitrary values of K parallel, where K parallel is the component of the wave vector in the direction of the external magnetic field assumed in the z direction. For higher temperatures the ion-cyclotron-like modes described earlier in the literature by Ganguli, Lee and Plamadesso are recovered. In this article, the integral equation is reduced to a second-order differential equation and a study is made of the kinetic Kelvin-Helmholtz and ion-cyclotron-like modes that constitute the two branches of oscillation in a magnetized plasma including a transverse inhomogeneous dc electric field.

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

1988-01-01

261

Impact of localized inhomogeneity on the surface-wave velocity and bulk-wave reflection in solids.  

PubMed

The effect of a weak surface, near-surface and interfacial inhomogeneity on the frequency dependence of the surface wave velocity and of the SH (shear horizontal) wave reflectivity in isotropic elastic media is studied analytically and numerically. The inhomogeneity is modeled as an infinite planar layer with continuously varying properties. Weak inhomogeneity may markedly affect the dispersion of the Rayleigh velocity and especially of the reflectivity. It is demonstrated how this effect, particularly pronounced at high frequency, depends on the extent of inhomogeneity. The material data for damaged and ideal concrete and several simple examples of inhomogeneity profiles are utilized for the numerical calculations based on the Peano expansion. The use of explicit low- and high-frequency approximations is also exemplified. Among these, simple WKB asymptotics are shown to be particularly helpful for the Rayleigh velocity in the case of a prominent inhomogeneity attached to the surface and for the reflection on weak interfaces. PMID:17064750

Baron, C; Shuvalov, A L; Poncelet, O

2007-03-01

262

Electromagnetic wave propagation with negative phase velocity in regular black holes  

SciTech Connect

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

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

2012-12-15

263

Variable aspect ratio method in the Xu-White model for shear-wave velocity estimation  

NASA Astrophysics Data System (ADS)

Shear-wave velocity logs are useful for various seismic interpretation applications, including bright spot analyses, amplitude-versus-offset analyses and multicomponent seismic interpretations. This paper presents a method for predicting the shear-wave velocity of argillaceous sandstone from conventional log data and experimental data, based on Gassmann's equations and the Xu-White model. This variable aspect ratio method takes into account all the influences of the matrix nature, shale content, porosity size and pore geometry, and the properties of pore fluid of argillaceous sandstone, replacing the fixed aspect ratio assumption in the conventional Xu-White model. To achieve this, we first use the Xu-White model to derive the bulk and shear modulus of dry rock in a sand-clay mixture. Secondly, we use Gassmann's equations to calculate the fluid-saturated elastic properties, including compressional and shear-wave velocities. Finally, we use the variable aspect ratio method to estimate the shear-wave velocity. The numerical results indicate that the variable aspect ratio method provides an important improvement in the application of the Xu-White model for sand-clay mixtures and allows for a variable aspect ratio log to be introduced into the Xu-White model instead of the constant aspect ratio assumption. This method shows a significant improvement in predicting velocities over the conventional Xu-White model.

Bai, Jun-Yu; Yue, Cheng-Qi; Liang, Yi-Qiang; Song, Zhi-Xiang; Ling, Su; Zhang, Yang; Wu, Wei

2013-06-01

264

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

USGS Publications Warehouse

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

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

2003-01-01

265

Correlations between seismic wave velocities and physical properties of near-surface geologic materials in the southern San Francisco Bay region, California  

USGS Publications Warehouse

To identify geologic units with distinctly different seismic responses for the purposes of seismic zonation, compressional and shear wave velocities have been measured in boreholes at 59 sites in the San Francisco Bay region in a wide range of near-surface (0-30m) geologic materials. Several physical parameters, which can be readily determined in the field, were found to correlate with the shear wave velocities and were used to define seismically distinct groups. For the unconsolidated to semiconsolidated sediments, texture, standard penetration resistance and depth were used to define eight seismically distinct groups. For the bedrock materials, fracture spacing and hardness were used to differentiate ten distinct categories. The correlation obtained between shear wave velocity and the physical parameters were used to regroup the map units defined for the San Francisco Bay region into seismically distinct units. The map units for the younger unconsolidated sediments can be really differentiated seismically. In contrast, the older semiconsolidated sedimentary deposits and bedrock units, which have experienced significant variations in post-depositial changes, show wider and overlapping velocity ranges. The map units for the sedimentary deposits have been regrouped into eight seismically distinct geotechnical units. The bedrock map units have been broadly regrouped into five distinct categories. Compressional wave velocities were not found to be well correlated with the physical parameters dependent on the soil or rock structure. For materials above the water table, the wide velocity variations found for each geotechnical group can be attributed to differences in degree of saturation. The strong correlations observed between shear wave velocity and other readily determine physical properties suggest that geologic maps which incorporate these parameters are most useful for seismic zonation.

Fumal, Thomas E.

1978-01-01

266

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

E-print Network

) 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...

Zeng, Chong

2011-05-18

267

Group velocity measurement from the propagation of the ionization front in a surface-wave-produced plasma  

SciTech Connect

During the first instant, previous to steady-state in a surface-wave-produced plasma, an ionization front advance front the launcher to the plasma column end. The velocity of the ionization front is much slower than the group velocity of the surface wave, this give a reflection of the incident signal on the moving ionization front. In this paper, the authors use this effect to calculate the surface wave group velocity.

Cotrino, J.; Gamero, A.; Sola, A.; Lao, C. (Departamento de Fisica Aplicada, Universidad de Cordoba, San Alberto Magno, s/n. 14071 Cordoba (ES))

1989-05-01

268

Uppermost Mantle S-wave Velocity Structure of the East Anatolian-Caucasus Region  

NASA Astrophysics Data System (ADS)

The East Anatolian-Caucasus region is a part of the orogenic belt which formed as the result of the closure of the Neo Tethys Ocean and the corresponding continental collision of Arabian and Eurasian plates. Our study region includes the southwestern part of the Caspian basin, the Kura basin, the Lesser and Greater Caucasus mountains, and the East Anatolian and North Iranian plateaus. We used the data from the Eastern Turkey Seismic Experiment network (1999-2001) in addition to the data which was recorded by 26 permanent broadband stations located in eastern Turkey and Azerbaijan during January of 2006 - July of 2008 in order to develop a 3D S-wave velocity model of the regional uppermost mantle. We selected total of 62 teleseismic events with surface wave magnitudes larger than 5.8 and with good signal-to-noise ratio to determine the fundamental mode Rayleigh wave phase velocities at 13 periods between 20 and 143 seconds. The phase velocity maps show a broad low velocity zone beneath East Anatolian and North Iranian plateaus and three high velocity zones located under the eastern part of the Greater Caucasus, the Talesh and Pontides. Furthermore, the regional uppermost mantle appears to be relatively isotropic. We inverted the Rayleigh wave phase velocities to obtain the regional 3D S-wave velocity model (0-350 km). The low velocity zone is observed starting right at the moho down to 150 km, which suggests asthenospheric material underlying a very thin lithosphere of eastern Anatolia where widespread Late Miocene - Quaternary calc-alkaline volcanic products of mantle origin are reported. Eastern Arabia and Black Sea each have lithospheric roots reaching depths of 150 km. The high velocity body beneath the eastern Greater Caucasus and Kura Basin lies at depths below the moho down to 180 km, and apparently represents either a thick lithospheric mantle root or a shallow subducting slab associated with the South Caspian block. The high velocity bodies beneath the Talesh and Pontides are observed below 160 km. We believe that these high velocity bodies represent remnant Neo Tethys slabs that broke off after the initiation of continental collision between Arabia and Eurasia and could serve as an evidence of two subduction zones beneath Pontide and Bitlis arcs during the Neogene.

Skobeltsyn, G.; Mellors, R.; Gok, R.; Turkelli, N.; Forsyth, D. W.; Sandvol, E. A.

2011-12-01

269

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

Microsoft Academic Search

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

M. K. Schreba

1972-01-01

270

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

271

Shear-wave velocity profile at the Texcoco strong-motion array site, Valley of Mexico  

Microsoft Academic Search

The shear-wave velocity profile at the Texcoco strong motion accelerograph array site was evaluated down to 39 m using Seismic Cone Penetrometry (SCPT). The hard layer from 28 m to 29 m was drilled to allow penetration to greater depths. The results are compared with those obtained from shear-wave reflection\\/refraction, and from the inversion of microtremor data. All the studies

Bill Stephenson; Cinna Lomnitz

2005-01-01

272

Shallow shear wave velocity and Q structures at the El Centro Strong Motion Accelerograph Array  

Microsoft Academic Search

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 (beta) and quality factor (Q) and a site dependent equivalent elastic source spectrum. This approach provides an inexpensive means for

Terrance G. Barker; Jeffry L. Stevens

1983-01-01

273

Asymptotic solutions of 2D wave equations with variable velocity and localized right-hand side  

Microsoft Academic Search

In the paper, we consider the Cauchy problem for the inhomogeneous wave equation with variable velocity and with a perturbation\\u000a in the form of a right-hand side localized in space (near the origin) and in time. In particular, this problem is connected\\u000a with the question about the creation of tsunami and Rayleigh waves. Using abstract operator theory and in particular

S. Yu. Dobrokhotov; V. E. Nazaikinskii; B. Tirozzi

2010-01-01

274

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

275

Ionospheric Doppler and electron velocities in the presence of ULF waves  

SciTech Connect

In this paper we refine the earlier work (Poole et al., 1988) in which we derived three mechanisms relating geomagnetic pulsations to simultaneous Doppler velocity oscillations in a vertically incident, ionospherically reflected radio wave. We show that the generally held belief that field-aligned electron velocities associated with the corresponding currents can be ignored is unfounded in the case of magnetic pulsations. These field-aligned velocities contribute significantly to two of the three mechanisms identified in the earlier work. Diagrams are presented to compare our results with those previously obtained by us and other workers using various simplifying assumptions. {copyright} American Geophysical Union 1989

Sutcliffe, P.R. (Magnetic Observatory, Foundation for Research Development, Hermanus, (South Africa)); Poole, A.W.V. (Hermann Ohlthaver Institute for Aeronomy, Rhodes University, Grahamstown, (South Africa))

1989-10-01

276

Investigation of local rayleigh wave velocity dispersion due to surface residual stress  

NASA Astrophysics Data System (ADS)

Residual stress induced in materials by surface modification techniques creates a stress gradient very near the top surface. The modified layer can lead to dispersion of Rayleigh surface wave (RSW) velocity. An analysis of the dispersion of RSW is expected to provide a means for nondestructive evaluation of the stress gradient. Small changes in RSW velocity has been measured using an acoustic interferometer based on a three-element focused transducer. Dispersion results on Ti-6Al-4V material of RSW velocity and dispersion in shot-peened samples are presented.

Martin, Richard W.; Reibel, Richard S.; Sathish, Shamachary; Blodgett, Mark P.

2004-02-01

277

The P-wave boundary of the Large-Low Shear Velocity Province beneath the Pacific  

NASA Astrophysics Data System (ADS)

The Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle represent volumetrically significant thermal or chemical or thermo-chemical heterogeneities. Their structure and boundaries have been widely studied, mainly using S-waves, but much less is known about their signature in the P-wavefield. We use an extensive dataset recorded at USArray to create, for the first time, a high-resolution map of the location, shape, sharpness, and extent of the boundary of the Pacific LLSVP using P(Pdiff)-waves. We find that the northern edge of the Pacific LLSVP is shallow dipping (26° relative to the horizontal) and diffuse (?120 km wide transition zone) whereas the eastern edge is steeper dipping (70°) and apparently sharp (?40 km wide). We trace the LLSVP boundary up to ?500 km above the CMB in most areas, and 700 km between 120° and 90°W at the eastern extent of the boundary. Apparent P-wave velocity drops are ?1-3% relative to PREM, indicating a strong influence of LLSVPs on P-wave velocity, at least in the high-frequency wavefield, in contrast to previous studies. A localised patch with a greater velocity drop of ?15-25% is detected, defined by large magnitude gradients of the travel-time residuals. We identify this as a likely location of an Ultra-Low Velocity Zone (ULVZ), matching the location of a previously detected ULVZ in this area. The boundary of a separate low velocity anomaly, of a similar height to the LLSVP, is detected in the north-west Pacific, matching tomographic images. This outlier appears to be connected to the main LLSVP through a narrow channel close to the CMB and may be in the process of joining or splitting from the main LLSVP. We also see strong velocity increases in the lower mantle to the east of the LLSVP, likely detecting subducted material beneath central America. The LLSVP P-wave boundary is similar to that determined in high-resolution S-wave studies and follows the -0.4% ?VS iso-velocity contour in the S40RTS tomography model. Additionally, the LLSVP boundary roughly matches the shape of the -0.4% ?VP iso-velocity contour of the P-wave model GyPSuM but defines an area more similar to that defined by the 0.0% VP iso-velocity contour. High resolution P-wave velocity determination allows for estimation of the ratio of P- and S-wave velocity anomalies (RS,P) which can be used to indicate dominantly thermal or chemical control of seismic velocities. Although the RS,P is found here to be approximately 2.4, which is indicative of a thermo-chemical anomaly. However, this result contains a large amount of uncertainty and the implications for the origin of LLSVPs likely remain inconclusive. Nonetheless, other observations of the Pacific LLSVP are consistent with a thermo-chemical anomaly whose shape and boundary sharpness are controlled by proximity to active and past subduction.

Frost, Daniel A.; Rost, Sebastian

2014-10-01

278

Mantle shear-wave velocity structure beneath the Hawaiian hot spot.  

PubMed

Defining the mantle structure that lies beneath hot spots is important for revealing their depth of origin. Three-dimensional images of shear-wave velocity beneath the Hawaiian Islands, obtained from a network of sea-floor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a parabola-shaped high-velocity anomaly. Low velocities continue downward to the mantle transition zone between 410 and 660 kilometers depth, a result that is in agreement with prior observations of transition-zone thinning. The inclusion of SKS observations extends the resolution downward to a depth of 1500 kilometers and reveals a several-hundred-kilometer-wide region of low velocities beneath and southeast of Hawaii. These images suggest that the Hawaiian hot spot is the result of an upwelling high-temperature plume from the lower mantle. PMID:19965755

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

2009-12-01

279

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

280

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

281

CELEBRATION 2000: P-wave velocity models of the Bohemian Massif  

NASA Astrophysics Data System (ADS)

Deep structure of the Bohemian Massif (BM), the largest stable outcrop of Variscan rocks in Central Europe, was studied along two refraction profiles, CEL09 that traverses the whole massif in the NW-SE direction, and CEL10 that extends along its eastern edge almost perpendicularly to CEL09. Good quality recordings with clear first arrivals of crustal and upper mantle phases show apparent velocity 5.9 km/s for the upper crust with slightly higher gradient in NW part of the BM and app. velocity 8.0 to 8.1 km/s for the upper mantle. Decrease of amplitudes of crustal phases visible in some sections may be connected with a specific upper crustal structure (zero to negative velocity gradient zone). Pronounced Moho reflections in central part of the BM suggest well-defined Moho in that part and not so clear Moho with smaller velocity contrast in other parts of the BM. For interpretation, the tomographic inversion routine of Hole (1992) was used as an efficient tool to determine seismic P-wave velocity distribution in the crust using first arrivals. Tomographic models were verified by forward ray tracing modelling based on well-established algorithm developed by Cerveny et al. (1983), where apart from first arrivals also further phases were included. 2-D velocity models of first arrivals and reflected phases show high P-wave velocity gradient zone reaching the depth of 5-7 km followed by small gradient and laterally homogeneous P-wave velocity distribution in the middle crust. Differences in velocity distribution in the lower crust delimit central part of the BM (sharp Moho discontinuity) from other tectonic units within the BM (lower crust high gradient transition zone). Position of Moho discontinuity ranging from 32 km to 40 km and reflectors within the crust complement the P-wave velocity distribution. Presented models also show the contact of the BM with its neighbouring units - Carpathians, Paleozoic Platform, Vienna Basin and the Alps. References: Cerveny, V., Psencik, I., 1983. Program SEIS83, Numerical Modelling of Seismic Wave Fields in 2-D Laterally Varying Layered Structures by the Ray Method, Charles University, Prague. Hole, J.A. 1992: Non-linear high-resolution three-dimensional seismic travel time tomography, J. Geophys. Res. 97, 6553-6562.

Hrubcova, P.

2003-04-01

282

Total-variation-based methods for gravitational wave denoising  

E-print Network

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

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

2014-09-28

283

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

284

Near-Surface Shear-Wave Velocity Measurements in Unlithified Sediment  

E-print Network

the effectiveness of the method shallower than 20 ft. First arrival analysis provides evidence of a velocity discontinuity within the first 10 feet of unconsolidated sediment. S-wave first arrivals were picked using impulsive sledgehammer data which were then used...

Rickards, Benjamin Thomas

2011-05-31

285

Control of the chaotic velocity dispersion of a cold electron beam interacting with electrostatic waves  

E-print Network

In this article we present an application of a method of control of Hamiltonian systems to the chaotic velocity diffusion of a cold electron beam interacting with electrostatic waves. We numerically show the efficiency and robustness of the additional small control term in restoring kinetic coherence of the injected electron beam.

Guido Ciraolo; Cristel Chandre; Ricardo Lima; Marco Pettini; Michel Vittot

2006-07-31

286

WAVE PROPAGATION IN VISCOACOUSTIC HETEROGENEOUS MEDIA: A VELOCITY-PRESSURE FINITE-DIFFERENCE METHOD  

Microsoft Academic Search

We present the panicle velocity-pressure hire-difference method 'or modelling Y~SCO~CDUS~~C wave propagation in heterogeneous media. In this method, the vismacoustic modulus is appmrimated hy a low-order rational function. Then, the convolution integral. pre- sent is the viscoacoutic consfirutive relation, is transformed into a differential form. The equations \\

ANTONIOS VAFIDIS; NANXUN Dd; ERNEST KANASEWICH

287

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

288

Elastic wave velocity and acoustic emission monitoring during Gypsum dehydration under triaxial stress conditions  

Microsoft Academic Search

Dehydration experiments were performed on natural Gypsum polycrystal samples coming from Volterra, Italy in order to study contemporaneously the evolution of P and S elastic wave velocities and acoustic emission (AE) triggering. During these experiments, temperature was slowly raised at 0.15 degrees C per minute under constant stress conditions. Two experiments were realized under quasi-hydrostatic stress (15 and 55 MPa

N. Brantut; E. C. David; E. Héripré; A. J. Schubnel; R. W. Zimmerman; Y. Gueguen

2010-01-01

289

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

E-print Network

Increase of shear wave velocity before the 1998 eruption of Merapi volcano (Indonesia) U. Wegler,1 of the edifice of Merapi volcano (Java, Indonesia) before its eruption in 1998 by analyzing multiply scattered eruption of Merapi volcano (Indonesia), Geophys. Res. Lett., 33, L09303, doi:10.1029/2006GL025928. 1

Snieder, Roel

290

Sound wave velocities in dry and lubricated granular packings packings: numerical simulations and experiments  

Microsoft Academic Search

Numerical simulations are used to investigate the origins of the different wave velocities measured in dense granular samples assembled with different methods. Glass bead packings are prepared in the lab either by pouring and vibrating the dry material in a container, or by mixing with a very small amount of a viscous lubricant. Lubricated samples, although less dense, exhibit significantly

Ivana Agnolin; Jean-Noël Roux; Pascal Massaad; Xiaoping Jia; Pierre Mills

2009-01-01

291

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

Microsoft Academic Search

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

Francis Birch

1961-01-01

292

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

USGS Publications Warehouse

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

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

1996-01-01

293

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

Microsoft Academic Search

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

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

294

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

295

A comprehensive dispersion model of surface wave phase and group velocity  

NASA Astrophysics Data System (ADS)

In order to complement our large datasets of surface wave group arrival times, we have adapted our cluster analysis method of group velocity measurement to measure phase velocity. When measuring phase, we are no longer working with waveform envelopes and we must be very careful to avoid cycle-skipping. We correct for source phase and the predicted phase shift due to 3d structure by using a nominal phase velocity map. This latter step is important at periods shorter than 100 seconds but is not necessary at longer periods. We have processed all the long period data from IRIS from 1988 to 2007. Currently, we have finished phase measurements for Love wave from 7mHz to 30mHz. This dataset contains about 200,000 measurements at 10mHz and about 100,000 measurements at 30mHz. The phase dataset for Rayleigh waves is complete from 5mHz to 25mHz, and has about 600,000 measurements at 10 mHz and 400,000 measurements at 25mHz. We see no difficulty in extending the Rayleigh wave measurements to, say, 40mHz to complement our group velocity measurements. In order to find a dispersion model that simultaneously matches both phase and group data, we use b-splines to parameterize the frequency dependence of the phase velocity. We find that a parameterization of equally spaced b-splines with an interval of 2mHz is sufficient to explain both our phase and group velocity maps consistently. At long periods, there is a strong tradeoff between the isotropic part of the Rayleigh wave phase velocity and azimuthal anisotropy (e.g. Ekstrom, 2011). This effect is mainly confined to the Pacific basin where azimuthal anisotropy is coherent over large distances and results in significant signal. We include the effect of azimuthal anisotropy in our inversions in order to obtain a reliable isotropic part of the phase velocity. Patterns in the fast directions of Rayleigh wave azimuthal anisotropy and their reliability will also be discussed.

Ma, Z.; Masters, G.; Laske, G.; Pasyanos, M. E.

2012-12-01

296

Azimuthal Anisotropy in Mexico from Rayleigh Wave Phase Velocity Maps and Shear-Wave Splitting  

Microsoft Academic Search

Most shear-wave splitting analysis performed in subduction zones display a fast direction of propagation for seismic waves oriented parallel to the trench near the trench, and trench-perpendicular in the backarc. The Mexico subduction zone, however, is an exception to this rule: shear-wave splitting analysis performed in that region show fast directions that are perpendicular to the trench with no significant

I. Stubailo; C. Beghein; P. M. Davis

2010-01-01

297

Shear-wave velocity compilation for Northridge strong-motion recording sites  

USGS Publications Warehouse

Borehole and other geotechnical information collected at the strong-motion recording sites of the Northridge earthquake of January 17, 1994 provide an important new basis for the characterization of local site conditions. These geotechnical data, when combined with analysis of strong-motion recordings, provide an empirical basis to evaluate site coefficients used in current versions of US building codes. Shear-wave-velocity estimates to a depth of 30 meters are derived for 176 strong-motion recording sites. The estimates are based on borehole shear-velocity logs, physical property logs, correlations with physical properties and digital geologic maps. Surface-wave velocity measurements and standard penetration data are compiled as additional constraints. These data as compiled from a variety of databases are presented via GIS maps and corresponding tables to facilitate use by other investigators.

Borcherdt, Roger D.; Fumal, Thomas E.

2002-01-01

298

Instabilities of magnetohydrodynamic waves driven by the velocity anisotropy of the energetic ions  

NASA Astrophysics Data System (ADS)

The destabilization of fast magnetoacoustic waves (FMWs) and Alfvén waves (AWs) in plasmas with energetic ions having strongly anisotropic velocity distribution is considered. It is shown that when the ratio of the energetic ion velocity to the Alfvén velocity is large enough, the growth rate of the low-frequency FMW instability has sharp maxima at certain angles of the wave propagation. On the other hand, low-frequency FMWs are always stable, whereas the AWs and high-frequency FMWs can be unstable when the waves propagate along the magnetic field. The developed theory is applied to a plasma of the flashing loops of the Solar Corona. It is found that the presence of a very small amount of the energetic ions can lead to instabilities with the growth rate, which considerably exceeds the inverse characteristic collisional times. It is concluded that the frequency spectrum of the destabilized waves can be rather wide, which may lead to the heating of the bulk plasma ions on the sub-harmonics of the ion gyrofrequency due to the mechanism suggested by Chen et al.

Kolesnychenko, Oleh Ya.; Yukhimuk, Adam K.

2004-04-01

299

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

300

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

NASA Technical Reports Server (NTRS)

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

Turner, T. N.

1979-01-01

301

Seasonal variation of solitary wave properties in Lake Constance  

NASA Astrophysics Data System (ADS)

The properties of internal solitary waves (ISWs) depend on the stratification of the water body. In most climatic regions the stratification in lakes and oceans varies during the year, and hence the properties of the ISWs can also be expected to change over the seasons. On the basis of a long-term temperature time series recorded over 6 years, this paper investigates seasonal changes in the characteristic properties of ISWs in Lake Überlingen, a subbasin of Lake Constance. A large number of ISWs with amplitudes ranging from 3 m to 30 m were identified. More than 15% of the leading ISWs of a wave train were associated with density inversions, often indicating shear instabilities or trapped cores. For all waves the propagation depth and the value of a nonlinearity index nlp providing the degree of nonlinearity were determined, propagation depth being the rest height of the isotherm undergoing maximum displacement and nlp the ratio between wave amplitude and propagation depth. The index nlp was found to be a good parameter for predicting the occurrence of inversions. The statistical analysis of the wave properties derived from the observations revealed that the degree of nonlinearity of the ISWs changes with season. Complementary to the statistical analysis, the seasonally averaged ISW properties were compared with wave prototypes obtained numerically from the Dubreil-Jacotin-Long (DJL) and the stratified Korteweg-deVries (KdV) models. The simulations indicate that the typical stratification and its seasonal variation are responsible for the degree and the seasonality of nonlinearity of the ISWs.

Preusse, M.; Freistühler, H.; Peeters, F.

2012-04-01

302

Solar cycle variation of gravity waves observed in OH airglow  

NASA Astrophysics Data System (ADS)

Airglow imaging provides a unique means by which to study many wave-related phenomena in the 80 to 100 km altitude regime. Two-dimensional image observations reveal quasi-monochromatic disturbances associated with atmospheric gravity waves (AGWs) as well as small-scale instabilities, often called ripples. Image-averaged temperature and intensity measurements can be used to study the response of the airglow layer to tides and planetary waves, as well as monitor longer-term climatological variations. Here we present results of low and mid-latitude OH airglow observations beginning near solar max of solar cycle 23 and continuing through solar max of cycle 24. Aerospace imagers deployed at Alice Springs (23o42'S, 133o53'E) and Adelaide (34o55'S, 138o36'E) have been operating nearly continuously since ~2001. The imagers employ filters measuring OH Meinel (6, 2) and O2 Atmospheric (0, 1) band emission intensities and temperatures, as well as atmospheric gravity wave parameters. The Aerospace Corporation's Infrared Camera deployed at Maui, HI (20.7N,156.3W), collected more than 700 nights of airglow images from 2002-2005. The camera measures the OH Meinel (4,2) emission at 1.6 um using a 1 second exposure at a 3 second cadence, which allows the study of AGW and ripple features over very short temporal and spatial scales. The camera was relocated to Cerro Pachon, Chile (30.1 S, 70.8 W) and has been operating continuously since 2010. Temperature, intensity and gravity wave climatologies derived from the two Australian airglow imagers span a full solar cycle (solar max to solar max). Emission intensities have been calibrated using background stars, and temperatures have been calibrated with respect to TIMED/SABER temperatures, reducing the influence of instrument degradation on the solar cycle climatology. An automated wave detection algorithm is used to identify quasi monochromatic wave features in the airglow data, including wavelength, wave period and propagation direction. Supplemental observations of OH emission brightness and gravity waves (including ripple climatology) at Maui (2002-2005) and Chile (2010-present) will be compared to the Australian observations.

Gelinas, L. J.; Hecht, J. H.; Walterscheid, R. L.; Reid, I. M.; Woithe, J.; Vincent, R. A.

2013-12-01

303

Landstreamer Use for Near-Surface P- and S-Wave Velocities and Poisson's Ratio  

NASA Astrophysics Data System (ADS)

Seismic landstreamer use is becomingly increasingly important in shallow seismic investigations where cost or time efficiency is critical. Applications range from locating underground voids to investigating archaeological sites, and recently to determining engineering parameters such as Poisson's ratio and shear modulus. The advantage of a landstreamer is the capability to drag a geophone array from location to location rather than planting individual geophones by hand each time the array is moved. Comparison studies have shown that data quality from landstreamer geophones and conventionally planted spiked geophones is similar and often practically indistinguishable. The Geophysical Engineering Department at Montana Tech has been using seismic landstreamers to aid data collection for projects that use both diving wave tomographic inversion to produce P-wave velocity images and surface wave dispersion inversion to estimate S-wave velocity images. We then use the P-wave and S-wave velocity images to calculate a Poisson's ratio image. All of these are produced from the same set of recorded shot gathers using vertical component, gimbaled geophones in a landstreamer configuration. This combined use of landstreamers and single mode geophones proves to be an extremely efficient method for determining subsurface parameters of interest. We used this approach to image an open-top buried cement structure at a local geophysical test site. The two velocity images and Poisson's ratio image show good agreement with expected values and clearly show the location of the buried structure. We also used this approach to characterize a leaking earthen mine tailings dam. Results from this investigation highlight areas interpreted to be weak zones associated to known leakage locations.

Link, C. A.; Speece, M. A.

2006-12-01

304

Using pressure and seismological broadband ocean data to model shear wave velocities in the north Atlantic.  

NASA Astrophysics Data System (ADS)

Seafloor compliance is the transfer function between pressure and vertical displacement at the seafloor Infragravity waves in the oceanic layer have long periods in the range of 30 - 500 s and obey a simple frequency-wavenumber relation. Seafloor compliance from infragravity waves can be analyzed with single station recordings to determinate sub-seafloor shear wave velocities. Previous studies in the Pacific Ocean have demonstrated that reliable near-surface shear wave profiles can be derived from infragravity wave compliance. However, these studies indicate that, beside the water depth the compliance measurements are limited by instrument sensitivity, calibration uncertainties and possibly other effects. In this work seafloor compliance and infragravity waves are observed at two different locations in the Atlantic Ocean: the Logatchev hydrothermal field at the Mid Atlantic Ridge and the Azores (Sao Miguel Island). The data was acquired with the broadband ocean compliance station developed at the University of Hamburg as well as ocean station from the German instrument pool for amphibian seismology (DEPAS) equipped with broadband seismometers and pressure sensors. Vertical velocity and pressure data were used to calculate power spectral densities and normalized compliance along two profiles (one in each location). Power spectral densities show a dominant peak at low frequencies (0.01-0.035Hz) limited by the expected cut-off frequency, which is dependent on the water depth at each station. The peak has been interpreted as a strong infragravity wave with values between 10-14 and 10-11 (m/s2)2/Hz and 104 and 106 (Pa2)2/Hz for acceleration and pressure respectively. The results show compliance values between 10-10 and 10-8 1/Pa and its estimations take into account the coherence between seismic and pressure signals in order to confirm that the seismic signals in the infragravity waves are caused by pressure sources. Shear wave velocity models, with depth resolution from 200 to 2500 m for the deep water stations, were derived from compliance. Preliminary results indicate shear wave velocity increasing from 200 to 3500 m/s.

Rios, Celia; Dahm, Torsten; Jegen, Marion

2010-05-01

305

Variational stereo imaging of oceanic waves with statistical constraints.  

PubMed

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

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

2013-11-01

306

Effects of Colistin on the Sensory Nerve Conduction Velocity and F-wave in Mice.  

PubMed

The aim of this study was to examine the changes of sensory nerve conduction velocity (SNCV) and F-wave for colistin-induced peripheral neurotoxicity using a mouse model. Mice were administered with colistin 5, 7.5 and 15 mg/kg/day via a 3-min. intravenous infusion. The sensory nerve conduction velocity (SNCV) and F-wave were measured using the bipolar recording electrodes. The SNCV and F-wave latency changed in a dose- and time-dependent manner. The significant increase of F-wave latency and significant decrease of SNCV appeared on day 3 (p < 0.05 and 0.01, respectively) in the 15 mg/kg/day group, and they were markedly changed on day 7 in the 7.5 mg/kg/day (p < 0.01 and 0.05, respectively) and 15 mg/kg/day groups (both p < 0.01). In addition, F-wave latency also significantly increased on day 7 in the 5 mg/kg/day group (p < 0.05) without any clinical signs. These results indicate that SNCV and F-wave latency were more sensitive in colistin-induced neurotoxicity in mice, which highlights the early monitoring tool of polymyxins neurotoxicity in the clinic. PMID:24861773

Dai, Chongshan; Tang, Shusheng; Li, Jichang; Wang, Jiping; Xiao, Xilong

2014-12-01

307

Shear wave velocity and radial anisotropy along the Rio Grande rift  

NASA Astrophysics Data System (ADS)

We have determined shear wave velocity and radial anisotropy beneath the Rio Grande rift by analyzing ambient seismic noise recorded at the USArray Transportable Array in New Mexico. The results reveal a variable degree of lithosphere extension along the rift. Magma chambers are imaged as significant low velocity anomalies under the Albuquerque volcano in the shallow crust and beneath Socorro in the mid-crust. The central and southern rift is characterized by high velocity anomaly in the lower crust and uppermost mantle, reflecting residual materials after the extraction of melt, and by strong radial anisotropy with VSH > VSV in the mid to lower crust, implying horizontal alignment of crustal minerals due to the vigorous extensional deformation. However, low velocity anomaly and small radial anisotropy are observed in the rift in northern New Mexico, suggesting the presence of partial melt in the lower crust and uppermost mantle and a weak lithosphere extension.

Li, A.; Fu, Y. V.

2011-12-01

308

Joint migration velocity analysis of PP-and PS-waves for VTI media Pengfei Cai1  

E-print Network

, , and are defined at each grid point. The objective function also includes the differences between the migratedJoint migration velocity analysis of PP- and PS-waves for VTI media Pengfei Cai1 and Ilya Tsvankin2 ABSTRACT Combining PP-waves with mode-converted PS reflections in migration velocity analysis (MVA) can

Tsvankin, Ilya

309

Fine S wave velocity structure beneath Iwate volcano, northeastern Japan, as derived from receiver functions and travel times  

Microsoft Academic Search

A genetic algorithm inversion of receiver functions derived from a dense seismic network around Iwate volcano, northeastern Japan, provides the fine S wave velocity structure of the crust and uppermost mantle. Since receiver functions are insensitive to an absolute velocity, travel times of P and S waves propagating vertically from earthquakes in the subducting slab beneath the volcano are involved

Haruhisa Nakamichi; Satoru Tanaka; Hiroyuki Hamaguchi

2002-01-01

310

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

NASA Astrophysics Data System (ADS)

The nonlinear theory of slow-wave electron cyclotron masers (ECM) with an initially straight electron beam is developed. The evolution equation of the nonlinear beam electron energy is derived. The numerical studies of the slow-wave ECM efficiency with inclusion of Gaussian beam velocity spread are presented. It is shown that the velocity spread reduces the interaction efficiency.

Kong, Ling-Bao; Wang, Hong-Yu; Hou, Zhi-Ling; Jin, Hai-Bo; Du, Chao-Hai

2013-12-01

311

Three-Dimensional P-Wave Velocity Structure and Precise Earthquake Relocation at Great Sitkin Volcano, Alaska  

E-print Network

Three-Dimensional P-Wave Velocity Structure and Precise Earthquake Relocation at Great Sitkin Volcano, Alaska by Jeremy D. Pesicek, Clifford H. Thurber, Heather R. DeShon,* Stephanie G. Prejean-difference tomography to increase the precision of earthquake locations and constrain regional 3D P-wave velocity

312

Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi-net  

E-print Network

Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi seismic noise. We applied this method to the recording of Hi-net tiltmeters in Japan at 679 stations from), Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi

Kawakatsu, Hitoshi

313

Coseismic velocity variations caused by static stress changes associated with the 2001 Mw = 4.3 Agios Ioanis earthquake in the Gulf of Corinth, Greece  

NASA Astrophysics Data System (ADS)

The analysis of temporal variations in the seismic velocity across faults can be used to estimate in situ stress changes. Seismic velocity of propagation depends on the fault stiffness, which is a function of stress. The coda wave interferometry technique is applied to seven families of repeating earthquakes (multiplets) recorded on the southern shore of the Gulf of Corinth, Greece, to estimate high precision velocity changes in the Earth's crust associated with the Mw = 4.3 Agios Ioanis earthquake. Results show that the Agios Ioanis event causes a perturbation in elastic properties at seismogenic depth, resulting in a reduction of 0.2% in the seismic velocity. The results are not consistent with either damage induced by dynamic stresses nor a fluid transient origin. In contrast, both the spatial distribution and magnitude of the velocity perturbation correlate well with modeled static stress variations. This suggests that the measured changes in the mechanical properties of the seismogenic crust can be attributed to a change in static stress field associated with the Mw = 4.3 Agios Ioanis earthquake. The velocity changes indicate an unclamping of the Pyrgaki fault at depth, which has local hazard implications.

Cociani, L.; Bean, C. J.; Lyon-Caen, H.; Pacchiani, F.; Deschamps, A.

2010-07-01

314

Three-dimensional Shear Wave Velocity Structure of The Upper Mantle Below Antarctica  

NASA Astrophysics Data System (ADS)

We measure fundamental-mode Rayleigh and Love surface wave group dispersion curves from seismograms recorded by stations in the Antarctic continent and neigh- boring lands at latitude below -30. Our growing regional dataset is merged with the global dataset of phase velocity measurements by Ekström et al. (1997, JGR 102, 8137-8157). Our inversion procedure is divided in two steps. The first is a linear to- mographic inversion of the dispersion measurements to model laterally heterogeneous group velocity at different periods. Wave slowness is parameterized by spline interpo- lation on a geographical grid, with knots equally spaced by 250 km in an orthographic projection. For each point in these maps we then compute the vertical profile of shear wave velocity vs. depth by iterative nonlinear inversion. Crustal properties are as- sumed to be known and follow the CRUST2.0 model (Bassin et al., 2000, EOS Trans AGU, 81 F897). The resulting vS model shows intense negative anomalies under oceanic ridges, at least down to 150 km. The strongest values are related to young oceanic crust near rapidly opening ridges. Shallow low velocity anomalies characterize volcanic provinces and hot-spots in Marie Byrd Land, Ross Sea, Kerguelen, Balleny and South Sandwich archipelagoes. Only few slow anomalies reach depths below 150km (West Antarctica, Ross Sea and the triple junction among Southeast Pacific-South Pacific- Indian Ridges). The East Antarctica archean craton has deep, fast (cold) continental roots reaching at least 200km in depth.

Danesi, S.; Morelli, A.

315

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

316

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

PubMed

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

Li, Li; Weidner, Donald J

2008-08-21

317

Low-frequency P-wave logging for improved compressional velocity in slow formation gas zones  

SciTech Connect

The ratio of compressional to shear velocity is commonly used as a gas indicator, for seismic correlation, and for rock mechanical properties evaluation in the oil industry. However, in soft rock gas zone, it is difficult to obtain a good measure of compressional velocity. Traditional monopole sonic logging tools operate with source frequencies above 10 kHz, giving rise to large amplitude borehole fluid modes which dominate compressional wave traveling with speed slower than the fluid speed. Numerical modeling of monopole sonic waveforms in a fluid-filled borehole in porous formations confirms these observations, but also shows that at significantly lower frequencies two important changes occur: (1) the non-Stoneley fluid modes becomes extremely small, and (2) the P-wave amplitude is greatly increased. Therefore, at low frequencies, logging of these very slow P-waves in gas zones should be feasible. A field example shows the value of this low frequency technique, resulting in measurements of P-wave velocities substantially lower than the fluid speed.

Wu, P.T.; Darling, H.L.; Scheibner, D.

1995-12-31

318

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

319

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

PubMed

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

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

2001-11-01

320

Carotid-femoral pulse wave velocity is negatively correlated with aortic diameter.  

PubMed

Cardiovascular events pose significant morbidity and mortality burden to abdominal aortic aneurysm (AAA) patients. Arterial stiffness as measured by pulse wave velocity (PWV) is an independent predictor of cardiovascular risk. We investigated the relationship between aortic diameter and PWV. Consecutive patients with AAA were invited to participate. Patients completed a health questionnaire, received aortic ultrasound and carotid-femoral PWV (cfPWV) recordings with a Vicorder. Thirty patients were used for reproducibility assessment. A linear regression model was used to identify significant predictors of cfPWV. Observer variation was assessed using Bland and Altman analysis and the intraclass correlation coefficient. Three hundred and nine patients were included-148 with AAA and 161 controls. The mean difference for repeated cfPWV between observers was 0.11?ms(-1). cfPWV was positively correlated with age (r=0.24, P<0.001) and systolic blood pressure (r=0.29, P<0.001) and negatively correlated with aortic diameter (r=-0.15, P=0.008). There was no difference in cfPWV between AAA and control groups (9.75±2.3?ms(-1) vs. 9.55±2.3?ms(-1), P=0.43). Aortic diameter (P=0.003) and systolic blood pressure (P<0.001) were significant predictors of cfPWV independent of age, aspirin usage and a history of myocardial infarction. Patients with large AAA (>5?cm) had decreased cfPWV compared with patients with small AAA (P=0.02) or normal diameter aorta (P=0.02). Vicorder measurements of cfPWV are repeatable. cfPWV is negatively associated with infra-renal aortic diameter and reduced in large AAA. cfPWV is likely invalid for accurate arterial stiffness assessment in patients with AAA owing to the apparent confounding effect of aortic size. PMID:24919482

Bailey, Marc A; Davies, Jennifer M; Griffin, Kathryn J; Bridge, Katherine I; Johnson, Anne B; Sohrabi, Soroush; Baxter, Paul D; Scott, D Julian A

2014-10-01

321

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

322

Shear wave velocity estimation in the metropolitan area of Málaga (S Spain)  

NASA Astrophysics Data System (ADS)

We carry out a seismic noise study based on array measurements at three sites in the Málaga basin, South Spain, for the further estimation of shear wave velocity profiles. For this purpose, we use both the H/V method and the f-k technique in order to characterize the different materials present in the zone, i.e., Quaternary sediments and Pliocene sedimentary rocks above the bedrock. The H/V analysis shows frequency peaks going from 1 Hz, in areas close to the border of the basin, to 0.3 Hz in places located toward the center of the formation. The f-k analysis allows obtaining the dispersion curves associated with each site and subsequently, estimating the Vs profiles by inversion of the respective group velocities. In this way, the basin basement can be characterized by S-wave velocities greater than 2000 m/s. Regarding the basin fill, it is divided into three layers defined by different wave velocity intervals. The shallowest one is featured by velocities ranging from 150 to 400 m/s and comprises the Quaternary sediments, while velocities going from 550-700 to1200-1600 m/s characterize the two underlying layers composed by Pliocene sediments. Finally, the information provided by the three Vs profiles is integrated in a 2D cross-section of the basin to have a spatial view of its sedimentary structure. The results obtained here, in addition to providing useful information about the infill of the basin near the metropolitan area of Málaga, will be very helpful for future seismic zonation studies in the region.

Clavero, D.; Rosa-Cintas, S.; López-Casado, C.; Delgado, J.; Galiana-Merino, J. J.

2014-10-01

323

Crustal P-wave velocity model for the central-western region of Mexico  

NASA Astrophysics Data System (ADS)

Several studies require a p-wave velocity model to obtain accurate results moreover such models could provide an insight of the tectonic structure of the study area. Accordingly, in this study we estimate the crustal 3D p-wave velocity model for the Jalisco Block located at the central-western region of Mexico. The Jalisco Block is limited on its eastern side by the Colima and Tepic-Zacoalcos Rifts, and the Trans-Mexican Volcanic Belt; while on its western side it is limited by the Mesoamerican Trench. Cocos and Rivera plates are subducting beneath the Jalisco Block conforming a tectonically complex region. We used earthquakes occurring within the limits of lithosphere volume from which we want to estimate the velocity model. Such events were registered by the Mapping the Rivera Subduction Zone experiment (MARS) and the Seismic and Acelerometric Network of Jalisco (RESAJ). During MARS experiment 51broadband stations active from January 2006 to June 2007 were deployed while RESAJ by July of 2012consists of nine active stations however more stations will be deployed until reach 30 stations. The velocity model is estimated using the Fast Marching Tomography (FMTOMO) software. FMTOMO uses the Fast Marching Method (FMM) in order to solve the forward problem; the FMM is a numerical algorithm that tracks the interfaces evolution along a nodes narrow band, and travel times are updated solving the eikonal equation. Finally , the inverse problem is about adjusting the model parameters (interface depth, velocity, hypocenter location) in order to try to satisfy the observed data (travel times). We perform a resolution test using several events that show good resolution results up to a 60 km depth. We present a 3D p-wave velocity model, we compare our results within the MARS data with previous results for greater depths, approximately the upper mantle, finally we also present studies towards the northern portion of the Jalisco Block using the RESAJ data.

Ochoa, J.; Escudero, C. R.; Perez, O. G.; Nunez-Cornu, F. J.

2012-12-01

324

Determination of surface-wave phase velocities across USArray from noise and Aki's spectral formulation  

NASA Astrophysics Data System (ADS)

We use expressions for the cross-correlation of stochastic surface waves originally derived by Aki (1957) to develop an algorithm for determining inter-station phase-velocity measurements from continuous seismic data. In the frequency domain, the cross correlation of azimuthally isotropic noise is described by a Bessel function, and we associate zeros in the observed spectrum with zeros of the Bessel function to obtain phase-velocity estimates at discrete frequencies. Phase velocities derived in this way at several frequencies are joined to form a dispersion curve, which is then sampled to obtain phase-velocity estimates at arbitrary frequencies. We collect a set of dispersion curves for more than 30,000 station pairs of the transportable component of USArray, and derive Rayleigh wave phase-velocity maps at periods of 12 and 24 s for the western United States. The spectral method lends itself well to automation, and avoids limitations at short inter-station distances associated with time-domain methods.

Ekström, Göran; Abers, Geoffrey A.; Webb, Spahr C.

2009-09-01

325

An anisotropic shear velocity model of the Earth's mantle using normal modes, body waves, surface waves and long-period waveforms  

NASA Astrophysics Data System (ADS)

We use normal-mode splitting functions in addition to surface wave phase anomalies, body wave traveltimes and long-period waveforms to construct a 3-D model of anisotropic shear wave velocity in the Earth's mantle. Our modelling approach inverts for mantle velocity and anisotropy as well as transition-zone discontinuity topographies, and incorporates new crustal corrections for the splitting functions that are consistent with the non-linear corrections we employ for the waveforms. Our preferred anisotropic model, S362ANI+M, is an update to the earlier model S362ANI, which did not include normal-mode splitting functions in its derivation. The new model has stronger isotropic velocity anomalies in the transition zone and slightly smaller anomalies in the lowermost mantle, as compared with S362ANI. The differences in the mid- to lowermost mantle are primarily restricted to features in the Southern Hemisphere. We compare the isotropic part of S362ANI+M with other recent global tomographic models and show that the level of agreement is higher now than in the earlier generation of models, especially in the transition zone and the lower mantle. The anisotropic part of S362ANI+M is restricted to the upper 300 km in the mantle and is similar to S362ANI. When radial anisotropy is allowed throughout the mantle, large-scale anisotropic patterns are observed in the lowermost mantle with vSV > vSH beneath Africa and South Pacific and vSH > vSV beneath several circum-Pacific regions. The transition zone exhibits localized anisotropic anomalies of ˜3 per cent vSH > vSV beneath North America and the Northwest Pacific and ˜2 per cent vSV > vSH beneath South America. However, small improvements in fits to the data on adding anisotropy at depth leave the question open on whether large-scale radial anisotropy is required in the transition zone and in the lower mantle. We demonstrate the potential of mode-splitting data in reducing the trade-offs between isotropic velocity and anisotropy in the lowermost mantle for the even-degree variations. Spurious anisotropic variations in the mid-mantle are also suppressed with the addition of mode-splitting data.

Moulik, P.; Ekström, G.

2014-12-01

326

A ``no-lid'' zone in the central chang-thang platform of tibet: Evidence from pure path phase velocity measurements of long period rayleigh waves  

Microsoft Academic Search

Pure path phase velocities of fundamental mode Rayleigh waves, in the period range 30-80s, are obtained for different paths across the Tibet Plateau, using a ``two-event'' method, with events located in Tibet and aligned with World-Wide Standard Seismograph Network stations at teleseismic distance. Evidence is found for significant lateral variations of structure of the crust and uppermost mantle in Tibet.

Catherine Brandon; Barbara Romanowicz

1986-01-01

327

Simulated annealing inversion of teleseismic P-wave slowness and azimuth for crustal velocity structure at Long Valley Caldera  

Microsoft Academic Search

Simulated annealing, a non-linear global search algorithm, is used to invert teleseismic P-wave slowness and azimuth data for crustal velocity structure. Synthetic tests show that simulated annealing is able to locate low P-wave velocity zones with reasonable accuracy in space, and recovers almost 70% of the target velocity perturbations. Testing suggests that significant results can be obtained with as few

Lee K. Steck

1995-01-01

328

Third-Order Elastic Constants and the Velocity of Small Amplitude Elastic Waves in Homogeneously Stressed Media  

Microsoft Academic Search

Third-order elastic constants can be determined from the velocity of small amplitude sound waves in statically stressed media. For this purpose exact expressions are derived for the sound velocity and for a natural velocity and their stress derivatives, evaluated at zero stress, in terms of second- and third-order elastic constants. The formulas apply to arbitrary crystal symmetry and to arbitrary

R. N. Thurston; K. Brugger

1964-01-01

329

Revision of P-wave velocity and thickness of hydrate layer in Shenhu Area, South China Sea  

NASA Astrophysics Data System (ADS)

To revise P-wave velocity and thickness of the hydrate layer in the Shenhu area of the South China Sea, acoustic and resistivity logging curves are reanalyzed. The waterlogging phenomenon is found in the shallow sediments of five drilling wells, which causes P-wave velocity to approximate the propagation velocity of sea water (about 1500 m s-1). This also affects the identification of the hydrate layer and results in the underestimate of its thickness. In addition, because there could be about a 5 m thick velocity ramp above or below the hydrate layer as interpreted by acoustic and resistivity logging curves, the recalibrated thickness of this layer is less than the original estimated thickness. The recalibrated P-wave velocity of the hydrate layer is also higher than the original estimated velocity. For the drilling well with a relatively thin hydrate layer, the velocity ramp plays a more important role in identifying and determining the thickness of the layer.

Gong, Jianming; Zhang, Xunhua; Zou, Changchun; Chen, Qiang; Wang, Lichen; Yuan, Chunfang; Hu, Gaowei; Jiang, Yubo

2014-10-01

330

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

331

wave velocity group velocity  

E-print Network

and momentum Umklapp: conserve energy but not momentum Impurity scattering charge carriers migrate in field, suggesting damage is possible The "melt" radius corresponds to the damage profile predicted by cascades [1

Walker, D. Greg

332

Characteristics of Light Reflected from a Dense Ionization Wave with a Tunable Velocity  

SciTech Connect

An optically dense ionization wave (IW) produced by two femtosecond (approx10/30 fs) laser pulses focused cylindrically and crossing each other may become an efficient coherent x-ray converter in accordance with the Semenova-Lampe theory. The resulting velocity of a quasiplane IW in the vicinity of pulse intersection changes with the angle between the pulses from the group velocity of ionizing pulses to infinity allowing a tuning of the wavelength of x rays and their bunching. The x-ray spectra after scattering of a lower frequency and long coherent light pulse change from the monochromatic to high order harmoniclike with the duration of the ionizing pulses.

Zhidkov, A.; Fujii, T.; Nemoto, K. [Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa, 240-0196 Japan (Japan); Esirkepov, T.; Koga, J.; Bulanov, S. V. [Kansai Photon Science Institute, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto, 619-0215 (Japan)

2009-11-20

333

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

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

334

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 (Q{sub E} 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.

2001-08-10

335

Travelling waves for a velocity-jump model of cell migration and proliferation.  

PubMed

Cell invasion, characterised by moving fronts of cells, is an essential aspect of development, repair and disease. Typically, mathematical models of cell invasion are based on the Fisher-Kolmogorov equation. These traditional parabolic models cannot be used to represent experimental measurements of individual cell velocities within the invading population since they imply that information propagates with infinite speed. To overcome this limitation we study combined cell motility and proliferation based on a velocity-jump process where information propagates with finite speed. The model treats the total population of cells as two interacting subpopulations: a subpopulation of left-moving cells, L(x,t), and a subpopulation of right-moving cells, R(x,t). This leads to a system of hyperbolic partial differential equations that includes a turning rate, ??0, describing the rate at which individuals in the population change direction of movement. We present exact travelling wave solutions of the system of partial differential equations for the special case where ?=0 and in the limit that ???. For intermediate turning rates, 0waves using the phase plane and we demonstrate a transition from smooth monotone travelling waves to smooth nonmonotone travelling waves as ? decreases through a critical value ?crit. We conclude by providing a qualitative comparison between the travelling wave solutions of our model and experimental observations of cell invasion. This comparison indicates that the small ? limit produces results that are consistent with experimental observations. PMID:23665453

Simpson, Matthew J; Foy, Brody H; McCue, Scott W

2013-08-01

336

On-line noninvasive one-point measurements of pulse wave velocity.  

PubMed

Pulse wave velocity (PWV) is a basic parameter in the dynamics of pressure and flow waves traveling in arteries. Conventional on-line methods of measuring PWV have mainly been based on "two-point" measurements, i.e., measurements of the time of travel of the wave over a known distance. This paper describes two methods by which on-line "one-point" measurements can be made, and compares the results obtained by the two methods. The principle of one method is to measure blood pressure and velocity at a point, and use the water-hammer equation for forward traveling waves. The principle of the other method is to derive PWV from the stiffness parameter of the artery. Both methods were realized by using an ultrasonic system which we specially developed for noninvasive measurements of wave intensity. We applied the methods to the common carotid artery in 13 normal humans. The regression line of the PWV (m/s) obtained by the former method on the PWV (m/s) obtained by the latter method was y = 1.03x - 0.899 (R(2) = 0.83). Although regional PWV in the human carotid artery has not been reported so far, the correlation between the PWVs obtained by the present two methods was so high that we are convinced of the validity of these methods. PMID:12541096

Harada, Akimitsu; Okada, Takashi; Niki, Kiyomi; Chang, Dehua; Sugawara, Motoaki

2002-12-01

337

Lateral variations of coda wave attenuation in the Alps  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

338

Validation of a new non-invasive portable tonometer for determining arterial pressure wave and pulse wave velocity: the PulsePen device  

Microsoft Academic Search

Objective To validate a new, small portable tonometer (PulsePen) that is able to assess carotid artery pressure and to measure pulse wave velocity (PWV) non-invasively. Its software provides absolute arterial pressure values, an assessment of arterial pulse wave contours, an estimation of reflection waves and measurements of PWV. Design and methods Two validation studies were carried out. The aim of

Paolo Salvi; Medicina Interna

339

The velocity structure of the Carpathian zone from the ambient noise surface wave tomography  

NASA Astrophysics Data System (ADS)

The dispersion curves of the Rayleigh wave group velocities are constructed along 60 interstation seismic paths in Central Europe based on the cross-correlation function of seismic noise. Together with the previous data (Yanovskaya and Lyskova, 2013), this information was used for reconstructing the three-dimensional distribution of S-wave velocities in the upper mantle of the Carpathian region. In the present work, the previous results are refined by expanding the data set by the additional seismic paths that intersect the Carpathian region and by modifying the procedure for constructing the locally averaged dispersion curves so as to obtain a more compact resolution. The results of the study suggest the complex, multidirectional character of the plate motion in the region.

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

2014-09-01

340

Interaction of a liquid film with a high-velocity gas flow behind a shock wave  

SciTech Connect

The paper examines heat- and mass-transfer processes between a liquid film and a turbulent gaseous boundary layer behind a shock wave. The study is important for understanding the mechanism of heterogeneous detonation in the liquid-film gaseous-oxidizer system. Under the conditions of a tubulent gaseous boundary layer, large-scale disturbances appear on the gas-liquid interface and the surface of the film becomes an irregular structure of waves and splashes, from whose crests drops of liquids are detached. The drops penetrate into the boundary layer and are carried away by the high-velocity gas flow downstream. Experiments show that in the presence of high dynamic velocities of the gas, the mass carried away by the drops is larger than the mass carried away by evaporation. At the present time, there is no justification for neglecting, as in the detachment of drops.

Frolov, S.M.; Gel'fand, B.E.; Timofeev, E.I.

1985-03-01

341

Wave Velocity Attenuation and Sediment Retention among Different Vegetation Types in a Pacific Northwest Estuary  

NASA Astrophysics Data System (ADS)

Feedbacks between vegetation, wave climate, and sedimentation create stable ecosystem states within estuaries that provide ecosystem services such as wildlife habitat, erosion control, and pollution filtration. Flume and field studies conducted with cordgrass (Spartina spp.) and sea grasses (Zostera spp., Halodule spp.) have demonstrated that the presence of vegetation reduces wave energy and increases sediment retention. Since the spatial distribution of plant species and the presence of unique plant species differ between estuaries, there is a need to understand how individual plant species, or groups of species with similar morphology, influence wave characteristics and sedimentation. Within Tillamook Bay, Oregon, three species of emergent vascular vegetation species (Carex lyngbyei, Eleocharis sp., Schoenoplectus pungens) and one species of submergent vascular vegetation species (Zostera marina) are present in the high wave energy portion of the estuary at the border of open water and the start of vegetation. These species represent three distinct growth forms (emergent reeds, emergent grasses, submergent grasses) and occur at varying densities relative to each other, as well as within the estuary. Using paired acoustic Doppler velocimeters (ADVs), we quantify the relative attenuation of wave velocity between vegetation types and densities within the estuary and compare these results with published attenuation rates from flume and field studies in different environments. The effect of decreased wave velocity on sediment retention is measured using permanent sediment markers within and outside of vegetation stands and paired with ADV data. Sediment retention is predicted to vary seasonally with seasonal vegetation composition changes and remain constant in unvegetated areas. From this experiment we expect to identify like groups of plant species whose attenuation characteristics are the same, allowing for models of wave-vegetation-sediment interaction to be created with multiple vegetation types.

Lemein, T.; Cox, D. T.; Albert, D.; Blackmar, P.

2012-12-01

342

Velocity ratio measurement using the frequency of the gyro backward wave  

Microsoft Academic Search

The operating diagram of a low quality factor, 8 GHz TE°01 gyrotron exhibits oscillations between 6.8 and 7.3 GHz. These oscillations are identified as the backward wave component of the TE°21 traveling mode. As the resonance condition of this mode depends on the average parallel velocity ?v∥? of the beam electrons (?BW&bartil;?c\\/??k∥?v∥?), the measurement of ?BW for given ?c and

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

1991-01-01

343

Piezooptic Coefficients and Acoustic Wave Velocities in Sn2P2S6 Crystals  

E-print Network

Piezooptic coefficients of Sn2P2S6 crystals are experimentally determined for l=623.8 nm and T=293 K with the aid of interferometric technique. The components of the elastic stiffness tensor for these crystals are calculated on the basis of studies for the acoustic wave velocities. It is shown that acoustooptic figure of merit can achieve extremely high values for Sn2P2S6 crystals (M2 - 2x10-12s3/kg2).

O. Mys; I. Martynyuk-Lototska; A. Grabar; Yu. Vysochanskii; R. Vlokh

2007-06-28

344

A wind tunnel study of air flow in waving wheat: Single-point velocity statistics  

Microsoft Academic Search

We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made

Y. Brunet; J. J. Finnigan; M. R. Raupach

1994-01-01

345

Influence of schistosity on the strength and wave velocity characteristics of a mica schist saprolite  

Microsoft Academic Search

Anisotropy in a mica schist saprolite is investigated by consideration of its unconfined compressive strength and wave velocity\\u000a characteristics for different inclinations of the planes of schistosity. In each locality, the differences for each characteristic\\u000a with respect to the inclinations of schistosity planes were significant. Microfabric studies aided the explanation for the\\u000a differences obtained. The pertinence of this anisotropy in

Olufemi Ogunsanwo

1991-01-01

346

Elastic wave velocities of silica glass at high temperatures and high pressures  

Microsoft Academic Search

The elastic compressional (P) and shear (S) wave velocities of silica glass at simultaneous high pressures and high temperatures were measured using a combination of ultrasonic interferometry, in situ synchrotron x-ray diffraction, and radiographic techniques in a large-volume Kawai-type multianvil apparatus. We found no first-order amorphous-amorphous transformation throughout the pressure and temperature conditions between 2 and 5 GPa and up

Ayako Yokoyama; Masanori Matsui; Yuji Higo; Yoshio Kono; Tetsuo Irifune; Ken-Ichi Funakoshi

2010-01-01

347

Elastic Wave Velocity Measurements for Clinohumite and Phase A to 11 GPa: Seismic and Petrologic Implications  

Microsoft Academic Search

Ultrasonic wave propagation velocities of polycrystalline aggregates of the dense hydrous magnesium silicate phases (DHMS) clinohumite (Mg9Si4O16(OH)2)and phase A (Mg7Si2O8(OH)6) have been measured at 1 to 11 GPa and room temperature condition. Starting materials were prepared by a sol-gel technique and synthesis experiments were conducted with a rocking piston cylinder and multi-anvil apparatus to produce homogenous fine-grained polycrystalline aggregates. Measurements

P. Ulmer; H. T. Phan; E. Reusser; L. Burlini

2009-01-01

348

Comparative experimental study on several methods for measuring elastic wave velocities in rocks at high pressure  

Microsoft Academic Search

To measure elastic wave velocities in rocks at high temperature and high pressure is an important way to acquire the mechanics\\u000a and thermodynamics data of rocks in the earth? interior and also a substantial approach to studying the structure and composition\\u000a of materials there. In recent years, a rapid progress has been made in methodology pertaining to the measurements of

Hongsen Xie; Wenge Zhou; Yonggang Liu; Jie Guo; Wei Hou; Zhidan Zhao

2002-01-01

349

Three-dimensional P wave velocity model for the San Francisco Bay region, California  

Microsoft Academic Search

A new three-dimensional P wave velocity model for the greater San Francisco Bay region has been derived using the double-difference seismic tomography method, using data from about 5,500 chemical explosions or air gun blasts and approximately 6,000 earthquakes. The model region covers 140 km NE-SW by 240 km NW-SE, extending from 20 km south of Monterey to Santa Rosa and

Clifford H. Thurber; Thomas M. Brocher; Haijiang Zhang; Victoria E. Langenheim

2007-01-01

350

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

E-print Network

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

Tom G. Mackay; Akhlesh Lakhtakia; Sandi Setiawan

2005-03-07

351

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

NASA Astrophysics Data System (ADS)

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

Dahm, Torsten; Fischer, Tomas

2014-02-01

352

Non-Doppler shift related experimental shock wave measurements using velocity interferometer systems for any reflector.  

PubMed

Velocity interferometer system for any reflectors (VISARs), are becoming increasingly popular in the measurement of shock waves in solids and liquids. VISAR techniques are used in measurements of transit time, speed of shock waves in flight in transparent media [L. C. Chhabildas and J. L. Wise, in Proceedings of the 4th APS Topical Conference on Shock Waves in Condensed Matter, Spokane, Washington, 1985, edited by Y. M. Gupta (Plenum, New York, 1986); P. M. Celliers et al., Appl. Phys. Lett. 73, 1320 (1998)], and in measurements of particle velocity. However, in cases where shock compression or release may change the index of refraction n+ik of the material being studied, the VISAR technique must be applied with care. Changes in n and k introduce phase shifts into the VISAR results that are not associated with changes in velocity. This paper presents a derivation of the theoretical output of a line VISAR that includes the effects of changing n and k and an experimental observation of a non-Doppler shift related effect. PMID:11415014

Forsman, A C; Kyrala, G A

2001-05-01

353

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

354

Measurement and Resolution of Global Rayleigh-wave Phase-velocity Maps  

NASA Astrophysics Data System (ADS)

In this study, we present our preliminary dataset of fundamental-mode Rayleigh-wave dispersion measurements made for earthquakes occurred between 2006 and 2011 recorded at ~600 stations. The dataset include both minor- and major-arc arrivals at periods between 25 s and 200 s. Our measurements in general agree with calculations based on published models, indicating cycle skips at short periods have been correctly corrected for. We apply finite-frequency phase-velocity kernels as well as ray theory to obtain global phase velocity maps. The 2D tomographic problems are solved based on singular value decomposition (SVD). Tomographic problems are often ill posed due to a combination of errors in tomographic theory, uncertainties in measurements as well as imperfect path coverage. The SVD-based inversions allow us to investigate resolution limits associated with tomographic theory using numerical simulations, where phase-velocity maps in the range of the model space are implemented as input model in wave propagation simulations using spectral element method (SEM). Because the input model has no component in the null space, the inverted model based on SEM measurements should be identical to the input model if tomographic theory fully represents wave propagation physics. This experiment allows us to analyze the magnitude and location of artifacts introduced by approximated tomographic theories.

Liu, K.; Zhou, Y.

2013-12-01

355

Radial velocity variations in EX Lup: hints for a low-mass close companion  

NASA Astrophysics Data System (ADS)

EXors are low-mass pre-main sequence objects producing repetitive optical outbursts attributed to highly enhanced accretion from the circumstellar disk onto the star. One type of outburst theories requires a close stellar or sub-stellar companion that perturbs the inner part of the disk and triggers the onset of the enhanced accretion. Here, we look for a possible companion to EX Lup, the prototype of the EXor class, using radial velocity (RV) observations. The RVs show large periodic variations that can be explained by the presence of a close companion in the brown dwarf mass range. Chromospheric activity or starspots are less likely to explain the observed RV curve.

Kóspál, Ágnes; Mohler-Fischer, Maren; Sicilia-Aguilar, Aurora; Ábrahám, Péter; Curé, Michel; Henning, Thomas; Kiss, Csaba; Launhardt, Ralf; Moór, Attila; Müller, André

2013-07-01

356

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

SciTech Connect

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

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

2012-07-15

357

Ultrasonic wave velocity measurement in small polymeric and cortical bone specimens  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

358

Ion heating and saturation in the presence of transverse velocity-shear-driven waves  

NASA Astrophysics Data System (ADS)

The nature of ion heating and saturation in the presence of waves driven by transverse-velocity shear is investigated in a Q-machine(N. Rynn and N. D'Angelo, Rev. Sci. Instrum. 31), 1326 (1960). using laser-induced-fluorescence (LIF)(D.N. Hill, S. Fornaca and M.G. Wickham, Rev. Sci. Instrum. 54), 301 (1983). techniques. Correlations in space and time between the waves and heating are presented. Ion heating for shear-driven waves is compared to that for current-driven waves, with the expectation that shear-driven waves are better suited for ion heating than the current-driven waves, since the shear-driven growth rate decreases substantially less than the current-driven growth rate as the ion-to-electron temperature ratio increases.(V. Gavrishchaka, M.E. Koepke and G. Ganguli, Phys. Plasmas 3), 3091 (1996). Relevance to ion heating in space plasmas will be addressed.(Norqvist et al.), J. Geophys. Res. 101, 13179 (1996); Ganguli et al., Geophys. Res. Lett. 12, 643 (1985). Work sponsored by ONR and NSF.

Zintl, M. W.; Koepke, M. E.; Carroll, J. J., III; Selcher, C. A.

1996-11-01

359

On-chip laser Doppler vibrometer for arterial pulse wave velocity measurement  

PubMed Central

Pulse wave velocity (PWV) is an important marker for cardiovascular risk. The Laser Doppler vibrometry has been suggested as a potential technique to measure the local carotid PWV by measuring the transit time of the pulse wave between two locations along the common carotid artery (CCA) from skin surface vibrations. However, the present LDV setups are still bulky and difficult to handle. We present in this paper a more compact LDV system integrated on a CMOS-compatible silicon-on-insulator substrate. In this system, a chip with two homodyne LDVs is utilized to simultaneously measure the pulse wave at two different locations along the CCA. Measurement results show that the dual-LDV chip can successfully conduct the PWV measurement. PMID:23847745

Li, Yanlu; Segers, Patrick; Dirckx, Joris; Baets, Roel

2013-01-01

360

Asymptotic solutions of 2D wave equations with variable velocity and localized right-hand side  

NASA Astrophysics Data System (ADS)

In the paper, we consider the Cauchy problem for the inhomogeneous wave equation with variable velocity and with a perturbation in the form of a right-hand side localized in space (near the origin) and in time. In particular, this problem is connected with the question about the creation of tsunami and Rayleigh waves. Using abstract operator theory and in particular Maslov's noncommutative analysis, we show that the solution is separated into two parts: the transient one, which is localized in a neighborhood of the origin and decreases in time and the propagating one, which propagates in space like the wave created by the momentary “equivalent source.” We present several examples covering a wide range of perturbation resulting in rather explicit formulas expressing the solutions it terms of the error function of complex argument.

Dobrokhotov, S. Yu.; Nazaikinskii, V. E.; Tirozzi, B.

2010-03-01

361

Crustal thickness variations in the Zagros continental collision zone (Iran) from joint inversion of receiver functions and surface wave dispersion  

NASA Astrophysics Data System (ADS)

Variations in crustal thickness in the Zagros determined by joint inversion of P wave receiver functions (RFs) and Rayleigh wave group and phase velocity dispersion. The time domain iterative deconvolution procedure was employed to compute RFs from teleseismic recordings at seven broadband stations of INSN network. Rayleigh wave phase velocity dispersion curves were estimated employing two-station method. Fundamental mode Rayleigh wave group velocities for each station is taken from a regional scale surface wave tomographic imaging. The main variations in crustal thickness that we observe are between stations located in the Zagros fold and thrust belt with those located in the Sanandaj-Sirjan zone (SSZ) and Urumieh-Dokhtar magmatic assemblage (UDMA). Our results indicate that the average crustal thickness beneath the Zagros Mountain Range varies from ˜46 km in Western and Central Zagros beneath SHGR and GHIR up to ˜50 km beneath BNDS located in easternmost of the Zagros. Toward NE, we observe an increase in Moho depth where it reaches ˜58 km beneath SNGE located in the SSZ. Average crustal thickness also varies beneath the UDMA from ˜50 km in western parts below ASAO to ˜58 in central parts below NASN. The observed variation along the SSZ and UDMA may be associated to ongoing slab steepening or break off in the NW Zagros, comparing under thrusting of the Arabian plate beneath Central Zagros. The results show that in Central Iran, the crustal thickness decrease again to ˜47 km below KRBR. There is not a significant crustal thickness difference along the Zagros fold and thrust belt. We found the same crystalline crust of ˜34 km thick beneath the different parts of the Zagros fold and thrust belt. The similarity of crustal structure suggests that the crust of the Zagros fold and thrust belt was uniform before subsidence and deposition of the sediments. Our results confirm that the shortening of the western and eastern parts of the Zagros basement is small and has only started recently.

Tatar, M.; Nasrabadi, A.

2013-10-01

362

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

SciTech Connect

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

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

2010-02-18

363

Solar wind driving of magnetospheric ULF waves: Pulsations driven by velocity shear at the magnetopause  

NASA Astrophysics Data System (ADS)

We present results from global, three-dimensional magnetohydrodynamic (MHD) simulations of the solar wind/magnetosphere interaction. These MHD simulations are used to study ultra low frequency (ULF) pulsations in the Earth's magnetosphere driven by shear instabilities at the flanks of the magnetopause. We drive the simulations with idealized, constant solar wind input parameters, ensuring that any discrete ULF pulsations generated in the simulation magnetosphere are not due to fluctuations in the solar wind. The simulations presented in this study are driven by purely southward interplanetary magnetic field (IMF) conditions, changing only the solar wind driving velocity while holding all of the other solar wind input parameters constant. We find surface waves near the dawn and dusk flank magnetopause and show that these waves are generated by the Kelvin-Helmholtz (KH) instability. We also find that two KH modes are generated near the magnetopause boundary. One mode, the magnetopause KH mode, propagates tailward along the magnetopause boundary. The other mode, the inner KH mode, propagates tailward along the inner edge of the boundary layer (IEBL). We find large vortical structures associated with the inner KH mode that are centered on the IEBL. The phase velocities, wavelengths, and frequencies of the two KH modes are computed. The KH waves are found to be fairly monochromatic with well-defined wavelengths. In addition, the inner and magnetopause KH modes are coupled and lead to a coupled oscillation of the low-latitude boundary layer. The boundary layer thickness, d, is computed and we find maximum wave growth for kd = 0.5-1.0, where k is the wave number, consistent with the linear theory of the KH instability. We comment briefly on the effectiveness of these KH waves in the energization and transport of radiation belt electrons.

Claudepierre, S. G.; Elkington, S. R.; Wiltberger, M.

2008-05-01

364

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

SciTech Connect

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

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

2013-12-15

365

Compressional wave velocity and attenuation at ultrasonic and sonic frequencies in near-surface sedimentary rocks  

SciTech Connect

Laboratory ultrasonic measurements of compressional wave velocity and attenuation were made as a function of effective pressure on samples of limestone, sandstone and siltstone taken from a shallow borehole test site. The results indicate that the sandstones are pervaded by grain contact microcracks which dramatically affect their compressional wave attenuations. Clean sandstone shows a compressional wave quality factor (Q{sub p}) of 24 {+-} 2 at 5 MPa effective pressure (close to the estimated in situ burial pressure) and a Q{sub p} of 83 {+-} 29 at 60 MPa. The Q{sub p} of limestones and siltstones at the site show negligible and small increases with pressure in the laboratory, respectively. The strong pressure dependence of Q{sub p} in clean sandstone was used to infer the presence of in situ microcracks. Sediment velocities measured in the laboratory at about 1 MHz were compared with those from the full waveform sonic log at about 10 kHz implies that they must also be highly attenuating over a significant part of the frequency range 10 kHz to 1 MHz, to account for the magnitude of the observed velocity dispersion. Assuming the laboratory Q{sub p} values measured at 5 MPa remain constant down to 10 kHz predicts the observed dispersion quite well. Furthermore, the sonic log velocities of sandstones, limestones and siltstones (after normalizing each lithology for porosity and clay content) were found to reflect the same pressure (depth) trends observed in the laboratory. The results provide evidence for the existence of in situ microcracks in near-surface sediments.

Best, A.I. [Univ. of Reading (United Kingdom). Postgraduate Research Inst. for Sedimentology] [Univ. of Reading (United Kingdom). Postgraduate Research Inst. for Sedimentology; Sams, M.S. [Imperial College of Science, Technology and Medicine, London (United Kingdom). Dept. of Geology] [Imperial College of Science, Technology and Medicine, London (United Kingdom). Dept. of Geology

1997-03-01

366

Shallow shear-wave velocity profiles and site response characteristics from microtremor array measurements in Metro Manila, the Philippines  

NASA Astrophysics Data System (ADS)

This paper presents the outcome of reconnaissance surveys in metropolitan Manila (Metro Manilla), the Philippines, with the aim of mapping shallow shear-wave velocity structures. Metro Manila is a seismically active and densely populated region that is in need of detailed investigation of the subsurface structures, to assess local site effects in seismic hazard estimation. We conducted microtremor array observations and used the spatial autocorrelation method to estimate the shear-wave profiles at 32 sites in major geological settings in Metro Manila. We applied a hybrid genetic simulated annealing algorithm to invert phase velocity data from the spatial autocorrelation method to generate shear-wave velocity models near the global best-fit solution. The comparison between the inferred shear-wave velocity profiles and PS logging showed good agreement in terms of the fundamental mode of Rayleigh waves and site responses. Then, we utilised the inferred shear-wave velocity profiles to compute the site amplifications with reference to the motion in engineering bedrock. Subsequently, the site amplifications have been grouped, based on NEHRP site classes. The amplification factor has also been compared with the average shear-wave velocity of the upper 30m at each site, to produce a power-law regression equation that can be used as a starting basis for further site-effects evaluation in the metropolis.

Grutas, Rhommel; Yamanaka, Hiroaki

2012-07-01

367

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

368

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

SciTech Connect

Finite difference methods for solving the wave equation more accurately capture the physics of waves propagating through the earth than asymptotic solution methods. Unfortunately. finite difference simulations for 3D elastic wave propagation are expensive. We model waves in a 3D isotropic elastic earth. The wave equation solution consists of three velocity components and six stresses. The partial derivatives are discretized using 2nd-order in time and 4th-order in space staggered finite difference operators. Staggered schemes allow one to obtain additional accuracy (via centered finite differences) without requiring additional storage. The serial code is most unique in its ability to model a number of different types of seismic sources. The parallel implementation uses the MP1 library, thus allowing for portability between platforms. Spatial parallelism provides a highly efficient strategy for parallelizing finite difference simulations. In this implementation, one can decompose the global problem domain into one-, two-, and three-dimensional processor decompositions with 3D decompositions generally producing the best parallel speed up. Because i/o is handled largely outside of the time-step loop (the most expensive part of the simulation) we have opted for straight-forward broadcast and reduce operations to handle i/o. The majority of the communication in the code consists of passing subdomain face information to neighboring processors for use as ''ghost cells''. When this communication is balanced against computation by allocating subdomains of reasonable size, we observe excellent scaled speed up. Allocating subdomains of size 25 x 25 x 25 on each node, we achieve efficiencies of 94% on 128 processors. Numerical examples for both a layered earth model and a homogeneous medium with a high-velocity blocky inclusion illustrate the accuracy of the parallel code.

MINKOFF,SUSAN E.

1999-12-09

369

Spatial parallelism of a 3D finite difference, velocity-stress elastic wave propagation code  

SciTech Connect

Finite difference methods for solving the wave equation more accurately capture the physics of waves propagating through the earth than asymptotic solution methods. Unfortunately, finite difference simulations for 3D elastic wave propagation are expensive. The authors model waves in a 3D isotropic elastic earth. The wave equation solution consists of three velocity components and six stresses. The partial derivatives are discretized using 2nd-order in time and 4th-order in space staggered finite difference operators. Staggered schemes allow one to obtain additional accuracy (via centered finite differences) without requiring additional storage. The serial code is most unique in its ability to model a number of different types of seismic sources. The parallel implementation uses the MPI library, thus allowing for portability between platforms. Spatial parallelism provides a highly efficient strategy for parallelizing finite difference simulations. In this implementation, one can decompose the global problem domain into one-, two-, and three-dimensional processor decompositions with 3D decompositions generally producing the best parallel speedup. Because I/O is handled largely outside of the time-step loop (the most expensive part of the simulation) the authors have opted for straight-forward broadcast and reduce operations to handle I/O. The majority of the communication in the code consists of passing subdomain face information to neighboring processors for use as ghost cells. When this communication is balanced against computation by allocating subdomains of reasonable size, they observe excellent scaled speedup. Allocating subdomains of size 25 x 25 x 25 on each node, they achieve efficiencies of 94% on 128 processors. Numerical examples for both a layered earth model and a homogeneous medium with a high-velocity blocky inclusion illustrate the accuracy of the parallel code.

Minkoff, S.E.

1999-12-01

370

An explicit relation for the apparent phase velocity of Rayleigh waves in a vertically heterogeneous elastic half-space  

NASA Astrophysics Data System (ADS)

This paper presents the mathematical derivation of an explicit relation for the apparent (or effective) phase velocity of Rayleigh waves in a vertically heterogeneous, isotropic elastic half-space for harmonic excitation. As a kinematical feature, the apparent phase velocity captures the superposition, in a spatial Fourier series, of the individual modes of propagation of Rayleigh waves and describes the speed of propagation of a composite waveform generated by a vertically oscillating point load. The relation, which is a function of the distance from the source, frequency and depth, depends explicitly on the modal phase and group velocities of Rayleigh waves, and their corresponding wavenumbers and eigenfunctions, which can be computed directly from the solution of the Rayleigh-wave eigenproblem. A practical scenario for the application of the notion of apparent Rayleigh-wave phase velocity is the modelling of the dispersion curve in the well-known surface wave measurement methods `spectral analysis of surface waves' (SASW) and `multichannel analysis of surface waves' (MASW). Apart from a theoretical motivation, the availability in surface wave testing of an explicit formula for the calculation of the apparent Rayleigh-wave phase velocity may lead to the development of a new class of inversion algorithms capable of taking into account the influence of all the modes of surface wave propagation. To demonstrate the exactness of the explicit relation, the predicted values of apparent phase velocity are compared to those computed synthetically from a numerical simulation of SASW and MASW testing for three case studies, which show both single as well as multiple mode dominance effects.

Lai, Carlo G.; Mangriotis, Maria-Daphne; Rix, Glenn J.

2014-11-01

371

Dust-acoustic solitary waves and double layers in a magnetized dusty plasma with nonthermal ions and dust charge variation  

SciTech Connect

The effect of nonthermal ions and variable dust charge on small-amplitude nonlinear dust-acoustic (DA) waves is investigated. It is found that both compressive and rarefactive solitons exist and depend on the nonthermal parameter a. Using a reductive perturbation theory, a Zakharov-Kuznetsov (ZK) equation is derived. At critical value of a, a{sub c}, a modified ZK equation with third- and fourth-order nonlinearities, is obtained. Depending on a, the solution of the evolution equation reveals whether there is coexistence of both compressive and rarefactive solitary waves or double layers (DLs) with the possibility of their two kinds. In addition, for certain plasma parameters, the solitary wave disappears and a DL is expected. The variation of dust charge number, wave velocity, and soliton amplitude and its width against system parameters is investigated for the DA solitary waves. It is shown that the incorporation of both the adiabatic dust-charge variation and the nonthermal distributed ions modifies significantly the nature of DA solitary waves and DA DLs. The findings of this investigation may be useful in understanding the ion acceleration mechanisms close to the Moon and also enhances our knowledge on pickup ions around unmagnetized bodies, such as comets, Mars, and Venus.

El-Taibany, W.F.; Sabry, R. [Department of Physics, Faculty of Science-Damietta, Mansoura University, Damietta El-Gedida, P.O. 34517 (Egypt)

2005-08-15

372

Evolution of microstructure and elastic wave velocities in dehydrated gypsum samples  

NASA Astrophysics Data System (ADS)

We report on changes in P and S-wave velocities and rock microstructure induced by devolatilization reactions using gypsum as a reference analog material. Cylindrical samples of natural alabaster were dehydrated in air, at ambient pressure, and temperatures between 378 and 423 K. Dehydration did not proceed homogeneously but via a reaction front moving sample inwards separating an outer highly porous rim from the remaining gypsum which, above approximately 393 (±5) K, concurrently decomposed into hemihydrate. Overall porosity was observed to continuously increase with reaction progress from approximately 2% for fully hydrated samples to 30% for completely dehydrated ones. Concurrently, P and S-wave velocities linearly decreased with porosity from 5.2 and 2.7 km/s to 1.0 and 0.7 km/s, respectively. It is concluded that a linearized empirical Raymer-type model extended by a critical porosity term and based on the respective time dependent mineral and pore volumes reasonably replicates the P and S-wave data in relation to reaction progress and porosity.

Milsch, Harald; Priegnitz, Mike

2012-12-01

373

Upper mantle S wave velocity structure of the East Anatolian-Caucasus region  

NASA Astrophysics Data System (ADS)

processes occurring in the upper mantle such as slab break off and lithosphere delamination often result in high rates of lithospheric deformation and rapid tectonic uplift of large areas. The continent-continent collision zone between Arabia and Eurasia has been widely studied in this context, but several different viable geodynamic models exist to explain the uplift and deformation of the Anatolian Plateau and the Caucasus Mountains. We have imaged the uppermost mantle shear wave velocity structure of the East Anatolian-Caucasus region using surface wave tomography to better understand the regional tectonic activity since the onset of the collision between the Arabian and Eurasian Plates. Furthermore, we used our tomographic models to better understand the processes, which are responsible for the formation of the 2 km high plateau and the widespread volcanism in eastern Turkey, as well as reactivation of deformation and deep seismicity in the eastern Greater Caucasus. Our model of regional upper mantle shear wave velocity structure supports subduction of the northern and southern branches of Neo-Tethys lithosphere between Eurasia and Gondwana and suggests a possible underthrusting of the Kura Basin lithosphere beneath the Greater Caucasus.

Skolbeltsyn, Gleb; Mellors, Robert; Gök, Rengin; Türkelli, Niyazi; Yetirmishli, Gurban; Sandvol, Eric

2014-03-01

374

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

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

375

P-wave Velocity Anisotropy and Shear-wave Splitting of Sheared Metasediments from the Flin-Flon Belt, Trans-Hudson Orogen  

NASA Astrophysics Data System (ADS)

Metasediments of the upper greenschist - lower amphibolite facies of metamorphism from two ductile shear zones of Flin-Flon Belt (FFB) of Trans-Hudson Orogen (THO) were used to carried out laboratory measurements of compressional wave (Vp), shear-wave (Vs) velocities and shear-wave splitting. The investigated metasediments vary in composition from felsic to mafic. Test sites with outcrops of sheared metasediments were correlated with a series of inclined seismic reflectors possibly extending from the midcrust and intersecting a well mapped shear zone at the surface. Determination of lithological and physical properties of highly deformed metasediments is essential for proper interpretation of the nature of observed seismic reflectors. To investigate anisotropic properties of the rocks compressional velocity was measured to confining pressures of 300 MPa in three mutually orthogonal directions with respect to the visible textural properties. In addition, on nine selected samples shear-wave velocity was measured at two orthogonal polarizations for each of three propagation directions to determine shear-wave splitting and correlate it with P-wave anisotropy. For most of the hand specimens seismic heterogeneity was investigated by measuring P- and S-wave velocities on several cores cut in the same direction. Elastic velocities were measured on the 147 core samples in total. Observed Vp anisotropy varied from quasi-isotropic to highly anisotropic (Ap=24%). Maximum observed shear wave splitting reaches the value of 0.77 km/sec at confining pressure of 300 MPa. An estimated splitting of the SKS wave propagating through the ten kilometres thick crustal slab of metasediments, characterized by the averaged value of laboratory observed shear-wave splitting, may reach value of 0.2 sec. Pressure invariance of observed P-wave anisotropy and shear-wave splitting indicates that lattice preferred orientation (LPO) of highly anisotropic minerals such as mica and hornblende is mainly responsible for measured seismic anisotropy.

Schmitt, D. R.; Cholach, P. Y.; Molyneux, J. B.

2003-12-01

376

Shear wave splitting and waveform complexity for lowermost mantle structures with low-velocity lamellae and transverse isotropy  

Microsoft Academic Search

Shear waves that traverse the lowermost mantle exhibit polarization anomalies and waveform complexities that indicate the presence of complex velocity structure above the core-mantle boundary. Synthetic seismograms for horizontally and vertically polarized shear waves (SH and SV, respectively) are computed using the reflectivity method for structures with low-velocity sheets (“lamellae”), and for comb-like models approximating long wavelength vertical transverse isotropy

Melissa M. Moore; Edward J. Garnero; Thorne Lay; Quentin Williams

2004-01-01

377

Teleseismic tomography of the compressional wave velocity structure beneath the Long Valley region, California  

SciTech Connect

In 1982 and 1984 the U.S. Geological Survey used several seismic networks, totaling over 90 stations, to record teleseismic P waves and measure travel time residuals in an area centered on the Long Valley caldera. The authors inverted the travel time residuals to obtain a three-dimensional image of the velocity structure with resolution of 5-6 km to depths of 70 km beneath the array. Direct inversion of these data indicates that the 2- to 4-km-thick low-velocity caldera fill contaminates the signal from any midcrustal velocity anomalies beneath the caldera. Thus two methods were used to strip the effects of the upper crust from the travel time residuals: (1) ray tracing through upper crustal velocity models provided by seismic refraction experiments and gravity surveys, and (2) an iterative stripping scheme using the inversion itself. The methods produce essentially identical results and adequately remove the effects of the shallowest crustal structures, including the caldera fill and hydrothermal alteration effects. The resulting stripped models show two well-resolved midcrustal low-velocity bodies in the Long Valley region. The first body is centered between 7 and 20 km depth beneath the resurgent dome of the Long Valley caldera and has a volume of 150-600 km{sup 3}. The second, with a similar volume, is centered between 10 and 20 km depth beneath the Mono Craters, about 10 km north of Long Valley. Velocity contrasts in both of these bodies are about 6-10%, and the features are interpreted as silicic magma chambers. This experiment does not preclude the presence of additional pockets of magma smaller than 5 km across in the upper crust, particularly beneath the resurgent dome of the caldera (which would be removed with the stripping methods). The high eruptive rate of the Mono Craters and these upper mantle structures suggest that the focus of volcanism is shifting north from Long Valley to the Mono Craters

Dawson, P.B.; Evans, J.R.; Iyer, H.M. (Geological Survey, Menlo Park, CA (USA))

1990-07-10

378

Mapping Deep Low Velocity Zones in Alaskan Arctic Coastal Permafrost using Seismic Surface Waves  

NASA Astrophysics Data System (ADS)

Permafrost degradation may be an important amplifier of climate change; Thawing of near-surface sediments holds the potential of increasing greenhouse gas emissions due to microbial decomposition of preserved organic carbon. Recently, the characterization of "deep" carbon pools (several meters below the surface) in circumpolar frozen ground has increased the estimated amount of soil carbon to three times higher than what was previously thought. It is therefore potentially important to include the characteristics and processes of deeper permafrost strata (on the orders of a few to tens of meters below surface) in climate models for improving future predictions of accessible carbon and climate feedbacks. This extension is particularly relevant if deeper formations are not completely frozen and may harbor on-going microbial activity despite sub-zero temperatures. Unfortunately, the characterization of deep permafrost systems is non-trivial; logistics and drilling constraints often limit direct characterization to relatively shallow units. Geophysical measurements, either surface or airborne, are often the most effective tools for evaluating these regions. Of the available geophysical techniques, the analysis of seismic surface waves (e.g. MASW) has several unique advantages, mainly the ability to provide field-scale information with good depth resolution as well as penetration (10s to 100s of m with small portable sources). Surface wave methods are also able to resolve low velocity regions, a class of features that is difficult to characterize using traditional P-wave refraction methods. As part of the Department of Energy (DOE) Next-Generation Ecosystem Experiments (NGEE-Arctic) project, we conducted a three-day seismic field survey (May 12 - 14, 2012) at the Barrow Environmental Observatory, which is located within the Alaskan Arctic Coastal Plain. Even though permafrost at the study site is continuous, ice-rich and thick (>= 350m), our Multichannel Analysis of Surface Waves (MASW) suggests the existence of pronounced low shear wave velocity zones that span the depth range of 2 - 30 meters; this zone has shear velocity values comparable to partially thawed soils. Such features coincide with previous findings of very low electrical resistivity structure (as low as ~10 Ohm*m at some locations) from measurements obtained in the first NGEE-Arctic geophysical field campaign (conducted in the week of September 24 - October 1, 2011). These low shear velocity zones are likely representative of regions with high unfrozen water content and thus have important implications on the rate of microbial activity and the vulnerability of deep permafrost carbon pools. Analysis of this dataset required development of a novel inversion approach based on waveform inversion. The existence of multiple closely spaced Rayleigh wave modes made traditional inversion based on mode picking virtually impossible; As a result, we selected a direct misfit evaluation based on comparing dispersion images in the phase velocity/frequency domain. The misfit function was optimized using a global search algorithm, in this case Huyer and Neumaier's Multi Coordinate Search algorithm (MCS). This combination of MCS and waveform misfit allowed recovery of the low velocity region despite the existence of closely spaced modes.

Dou, S.; Ajo Franklin, J. B.; Dreger, D. S.

2012-12-01

379

A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations  

USGS Publications Warehouse

It is known that GPS time series contain a seasonal variation that is not due to tectonic motions, and it has recently been shown that crustal seismic velocities may also vary seasonally. In order to explain these changes, a number of hypotheses have been given, among which thermoelastic and hydrology-induced stresses and strains are leading candidates. Unfortunately, though, since a general framework does not exist for understanding such seasonal variations, it is currently not possible to quickly evaluate the plausibility of these hypotheses. To fill this gap in the literature, I generalize a two-dimensional thermoelastic strain model to provide an analytic solution for the displacements and wave speed changes due to either thermoelastic stresses or hydrologic loading, which consists of poroelastic stresses and purely elastic stresses. The thermoelastic model assumes a periodic surface temperature, and the hydrologic models similarly assume a periodic near-surface water load. Since all three models are two-dimensional and periodic, they are expected to only approximate any realistic scenario; but the models nonetheless provide a quantitative framework for estimating the effects of thermoelastic and hydrologic variations. Quantitative comparison between the models and observations is further complicated by the large uncertainty in some of the relevant parameters. Despite this uncertainty, though, I find that maximum realistic thermoelastic effects are unlikely to explain a large fraction of the observed annual variation in a typical GPS displacement time series or of the observed annual variations in seismic wave speeds in southern California. Hydrologic loading, on the other hand, may be able to explain a larger fraction of both the annual