Luo, Y.; Xia, J.; Liu, J.; Xu, Y.; Liu, Q.
2008-01-01
Multichannel Analysis of Surface Waves utilizes a multichannel recording system to estimate near-surface shear (S)-wave velocities from high-frequency Rayleigh waves. A pseudo-2D S-wave velocity (vS) section is constructed by aligning 1D models at the midpoint of each receiver spread and using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. The receiver spread length sets the theoretical lower limit and any vS structure with its lateral dimension smaller than this length will not be properly resolved in the final vS section. A source interval smaller than the spread length will not improve the horizontal resolution because spatial smearing has already been introduced by the receiver spread. In this paper, we first analyze the horizontal resolution of a pair of synthetic traces. Resolution analysis shows that (1) a pair of traces with a smaller receiver spacing achieves higher horizontal resolution of inverted S-wave velocities but results in a larger relative error; (2) the relative error of the phase velocity at a high frequency is smaller than at a low frequency; and (3) a relative error of the inverted S-wave velocity is affected by the signal-to-noise ratio of data. These results provide us with a guideline to balance the trade-off between receiver spacing (horizontal resolution) and accuracy of the inverted S-wave velocity. We then present a scheme to generate a pseudo-2D S-wave velocity section with high horizontal resolution using multichannel records by inverting high-frequency surface-wave dispersion curves calculated through cross-correlation combined with a phase-shift scanning method. This method chooses only a pair of consecutive traces within a shot gather to calculate a dispersion curve. We finally invert surface-wave dispersion curves of synthetic and real-world data. Inversion results of both synthetic and real-world data demonstrate that
NASA Astrophysics Data System (ADS)
Coccia, S.; Del Gaudio, V.; Venisti, N.; Wasowski, J.
2009-04-01
In the context of an ongoing study on seismic response of landslide-prone hill-slopes in Central Italy (area of Caramanico Terme), we tested the applicability of the Refraction Microtremor (ReMi) analysis technique (Louie, 2001) to obtain geometrical and physical parameters needed for numerical modelling. In particular, we used this technique to determine one-dimensional shear-wave velocity profiles (Vs) at sites located on and close to a recent landslide that mobilized 30-40 m thick Quaternary colluvium overlying Pliocene mudstones. The use of this technique in unstable slope areas presents difficulties related to rough topography and lateral lithological heterogeneities, which prevent the extension of geophone array up to the minimum lengths (100 - 200 m) commonly adopted in standard applications. Moreover, sites distant from anthropic sources of microtremors can have unfavourable noise conditions in comparison with other well established cases of application. To check the stability of the ReMi data in these operative conditions and the confidence level of the results, three ReMi campaigns were conducted at different times using different acquisition parameters (seismograph channel number, geophone frequency and spacing). We also tested simultaneous noise recording along orthogonal arrays to investigate a possible presence of directional variations of soil properties. The Rayleigh wave velocity dispersion data derived from picking carried out on p (slowness)-f (frequency) matrix showed the presence in noise recordings of different Rayleigh wave vibration modes (fundamental and first two higher modes), which prevail at different frequency intervals. This indicates that it is essential to correctly identify the different vibration modes to avoid erroneous data interpretation (e.g. fictitious identification of velocity decrease with depth). An analysis of the influence of changing environmental conditions and of different acquisition parameters was conducted through
Ultrasonic shear wave couplant
Kupperman, David S.; Lanham, Ronald N.
1985-01-01
Ultrasonically testing of an article at high temperatures is accomplished by the use of a compact layer of a dry ceramic powder as a couplant in a method which involves providing an ultrasonic transducer as a probe capable of transmitting shear waves, coupling the probe to the article through a thin compact layer of a dry ceramic powder, propagating a shear wave from the probe through the ceramic powder and into the article to develop echo signals, and analyzing the echo signals to determine at least one physical characteristic of the article.
Ultrasonic shear wave couplant
Kupperman, D.S.; Lanham, R.N.
1984-04-11
Ultrasonically testing of an article at high temperatures is accomplished by the use of a compact layer of a dry ceramic powder as a couplant in a method which involves providing an ultrasonic transducer as a probe capable of transmitting shear waves, coupling the probe to the article through a thin compact layer of a dry ceramic powder, propagating a shear wave from the probe through the ceramic powder and into the article to develop echo signals, and analyzing the echo signals to determine at least one physical characteristic of the article.
Shear wave transmissivity measurement by color Doppler shear wave imaging
NASA Astrophysics Data System (ADS)
Yamakoshi, Yoshiki; Yamazaki, Mayuko; Kasahara, Toshihiro; Sunaguchi, Naoki; Yuminaka, Yasushi
2016-07-01
Shear wave elastography is a useful method for evaluating tissue stiffness. We have proposed a novel shear wave imaging method (color Doppler shear wave imaging: CD SWI), which utilizes a signal processing unit in ultrasound color flow imaging in order to detect the shear wave wavefront in real time. Shear wave velocity is adopted to characterize tissue stiffness; however, it is difficult to measure tissue stiffness with high spatial resolution because of the artifact produced by shear wave diffraction. Spatial average processing in the image reconstruction method also degrades the spatial resolution. In this paper, we propose a novel measurement method for the shear wave transmissivity of a tissue boundary. Shear wave wavefront maps are acquired by changing the displacement amplitude of the shear wave and the transmissivity of the shear wave, which gives the difference in shear wave velocity between two mediums separated by the boundary, is measured from the ratio of two threshold voltages required to form the shear wave wavefronts in the two mediums. From this method, a high-resolution shear wave amplitude imaging method that reconstructs a tissue boundary is proposed.
Viscous shear heating instabilities in a 1-D viscoelastic shear zone
NASA Astrophysics Data System (ADS)
Homburg, J. M.; Coon, E. T.; Spiegelman, M.; Kelemen, P. B.; Hirth, G.
2010-12-01
Viscous shear instabilities may provide a possible mechanism for some intermediate depth earthquakes where high confining pressure makes it difficult to achieve frictional failure. While many studies have explored the feedback between temperature-dependent strain rate and strain-rate dependent shear heating (e.g. Braeck and Podladchikov, 2007), most have used thermal anomalies to initiate a shear instability or have imposed a low viscosity region in their model domain (John et al., 2009). By contrast, Kelemen and Hirth (2007) relied on an initial grain size contrast between a predetermined fine-grained shear zone and coarse grained host rock to initiate an instability. This choice is supported by observations of numerous fine grained ductile shear zones in shallow mantle massifs as well as the possibility that annealed fine grained fault gouge, formed at oceanic transforms, subduction related thrusts and ‘outer rise’ faults, could be carried below the brittle/ductile transition by subduction. Improving upon the work of Kelemen and Hirth (2007), we have developed a 1-D numerical model that describes the behavior of a Maxwell viscoelastic body with the rheology of dry olivine being driven at a constant velocity at its boundary. We include diffusion and dislocation creep, dislocation accommodated grain boundary sliding, and low-temperature plasticity (Peierls mechanism). Initial results suggest that including low-temperature plasticity inhibits the ability of the system to undergo an instability, similar to the results of Kameyama et al. (1999). This is due to increased deformation in the background allowing more shear heating to take place, and thus softening the system prior to reaching the peak stress. However if the applied strain rate is high enough (e.g. greater than 0.5 x 10-11 s-1 for a domain size of 2 km, an 8 m wide shear zone, a background grain size of 1 mm, a shear zone grain size of 150 μm, and an initial temperature of 650°C) dramatic
A new method for shear wave speed estimation in shear wave elastography.
Engel, Aaron J; Bashford, Gregory R
2015-12-01
Visualization of mechanical properties of tissue can aid in noninvasive pathology diagnosis. Shear wave elastography (SWE) measures the elastic properties of soft tissues by estimation of local shear wave propagation speed. In this paper, a new robust method for estimation of shear wave speed is introduced which has the potential for simplifying continuous filtering and real-time elasticity processing. Shear waves were generated by external mechanical excitation and imaged at a high frame rate. Three homogeneous phantoms of varying elastic moduli and one inclusion phantom were imaged. Waves propagating in separate directions were filtered and shear wave speed was estimated by inversion of the 1-D first-order wave equation. Final 2-D shear wave speed maps were constructed by weighted averaging of estimates from opposite traveling directions. Shear wave speed results for phantoms with gelatin concentrations of 5%, 7%, and 9% were 1.52 ± 0.10 m/s, 1.86 ± 0.10 m/s, and 2.37 ± 0.15 m/s, respectively, which were consistent with estimates computed from three other conventional methods, as well as compression tests done with a commercial texture analyzer. The method was shown to be able to reconstruct a 2-D speed map of an inclusion phantom with good image quality and variance comparable to conventional methods. Suggestions for further work are given.
NASA Technical Reports Server (NTRS)
Bechert, D. W.
1982-01-01
The generation of instability waves in free shear layers is investigated. The model assumes an infinitesimally thin shear layer shed from a semi-infinite plate which is exposed to sound excitation. The acoustical shear layer excitation by a source further away from the plate edge in the downstream direction is very weak while upstream from the plate edge the excitation is relatively efficient. A special solution is given for the source at the plate edge. The theory is then extended to two streams on both sides of the shear layer having different velocities and densities. Furthermore, the excitation of a shear layer in a channel is calculated. A reference quantity is found for the magnitude of the excited instability waves. For a comparison with measurements, numerical computations of the velocity field outside the shear layer were carried out.
Shear-wave splitting near Guam
NASA Astrophysics Data System (ADS)
Xie, Jiakang
1992-08-01
Polarities of shear waves from intermediate-focus events underneath Guam are studied. For records from a group of ten events, shear-wave splitting with faster-arriving E-W components are observed. This event group occurred within, or above, one geographic portion of the Wadati-Benioff zone, with depths ranging between 57 and 148 km. Ray tracing calculations were performed for 3-D and 1-D velocity models constructed for the region to determine expected S-wave polarities and ray patterns, as well as their sensitivities to variations in velocity structure. These were used to infer the probable existence of intrinsic anisotropy at depth and to determine the location and magnitude of anisotropy which can explain the observed shear-wave splitting. The most probable location of the anisotropy is beneath the crust and above, or partially within, the subducting slab. Assuming a maximum depth range of 10-120 km for the location of the anisotropy, its amount is about 1%, which may be viewed as a lower bound. Plausible causes of the anisotropy include mantle flow and thin, sheet-like channels filled with lava, or water vapor migrating upward from the subducting slab.
GIS-BASED 1-D DIFFUSIVE WAVE OVERLAND FLOW MODEL
KALYANAPU, ALFRED; MCPHERSON, TIMOTHY N.; BURIAN, STEVEN J.
2007-01-17
This paper presents a GIS-based 1-d distributed overland flow model and summarizes an application to simulate a flood event. The model estimates infiltration using the Green-Ampt approach and routes excess rainfall using the 1-d diffusive wave approximation. The model was designed to use readily available topographic, soils, and land use/land cover data and rainfall predictions from a meteorological model. An assessment of model performance was performed for a small catchment and a large watershed, both in urban environments. Simulated runoff hydrographs were compared to observations for a selected set of validation events. Results confirmed the model provides reasonable predictions in a short period of time.
Coded excitation plane wave imaging for shear wave motion detection.
Song, Pengfei; Urban, Matthew W; Manduca, Armando; Greenleaf, James F; Chen, Shigao
2015-07-01
Plane wave imaging has greatly advanced the field of shear wave elastography thanks to its ultrafast imaging frame rate and the large field-of-view (FOV). However, plane wave imaging also has decreased penetration due to lack of transmit focusing, which makes it challenging to use plane waves for shear wave detection in deep tissues and in obese patients. This study investigated the feasibility of implementing coded excitation in plane wave imaging for shear wave detection, with the hypothesis that coded ultrasound signals can provide superior detection penetration and shear wave SNR compared with conventional ultrasound signals. Both phase encoding (Barker code) and frequency encoding (chirp code) methods were studied. A first phantom experiment showed an approximate penetration gain of 2 to 4 cm for the coded pulses. Two subsequent phantom studies showed that all coded pulses outperformed the conventional short imaging pulse by providing superior sensitivity to small motion and robustness to weak ultrasound signals. Finally, an in vivo liver case study on an obese subject (body mass index = 40) demonstrated the feasibility of using the proposed method for in vivo applications, and showed that all coded pulses could provide higher SNR shear wave signals than the conventional short pulse. These findings indicate that by using coded excitation shear wave detection, one can benefit from the ultrafast imaging frame rate and large FOV provided by plane wave imaging while preserving good penetration and shear wave signal quality, which is essential for obtaining robust shear elasticity measurements of tissue.
Li, Sinan; Cheng, Yi; Eckersley, Robert J; Elson, Daniel S; Tang, Meng-Xing
2015-01-01
Shear wave speed is quantitatively related to tissue viscoelasticity. Previously we reported shear wave tracking at centimetre depths in a turbid optical medium using laser speckle contrast detection. Shear wave progression modulates displacement of optical scatterers and therefore modulates photon phase and changes the laser speckle patterns. Time-resolved charge-coupled device (CCD)-based speckle contrast analysis was used to track shear waves and measure the time-of-flight of shear waves for speed measurement. In this manuscript, we report a new observation of the laser speckle contrast difference signal for dual shear waves. A modulation of CCD speckle contrast difference was observed and simulation reproduces the modulation pattern, suggesting its origin. Both experimental and simulation results show that the dual shear wave approach generates an improved definition of temporal features in the time-of-flight optical signal and an improved signal to noise ratio with a standard deviation less than 50% that of individual shear waves. Results also show that dual shear waves can correct the bias of shear wave speed measurement caused by shear wave reflections from elastic boundaries. PMID:26114021
The many faces of shear Alfven waves
Gekelman, W.; Vincena, S.; Van Compernolle, B.; Morales, G. J.; Maggs, J. E.; Pribyl, P.; Carter, T. A.
2011-05-15
One of the fundamental waves in magnetized plasmas is the shear Alfven wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfven waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvenic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfven waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern
Continuous wave laser for wind shear detection
NASA Technical Reports Server (NTRS)
Nelson, Loren
1991-01-01
Details of the design and development of a continuous-wave heterodyne carbon dioxide laser which has wind shear detection capabilities are given in viewgraph form. The goal of the development was to investigate the lower cost CW (rather than pulsed) lidar option for look-ahead wind shear detection from aircraft. The device has potential utility for ground based wind shear detection at secondary airports where the high cost of a Terminal Doppler Weather Radar system is not justifiable.
4-D ultrafast shear-wave imaging.
Gennisson, Jean-Luc; Provost, Jean; Deffieux, Thomas; Papadacci, Clément; Imbault, Marion; Pernot, Mathieu; Tanter, Mickael
2015-06-01
Over the last ten years, shear wave elastography (SWE) has seen considerable development and is now routinely used in clinics to provide mechanical characterization of tissues to improve diagnosis. The most advanced technique relies on the use of an ultrafast scanner to generate and image shear waves in real time in a 2-D plane at several thousands of frames per second. We have recently introduced 3-D ultrafast ultrasound imaging to acquire with matrix probes the 3-D propagation of shear waves generated by a dedicated radiation pressure transducer in a single acquisition. In this study, we demonstrate 3-D SWE based on ultrafast volumetric imaging in a clinically applicable configuration. A 32 × 32 matrix phased array driven by a customized, programmable, 1024-channel ultrasound system was designed to perform 4-D shear-wave imaging. A matrix phased array was used to generate and control in 3-D the shear waves inside the medium using the acoustic radiation force. The same matrix array was used with 3-D coherent plane wave compounding to perform high-quality ultrafast imaging of the shear wave propagation. Volumetric ultrafast acquisitions were then beamformed in 3-D using a delay-and-sum algorithm. 3-D volumetric maps of the shear modulus were reconstructed using a time-of-flight algorithm based on local multiscale cross-correlation of shear wave profiles in the three main directions using directional filters. Results are first presented in an isotropic homogeneous and elastic breast phantom. Then, a full 3-D stiffness reconstruction of the breast was performed in vivo on healthy volunteers. This new full 3-D ultrafast ultrasound system paves the way toward real-time 3-D SWE. PMID:26067040
Longitudinal shear wave and transverse dilatational wave in solids.
Catheline, S; Benech, N
2015-02-01
Dilatation wave involves compression and extension and is known as the curl-free solution of the elastodynamic equation. Shear wave on the contrary does not involve any change in volume and is the divergence-free solution. This letter seeks to examine the elastodynamic Green's function through this definition. By separating the Green's function in divergence-free and curl-free terms, it appears first that, strictly speaking, the longitudinal wave is not a pure dilatation wave and the transverse wave is neither a pure shear wave. Second, not only a longitudinal shear wave but also a transverse dilatational wave exists. These waves are shown to be a part of the solution known as coupling terms. Their special motion is carefully described and illustrated.
Magnetized stratified rotating shear waves
NASA Astrophysics Data System (ADS)
Salhi, A.; Lehner, T.; Godeferd, F.; Cambon, C.
2012-02-01
We present a spectral linear analysis in terms of advected Fourier modes to describe the behavior of a fluid submitted to four constraints: shear (with rate S), rotation (with angular velocity Ω), stratification, and magnetic field within the linear spectral theory or the shearing box model in astrophysics. As a consequence of the fact that the base flow must be a solution of the Euler-Boussinesq equations, only radial and/or vertical density gradients can be taken into account. Ertel's theorem no longer is valid to show the conservation of potential vorticity, in the presence of the Lorentz force, but a similar theorem can be applied to a potential magnetic induction: The scalar product of the density gradient by the magnetic field is a Lagrangian invariant for an inviscid and nondiffusive fluid. The linear system with a minimal number of solenoidal components, two for both velocity and magnetic disturbance fields, is eventually expressed as a four-component inhomogeneous linear differential system in which the buoyancy scalar is a combination of solenoidal components (variables) and the (constant) potential magnetic induction. We study the stability of such a system for both an infinite streamwise wavelength (k1=0, axisymmetric disturbances) and a finite one (k1≠0, nonaxisymmetric disturbances). In the former case (k1=0), we recover and extend previous results characterizing the magnetorotational instability (MRI) for combined effects of radial and vertical magnetic fields and combined effects of radial and vertical density gradients. We derive an expression for the MRI growth rate in terms of the stratification strength, which indicates that purely radial stratification can inhibit the MRI instability, while purely vertical stratification cannot completely suppress the MRI instability. In the case of nonaxisymmetric disturbances (k1≠0), we only consider the effect of vertical stratification, and we use Levinson's theorem to demonstrate the stability of the
Magnetized stratified rotating shear waves.
Salhi, A; Lehner, T; Godeferd, F; Cambon, C
2012-02-01
We present a spectral linear analysis in terms of advected Fourier modes to describe the behavior of a fluid submitted to four constraints: shear (with rate S), rotation (with angular velocity Ω), stratification, and magnetic field within the linear spectral theory or the shearing box model in astrophysics. As a consequence of the fact that the base flow must be a solution of the Euler-Boussinesq equations, only radial and/or vertical density gradients can be taken into account. Ertel's theorem no longer is valid to show the conservation of potential vorticity, in the presence of the Lorentz force, but a similar theorem can be applied to a potential magnetic induction: The scalar product of the density gradient by the magnetic field is a Lagrangian invariant for an inviscid and nondiffusive fluid. The linear system with a minimal number of solenoidal components, two for both velocity and magnetic disturbance fields, is eventually expressed as a four-component inhomogeneous linear differential system in which the buoyancy scalar is a combination of solenoidal components (variables) and the (constant) potential magnetic induction. We study the stability of such a system for both an infinite streamwise wavelength (k(1) = 0, axisymmetric disturbances) and a finite one (k(1) ≠ 0, nonaxisymmetric disturbances). In the former case (k(1) = 0), we recover and extend previous results characterizing the magnetorotational instability (MRI) for combined effects of radial and vertical magnetic fields and combined effects of radial and vertical density gradients. We derive an expression for the MRI growth rate in terms of the stratification strength, which indicates that purely radial stratification can inhibit the MRI instability, while purely vertical stratification cannot completely suppress the MRI instability. In the case of nonaxisymmetric disturbances (k(1) ≠ 0), we only consider the effect of vertical stratification, and we use Levinson's theorem to demonstrate the
Seismic shear waves as Foucault pendulum
NASA Astrophysics Data System (ADS)
Snieder, Roel; Sens-Schönfelder, Christoph; Ruigrok, Elmer; Shiomi, Katsuhiko
2016-03-01
Earth's rotation causes splitting of normal modes. Wave fronts and rays are, however, not affected by Earth's rotation, as we show theoretically and with observations made with USArray. We derive that the Coriolis force causes a small transverse component for P waves and a small longitudinal component for S waves. More importantly, Earth's rotation leads to a slow rotation of the transverse polarization of S waves; during the propagation of S waves the particle motion behaves just like a Foucault pendulum. The polarization plane of shear waves counteracts Earth's rotation and rotates clockwise in the Northern Hemisphere. The rotation rate is independent of the wave frequency and is purely geometric, like the Berry phase. Using the polarization of ScS and ScS2 waves, we show that the Foucault-like rotation of the S wave polarization can be observed. This can affect the determination of source mechanisms and the interpretation of observed SKS splitting.
Estimation of seabed shear-wave velocity profiles using shear-wave source data.
Dong, Hefeng; Nguyen, Thanh-Duong; Duffaut, Kenneth
2013-07-01
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
Shear surface waves in phononic crystals.
Kutsenko, A A; Shuvalov, A L
2013-02-01
The existence of shear horizontal (SH) surface waves in two-dimensional periodic phononic crystals with an asymmetric depth-dependent profile is theoretically reported. Examples of dispersion spectra with bandgaps for subsonic and supersonic SH surface waves are demonstrated. The link between the effective (quasistatic) speeds of the SH bulk and surface waves is established. Calculation and analysis is based on the integral form of a projector on the subspace of evanescent modes which means no need for their explicit finding. This method can be extended to the vector waves and the three-dimensional case.
Fan-structure waves in shear ruptures
NASA Astrophysics Data System (ADS)
Tarasov, Boris
2016-04-01
This presentation introduces a recently identified shear rupture mechanism providing a paradoxical feature of hard rocks - the possibility of shear rupture propagation through the highly confined intact rock mass at shear stress levels significantly less than frictional strength. According to the fan-mechanism the shear rupture propagation is associated with consecutive creation of small slabs in the fracture tip which, due to rotation caused by shear displacement of the fracture interfaces, form a fan-structure representing the fracture head. The fan-head combines such unique features as: extremely low shear resistance (below the frictional strength), self-sustaining stress intensification in the rupture tip (providing easy formation of new slabs), and self-unbalancing conditions in the fan-head (making the failure process inevitably spontaneous and violent). An important feature of the fan-mechanism is the fact that for the initial formation of the fan-structure an enhanced local shear stress is required, however, after completion of the fan-structure it can propagate as a dynamic wave through intact rock mass at shear stresses below the frictional strength. Paradoxically low shear strength of pristine rocks provided by the fan-mechanism determines the correspondingly low transient strength of the lithosphere, which favours generation of new earthquake faults in the intact rock mass adjoining pre-existing faults in preference to frictional stick-slip instability along these faults. The new approach reveals an alternative role of pre-existing faults in earthquake activity: they represent local stress concentrates in pristine rock adjoining the fault where special conditions for the fan-mechanism nucleation are created, while further dynamic propagation of the new fault (earthquake) occurs at low field stresses even below the frictional strength.
Hammering Yucca Flat, Part Two: Shear-Wave Velocity
NASA Astrophysics Data System (ADS)
Finlay, T. S.; Abbott, R. E.; Knox, H. A.; Tang, D. G.; James, S. R.; Haney, M. M.; Hampshire, J. B., II
2015-12-01
In preparation for the next phase of the Source Physics Experiment (SPE), we conducted an active-source seismic survey of Yucca Flat, Nevada, on the Nevada National Security Site. Results from this survey will be used to inform the geologic models associated with the SPE project. For this study, we used a novel 13,000 kilogram weight-drop seismic source to interrogate an 18-km North-South transect of Yucca Flat. Source points were spaced every 200 meters and were recorded by 350 to 380 3-component 2-Hz geophones with variable spacings of 10, 20, and 100 meters. We utilized the Refraction-Microtremor (ReMi) technique to create multiple 1D dispersion curves, which were then inverted for shear-wave velocity profiles using the Dix inversion method (Tsai and Haney, 2015). Each of these 1D velocity models was subsequently stitched together to create a 2D profile over the survey area. The dispersion results indicate a general decrease in surface-wave phase velocity to the south. This result is supported by slower shear-wave velocity sediments and increasing basin depth towards the survey's southern extent. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Shear wave velocities in the earth's mantle.
NASA Technical Reports Server (NTRS)
Robinson, R.; Kovach, R. L.
1972-01-01
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.
Waves in Turbulent Stably Stratified Shear Flow
NASA Technical Reports Server (NTRS)
Jacobitz, F. G.; Rogers, M. M.; Ferziger, J. H.; Parks, John W. (Technical Monitor)
2002-01-01
Two approaches for the identification of internal gravity waves in sheared and unsheared homogeneous stratified turbulence are investigated. First, the phase angle between the vertical velocity and density fluctuations is considered. It was found, however, that a continuous distribution of the phase angle is present in weakly and strongly stratified flow. Second, a projection onto the solution of the linearized inviscid equations of motion of unsheared stratified flow is investigated. It was found that a solution of the fully nonlinear viscous Navier-Stokes equations can be represented by the linearized inviscid solution. The projection yields a decomposition into vertical wave modes and horizontal vortical modes.
Horizontal Shear Wave Imaging of Large Optics
Quarry, M J
2007-09-05
When complete the National Ignition Facility (NIF) will be the world's largest and most energetic laser and will be capable of achieving for the first time fusion ignition in the laboratory. Detecting optics features within the laser beamlines and sizing them at diameters of 0.1 mm to 10 mm allows timely decisions concerning refurbishment and will help with the routine operation of the system. Horizontally polarized shear waves at 10 MHz were shown to accurately detect, locate, and size features created by laser operations from 0.5 mm to 8 mm by placing sensors at the edge of the optic. The shear wave technique utilizes highly directed beams. The outer edge of an optic can be covered with shear wave transducers on four sides. Each transducer sends a pulse into the optic and any damage reflects the pulse back to the transmitter. The transducers are multiplexed, and the collected time waveforms are enveloped and replicated across the width of the element. Multiplying the data sets from four directions produces a map of reflected amplitude to the fourth power, which images the surface of the optic. Surface area can be measured directly from the image, and maximum depth was shown to be correlated to maximum amplitude of the reflected waveform.
Generation of 1D interference patterns of Bloch surface waves
NASA Astrophysics Data System (ADS)
Kadomina, E. A.; Bezus, E. A.; Doskolovich, L. L.
2016-09-01
Interference patterns of Bloch surface waves with a period that is significantly less than the wavelength of incident radiation are formed using dielectric diffraction gratings located on the surface of photonic crystal. The simulation based on electromagnetic diffraction theory is used to demonstrate the possibility of high-quality interference patterns due to resonant enhancement of higher evanescent diffraction orders related to the excitation of the Bloch surface waves. The contrast of the interference patterns is close to unity, and the period is less than the period of the diffraction structure by an order of magnitude.
Explosion Shear Wave Generation and Scattering
NASA Astrophysics Data System (ADS)
Baker, G. E.; Stevens, J. L.; Xu, H.
2004-12-01
We use observations of explosion-generated Lg together with three separate types of numerical models to determine how underground nuclear explosions generate shear wave phases. This question is fundamental to how Lg phases are interpreted for use in explosion yield estimation and earthquake/explosion discrimination. A simple point explosion in a uniform medium generates no shear waves, so the Lg phase is generated entirely by non-spherical components of the source and conversions through reflections and scattering. Our results indicate that the most important sources of high frequency explosion shear waves are P to S conversions at the free surface and S waves generated directly by a realistic distributed explosion source including nonlinear effects due to the free surface and gravity. In addition, Rg scattering may contribute to lower frequency Lg. Near source S is observed on both radial and tangential component records from a diverse set of explosion data. The data sets include 1) Degelen Mountain explosions recorded at distances less than 100 km and corresponding recordings at Borovoye (BOR) at 650 km; 2) recordings from Russian deep seismic sounding experiments; 3) Nevada Test Site (NTS) explosion sources including the Nonproliferation Experiment (NPE) and nuclear tests covering a range of source depths and media properties. We model the overburied NPE, and underburied and overburied Degelen explosions, using point sources and two-dimensional nonlinear finite difference calculations to quantify the source effects. We use energy conservation to determine an upper bound on Rg to Lg scattering. Results indicate that Rg to Lg scattering may be important at frequencies less than 1 Hz, and in Lg coda, but is less than Lg generated directly by the explosion at higher frequencies. We use 2D and 3D finite difference calculations, using the known topography and velocity structure at Degelen Mt. and lateral heterogeneities within the crust, to estimate the effect of
Shear waves in inhomogeneous, compressible fluids in a gravity field.
Godin, Oleg A
2014-03-01
While elastic solids support compressional and shear waves, waves in ideal compressible fluids are usually thought of as compressional waves. Here, a class of acoustic-gravity waves is studied in which the dilatation is identically zero, and the pressure and density remain constant in each fluid particle. These shear waves are described by an exact analytic solution of linearized hydrodynamics equations in inhomogeneous, quiescent, inviscid, compressible fluids with piecewise continuous parameters in a uniform gravity field. It is demonstrated that the shear acoustic-gravity waves also can be supported by moving fluids as well as quiescent, viscous fluids with and without thermal conductivity. Excitation of a shear-wave normal mode by a point source and the normal mode distortion in realistic environmental models are considered. The shear acoustic-gravity waves are likely to play a significant role in coupling wave processes in the ocean and atmosphere.
Shear waves in inhomogeneous, compressible fluids in a gravity field.
Godin, Oleg A
2014-03-01
While elastic solids support compressional and shear waves, waves in ideal compressible fluids are usually thought of as compressional waves. Here, a class of acoustic-gravity waves is studied in which the dilatation is identically zero, and the pressure and density remain constant in each fluid particle. These shear waves are described by an exact analytic solution of linearized hydrodynamics equations in inhomogeneous, quiescent, inviscid, compressible fluids with piecewise continuous parameters in a uniform gravity field. It is demonstrated that the shear acoustic-gravity waves also can be supported by moving fluids as well as quiescent, viscous fluids with and without thermal conductivity. Excitation of a shear-wave normal mode by a point source and the normal mode distortion in realistic environmental models are considered. The shear acoustic-gravity waves are likely to play a significant role in coupling wave processes in the ocean and atmosphere. PMID:24606251
1D profiling using highly dispersive guided waves
Volker, Arno; Zon, Tim van; Enthoven, Daniel; Verburg, Wesley
2015-03-31
Corrosion is one of the industries major issues regarding the integrity of assets. Currently inspections are conducted at regular intervals to ensure a sufficient integrity level of these assets. Cost reduction while maintaining a high level of reliability and safety of installations is a major challenge. There are many situations where the actual defect location is not accessible, e.g., a pipe support or a partially buried pipe. Guided wave tomography has been developed to reconstruct the wall thickness. In case of bottom of the line corrosion, i.e., a single corrosion pit, a simpler approach may be followed. Data is collected in a pit-catch configuration at the 12 o'clock position using highly dispersive guided waves. The phase spectrum is used to invert for a wall thickness profile in the circumferential direction, assuming a Gaussian defect profile. An EMAT sensor design has been made to measure at the 12 o'clock position of a pipe. The concept is evaluated on measured data, showing good sizing capabilities on a variety simple defect profiles.
1-D profiling using highly dispersive guided waves
Volker, Arno; Zon, Tim van
2014-02-18
Corrosion is one of the industries major issues regarding the integrity of assets. Currently, inspections are conducted at regular intervals to ensure a sufficient integrity level of these assets. Cost reduction while maintaining a high level of reliability and safety of installations is a major challenge. There are many situations where the actual defect location is not accessible, e.g., a pipe support or a partially buried pipe. Guided wave tomography has been developed to reconstruct the wall thickness of steel pipes. In case of bottom of the line corrosion, i.e., a single corrosion pit, a simpler approach may be followed. Data is collected in a pitch-catch configuration at the 12 o'clock position using highly dispersive guided waves. After dispersion correction the data collapses to a short pulse, any residual dispersion indicates wall loss. The phase spectrum is used to invert for the wall thickness profile in the circumferential direction, assuming a Gaussian defect profile. The approach is evaluated on numerically simulated and on measured data. The method is intended for rapid, semi-quantitative screening of pipes.
1D profiling using highly dispersive guided waves
NASA Astrophysics Data System (ADS)
Volker, Arno; van Zon, Tim; Hsu, Mick; Boogert, Lennart
2016-02-01
Corrosion is one of the industries major issues regarding the integrity of assets. Currently inspections are conducted at regular intervals to ensure a sufficient integrity level of these assets. Cost reduction while maintaining a high level of reliability and safety of installations is a major challenge. There are many situations where the actual defect location is not accessible, e.g., a pipe support or a partially buried pipe. In case of bottom of the line corrosion, i.e., a single corrosion pit, a simpler approach may be followed. Guided waves are propagated around the circumference of a pipe. In case of wall loss, the phase of the signal changes which is used to estimate the local wall thickness profile. A special EMAT sensor has been developed, which works in a pit-catch configuration at the 12 o'clock position using highly dispersive guided waves. In order to improve the sensitivity, an inversion in performed on multiple orders of circumferential passes. Experimental results are presented on different pipes containing artificial and real defects.
1D profiling using highly dispersive guided waves
NASA Astrophysics Data System (ADS)
Volker, Arno; van Zon, Tim; Enthoven, Daniel; Verburg, Wesley
2015-03-01
Corrosion is one of the industries major issues regarding the integrity of assets. Currently inspections are conducted at regular intervals to ensure a sufficient integrity level of these assets. Cost reduction while maintaining a high level of reliability and safety of installations is a major challenge. There are many situations where the actual defect location is not accessible, e.g., a pipe support or a partially buried pipe. Guided wave tomography has been developed to reconstruct the wall thickness. In case of bottom of the line corrosion, i.e., a single corrosion pit, a simpler approach may be followed. Data is collected in a pit-catch configuration at the 12 o'clock position using highly dispersive guided waves. The phase spectrum is used to invert for a wall thickness profile in the circumferential direction, assuming a Gaussian defect profile. An EMAT sensor design has been made to measure at the 12 o'clock position of a pipe. The concept is evaluated on measured data, showing good sizing capabilities on a variety simple defect profiles.
1-D profiling using highly dispersive guided waves
NASA Astrophysics Data System (ADS)
Volker, Arno; van Zon, Tim
2014-02-01
Corrosion is one of the industries major issues regarding the integrity of assets. Currently, inspections are conducted at regular intervals to ensure a sufficient integrity level of these assets. Cost reduction while maintaining a high level of reliability and safety of installations is a major challenge. There are many situations where the actual defect location is not accessible, e.g., a pipe support or a partially buried pipe. Guided wave tomography has been developed to reconstruct the wall thickness of steel pipes. In case of bottom of the line corrosion, i.e., a single corrosion pit, a simpler approach may be followed. Data is collected in a pitch-catch configuration at the 12 o'clock position using highly dispersive guided waves. After dispersion correction the data collapses to a short pulse, any residual dispersion indicates wall loss. The phase spectrum is used to invert for the wall thickness profile in the circumferential direction, assuming a Gaussian defect profile. The approach is evaluated on numerically simulated and on measured data. The method is intended for rapid, semi-quantitative screening of pipes.
Shear-Alfven Waves in Gyrokinetic Plasmas
W.W.Lee; J.L.V.Lewandowski; T.S. Hahm; Z. Lin
2000-10-18
It is found that the thermal fluctuation level of the shear-Alfven waves in a gyrokinetic plasma decreases with plasma b(* cs2/uA2), where cs is the ion acoustic speed and uA is the Alfven velocity. This unique thermodynamic property based on the fluctuation-dissipation theorem is verified in this paper using a new gyrokinetic particle simulation scheme, which splits the particle distribution function into the equilibrium part as well as the adiabatic and nonadiabatic parts.
From supersonic shear wave imaging to full-field optical coherence shear wave elastography
NASA Astrophysics Data System (ADS)
Nahas, Amir; Tanter, Mickaël; Nguyen, Thu-Mai; Chassot, Jean-Marie; Fink, Mathias; Claude Boccara, A.
2013-12-01
Elasticity maps of tissue have proved to be particularly useful in providing complementary contrast to ultrasonic imaging, e.g., for cancer diagnosis at the millimeter scale. Optical coherence tomography (OCT) offers an endogenous contrast based on singly backscattered optical waves. Adding complementary contrast to OCT images by recording elasticity maps could also be valuable in improving OCT-based diagnosis at the microscopic scale. Static elastography has been successfully coupled with full-field OCT (FF-OCT) in order to realize both micrometer-scale sectioning and elasticity maps. Nevertheless, static elastography presents a number of drawbacks, mainly when stiffness quantification is required. Here, we describe the combination of two methods: transient elastography, based on speed measurements of shear waves induced by ultrasonic radiation forces, and FF-OCT, an en face OCT approach using an incoherent light source. The use of an ultrafast ultrasonic scanner and an ultrafast camera working at 10,000 to 30,000 images/s made it possible to follow shear wave propagation with both modalities. As expected, FF-OCT is found to be much more sensitive than ultrafast ultrasound to tiny shear vibrations (a few nanometers and micrometers, respectively). Stiffness assessed in gel phantoms and an ex vivo rat brain by FF-OCT is found to be in good agreement with ultrasound shear wave elastography.
Shear wave elastography with a new reliability indicator
Dong, Yi
2016-01-01
Non-invasive methods for liver stiffness assessment have been introduced over recent years. Of these, two main methods for estimating liver fibrosis using ultrasound elastography have become established in clinical practice: shear wave elastography and quasi-static or strain elastography. Shear waves are waves with a motion perpendicular (lateral) to the direction of the generating force. Shear waves travel relatively slowly (between 1 and 10 m/s). The stiffness of the liver tissue can be assessed based on shear wave velocity (the stiffness increases with the speed). The European Federation of Societies for Ultrasound in Medicine and Biology has published Guidelines and Recommendations that describe these technologies and provide recommendations for their clinical use. Most of the data available to date has been published using the Fibroscan (Echosens, France), point shear wave speed measurement using an acoustic radiation force impulse (Siemens, Germany) and 2D shear wave elastography using the Aixplorer (SuperSonic Imagine, France). More recently, also other manufacturers have introduced shear wave elastography technology into the market. A comparison of data obtained using different techniques for shear wave propagation and velocity measurement is of key interest for future studies, recommendations and guidelines. Here, we present a recently introduced shear wave elastography technology from Hitachi and discuss its reproducibility and comparability to the already established technologies. PMID:27679731
Shear wave elastography with a new reliability indicator.
Dietrich, Christoph F; Dong, Yi
2016-09-01
Non-invasive methods for liver stiffness assessment have been introduced over recent years. Of these, two main methods for estimating liver fibrosis using ultrasound elastography have become established in clinical practice: shear wave elastography and quasi-static or strain elastography. Shear waves are waves with a motion perpendicular (lateral) to the direction of the generating force. Shear waves travel relatively slowly (between 1 and 10 m/s). The stiffness of the liver tissue can be assessed based on shear wave velocity (the stiffness increases with the speed). The European Federation of Societies for Ultrasound in Medicine and Biology has published Guidelines and Recommendations that describe these technologies and provide recommendations for their clinical use. Most of the data available to date has been published using the Fibroscan (Echosens, France), point shear wave speed measurement using an acoustic radiation force impulse (Siemens, Germany) and 2D shear wave elastography using the Aixplorer (SuperSonic Imagine, France). More recently, also other manufacturers have introduced shear wave elastography technology into the market. A comparison of data obtained using different techniques for shear wave propagation and velocity measurement is of key interest for future studies, recommendations and guidelines. Here, we present a recently introduced shear wave elastography technology from Hitachi and discuss its reproducibility and comparability to the already established technologies. PMID:27679731
Shear wave elastography with a new reliability indicator
Dong, Yi
2016-01-01
Non-invasive methods for liver stiffness assessment have been introduced over recent years. Of these, two main methods for estimating liver fibrosis using ultrasound elastography have become established in clinical practice: shear wave elastography and quasi-static or strain elastography. Shear waves are waves with a motion perpendicular (lateral) to the direction of the generating force. Shear waves travel relatively slowly (between 1 and 10 m/s). The stiffness of the liver tissue can be assessed based on shear wave velocity (the stiffness increases with the speed). The European Federation of Societies for Ultrasound in Medicine and Biology has published Guidelines and Recommendations that describe these technologies and provide recommendations for their clinical use. Most of the data available to date has been published using the Fibroscan (Echosens, France), point shear wave speed measurement using an acoustic radiation force impulse (Siemens, Germany) and 2D shear wave elastography using the Aixplorer (SuperSonic Imagine, France). More recently, also other manufacturers have introduced shear wave elastography technology into the market. A comparison of data obtained using different techniques for shear wave propagation and velocity measurement is of key interest for future studies, recommendations and guidelines. Here, we present a recently introduced shear wave elastography technology from Hitachi and discuss its reproducibility and comparability to the already established technologies.
Shear wave elastography with a new reliability indicator.
Dietrich, Christoph F; Dong, Yi
2016-09-01
Non-invasive methods for liver stiffness assessment have been introduced over recent years. Of these, two main methods for estimating liver fibrosis using ultrasound elastography have become established in clinical practice: shear wave elastography and quasi-static or strain elastography. Shear waves are waves with a motion perpendicular (lateral) to the direction of the generating force. Shear waves travel relatively slowly (between 1 and 10 m/s). The stiffness of the liver tissue can be assessed based on shear wave velocity (the stiffness increases with the speed). The European Federation of Societies for Ultrasound in Medicine and Biology has published Guidelines and Recommendations that describe these technologies and provide recommendations for their clinical use. Most of the data available to date has been published using the Fibroscan (Echosens, France), point shear wave speed measurement using an acoustic radiation force impulse (Siemens, Germany) and 2D shear wave elastography using the Aixplorer (SuperSonic Imagine, France). More recently, also other manufacturers have introduced shear wave elastography technology into the market. A comparison of data obtained using different techniques for shear wave propagation and velocity measurement is of key interest for future studies, recommendations and guidelines. Here, we present a recently introduced shear wave elastography technology from Hitachi and discuss its reproducibility and comparability to the already established technologies.
Standing shear waves in anisotropic viscoelastic media
NASA Astrophysics Data System (ADS)
Krit, T.; Golubkova, I.; Andreev, V.
2015-10-01
We studied standing shear waves in anisotropic resonator represented by a rectangular parallelepiped (layer) fixed without slipping between two wooden plates of finite mass. The viscoelastic layer with edges of 70 mm × 40 mm × 15 mm was made of a rubber-like polymer plastisol with rubber bands inside. The bands were placed vertical between the top and the bottom plate. Mechanical properties of the plastisol itself were carefully measured previously. It was found that plastisol shows a cubic nonlinear behavior, i.e. the stress-strain curve could be represented as: σ = μɛ + βμɛ3, where ɛ stands for shear strain and σ is an applied shear stress. The value of shear modulus μ depends on frequency and was found to be several kilopascals which is common for such soft solids. Nonlinear parameter β is frequency dependent too and varies in range from tenths to unity at 1-100 Hz frequency range, decreasing with frequency growth. Stretching the rubber bands inside the layer leads to change of elastic properties in resonator. Such effect could be noticed due to frequency response of the resonator. The numerical model of the resonator was based on finite elements method (FEM) and performed in MatLab. The resonator was cut in hundreds of right triangular prisms. Each prism was provided with viscoelastic properties of the layer except for the top prisms provided with the wooden plate properties and the prisms at the site of the rubber bands provided with the rubber properties. The boundary conditions on each prism satisfied the requirements that resonator is inseparable and all its boundaries but bottom are free. The bottom boundary was set to move horizontally with constant acceleration amplitude. It was shown numerically that the resonator shows anisotropic behavior expressed in different frequency response to oscillations applied to a bottom boundary in different directions.
Shear Wave Splitting Beneath the Galapagos Archipelago
NASA Astrophysics Data System (ADS)
Fontaine, F. R.; Burkett, P. G.; Hooft, E. E.; Toomey, D. R.; Solomon, S. C.; Silver, P. G.
2004-12-01
We report measurements of teleseismic shear wave splitting in the Galápagos Archipelago. The inferred lateral variations in azimuthal anisotropy allow us to examine the dynamics of an evolving hotspot-ridge system. The data are from SKS and SKKS phases, as well as S waves from deep sources, recorded by a relatively dense network of 10 portable broadband seismometers deployed from 1999 to 2003 for the IGUANA (Imaging Galápagos Upwelling and Neotectonics of the Archipelago) experiment and from the GSN broadband station in Santa Cruz (PAYG). We find a delay time between fast and slow shear waves of 0.4 to 0.9 s and fast polarization directions of N85-90° E beneath five stations at the leading and southern edge of the archipelago. Despite clear seismic signals, we did not find any anisotropy at the six stations located in the interior of the archipelago. For those stations that show shear wave splitting, there is an increase in the delay time toward the expected location of the Galápagos hotspot at the western edge of the archipelago. With the exception of Española, fast polarization directions (N85-90° E) are close to the current direction of absolute plate motion of the overlying Nazca plate (N91° E). The lack of azimuthal anisotropy in the interior of the archipelago is interpreted as an absence of strongly oriented mantle fabric beneath these stations. The apparent isotropy in this dynamic region, where we expect considerable mantle strain, is surprising. It is not likely that the olivine a-axis is oriented vertically beneath the interior of the archipelago as the Galápagos plume is thought to lie at the western edge. It is also unlikely that there are two layers of perpendicularly-oriented anisotropy which are solely confined to the center of the archipelago. However, there appears to be some correlation between the region of apparent isotropy and a zone of anomalously low upper mantle velocities imaged beneath Santiago and Marchena from surface waves by
NASA Astrophysics Data System (ADS)
Xing, Guangchi; Niu, Fenglin; Chen, Min; Yang, Yingjie
2016-05-01
Surface wave tomography routinely uses empirically scaled density model in the inversion of dispersion curves for shear wave speeds of the crust and uppermost mantle. An improperly selected empirical scaling relationship between density and shear wave speed can lead to unrealistic density models beneath certain tectonic formations such as sedimentary basins. Taking the Sichuan basin east to the Tibetan plateau as an example, we investigate the differences between density profiles calculated from four scaling methods and their effects on Rayleigh wave phase velocities. Analytical equations for 1-D layered models and adjoint tomography for 3-D models are used to examine the trade-off between density and S-wave velocity structures at different depth ranges. We demonstrate that shallow density structure can significantly influence phase velocities at short periods, and thereby affect the shear wave speed inversion from phase velocity data. In particular, a deviation of 25 per cent in the initial density model can introduce an error up to 5 per cent in the inverted shear velocity at middle and lower crustal depths. Therefore one must pay enough attention in choosing a proper velocity-density scaling relationship in constructing initial density model in Rayleigh wave inversion for crustal shear velocity structure.
Fluid Effects on Shear Waves in FInely Layered Porous Media
Berryman, J G
2004-05-21
Although there are five effective shear moduli for any layered VTI medium, one and only one effective shear modulus for the layered system contains all the dependence of pore fluids on the elastic or poroelastic constants that can be observed in vertically polarized shear waves. Pore fluids can increase the magnitude the shear energy stored by this modulus by a term that ranges from the smallest to the largest shear moduli of the VTI system. But, since there are five shear moduli in play, the increase in shear energy overall is reduced by a factor of about 5 in general. We can therefore give definite bounds on the maximum increase of shear modulus, being about 20% of the permitted range, when gas is fully replaced by liquid. An attendant increase of density (depending on porosity and fluid density) by approximately 5 to 10% partially offsets the effect of this shear modulus increase. Thus, an increase of shear wave speed on the order of 5 to 10% is shown to be possible when circumstances are favorable - i.e., when the shear modulus fluctuations are large (resulting in strong anisotropy), and the medium behaves in an undrained fashion due to fluid trapping. At frequencies higher than seismic (such as sonic and ultrasonic waves for well-logging or laboratory experiments), short response times also produce the requisite undrained behavior and, therefore, fluids also affect shear waves at high frequencies by increasing rigidity.
Shear wavelength estimation based on inverse filtering and multiple-point shear wave generation
NASA Astrophysics Data System (ADS)
Kitazaki, Tomoaki; Kondo, Kengo; Yamakawa, Makoto; Shiina, Tsuyoshi
2016-07-01
Elastography provides important diagnostic information because tissue elasticity is related to pathological conditions. For example, in a mammary gland, higher grade malignancies yield harder tumors. Estimating shear wave speed enables the quantification of tissue elasticity imaging using time-of-flight. However, time-of-flight measurement is based on an assumption about the propagation direction of a shear wave which is highly affected by reflection and refraction, and thus might cause an artifact. An alternative elasticity estimation approach based on shear wavelength was proposed and applied to passive configurations. To determine the elasticity of tissue more quickly and more accurately, we proposed a new method for shear wave elasticity imaging that combines the shear wavelength approach and inverse filtering with multiple shear wave sources induced by acoustic radiation force (ARF). The feasibility of the proposed method was verified using an elasticity phantom with a hard inclusion.
Electron trapping in shear Alfvén waves that power the aurora.
Watt, Clare E J; Rankin, Robert
2009-01-30
Results from 1D Vlasov drift-kinetic plasma simulations reveal how and where auroral electrons are accelerated along Earth's geomagnetic field. In the warm plasma sheet, electrons become trapped in shear Alfvén waves, preventing immediate wave damping. As waves move to regions with larger v(Te)/v(A), their parallel electric field decreases, and the trapped electrons escape their influence. The resulting electron distribution functions compare favorably with in situ observations, demonstrating for the first time a self-consistent link between Alfvén waves and electrons that form aurora.
Miyamoto, Naokazu; Hirata, Kosuke; Kanehisa, Hiroaki; Yoshitake, Yasuhide
2015-01-01
Ultrasound shear wave elastography is becoming a valuable tool for measuring mechanical properties of individual muscles. Since ultrasound shear wave elastography measures shear modulus along the principal axis of the probe (i.e., along the transverse axis of the imaging plane), the measured shear modulus most accurately represents the mechanical property of the muscle along the fascicle direction when the probe's principal axis is parallel to the fascicle direction in the plane of the ultrasound image. However, it is unclear how the measured shear modulus is affected by the probe angle relative to the fascicle direction in the same plane. The purpose of the present study was therefore to examine whether the angle between the principal axis of the probe and the fascicle direction in the same plane affects the measured shear modulus. Shear modulus in seven specially-designed tissue-mimicking phantoms, and in eleven human in-vivo biceps brachii and medial gastrocnemius were determined by using ultrasound shear wave elastography. The probe was positioned parallel or 20° obliquely to the fascicle across the B-mode images. The reproducibility of shear modulus measurements was high for both parallel and oblique conditions. Although there was a significant effect of the probe angle relative to the fascicle on the shear modulus in human experiment, the magnitude was negligibly small. These findings indicate that the ultrasound shear wave elastography is a valid tool for evaluating the mechanical property of pennate muscles along the fascicle direction. PMID:25853777
Miyamoto, Naokazu; Hirata, Kosuke; Kanehisa, Hiroaki; Yoshitake, Yasuhide
2015-01-01
Ultrasound shear wave elastography is becoming a valuable tool for measuring mechanical properties of individual muscles. Since ultrasound shear wave elastography measures shear modulus along the principal axis of the probe (i.e., along the transverse axis of the imaging plane), the measured shear modulus most accurately represents the mechanical property of the muscle along the fascicle direction when the probe's principal axis is parallel to the fascicle direction in the plane of the ultrasound image. However, it is unclear how the measured shear modulus is affected by the probe angle relative to the fascicle direction in the same plane. The purpose of the present study was therefore to examine whether the angle between the principal axis of the probe and the fascicle direction in the same plane affects the measured shear modulus. Shear modulus in seven specially-designed tissue-mimicking phantoms, and in eleven human in-vivo biceps brachii and medial gastrocnemius were determined by using ultrasound shear wave elastography. The probe was positioned parallel or 20° obliquely to the fascicle across the B-mode images. The reproducibility of shear modulus measurements was high for both parallel and oblique conditions. Although there was a significant effect of the probe angle relative to the fascicle on the shear modulus in human experiment, the magnitude was negligibly small. These findings indicate that the ultrasound shear wave elastography is a valid tool for evaluating the mechanical property of pennate muscles along the fascicle direction.
Method of making a piezoelectric shear wave resonator
Wang, Jin S.; Lakin, Kenneth M.; Landin, Allen R.
1987-02-03
An acoustic shear wave resonator comprising a piezoelectric film having its C-axis substantially inclined from the film normal such that the shear wave coupling coefficient significantly exceeds the longitudinal wave coupling coefficient, whereby the film is capable of shear wave resonance, and means for exciting said film to resonate. The film is prepared by deposition in a dc planar magnetron sputtering system to which a supplemental electric field is applied. The resonator structure may also include a semiconductor material having a positive temperature coefficient of resonance such that the resonator has a temperature coefficient of resonance approaching 0 ppm/.degree.C.
Piezoelectric shear wave resonator and method of making same
Wang, J.S.; Lakin, K.M.; Landin, A.R.
1983-10-25
An acoustic shear wave resonator comprising a piezoelectric film having its C-axis substantially inclined from the film normal such that the shear wave coupling coefficient significantly exceeds the longitudinal wave coupling coefficient, whereby the film is capable of shear wave resonance, and means for exciting said film to resonate. The film is prepared by deposition in a dc planar magnetron sputtering system to which a supplemental electric field is applied. The resonator structure may also include a semiconductor material having a positive temperature coefficient of resonance such that the resonator has a temperature coefficient of resonance approaching 0 ppM//sup 0/C.
Piezoelectric shear wave resonator and method of making same
Wang, Jin S.; Lakin, Kenneth M.; Landin, Allen R.
1988-01-01
An acoustic shear wave resonator comprising a piezoelectric film having its C-axis substantially inclined from the film normal such that the shear wave coupling coefficient significantly exceeds the longitudinal wave coupling coefficient, whereby the film is capable of shear wave resonance, and means for exciting said film to resonate. The film is prepared by deposition in a dc planar magnetron sputtering system to which a supplemental electric field is applied. The resonator structure may also include a semiconductor material having a positive temperature coefficient of resonance such that the resonator has a temperature coefficient of resonance approaching 0 ppm/.degree.C.
Piezoelectric shear wave resonator and method of making same
Wang, J.S.; Lakin, K.M.; Landin, A.R.
1985-05-20
An acoustic shear wave resonator comprising a piezoelectric film having its C-axis substantially inclined from the film normal such that the shear wave coupling coefficient significantly exceeds the longitudinal wave coupling coefficient, whereby the film is capable of shear wave resonance, and means for exciting said film to resonate. The film is prepared by deposition in a dc planar magnetron sputtering system to which a supplemental electric field is applied. The resonator structure may also include a semiconductor material having a positive temperature coefficient of resonance such that the resonator has a temperature coefficient of resonance approaching 0 ppM//sup 0/C.
Spatial variations in Achilles tendon shear wave speed
DeWall, Ryan J.; Slane, Laura C.; Lee, Kenneth S.; Thelen, Darryl G.
2014-01-01
Supersonic shear imaging (SSI) is an ultrasound imaging modality that can provide insight into tissue mechanics by measuring shear wave propagation speed, a property that depends on tissue elasticity. SSI has previously been used to characterize the increase in Achilles tendon shear wave speed that occurs with loading, an effect attributable to the strain-stiffening behavior of the tissue. However, little is known about how shear wave speed varies spatially, which is important, given the anatomical variation that occurs between the calcaneus insertion and the gastrocnemius musculotendon junction. The purpose of this study was to investigate spatial variations in shear wave speed along medial and lateral paths of the Achilles tendon for three different ankle postures: resting ankle angle (R, i.e. neutral), plantarflexed (P; R − 15 deg), and dorsiflexed (D; R + 15 deg). We observed significant spatial and posture variations in tendon shear wave speed in ten healthy young adults. Shear wave speeds in the Achilles free tendon averaged 12 ± 1.2 m/s in a resting position, but decreased to 7.2 ± 1.8 m/s with passive plantarflexion. Distal tendon shear wave speeds often reached the maximum tracking limit (16.3 m/s) of the system when the ankle was in the passively dorsiflexed posture (+15 deg from R). At a fixed posture, shear wave speeds decreased significantly from the free tendon to the gastrocnemius musculotendon junction, with slightly higher speeds measured on the medial side than on the lateral side. Shear wave speeds were only weakly correlated with the thickness and depth of the tendon, suggesting that the distal-to-proximal variations may reflect greater compliance in the aponeurosis relative to the free tendon. The results highlight the importance of considering both limb posture and transducer positioning when using SSI for biomechanical and clinical assessments of the Achilles tendon. PMID:24933528
Ismail, A.; Anderson, N.
2007-01-01
Shear-wave velocity (Vs) as a function of soil stiffness is an essential parameter in geotechnical characterization of the subsurface. In this study, multichannel analysis of surface wave (MASW) and downhole methods were used to map the shear-wave velocity-structure and depth to the bed-rock surface at a 125m ?? 125m geotechnical site in Missouri. The main objective was to assess the suitability of the site for constructing a large, heavy building. The acquired multichannel surface wave data were inverted to provide 1D shear-wave velocity profile corresponding to each shot gather. These 1D velocity profiles were interpolated and contoured to generate a suite of 2D shear-wave velocity sections. Integrating the shear-wave velocity data from the MASW method with the downhole velocity data and the available borehole lithologic information enabled us to map shear-wave velocity-structure to a depth on the order of 20m. The bedrock surface, which is dissected by a significant cut-and-fill valley, was imaged. The results suggest that the study site will require special consideration prior to construction. The results also demonstrate the successful use of MASW methods, when integrated with downhole velocity measurements and borehole lithologic information, in the characterization of the near surface at the geotechnical sites. ?? 2007 European Association of Geoscientists & Engineers.
Triad resonance between gravity and vorticity waves in vertical shear
NASA Astrophysics Data System (ADS)
Drivas, Theodore D.; Wunsch, Scott
2016-07-01
Weakly nonlinear theory is used to explore the effect of vertical shear on surface gravity waves in three dimensions. An idealized piecewise-linear shear profile motivated by wind-driven profiles and ambient currents in the ocean is used. It is shown that shear may mediate weakly nonlinear resonant triad interactions between gravity and vorticity waves. The triad results in energy exchange between gravity waves of comparable wavelengths propagating in different directions. For realistic ocean shears, shear-mediated energy exchange may occur on timescales of minutes for shorter wavelengths, but slows as the wavelength increases. Hence this triad mechanism may contribute to the larger angular spreading (relative to wind direction) for shorter wind-waves observed in the oceans.
Shear wave splitting and shear wave splitting tomography of the southern Puna plateau
NASA Astrophysics Data System (ADS)
Calixto, Frank J.; Robinson, Danielle; Sandvol, Eric; Kay, Suzanne; Abt, David; Fischer, Karen; Heit, Ben; Yuan, Xiaohui; Comte, Diana; Alvarado, Patricia
2014-11-01
We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by applying shear wave splitting analysis and shear wave splitting tomography to local S waves and teleseismic SKS, SKKS and PKS phases. Overall, a very complex pattern of fast directions throughout the southern Puna plateau region and a circular pattern of fast directions around the region of the giant Cerro Galan ignimbrite complex are observed. In general, teleseismic lag times are much greater than those for local events which are interpreted to reflect a significant amount of sub and inner slab anisotropy. The complex pattern observed from shear wave splitting analysis alone is the result of a complex 3-D anisotropic structure under the southern Puna plateau. Our application of shear wave splitting tomography provides a 3-D model of anisotropy in the southern Puna plateau that shows different patterns depending on the driving mechanism of upper-mantle flow and seismic anisotropy. The trench parallel a-axes in the continental lithosphere above the slab east of 68W may be related to deformation of the overriding continental lithosphere since it is under compressive stresses which are orthogonal to the trench. The more complex pattern below the Cerro Galan ignimbrite complex and above the slab is interpreted to reflect delamination of continental lithosphere and upwelling of hot asthenosphere. The a-axes beneath the Cerro Galan, Cerro Blanco and Carachi Pampa volcanic centres at 100 km depth show some weak evidence for vertically orientated fast directions, which could be due to vertical asthenospheric flow around a delaminated block. Additionally, our splitting tomographic model shows that there is a significant amount of seismic anisotropy beneath the slab. The subslab mantle west of 68W shows roughly trench parallel horizontal a-axes that are probably driven by slab roll back and the relatively small coupling between the Nazca slab and the underlying mantle. In
Latorre-Ossa, Heldmuth; Gennisson, Jean-Luc; De Brosses, Emilie; Tanter, Mickaël
2012-04-01
The study of new tissue mechanical properties such as shear nonlinearity could lead to better tissue characterization and clinical diagnosis. This work proposes a method combining static elastography and shear wave elastography to derive the nonlinear shear modulus by applying the acoustoelasticity theory in quasi-incompressible soft solids. Results demonstrate that by applying a moderate static stress at the surface of the investigated medium, and by following the quantitative evolution of its shear modulus, it is possible to accurately and quantitatively recover the local Landau (A) coefficient characterizing the shear nonlinearity of soft tissues.
Wavefield Analysis of Rayleigh Waves for Near-Surface Shear-Wave Velocity
NASA Astrophysics Data System (ADS)
Zeng, Chong
2011-12-01
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
[Propagation of shear waves in the muscle tissue].
Afanas'eva, D A; Tsaturian, A K
2010-01-01
A mathematical model of the propagation of acoustic shear waves in muscle tissue is considered. The muscle is modelled by an incompressible transversely isotropic viscoelastic continuum with quasi-one-dimensional active tension. Two types of shear waves in an infinite medium have been established. The waves of the second type (transverse) propagate without attenuation even when myofibril viscosity is taken into account. A problem of standing transverse waves in a rectangular layer has been investigated numerically. The values of the problem parameters have been found for which the active tension or muscle tonus is easily estimated from the characteristics of standing waves. This value is informative for the diagnosis of muscle state.
Shear waves in vegetal tissues at ultrasonic frequencies
NASA Astrophysics Data System (ADS)
Fariñas, M. D.; Sancho-Knapik, D.; Peguero-Pina, J. J.; Gil-Pelegrín, E.; Gómez Álvarez-Arenas, T. E.
2013-03-01
Shear waves are investigated in leaves of two plant species using air-coupled ultrasound. Magnitude and phase spectra of the transmission coefficient around the first two orders of the thickness resonances (normal and oblique incidence) have been measured. A bilayer acoustic model for plant leaves (comprising the palisade parenchyma and the spongy mesophyll) is proposed to extract, from measured spectra, properties of these tissues like: velocity and attenuation of longitudinal and shear waves and hence Young modulus, rigidity modulus, and Poisson's ratio. Elastic moduli values are typical of cellular solids and both, shear and longitudinal waves exhibit classical viscoelastic losses. Influence of leaf water content is also analyzed.
Excitation of fundamental shear horizontal wave by using face-shear (d36) piezoelectric ceramics
NASA Astrophysics Data System (ADS)
Miao, Hongchen; Dong, Shuxiang; Li, Faxin
2016-05-01
The fundamental shear horizontal (SH0) wave in plate-like structures is extremely useful for non-destructive testing (NDT) and structural health monitoring (SHM) as it is non-dispersive. However, currently, the SH0 wave is usually excited by electromagnetic acoustic transducers (EMAT) whose energy conversion efficiency is fairly low. The face-shear ( d 36 ) mode piezoelectrics is more promising for SH0 wave excitation, but this mode cannot appear in conventional piezoelectric ceramics. Recently, by modifying the symmetry of poled PbZr1-xTixO3 (PZT) ceramics via ferroelastic domain engineering, we realized the face-shear d 36 mode in both soft and hard PZT ceramics. In this work, we further improved the face-shear properties of PZT-4 and PZT-5H ceramics via lateral compression under elevated temperature. It was found that when bonded on a 1 mm-thick aluminum plate, the d 36 type PZT-4 exhibited better face-shear performance than PZT-5H. We then successfully excite SH0 wave in the aluminum plate using a face-shear PZT-4 square patch and receive the wave using a face-shear 0.72[Pb(Mg1/3Nb2/3)O3]-0.28[PbTiO3] (PMN-PT) patch. The frequency response and directionality of the excited SH0 wave were also investigated. The SH0 wave can be dominated over the Lamb waves (S0 and A0 waves) from 160 kHz to 280 kHz. The wave amplitude reaches its maxima along the two main directions (0° and 90°). The amplitude can keep over 80% of the maxima when the deviate angle is less than 30°, while it vanishes quickly at the 45° direction. The excited SH0 wave using piezoelectric ceramics could be very promising in the fields of NDT and SHM.
On acoustic wave generation in uniform shear flow
NASA Astrophysics Data System (ADS)
Gogoberidze, G.
2016-07-01
The linear dynamics of acoustic waves and vortices in uniform shear flow is studied. For flows with very low shear rates, the dynamics of perturbations is adiabatic and can be described by the WKB approximation. However, for flows with moderate and high shear rates the WKB approximation is not appropriate, and alternative analysis shows that two important phenomena occur: acoustic wave over-reflection and wave generation by vortices. The later phenomenon is a known linear mechanisms for sound generation in shear flows, a mechanism that is related to the continuous spectrum that arises in linear shear flow dynamics. A detailed analytical study of these phenomena is performed and the main quantitative and qualitative characteristics of the radiated acoustic field are obtained and analyzed.
Optimized shear wave generation using hybrid beamforming methods.
Nabavizadeh, Alireza; Greenleaf, James F; Fatemi, Mostafa; Urban, Matthew W
2014-01-01
Elasticity imaging is a medical imaging modality that measures tissue elasticity as an aid in the diagnosis of certain diseases. Shear wave-based methods have been developed to perform elasticity measurements in soft tissue. These methods often use the radiation force mechanism of focused ultrasound to induce shear waves in soft tissue such as liver, kidney, breast, thyroid and skeletal muscle. The efficiency of the ultrasound beam in producing broadband extended shear waves in soft tissue is very important to the widespread use of this modality. Hybrid beamforming combines two types of focusing, conventional spherical focusing and axicon focusing, to produce a beam for generating a shear wave that has increased depth-of-field (DOF) so that measurements can be made with a shear wave with a consistent wave front. Spherical focusing is used in many applications to achieve high lateral resolution, but has low DOF. Axicon focusing, with a cone-shaped transducer, can provide good lateral resolution with large DOF. We describe our linear aperture design and beam optimization performed using angular spectrum simulations. We performed a large parametric simulation study in which we varied the focal depth for the spherical focusing portion of the aperture, the numbers of elements devoted to the spherical and axicon focusing portions of the aperture and the opening angle used for axicon focusing. The hybrid beamforming method was experimentally tested in two phantoms, and shear wave speed measurement accuracy and DOF for each hybrid beam were evaluated. We compared our results with those for shear waves generated using only spherical focusing. The results of this study indicate that hybrid beamforming is capable of producing a beam with increased DOF over which accurate shear wave speed measurements can be made for different-size apertures and at different focal depths.
Convertion Shear Wave Velocity to Standard Penetration Resistance
NASA Astrophysics Data System (ADS)
Madun, A.; Tajuddin, S. A. A.; Abdullah, M. E.; Abidin, M. H. Z.; Sani, S.; Siang, A. J. L. M.; Yusof, M. F.
2016-07-01
Multichannel Analysis Surface Wave (MASW) measurement is one of the geophysics exploration techniques to determine the soil profile based on shear wave velocity. Meanwhile, borehole intrusive technique identifies the changes of soil layer based on soil penetration resistance, i.e. standard penetration test-number of blows (SPT-N). Researchers across the world introduced many empirical conversions of standard penetration test blow number of borehole data to shear wave velocity or vice versa. This is because geophysics test is a non-destructive and relatively fast assessment, and thus should be promoted to compliment the site investigation work. These empirical conversions of shear wave velocity to SPT-N blow can be utilised, and thus suitable geotechnical parameters for design purposes can be achieved. This study has demonstrated the conversion between MASW and SPT-N value. The study was conducted at the university campus and Sejagung Sri Medan. The MASW seismic profiles at the University campus test site and Sejagung were at a depth of 21 m and 13 m, respectively. The shear wave velocities were also calculated empirically using SPT-N value, and thus both calculated and measured shear wave velocities were compared. It is essential to note that the MASW test and empirical conversion always underestimate the actual shear wave velocity of hard layer or rock due to the effect of soil properties on the upper layer.
Use of shear wave reflection amplitude in geotechnial investigations: new concepts
NASA Astrophysics Data System (ADS)
Ghose, R.
2003-04-01
Shear waves are important to the geotechnical engineers because shear-wave velocity (V_S) offers the small-strain (˜10-6) rigidity (G_0) of the subsoil layers. G_0 is the key parameter used in evaluating the soil behaviour under any kind of dynamic loading e.g., vibrations, earthquakes, etc. Traditionally, the one-way traveltime of shear-wave is measured in a borehole as a function of depth, and the profile of in-situ G_o is obtained. As an alternative, the 1-D V_S structure is derived by the inversion of surface wave dispersion curves. In geotechnical engineering, surface seismic using shear waves has remained restricted to refraction surveys and some reflection works using large sledgehammer sources to map laterally the soil layers. The amplitude of shear waves has not yet been used in geotechnical site investigation. The difficulties to obtain reliable amplitudes of the shallow reflection events and the field acquisition challenges that are specific to shear-wave surveys have been the main obstacles. Recently, we have investigated the information potential of the shear-wave reflection amplitudes in the shallow subsoil, and evaluated the geotechnical merits. The use of an electromagnetic vibrator recently developed for generating high-frequency shear waves has been crucial to make breakthrough progress in our understanding of the potential of shear-wave reflections. Special attention has been paid to accurately monitor the amplitude and the phase of the shear-wave source. This, in turn, has allowed us to perform deterministic source signature deconvolution of the raw vibrograms. The resolution is significantly improved. Shot-to-shot variation is minimized. The receiver coupling effect still needs to be corrected for. However, once the source function is uniformly removed from the raw data, the amplitude information of the high-resolution reflection events reveal remarkable, new features of the subsoil that were otherwise not visible. Further, from the angle
Scattering of obliquely incident shear waves from a cylindrical cavity.
Aldrin, John C; Blodgett, Mark P; Lindgren, Eric A; Steffes, Gary J; Knopp, Jeremy S
2011-06-01
Prior work has proposed the use of ultrasonic angle-beam shear wave techniques to detect cracks of varying angular location around fastener sites by generating and detecting creeping waves. To better understand the nature of the scattering problem and quantify the role of creeping waves in fastener site inspections, a 3D analytical model was developed for the propagation and scattering of an obliquely incident plane shear wave from a cylindrical cavity with arbitrary shear wave polarization. The generation and decay of the spiral creeping waves was found to be dependent on both the angle of incidence and polarization of the plane shear wave. A difference between the angle of displacement in 3D and the direction of propagation for the spiral creeping wave was observed and attributed to differences in the curvature of the cavity surface for the tangential and vertical (z) directions. Using the model, practical insight was presented on measuring the displacement response in the far-field from the hole. Both analytical and experimental results highlighted the value of the diffracted and leaky spiral creeping wave signals for nondestructive evaluation of a crack located on the cavity. Last, array and signal processing methods are discussed to improve the resolution of the weaker creeping wave signals in the presence of noise.
A shear horizontal surface wave in magnetoelectric materials.
Liu, Jinxi; Fang, Daining; Liu, Xiangling
2007-07-01
We show that a semi-infinite magnetoelectric (ME) material adjoining a vacuum sustains the propagation of a shear horizontal wave accompanied by electromagnetic waves. The ME material is assumed to possess hexagonal (6 mm) symmetry. The expression for the phase velocity of this wave is obtained explicitly. The result is helpful for applications of piezoelectric-piezomagnetic composites to acoustic wave and microwave devices.
A pitfall in shallow shear-wave refraction surveying
Xia, J.; Miller, R.D.; Park, C.B.; Wightman, E.; Nigbor, R.
2002-01-01
The shallow shear-wave refraction method works successfully in an area with a series of horizontal layers. However, complex near-surface geology may not fit into the assumption of a series of horizontal layers. That a plane SH-wave undergoes wave-type conversion along an interface in an area of nonhorizontal layers is theoretically inevitable. One real example shows that the shallow shear-wave refraction method provides velocities of a converted wave rather than an SH- wave. Moreover, it is impossible to identify the converted wave by refraction data itself. As most geophysical engineering firms have limited resources, an additional P-wave refraction survey is necessary to verify if velocities calculated from a shear-wave refraction survey are velocities of converted waves. The alternative at this time may be the surface wave method, which can provide reliable S-wave velocities, even in an area of velocity inversion (a higher velocity layer underlain by a lower velocity layer). ?? 2002 Elsevier Science B.V. All rights reserved.
Measurement of Oblique Impact-generated Shear Waves
NASA Technical Reports Server (NTRS)
Dahl, J. M.; Schultz, P. H.
2001-01-01
Experimental strain measurements reveal that oblique impacts can generate shear waves with displacements as large as those in the P-wave. Large oblique impacts may thus be more efficient sources of surface disruption than vertical impacts. Additional information is contained in the original extended abstract.
1D GAS-DYNAMIC SIMULATION OF SHOCK-WAVE PROCESSES VIA INTERNET
Khishchenko, K. V.; Levashov, P. R.; Povarnitsyn, M. E.; Zakharenkov, A. S.
2009-12-28
We present a Web-interface for 1D simulation of different shock-wave experiments. The choosing of initial parameters, the modeling itself and output data treatment can be made directly via the Internet. The interface is based upon the expert system on shock-wave data and equations of state and contains both the Eulerian and Lagrangian Godunov hydrocodes. The availability of equations of state for a broad set of substances makes this system a useful tool for planning and interpretation of shock-wave experiments. As an example of simulation with the system, results of modeling of multistep shock loading of potassium between polytetrafluoroethylene and stainless steel plates are presented in comparison with experimental data from Shakhray et al.(2005).
Mountain wave drag for wind profiles with shear and curvature
NASA Astrophysics Data System (ADS)
Teixeira, M.
2003-04-01
Gravity wave drag produced by stratified flow over orography is one of the physical processes that must be parameterized in large-scale atmospheric models. While in recent studies attention has been given to the way in which the reaction force of the orography on the atmosphere is distributed in height (because this is what has a direct impact on the atmospheric circulation), it is also important to know the total surface drag, and how it depends on the characteristics of the incoming atmospheric flow, because this gives the total momentum flux that is available to be deposited at critical levels. Most available formulae for the surface drag, including those that are used in operational weather forecast models, ignore the effects of wind variation with height, but the few studies that address this problem have shown that these effects are significant and therefore deserve detailed investigation. In this study, a simple analytical model is developed to calculate the surface gravity wave drag exerted by a stratified flow on a bell-shaped mountain, when the wind varies with height in a complicated way. The model is linear with respect to the orography slope, as is required for the equations of motion to be solvable analytically, but is weakly nonlinear with respect to the incoming wind velocity, (U(z),V(z)), which is assumed to vary slowly with height. The solutions to the equations of motion are expanded as power series of a small parameter ɛ, inversely proportional to the square root of the Richardson number of the flow, Ri. By retaining terms in the solutions up to second order, it is found that the drag generally depends on the curvature of the wind profile, as well as on the shear. In the hydrostatic case, for a wind profile where the x velocity component varies linearly with height while the y velocity component is constant, the drag is given by D_x=D0x (1-(3/32) Ri-1), D_y=D0y (1-(1/32) Ri-1) (where (D0y,D0y) is the corresponding drag for a constant
Wave-current interaction, experiments with controlled uniform shear
NASA Astrophysics Data System (ADS)
Simon, Bruno; Touboul, Julien; Rey, Vincent
2016-04-01
Vertically varying currents have a non negligible impact on the propagation of waves. Even though the analytical aspect of the interaction between wave and sheared current is being an active subject of research, experimental data remain rare. Here, the effects of a uniformly shear were investigated in the 10 m long by 0.3 m wide wave flume of the Université de Toulon, France. The main difficulty of the study was to produce several conditions of current with constant shear (du/dz = cst) that would persist along the channel. This was achieved by using curved wire screens upstream the channel (Dunn and Tavoularis, 2007). The geometry and properties of the screens were adjusted to deflect the streamline towards the channel bed or the free surface in order to change the velocity profile. The study focused on regular wave propagating against the current for several wave frequencies and amplitudes. Properties of the free surface and flow velocity are discussed for current with positive and negative shear in order to quantify the influence of the current on the waves. ACKNOWLEDGEMENTS The DGA (Direction Générale de l'Armement, France) is acknowledged for its financial support through the ANR grant N° ANR-13-ASTR-0007.
Monolithic bulk shear-wave acousto-optic tunable filter.
Gnewuch, Harald; Pannell, Christopher N
2002-12-01
We demonstrate a monolithic bulk shear-wave acousto-optic tunable filter combining a piezoelectric transducer array and the acoustic interaction medium in a single crystal. An X-propagating acoustic longitudinal wave is excited in the "crossed-field" scheme by an rf-Ey-field in a chirped acoustic superlattice formed by domain-inversion in lithium niobate (LiNbO3). The acoustic longitudinal wave is efficiently (97.5%) converted at a mechanically free boundary into a Y-propagating acoustic slow-shear wave that couples collinearly propagating e- and o-polarized optical waves. A relative conversion efficiency of 80%/W was measured at 980 nm. PMID:12546145
Shear wave propagation in anisotropic soft tissues and gels.
Namani, Ravi; Bayly, Philip V
2009-01-01
The propagation of shear waves in soft tissue can be visualized by magnetic resonance elastography (MRE) to characterize tissue mechanical properties. Dynamic deformation of brain tissue arising from shear wave propagation may underlie the pathology of blast-induced traumatic brain injury. White matter in the brain, like other biological materials, exhibits a transversely isotropic structure, due to the arrangement of parallel fibers. Appropriate mathematical models and well-characterized experimental systems are needed to understand wave propagation in these structures. In this paper we review the theory behind waves in anisotropic, soft materials, including small-amplitude waves superimposed on finite deformation of a nonlinear hyperelastic material. Some predictions of this theory are confirmed in experimental studies of a soft material with controlled anisotropy: magnetically-aligned fibrin gel. PMID:19963987
Terrane-controlled crustal shear wave splitting in Taiwan
NASA Astrophysics Data System (ADS)
Okaya, David; Christensen, Nikolas I.; Ross, Zachary E.; Wu, Francis T.
2016-01-01
Taiwan is the result of arc-continent collision associated with the convergence of the Philippine Sea plate with the eastern Eurasian plate continental margin. The locus of deformation is found in eastern Taiwan in the form of mountain building (Central Range) with underlying thickened lithosphere. Rapid tectonic exhumation in the Central Range has uncovered low-to-high-grade metamorphic rocks marked by steep cleavage. We carried out a crustal seismic anisotropy study across Taiwan, producing a database of over 27,000 local earthquake shear wave splitting measurements. Additionally, we carried out rock physics measurements of metamorphic outcrop samples to quantify shear wave rock anisotropy. We produced a map of station-averaged splitting measurements across Taiwan. Patterns of fast shear wave directions correlate with tectonic terranes produced by plate convergence. Deformation-related mineral-preferred orientation in the metamorphic rocks produces a significant amount of the crustal anisotropy in the Taiwan collision zone.
Lithology and shear-wave velocity in Memphis, Tennessee
Gomberg, J.; Waldron, B.; Schweig, E.; Hwang, H.; Webbers, A.; Van Arsdale, R.; Tucker, K.; Williams, R.; Street, R.; Mayne, P.; Stephenson, W.; Odum, J.; Cramer, C.; Updike, R.; Hutson, S.; Bradley, M.
2003-01-01
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.
Could linear hysteresis contribute to shear wave losses in tissues?
Parker, Kevin J
2015-04-01
For nearly 100 y in the study of cyclical motion in materials, a particular phenomenon called "linear hysteresis" or "ideal hysteretic damping" has been widely observed. More recently in the field of shear wave elastography, the basic mechanisms underlying shear wave losses in soft tissues are in question. Could linear hysteresis play a role? An underlying theoretical question must be answered: Is there a real and causal physical model that is capable of producing linear hysteresis over a band of shear wave frequencies used in diagnostic imaging schemes? One model that can approximately produce classic linear hysteresis behavior, by examining a generalized Maxwell model with a specific power law relaxation spectrum, is described here. This provides a theoretical plausibility for the phenomenon as a candidate for models of tissue behavior.
Wave-particle transport by weak electrostatic flow shear fluctuations
NASA Technical Reports Server (NTRS)
Gary, S. P.; Schwartz, S. J.
1981-01-01
A description is presented of the first consistent theoretical treatment of transport due to weak electrostatic fluctuations from microinstabilities driven by a shear in plasma flow parallel to a uniform magnetic field. The model used considers electrostatic fluctuations in a Vlasov plasma with sheared bulk velocity parallel to a uniform magnetic field. The linear stability theory for the model has been studied by Gary and Schwartz (1980). In the current investigation, a calculation is performed of the wave-particle transport associated with the electrostatic flow shear instability.
KAM Tori for 1D Nonlinear Wave Equationswith Periodic Boundary Conditions
NASA Astrophysics Data System (ADS)
Chierchia, Luigi; You, Jiangong
In this paper, one-dimensional (1D) nonlinear wave equations
Supersymmetric configurations in the rotating D1-D5 system andpp-waves
NASA Astrophysics Data System (ADS)
Maoz, Liat
Two families of supersymmetric configurations are considered. One is the 1/4 supersymmetric D1--D5 system with angular momentum, and the other is a family of pp-waves of type IIB string theory with some supersymmetry. In the first part of the thesis some configurations of the D1--D5 system are examined which give conical singularities in AdS 3 as their near horizon limit. It is shown that they can be made non-singular by adding angular momentum to the brane system. The smooth asymptotically flat solutions constructed this way are used to obtain global AdS 3 as the near horizon geometry. Using the relation of the D1--D5 system to the oscillating string, a large family of supergravity solutions is constructed which describe BPS excitations on AdS3 x S 3 with angular momentum on S3. These solutions take into account the full back reaction on the metric, and can be viewed as Kaluza-Klein monopole "supertubes", which are completely non-singular geometries. The different chiral primaries of the dual CFT are identified with these different supergravity solutions. This part is adapted from the papers [1], [2]. In its second part, a general class of supersymmetric pp-wave solutions of type IIB string theory is constructed, such that the superstring worldsheet action in light cone gauge is that of an interacting massive field theory. It is shown that when the light cone Lagrangian has (2.2) supersymmetry, one can find backgrounds that lead to arbitrary superpotentials on the worldsheet. Both flat and curved transverse spaces are considered. In particular, the background giving rise to the N = 2 sine Gordon theory on the worldsheet is analyzed. Massive mirror symmetry relates it to the deformed CP1 model (or sausage model) which seems to elude a purely supergravity target space interpretation. These are results which appeared in the paper [3].
Shear wave elastography using phase sensitive optical coherence tomography
NASA Astrophysics Data System (ADS)
Song, Shaozhen; Huang, Zhihong; Nguyen, Thu-Mai; Wong, Emily Y.; Arnal, Bastien; O'Donnell, Matthew; Wang, Ruikang K.
2014-03-01
Optical coherence tomography (OCT) provides high spatial resolution and sensitivity that are ideal for imaging the cornea and lens. Quantifying the biomechanical properties of these tissues could add clinically valuable information. Thus, we propose a dynamic elastography method combining OCT detection and a mechanical actuator to map the shear modulus of soft tissues. We used a piezoelectric actuator driven in the kHz range and we used phase-sensitive OCT (PhS-OCT) to track the resulting shear waves at an equivalent frame rate of 47 kHz. We mapped the shear wave speed of anesthetized mice cornea using monochromatic excitations. We found a significant difference between a group of knock-out (3.92 +/- 0.35 m/s, N=4) and wild-type mice (5.04 +/- 0.51 m/s, N=3). These preliminary results demonstrate the feasibility of using PhS-OCT to perform in vivo shear wave elastography of the cornea. We then implemented a shear pulse compression approach on ex vivo human cornea. For that purpose, frequency- modulated excitations were used and the resulting displacement field was digitally compressed in a short broadband pulse with a 7 dB gain in signal-to-noise ratio (SNR).
Stiffener bond line monitoring using ultrasonic shear guided waves
NASA Astrophysics Data System (ADS)
Fan, Z.; Castaings, M.; Lowe, M. J. S.; Fromme, P.; Biateau, C.
2012-05-01
Adhesively bonded stiffeners are employed in aerospace applications to increase structural stiffness. The potential of shear guided wave modes for the verification of adhesion and bond line thickness in difficult to access regions has been investigated. The properties of guided wave modes propagating along a T-shaped stiffener bonded to an aluminium plate were calculated using the Semi-Analytical Finite Element (SAFE) method. Shear modes were identified as well suited with energy concentrated at the stiffener and bond line, limiting energy radiation into the plate and thus achieving increased inspection length. The influence of bond line properties and thickness was investigated from SAFE and 3D Finite Element calculations and a significant influence of the epoxy shear (Coulomb) modulus on the phase velocity found. Experiments were conducted during the curing of an epoxy adhesive, bonding a stiffener to the plate with bond strength and stiffness increasing over time. The excited shear mode was measured using a laser interferometer. The measured phase velocity changed significantly during curing. The frequency dependency matches well with the SAFE calculations for a variation of the Coulomb's modulus of the adhesive layer. The potential of the shear guided wave mode for bond line inspection and monitoring has been shown.
A Hammer-Impact, Aluminum, Shear-Wave Seismic Source
Haines, Seth S.
2007-01-01
Near-surface seismic surveys often employ hammer impacts to create seismic energy. Shear-wave surveys using horizontally polarized waves require horizontal hammer impacts against a rigid object (the source) that is coupled to the ground surface. I have designed, built, and tested a source made out of aluminum and equipped with spikes to improve coupling. The source is effective in a variety of settings, and it is relatively simple and inexpensive to build.
Density gradient effects on transverse shear driven lower hybrid waves
DuBois, Ami M.; Thomas, Edward; Amatucci, William E.; Ganguli, Gurudas
2014-06-15
Shear driven instabilities are commonly observed in the near-Earth space, particularly in boundary layer plasmas. When the shear scale length (L{sub E}) is much less than the ion gyro-radius (ρ{sub i}) but greater than the electron gyro-radius (ρ{sub e}), the electrons are magnetized in the shear layer, but the ions are effectively un-magnetized. The resulting shear driven instability, the electron-ion hybrid (EIH) instability, is investigated in a new interpenetrating plasma configuration in the Auburn Linear EXperiment for Instability Studies. In order to understand the dynamics of magnetospheric boundary layers, the EIH instability is studied in the presence of a density gradient located at the boundary layer between two plasmas. This paper reports on a recent experiment in which electrostatic lower hybrid waves are identified as the EIH instability, and the effect of a density gradient on the instability properties are investigated.
Shear wave arrival time estimates correlate with local speckle pattern.
Mcaleavey, Stephen A; Osapoetra, Laurentius O; Langdon, Jonathan
2015-12-01
We present simulation and phantom studies demonstrating a strong correlation between errors in shear wave arrival time estimates and the lateral position of the local speckle pattern in targets with fully developed speckle. We hypothesize that the observed arrival time variations are largely due to the underlying speckle pattern, and call the effect speckle bias. Arrival time estimation is a key step in quantitative shear wave elastography, performed by tracking tissue motion via cross-correlation of RF ultrasound echoes or similar methods. Variations in scatterer strength and interference of echoes from scatterers within the tracking beam result in an echo that does not necessarily describe the average motion within the beam, but one favoring areas of constructive interference and strong scattering. A swept-receive image, formed by fixing the transmit beam and sweeping the receive aperture over the region of interest, is used to estimate the local speckle pattern. Metrics for the lateral position of the speckle are found to correlate strongly (r > 0.7) with the estimated shear wave arrival times both in simulations and in phantoms. Lateral weighting of the swept-receive pattern improved the correlation between arrival time estimates and speckle position. The simulations indicate that high RF echo correlation does not equate to an accurate shear wave arrival time estimate-a high correlation coefficient indicates that motion is being tracked with high precision, but the location tracked is uncertain within the tracking beam width. The presence of a strong on-axis speckle is seen to imply high RF correlation and low bias. The converse does not appear to be true-highly correlated RF echoes can still produce biased arrival time estimates. The shear wave arrival time bias is relatively stable with variations in shear wave amplitude and sign (-20 μm to 20 μm simulated) compared with the variation with different speckle realizations obtained along a given tracking
Mellema, Daniel C; Song, Pengfei; Kinnick, Randall R; Urban, Matthew W; Greenleaf, James F; Manduca, Armando; Chen, Shigao
2016-09-01
Ultrasound shear wave elastography (SWE) utilizes the propagation of induced shear waves to characterize the shear modulus of soft tissue. Many methods rely on an acoustic radiation force (ARF) "push beam" to generate shear waves. However, specialized hardware is required to generate the push beams, and the thermal stress that is placed upon the ultrasound system, transducer, and tissue by the push beams currently limits the frame-rate to about 1 Hz. These constraints have limited the implementation of ARF to high-end clinical systems. This paper presents Probe Oscillation Shear Elastography (PROSE) as an alternative method to measure tissue elasticity. PROSE generates shear waves using a harmonic mechanical vibration of an ultrasound transducer, while simultaneously detecting motion with the same transducer under pulse-echo mode. Motion of the transducer during detection produces a "strain-like" compression artifact that is coupled with the observed shear waves. A novel symmetric sampling scheme is proposed such that pulse-echo detection events are acquired when the ultrasound transducer returns to the same physical position, allowing the shear waves to be decoupled from the compression artifact. Full field-of-view (FOV) two-dimensional (2D) shear wave speed images were obtained by applying a local frequency estimation (LFE) technique, capable of generating a 2D map from a single frame of shear wave motion. The shear wave imaging frame rate of PROSE is comparable to the vibration frequency, which can be an order of magnitude higher than ARF based techniques. PROSE was able to produce smooth and accurate shear wave images from three homogeneous phantoms with different moduli, with an effective frame rate of 300 Hz. An inclusion phantom study showed that increased vibration frequencies improved the accuracy of inclusion imaging, and allowed targets as small as 6.5 mm to be resolved with good contrast (contrast-to-noise ratio ≥ 19 dB) between the target and
Mellema, Daniel C; Song, Pengfei; Kinnick, Randall R; Urban, Matthew W; Greenleaf, James F; Manduca, Armando; Chen, Shigao
2016-09-01
Ultrasound shear wave elastography (SWE) utilizes the propagation of induced shear waves to characterize the shear modulus of soft tissue. Many methods rely on an acoustic radiation force (ARF) "push beam" to generate shear waves. However, specialized hardware is required to generate the push beams, and the thermal stress that is placed upon the ultrasound system, transducer, and tissue by the push beams currently limits the frame-rate to about 1 Hz. These constraints have limited the implementation of ARF to high-end clinical systems. This paper presents Probe Oscillation Shear Elastography (PROSE) as an alternative method to measure tissue elasticity. PROSE generates shear waves using a harmonic mechanical vibration of an ultrasound transducer, while simultaneously detecting motion with the same transducer under pulse-echo mode. Motion of the transducer during detection produces a "strain-like" compression artifact that is coupled with the observed shear waves. A novel symmetric sampling scheme is proposed such that pulse-echo detection events are acquired when the ultrasound transducer returns to the same physical position, allowing the shear waves to be decoupled from the compression artifact. Full field-of-view (FOV) two-dimensional (2D) shear wave speed images were obtained by applying a local frequency estimation (LFE) technique, capable of generating a 2D map from a single frame of shear wave motion. The shear wave imaging frame rate of PROSE is comparable to the vibration frequency, which can be an order of magnitude higher than ARF based techniques. PROSE was able to produce smooth and accurate shear wave images from three homogeneous phantoms with different moduli, with an effective frame rate of 300 Hz. An inclusion phantom study showed that increased vibration frequencies improved the accuracy of inclusion imaging, and allowed targets as small as 6.5 mm to be resolved with good contrast (contrast-to-noise ratio ≥ 19 dB) between the target and
Hoyt, Kenneth
2011-03-01
Acoustic radiation forces associated with high intensity focused ultrasound stimulate shear wave propagation allowing shear wave speed and shear viscosity estimation of tissue structures. As wave speeds are meters per second, real time displacement tracking over an extend field-of-view using ultrasound is problematic due to very high frame rate requirements. However, two spatially separated dynamic external sources can stimulate shear wave motion leading to shear wave interference patterns. Advantages are shear waves can be imaged at lower frame rates and local interference pattern spatial properties reflect tissue's viscoelastic properties. Here a theoretical analysis of shear wave interference patterns by means of dynamic acoustic radiation forces is detailed. Using a viscoelastic Green's function analysis, tissue motion due to a pair of focused ultrasound beams and associated radiation forces are presented. Overall, this paper theoretically demonstrates shear wave interference patterns can be stimulated using dynamic acoustic radiation forces and tracked using conventional ultrasound imaging.
Minimum 1-D P-wave velocity reference model for Northern Iran
NASA Astrophysics Data System (ADS)
Rezaeifar, Meysam; Diehl, Tobias; Kissling, Edi
2016-04-01
Uniform high-precision earthquake location is of importance in a seismically active area like northern Iran where the earthquake catalogue is a prerequisite for seismic hazard assessment and tectonic interpretation. We compile a complete and consistent local earthquake data set for the northern Iran region, using information from two independently operating seismological networks, Iran Seismological Center (IRSC) network, administered by the Geophysical Institute of Tehran University, and Iran Broadband network administered by International Institute of Engineering Earthquake and Seismology (IIEES). Special care is taken during the merging process to reduce the number of errors in the data, including station parameters, event pairing, phase identification, and to the assessment of quantitative observation uncertainties. The derived P-wave 1D-velocity model for Northern Iran may serve for consistent routine high-precision earthquake location and as initial reference model for 3D seismic tomography.
High-frequency shear-horizontal surface acoustic wave sensor
Branch, Darren W
2013-05-07
A Love wave sensor uses a single-phase unidirectional interdigital transducer (IDT) on a piezoelectric substrate for leaky surface acoustic wave generation. The IDT design minimizes propagation losses, bulk wave interferences, provides a highly linear phase response, and eliminates the need for impedance matching. As an example, a high frequency (.about.300-400 MHz) surface acoustic wave (SAW) transducer enables efficient excitation of shear-horizontal waves on 36.degree. Y-cut lithium tantalate (LTO) giving a highly linear phase response (2.8.degree. P-P). The sensor has the ability to detect at the pg/mm.sup.2 level and can perform multi-analyte detection in real-time. The sensor can be used for rapid autonomous detection of pathogenic microorganisms and bioagents by field deployable platforms.
High-frequency shear-horizontal surface acoustic wave sensor
Branch, Darren W
2014-03-11
A Love wave sensor uses a single-phase unidirectional interdigital transducer (IDT) on a piezoelectric substrate for leaky surface acoustic wave generation. The IDT design minimizes propagation losses, bulk wave interferences, provides a highly linear phase response, and eliminates the need for impedance matching. As an example, a high frequency (.about.300-400 MHz) surface acoustic wave (SAW) transducer enables efficient excitation of shear-horizontal waves on 36.degree. Y-cut lithium tantalate (LTO) giving a highly linear phase response (2.8.degree. P-P). The sensor has the ability to detect at the pg/mm.sup.2 level and can perform multi-analyte detection in real-time. The sensor can be used for rapid autonomous detection of pathogenic microorganisms and bioagents by field deployable platforms.
NASA Astrophysics Data System (ADS)
Chanishvili, R.; Chagelishvili, G.; Uchava, E.; Kharshiladze, O.
2016-04-01
Our goal is to gain new insight into the physics of wave dynamics in ionospheric zonal shear flows. We study the shear flow non-normality induced linear coupling of planetary scale (slow) modified Rossby waves and westward propagating fast magnetized (Khantadze) waves using an approach different from the existing one to the linear wave dynamics. The performed analysis allows us to separate from each other different physical processes, grasp their interplay, and, by this way, construct the basic physics of the linear coupling of the slow and fast waves in an ionospheric zonal flow with linear shear of mean velocity, U0=(S y ,0 ) . It should be noted from the beginning that we consider incompressible flow and the classified "slow" and "fast" waves are not connected with the similarly labeled magnetosonic waves in compressible heliosphere. We show that: the modified Rossby waves generate fast magnetized waves due to the coupling for a quite wide range of ionospheric and shear flow parameters; the linear transient processes are highly anisotropic in wavenumber plane; the generation of the magnetized waves/oscillations is most efficient/optimal for S ≃0.1 (S is the shear rate normalized to the combination of the angular velocity and latitude, Ω0 cos θ0 ); the streamwise wave number of the optimally generated magnetized wave harmonics decreases (the length scale increases) with increasing the Hall parameter, α. At the end, we discuss nonlinear consequences of the described anisotropic linear dynamics—they should lead to an anisotropy of nonlinear cascade processes (in wavenumber plane). In turn, an interplay of the analyzed quite strong transient growth of the fast magnetic waves with anisotropic nonlinear processes should ensure self-sustenance of (stochastic or regular) magnetic perturbations.
Instability of subharmonic resonances in magnetogravity shear waves
NASA Astrophysics Data System (ADS)
Salhi, A.; Nasraoui, S.
2013-12-01
We study analytically the instability of the subharmonic resonances in magnetogravity waves excited by a (vertical) time-periodic shear for an inviscid and nondiffusive unbounded conducting fluid. Due to the fact that the magnetic potential induction is a Lagrangian invariant for magnetohydrodynamic Euler-Boussinesq equations, we show that plane-wave disturbances are governed by a four-dimensional Floquet system in which appears, among others, the parameter ɛ representing the ratio of the periodic shear amplitude to the vertical Brunt-Väisälä frequency N3. For sufficiently small ɛ and when the magnetic field is horizontal, we perform an asymptotic analysis of the Floquet system following the method of Lebovitz and Zweibel [Astrophys. J. 609, 301 (2004), 10.1086/420972]. We determine the width and the maximal growth rate of the instability bands associated with subharmonic resonances. We show that the instability of subharmonic resonance occurring in gravity shear waves has a maximal growth rate of the form Δm=(3√3 /16)ɛ. This instability persists in the presence of magnetic fields, but its growth rate decreases as the magnetic strength increases. We also find a second instability involving a mixing of hydrodynamic and magnetic modes that occurs for all magnetic field strengths. We also elucidate the similarity between the effect of a vertical magnetic field and the effect of a vertical Coriolis force on the gravity shear waves considering axisymmetric disturbances. For both cases, plane waves are governed by a Hill equation, and, when ɛ is sufficiently small, the subharmonic instability band is determined by a Mathieu equation. We find that, when the Coriolis parameter (or the magnetic strength) exceeds N3/2, the instability of the subharmonic resonance vanishes.
Instability of subharmonic resonances in magnetogravity shear waves.
Salhi, A; Nasraoui, S
2013-12-01
We study analytically the instability of the subharmonic resonances in magnetogravity waves excited by a (vertical) time-periodic shear for an inviscid and nondiffusive unbounded conducting fluid. Due to the fact that the magnetic potential induction is a Lagrangian invariant for magnetohydrodynamic Euler-Boussinesq equations, we show that plane-wave disturbances are governed by a four-dimensional Floquet system in which appears, among others, the parameter ɛ representing the ratio of the periodic shear amplitude to the vertical Brunt-Väisälä frequency N(3). For sufficiently small ɛ and when the magnetic field is horizontal, we perform an asymptotic analysis of the Floquet system following the method of Lebovitz and Zweibel [Astrophys. J. 609, 301 (2004)]. We determine the width and the maximal growth rate of the instability bands associated with subharmonic resonances. We show that the instability of subharmonic resonance occurring in gravity shear waves has a maximal growth rate of the form Δ(m)=(3√[3]/16)ɛ. This instability persists in the presence of magnetic fields, but its growth rate decreases as the magnetic strength increases. We also find a second instability involving a mixing of hydrodynamic and magnetic modes that occurs for all magnetic field strengths. We also elucidate the similarity between the effect of a vertical magnetic field and the effect of a vertical Coriolis force on the gravity shear waves considering axisymmetric disturbances. For both cases, plane waves are governed by a Hill equation, and, when ɛ is sufficiently small, the subharmonic instability band is determined by a Mathieu equation. We find that, when the Coriolis parameter (or the magnetic strength) exceeds N(3)/2, the instability of the subharmonic resonance vanishes. PMID:24483566
Instability of subharmonic resonances in magnetogravity shear waves.
Salhi, A; Nasraoui, S
2013-12-01
We study analytically the instability of the subharmonic resonances in magnetogravity waves excited by a (vertical) time-periodic shear for an inviscid and nondiffusive unbounded conducting fluid. Due to the fact that the magnetic potential induction is a Lagrangian invariant for magnetohydrodynamic Euler-Boussinesq equations, we show that plane-wave disturbances are governed by a four-dimensional Floquet system in which appears, among others, the parameter ɛ representing the ratio of the periodic shear amplitude to the vertical Brunt-Väisälä frequency N(3). For sufficiently small ɛ and when the magnetic field is horizontal, we perform an asymptotic analysis of the Floquet system following the method of Lebovitz and Zweibel [Astrophys. J. 609, 301 (2004)]. We determine the width and the maximal growth rate of the instability bands associated with subharmonic resonances. We show that the instability of subharmonic resonance occurring in gravity shear waves has a maximal growth rate of the form Δ(m)=(3√[3]/16)ɛ. This instability persists in the presence of magnetic fields, but its growth rate decreases as the magnetic strength increases. We also find a second instability involving a mixing of hydrodynamic and magnetic modes that occurs for all magnetic field strengths. We also elucidate the similarity between the effect of a vertical magnetic field and the effect of a vertical Coriolis force on the gravity shear waves considering axisymmetric disturbances. For both cases, plane waves are governed by a Hill equation, and, when ɛ is sufficiently small, the subharmonic instability band is determined by a Mathieu equation. We find that, when the Coriolis parameter (or the magnetic strength) exceeds N(3)/2, the instability of the subharmonic resonance vanishes.
Shear waves in an inhomogeneous strongly coupled dusty plasma
Janaki, M. S.; Banerjee, D.; Chakrabarti, N.
2011-09-15
The properties of electrostatic transverse shear waves propagating in a strongly coupled dusty plasma with an equilibrium density gradient are examined using the generalized hydrodynamic (GH) equation. In the usual kinetic limit, the resulting equation has similarity to zero energy Schrodinger's equation. This has helped in obtaining some exact eigenmode solutions in both Cartesian and cylindrical geometries for certain nontrivial density profiles. The corresponding velocity profiles and the discrete eigenfrequencies are obtained for several interesting situations and their physics discussed.
Phase Aberration and Attenuation Effects on Acoustic Radiation Force-Based Shear Wave Generation.
Carrascal, Carolina Amador; Aristizabal, Sara; Greenleaf, James F; Urban, Matthew W
2016-02-01
Elasticity is measured by shear wave elasticity imaging (SWEI) methods using acoustic radiation force to create the shear waves. Phase aberration and tissue attenuation can hamper the generation of shear waves for in vivo applications. In this study, the effects of phase aberration and attenuation in ultrasound focusing for creating shear waves were explored. This includes the effects of phase shifts and amplitude attenuation on shear wave characteristics such as shear wave amplitude, shear wave speed, shear wave center frequency, and bandwidth. Two samples of swine belly tissue were used to create phase aberration and attenuation experimentally. To explore the phase aberration and attenuation effects individually, tissue experiments were complemented with ultrasound beam simulations using fast object-oriented C++ ultrasound simulator (FOCUS) and shear wave simulations using finite-element-model (FEM) analysis. The ultrasound frequency used to generate shear waves was varied from 3.0 to 4.5 MHz. Results: The measured acoustic pressure and resulting shear wave amplitude decreased approximately 40%-90% with the introduction of the tissue samples. Acoustic intensity and shear wave displacement were correlated for both tissue samples, and the resulting Pearson's correlation coefficients were 0.99 and 0.97. Analysis of shear wave generation with tissue samples (phase aberration and attenuation case), measured phase screen, (only phase aberration case), and FOCUS/FEM model (only attenuation case) showed that tissue attenuation affected the shear wave generation more than tissue aberration. Decreasing the ultrasound frequency helped maintain a focused beam for creation of shear waves in the presence of both phase aberration and attenuation.
Shear flow induced wave couplings in the solar wind
Poedts, S.; Rogava, A.D. |; Mahajan, S.M. |
1998-01-01
A sheared background flow in a plasma induces coupling between different MHD wave modes, resulting in their mutual transformations with corresponding energy redistributing between the modes. In this way, the energy can be transfered from one wave mode to the other, but energy can also be added to or extracted from the background flow. In the present paper it is investigated whether the wave coupling and energy transfer mechanisms can operate under solar wind conditions. It is shown that this is indeed the case. Hence, the long-period waves observed in the solar wind at r > 0.3 AU might be generated by much faster periodic oscillations in the photosphere of the Sun. Other possible consequences for observable beat phenomena in the wind and the acceleration of the solar wind particles are also discussed.
Laboratory measurements of compressional and shear wave speeds through methane hydrate
Waite, W.F.; Helgerud, M.B.; Nur, A.; Pinkston, J.C.; Stern, L.A.; Kirby, S.H.; Durham, W.B.
2000-01-01
Simultaneous measurements of compressional and shear wave speeds through polycrystalline methane hydrate have been made. Methane hydrate, grown directly in a wave speed measurement chamber, was uniaxially compacted to a final porosity below 2%. At 277 K, the compacted material had a compressional wave speed of 3650 ?? 50 m/s. The shear wave speed, measured simultaneously, was 1890 ?? 30 m/s. From these wave speed measurements, we derive V(p)/V(s), Poisson's ratio, bulk, shear, and Young's moduli.
1-D and 2-D Probabilistic Inversions of Fault Zone Guided Waves
NASA Astrophysics Data System (ADS)
Gulley, A.; Eccles, J. D.; Kaipio, J. P.; Malin, P. E.
2015-12-01
Fault Zone Guided Waves (FZGWs) are seismic coda that are trapped by the low velocity damage zone of faults. Inversions of these phases can be carried out using their measured dispersion and a Bayesian probability approach. This method utilises a Markov chain Monte Carlo which allows uncertainties and trade-offs to be quantified. Accordingly we have developed a scheme that estimates the dispersion curve and amplitude response variability from a FZGW record. This method allows the computation of both the point estimates and the covariance of the dispersion curve. The subsequent estimation of fault zone parameters is then based on a Gaussian model for the dispersion curve. We then show that inversions using FZGW dispersion data can only resolve fault zone velocity contrast and fault zone width - it leaves densities, absolute country rock velocities and the earthquake location unresolved. We show that they do however significantly affect the estimated fault zone velocities and widths. As these parameters cannot be resolved, we allow for their effects on the estimates of fault zone width and velocity contrast by using the Bayesian approximation error method. We show that using this method reduces computational time from days to minutes and the associated loss of accuracy is insignificant compared to carrying out the inversion on all parameters. We have extended our scheme to 2-D using 1-D slices. The Bayesian approximation error methodology is further employed to provide a 'correction term' with uncertainty for the 1-D slice approximation. We investigate these features with both synthetic data and FZGW data from the Alpine Fault of New Zealand.
Shear wave focusing for three-dimensional sonoelastography.
Wu, Zhe; Taylor, Lawrence S; Rubens, Deborah J; Parker, Kevin J
2002-01-01
A new vibration scheme is shown to provide localized vibration fields for three-dimensional sonoelastography imaging. The theoretical vibration distributions of double strip loads vibrating normally to the surface of a semi-infinite elastic space are calculated. A localization or focusing of shear waves inbetween the double-strip loads is predicted. Experimentally, two parallel rigid rectangular cross-section bars are mounted on an electromagnetic shaker. Driven by the signal source, the bars vibrate against the surface of a tissue-mimicking phantom. The double-bar source is also used to propagate shear wave into an ex vivo prostate phantom with a 6 mm "tumor" in it. A combination of high frequencies (400-600 Hz) is used to drive the double-bar applicator. In the phantom experiments, a shear wave focal zone with higher vibration amplitude and uniformity predicted by the theory was confirmed. The position of the focal zone is controllable when adjusting the separation of the bars as the theory shows. When this vibration scheme was used in a prostate phantom experiment, high-resolution tumor images with clear boundaries are obtained. The parallel bar is an ideal applicator to create more uniform vibration within a controllable localized volume. The field has uniformity especially in the direction along the bars. PMID:11831818
Shear-driven Dynamo Waves in the Fully Nonlinear Regime
NASA Astrophysics Data System (ADS)
Pongkitiwanichakul, P.; Nigro, G.; Cattaneo, F.; Tobias, S. M.
2016-07-01
Large-scale dynamo action is well understood when the magnetic Reynolds number (Rm) is small, but becomes problematic in the astrophysically relevant large Rm limit since the fluctuations may control the operation of the dynamo, obscuring the large-scale behavior. Recent works by Tobias & Cattaneo demonstrated numerically the existence of large-scale dynamo action in the form of dynamo waves driven by strongly helical turbulence and shear. Their calculations were carried out in the kinematic regime in which the back-reaction of the Lorentz force on the flow is neglected. Here, we have undertaken a systematic extension of their work to the fully nonlinear regime. Helical turbulence and large-scale shear are produced self-consistently by prescribing body forces that, in the kinematic regime, drive flows that resemble the original velocity used by Tobias & Cattaneo. We have found four different solution types in the nonlinear regime for various ratios of the fluctuating velocity to the shear and Reynolds numbers. Some of the solutions are in the form of propagating waves. Some solutions show large-scale helical magnetic structure. Both waves and structures are permanent only when the kinetic helicity is non-zero on average.
Shear wave velocity structure in West Java, Indonesia as inferred from surface wave dispersion
NASA Astrophysics Data System (ADS)
Anggono, Titi; Syuhada
2016-02-01
We investigated the crust and upper mantle of West Java, Indonesia by measuring the group velocity dispersion of surface waves. We analyzed waveform from four teleseismic earthquake recorded at three 3-component broadband seismometers. We analyzed fundamental mode of Rayleigh and Love waves from vertical, radial, and transverse components using multiple filter technique. We inverted the measured group velocity to obtain shear wave velocity profile down to 200 km depth. We observed low shear wave velocity zone at depth of about 20 km. Shear velocity reduction is estimated to be 18% compared to the upper and lower velocity layer. The low velocity zone might be associated with the subducting slab of Indo-Australian Plate as similar characteristics of low velocity zones also observed at other subducting regions.
NASA Astrophysics Data System (ADS)
Salacka, T. F.
1985-12-01
The QAZ1D method is redeveloped in detail and implemented in a first order, one-dimensional FORTRAN program, EULER-1. The program is tested on the shock tube problem and results are presented for various computational meshes and initial conditions. Based on good results of the EULER-1 code, recommendations are made for future extensions and testing to validate the suitability of the QAZ1D method for wave rotor applications.
Laser-based linear and nonlinear guided elastic waves at surfaces (2D) and wedges (1D).
Hess, Peter; Lomonosov, Alexey M; Mayer, Andreas P
2014-01-01
The characteristic features and applications of linear and nonlinear guided elastic waves propagating along surfaces (2D) and wedges (1D) are discussed. Laser-based excitation, detection, or contact-free analysis of these guided waves with pump-probe methods are reviewed. Determination of material parameters by broadband surface acoustic waves (SAWs) and other applications in nondestructive evaluation (NDE) are considered. The realization of nonlinear SAWs in the form of solitary waves and as shock waves, used for the determination of the fracture strength, is described. The unique properties of dispersion-free wedge waves (WWs) propagating along homogeneous wedges and of dispersive wedge waves observed in the presence of wedge modifications such as tip truncation or coatings are outlined. Theoretical and experimental results on nonlinear wedge waves in isotropic and anisotropic solids are presented.
Schmidt, J L; Tweten, D J; Benegal, A N; Walker, C H; Portnoi, T E; Okamoto, R J; Garbow, J R; Bayly, P V
2016-05-01
Mechanical anisotropy is an important property of fibrous tissues; for example, the anisotropic mechanical properties of brain white matter may play a key role in the mechanics of traumatic brain injury (TBI). The simplest anisotropic material model for small deformations of soft tissue is a nearly incompressible, transversely isotropic (ITI) material characterized by three parameters: minimum shear modulus (µ), shear anisotropy (ϕ=µ1µ-1) and tensile anisotropy (ζ=E1E2-1). These parameters can be determined using magnetic resonance elastography (MRE) to visualize shear waves, if the angle between the shear-wave propagation direction and fiber direction is known. Most MRE studies assume isotropic material models with a single shear (µ) or tensile (E) modulus. In this study, two types of shear waves, "fast" and "slow", were analyzed for a given propagation direction to estimate anisotropic parameters µ, ϕ, and ζ in two fibrous soft materials: turkey breast ex vivo and aligned fibrin gels. As expected, the speed of slow shear waves depended on the angle between fiber direction and propagation direction. Fast shear waves were observed when the deformations due to wave motion induced stretch in the fiber direction. Finally, MRE estimates of anisotropic mechanical properties in turkey breast were compared to estimates from direct mechanical tests. PMID:26920505
Soloway, Alexander G; Dahl, Peter H; Odom, Robert I
2015-10-01
Experimental measurements of Scholte waves from underwater explosions collected off the coast of Virginia Beach, VA in shallow water are presented. It is shown here that the dispersion of these explosion-generated Scholte waves traveling in the sandy seabed can be modeled using a power-law dependent shear wave speed profile and an empirical source model that determines the pressure time-series at 1 m from the source as a function of TNT-equivalent charge weight.
Finite-amplitude waves in inviscid shear flows
NASA Astrophysics Data System (ADS)
Moore, D. W.; Saffman, P. G.
1982-08-01
This paper examines the existence and properties of steady finite-amplitude waves of cats-eye form superposed on a unidirectional inviscid, incompressible shear flow. The problem is formulated as the solution of nonlinear Poisson equations for the stream function with boundary conditions on the unknown edges of the cats-eyes. The dependence of vorticity on stream function is assumed outside the cats-eyes to be as in the undisturbed flow, and uniform unknown vorticity is assumed inside. It is argued on the basis of a finite difference discretization that the problem is determinate, and numerical solutions are obtained for Couette-Poiseuille channel flow. These are compared with the predictions of a weakly nonlinear theory based on the approach of Benney and Bergeron (1969) and Davis (1969). The phase speed of the waves is found to be linear in the wave amplitude.
Single-sided Marchenko focusing of compressional and shear waves.
Wapenaar, Kees
2014-12-01
In time-reversal acoustics, waves recorded at the boundary of a strongly scattering medium are sent back into the medium to focus at the original source position. This requires that the medium can be accessed from all sides. We discuss a focusing method for media that can be accessed from one side only. We show how complex focusing functions, emitted from the top surface into the medium, cause independent foci for compressional and shear waves. The focused fields are isotropic and act as independent virtual sources for these wave types inside the medium. We foresee important applications in nondestructive testing of construction materials and seismological monitoring of processes inside the Earth. PMID:25615213
NASA Astrophysics Data System (ADS)
Helfenstein-Didier, C.; Andrade, R. J.; Brum, J.; Hug, F.; Tanter, M.; Nordez, A.; Gennisson, J.-L.
2016-03-01
The shear wave velocity dispersion was analyzed in the Achilles tendon (AT) during passive dorsiflexion using a phase velocity method in order to obtain the tendon shear modulus (C 55). Based on this analysis, the aims of the present study were (i) to assess the reproducibility of the shear modulus for different ankle angles, (ii) to assess the effect of the probe locations, and (iii) to compare results with elasticity values obtained with the supersonic shear imaging (SSI) technique. The AT shear modulus (C 55) consistently increased with the ankle dorsiflexion (N = 10, p < 0.05). Furthermore, the technique showed a very good reproducibility (all standard error of the mean values <10.7 kPa and all coefficient of variation (CV) values ⩽0.05%). In addition, independently from the ankle dorsiflexion, the shear modulus was significantly higher in the proximal location compared to the more distal one. The shear modulus provided by SSI was always lower than C55 and the difference increased with the ankle dorsiflexion. However, shear modulus values provided by both methods were highly correlated (R = 0.84), indicating that the conventional shear wave elastography technique (SSI technique) can be used to compare tendon mechanical properties across populations. Future studies should determine the clinical relevance of the shear wave dispersion analysis, for instance in the case of tendinopathy or tendon tear.
NASA Astrophysics Data System (ADS)
Song, Shaozhen; Joy, Joyce; Wang, Ruikang K.; Huang, Zhihong
2015-03-01
A quantitative measurement of the mechanical properties of biological tissue is a useful assessment of its physiologic conditions, which may aid medical diagnosis and treatment of, e.g., scleroderma and skin cancer. Traditional elastography techniques such as magnetic resonance elastography and ultrasound elastography have limited scope of application on skin due to insufficient spatial resolution. Recently, dynamic / transient elastography are attracting more applications with the advantage of non-destructive measurements, and revealing the absolute moduli values of tissue mechanical properties. Shear wave optical coherence elastography (SW-OCE) is a novel transient elastography method, which lays emphasis on the propagation of dynamic mechanical waves. In this study, high speed shear wave imaging technique was applied to a range of soft-embalmed mouse skin, where 3 kHz shear waves were launched with a piezoelectric actuator as an external excitation. The shear wave velocity was estimated from the shear wave images, and used to recover a shear modulus map in the same OCT imaging range. Results revealed significant difference in shear modulus and structure in compliance with gender, and images on fresh mouse skin are also compared. Thiel embalming technique is also proven to present the ability to furthest preserve the mechanical property of biological tissue. The experiment results suggest that SW-OCE is an effective technique for quantitative estimation of skin tissue biomechanical status.
Shear wave induced resonance elastography of spherical masses with polarized torsional waves
NASA Astrophysics Data System (ADS)
Hadj Henni, Anis; Schmitt, Cédric; Trop, Isabelle; Cloutier, Guy
2012-03-01
Shear wave induced resonance (SWIR) is a technique for dynamic ultrasound elastography of confined mechanical inclusions. It was developed for breast tumor imaging and tissue characterization. This method relies on the polarization of torsional shear waves modeled with the Helmholtz equation in spherical coordinates. To validate modeling, an invitro set-up was used to measure and image the first three eigenfrequencies and eigenmodes of a soft sphere. A preliminary invivo SWIR measurement on a breast fibroadenoma is also reported. Results revealed the potential of SWIR elastography to detect and mechanically characterize breast lesions for early cancer detection.
ML shear wave velocity tomography for the Iranian Plateau
NASA Astrophysics Data System (ADS)
Maheri-Peyrov, Mehdi; Ghods, Abdolreza; Abbasi, Madjid; Bergman, Eric; Sobouti, Farhad
2016-04-01
Iranian Plateau reflects several different tectonic styles of collision, and large-scale strike-slip faults. We calculate a high-resolution 2-D ML shear velocity map for the Iranian Plateau to detect lateral crustal thickness changes associated with different tectonic boundaries. The ML velocity is very sensitive to strong lateral variations of crustal thickness and varies between the velocity of Lg and Sn phases. Our data set consists of 65 795 ML amplitude velocity measurements from 2531 precisely relocated events recorded by Iranian networks in the period 1996-2014. Using a constrained least-squares inversion scheme, we inverted the ML velocities for a 2-D shear velocity map of Iran. Our results show that the Zagros and South Caspian Basin (SCB) have shear wave velocities close to the Sn phase, and are thus Lg-blocking regions. High velocities in the High Zagros and the Simply Folded Belt imply significant crustal undulations within these zones. We note that in the central and south Zagros, the velocity border between the Zagros and central Iran is not coincident with the Zagros suture line that marks underthrusting of the Arabian plate beneath central Iran. The low plains of Gilan and Gorgan to the south of the Caspian Sea show high shear velocities similar to the SCB, implying that they are either underlain by an oceanic type crust or a transitional crust with a strong lateral crustal thickness gradient. The Lut block is an Lg-passing block implying that it is not surrounded by any sudden crustal thickness changes along its borders with central Iran. In the Alborz, NW Iran, Kopeh-Dagh, Binalud and most of the central Iran, low shear velocity near the Lg velocity is attributed to smooth or minor Moho undulations within these regions.
Error in Estimates of Tissue Material Properties from Shear Wave Dispersion Ultrasound Vibrometry
Urban, Matthew W.; Chen, Shigao; Greenleaf, James F.
2009-01-01
Shear wave velocity measurements are used in elasticity imaging to find the shear elasticity and viscosity of tissue. A technique called shear wave dispersion ultrasound vibrometry (SDUV) has been introduced to use the dispersive nature of shear wave velocity to locally estimate the material properties of tissue. Shear waves are created using a multifrequency ultrasound radiation force, and the propagating shear waves are measured a few millimeters away from the excitation point. The shear wave velocity is measured using a repetitive pulse-echo method and Kalman filtering to find the phase of the harmonic shear wave at 2 different locations. A viscoelastic Voigt model and the shear wave velocity measurements at different frequencies are used to find the shear elasticity (μ1) and viscosity (μ2) of the tissue. The purpose of this paper is to report the accuracy of the SDUV method over a range of different values of μ1 and μ2. A motion detection model of a vibrating scattering medium was used to analyze measurement errors of vibration phase in a scattering medium. To assess the accuracy of the SDUV method, we modeled the effects of phase errors on estimates of shear wave velocity and material properties while varying parameters such as shear stiffness and viscosity, shear wave amplitude, the distance between shear wave measurements (Δr), signal-to-noise ratio (SNR) of the ultrasound pulse-echo method, and the frequency range of the measurements. We performed an experiment in a section of porcine muscle to evaluate variation of the aforementioned parameters on the estimated shear wave velocity and material property measurements and to validate the error prediction model. The model showed that errors in the shear wave velocity and material property estimates were minimized by maximizing shear wave amplitude, pulse-echo SNR, Δr, and the bandwidth used for shear wave measurements. The experimental model showed optimum performance could be obtained for Δr = 3-6 mm
Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves
Xia, J.; Miller, R.D.; Park, C.B.
1999-01-01
The shear-wave (S-wave) velocity of near-surface materials (soil, rocks, pavement) and its effect on seismic-wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Rayleigh-wave phase velocity of a layered-earth model is a function of frequency and four groups of earth properties: P-wave velocity, S-wave velocity, density, and thickness of layers. Analysis of the Jacobian matrix provides a measure of dispersion-curve sensitivity to earth properties. S-wave velocities are the dominant influence on a dispersion curve in a high-frequency range (>5 Hz) followed by layer thickness. An iterative solution technique to the weighted equation proved very effective in the high-frequency range when using the Levenberg-Marquardt and singular-value decomposition techniques. Convergence of the weighted solution is guaranteed through selection of the damping factor using the Levenberg-Marquardt method. Synthetic examples demonstrated calculation efficiency and stability of inverse procedures. We verify our method using borehole S-wave velocity measurements.Iterative solutions to the weighted equation by the Levenberg-Marquardt and singular-value decomposition techniques are derived to estimate near-surface shear-wave velocity. Synthetic and real examples demonstrate the calculation efficiency and stability of the inverse procedure. The inverse results of the real example are verified by borehole S-wave velocity measurements.
Nonlinear physics of shear Alfvén waves
NASA Astrophysics Data System (ADS)
Zonca, Fulvio; Chen, Liu
2014-02-01
Shear Alfvén waves (SAW) play fundamental roles in thermonuclear plasmas of fusion interest, since they are readily excited by energetic particles in the MeV range as well as by the thermal plasma components. Thus, understanding fluctuation induced transport in burning plasmas requires understanding nonlinear SAW physics. There exist two possible routes to nonlinear SAW physics: (i) wave-wave interactions and the resultant spectral energy transfer; (ii) nonlinear wave-particle interactions of SAW instabilities with energetic particles. Within the first route, it is advantageous to understand and describe nonlinear processes in term of proximity of the system to the Alfvénic state, where wave-wave interactions are minimized due to the cancellation of Reynolds and Maxwell stresses. Here, various wave-wave nonlinear dynamics are elucidated in terms of how they break the Alfvénic state. In particular, we discuss the qualitative and quantitative modification of the SAW parametric decay process due to finite ion compressibility and finite ion Larmor radius. We also show that toroidal geometry plays a crucial role in the nonlinear excitation of zonal structures by Alfvén eigenmodes. Within the second route, the coherent nonlinear dynamics of structures in the energetic particle phase space, by which secular resonant particle transport can occur on meso- and macro-scales, must be addressed and understood. These "nonlinear equilibria" or "phase-space zonal structures" dynamically evolve on characteristic (fluctuation induced) turbulent transport time scales, which are generally of the same order of the nonlinear time scale of the underlying fluctuations. In this work, we introduce the general structure of nonlinear Schrödinger equations with complex integro-differential nonlinear terms, which govern these physical processes. To elucidate all these aspects, theoretical analyses are presented together with numerical simulation results.
Nonlinear physics of shear Alfvén waves
Zonca, Fulvio; Chen, Liu
2014-02-12
Shear Alfvén waves (SAW) play fundamental roles in thermonuclear plasmas of fusion interest, since they are readily excited by energetic particles in the MeV range as well as by the thermal plasma components. Thus, understanding fluctuation induced transport in burning plasmas requires understanding nonlinear SAW physics. There exist two possible routes to nonlinear SAW physics: (i) wave-wave interactions and the resultant spectral energy transfer; (ii) nonlinear wave-particle interactions of SAW instabilities with energetic particles. Within the first route, it is advantageous to understand and describe nonlinear processes in term of proximity of the system to the Alfvénic state, where wave-wave interactions are minimized due to the cancellation of Reynolds and Maxwell stresses. Here, various wave-wave nonlinear dynamics are elucidated in terms of how they break the Alfvénic state. In particular, we discuss the qualitative and quantitative modification of the SAW parametric decay process due to finite ion compressibility and finite ion Larmor radius. We also show that toroidal geometry plays a crucial role in the nonlinear excitation of zonal structures by Alfvén eigenmodes. Within the second route, the coherent nonlinear dynamics of structures in the energetic particle phase space, by which secular resonant particle transport can occur on meso- and macro-scales, must be addressed and understood. These 'nonlinear equilibria' or 'phase-space zonal structures' dynamically evolve on characteristic (fluctuation induced) turbulent transport time scales, which are generally of the same order of the nonlinear time scale of the underlying fluctuations. In this work, we introduce the general structure of nonlinear Schrödinger equations with complex integro-differential nonlinear terms, which govern these physical processes. To elucidate all these aspects, theoretical analyses are presented together with numerical simulation results.
Transition from 1D to 2D Laser-Induced Ultrasonic Wave Propagation in an Extended Plate
NASA Astrophysics Data System (ADS)
Laloš, Jernej; Požar, Tomaž; Možina, Janez
2016-05-01
Optodynamic interaction between a laser pulse and the surface of an opaque, solid elastic object produces transient waves that propagate and reverberate within the object. They can be, in general, categorized into three distinctive types which are all formed through different mechanisms: ablation-induced waves, light-pressure-induced waves, and thermoelastic waves. In this paper, out-of-plane displacements of such waves are simulated at the epicentral position on the opposite side of an extended plane-parallel elastic plate. Wave propagation is mathematically described by Green's transfer functions convolved with suitable time profiles of the incoming laser pulses. The simulated size of the circularly symmetric laser-illuminated area on the plate surface is varied to show the limit-to-limit transition of the displacement waveforms: from a 2D point source to an infinite 1D source.
Explicit wave action conservation for water waves on vertically sheared flows
NASA Astrophysics Data System (ADS)
Quinn, Brenda; Toledo, Yaron; Shrira, Victor
2016-04-01
Water waves almost always propagate on currents with a vertical structure such as currents directed towards the beach accompanied by an under-current directed back toward the deep sea or wind-induced currents which change magnitude with depth due to viscosity effects. On larger scales they also change their direction due to the Coriolis force as described by the Ekman spiral. This implies that the existing wave models, which assume vertically-averaged currents, is an approximation which is far from realistic. In recent years, ocean circulation models have significantly improved with the capability to model vertically-sheared current profiles in contrast with the earlier vertically-averaged current profiles. Further advancements have coupled wave action models to circulation models to relate the mutual effects between the two types of motion. Restricting wave models to vertically-averaged non-turbulent current profiles is obviously problematic in these cases and the primary goal of this work is to derive and examine a general wave action equation which accounts for these shortcoming. The formulation of the wave action conservation equation is made explicit by following the work of Voronovich (1976) and using known asymptotic solutions of the boundary value problem which exploit the smallness of the current magnitude compared to the wave phase velocity and/or its vertical shear and curvature. The adopted approximations are shown to be sufficient for most of the conceivable applications. This provides correction terms to the group velocity and wave action definition accounting for the shear effects, which are fitting for application to operational wave models. In the limit of vanishing current shear, the new formulation reduces to the commonly used Bretherton & Garrett (1968) no-shear wave action equation where the invariant is calculated with the current magnitude taken at the free surface. It is shown that in realistic oceanic conditions, the neglect of the vertical
Berryman, J G
2004-02-24
Layered earth models are well justified by experience, and provide a simple means of studying fairly general behavior of the elastic and poroelastic characteristics of seismic waves in the earth. Thomsen's anisotropy parameters for weak elastic and poroelastic anisotropy are now commonly used in exploration, and can be conveniently expressed in terms of the layer averages of Backus. Since our main interest is usually in the fluids underground, it would be helpful to have a set of general equations relating the Thomsen parameters as directly as possible to the fluid properties. This end can be achieved in a rather straightforward fashion for these layered earth models, and the present paper develops and then discusses these relations. Furthermore, it is found that, although there are five effective shear moduli for any layered VTI medium, one and only one effective shear modulus for the layered system contains all the dependence of pore fluids on the elastic or poroelastic constants that can be observed in vertically polarized shear waves in VTI media. The effects of the pore fluids on this effective shear modulus can be substantial - an increase of shear wave speed on the order of 10% is shown to be possible when circumstances are favorable -when the medium behaves in an undrained fashion, and the shear modulus fluctuations are large (resulting in strong anisotropy). These effects are expected to be seen at higher frequencies such as sonic and ultrasonic waves for well-logging or laboratory experiments, or at seismic wave frequencies for low permeability regions of reservoirs, prior to hydrofracing. Results presented are strictly for velocity analysis.
The upper mantle shear wave velocity structure of East Africa derived from Rayleigh wave tomography
NASA Astrophysics Data System (ADS)
O'Donnell, J.; Nyblade, A.; Adams, A. N.; Weeraratne, D. S.; Mulibo, G.; Tugume, F.
2012-12-01
An expanded model of the three-dimensional shear wave velocity structure of the upper mantle beneath East Africa has been developed using data from the latest phases of the AfricaArray East African Seismic Experiment in conjunction with data from preceding studies. The combined dataset consists of 331 events recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this latest study, 149 events were used to determine fundamental mode Rayleigh wave phase velocities at periods ranging from 20 to 182 seconds using the two-plane-wave method. These were subsequently combined with the similarly processed published measurements and inverted for an updated upper mantle three-dimensional shear wave velocity model. Newly imaged features include a substantial fast anomaly in eastern Zambia that may have exerted a controlling influence on the evolution of the Western Rift Branch. Furthermore, there is a suggestion that the Eastern Rift Branch trends southeastward offshore eastern Tanzania.
An experimental phantom study on the effect of calcifications on ultrasound shear wave elastography.
Gregory, Adriana; Bayat, Mahdi; Denis, Max; Mehrmohammadi, Mohammad; Fatemi, Mostafa; Alizad, Azra
2015-08-01
In this study, we investigated the effects of single macrocalcifications and groups of microcalcifications on shear wave elastography. Supersonic shear imaging (SSI) and comb-push ultrasound shear elastography (CUSE) were performed on three sets of phantoms to investigate how calcifications of different sizes and distributions influence measured elasticity. Our results demonstrate that the presence of large isolated macrocalcifications and highly concentrated clusters of microcalcifications can introduce areas with apparent high elasticity when they are evaluated by shear wave elastography. PMID:26737132
Shear Wave Splitting Observations Beneath Uturuncu Volcano, Bolivia
NASA Astrophysics Data System (ADS)
Sims, N. E.; Christensen, D. H.; Moore-Driskell, M. M.
2015-12-01
Anisotropy in the upper mantle is often associated with mantle flow direction through the lattice preferred orientation of anisotropic minerals such as olivine in the upper mantle material. The flow of the mantle around subduction zones can be particularly complex, and thus difficult to explain. Because of its relationship to anisotropy, analysis of shear wave splitting measurements can help to answer questions regarding the upper mantle flow that surrounds subducting slabs. Here we present SK(K)S shear wave splitting measurements from a temporary broadband network (PLUTONS) of 33 stations deployed from April 2009 to October 2012 on the Altiplano plateau around Uturuncu volcano in Bolivia. The stations are spaced 10-20 km apart, providing a high spatial resolution of the region of the mantle directly below Uturuncu volcano. Despite the lack of numerous splitting results to analyze, preliminary measurements indicate a relatively consistent pattern of fast-polarization directions in a NW-SE orientation of about N80ºW. We think that it is likely that these observations come from anisotropy in the mantle wedge above the subducting Nazca plate indicating a direction of flow in the mantle wedge that is sub-parallel to the subduction direction of the Nazca plate. Although W-E flow beneath the subducting Nazca plate cannot be completely ruled out, these results appear to be consistent with the simple model of two-dimensional corner flow in the mantle wedge and slab-entrained mantle flow beneath the slab.
Utility of Shear Wave Elastography for Diagnosing Chronic Autoimmune Thyroiditis.
Fukuhara, Takahiro; Matsuda, Eriko; Izawa, Shoichiro; Fujiwara, Kazunori; Kitano, Hiroya
2015-01-01
The aims of this study were to evaluate the utility of shear wave elastography (SWE) using acoustic radiation force impulse (ARFI) for diagnosing chronic autoimmune thyroiditis (CAT) and to verify the effect of fibrotic thyroid tissue on shear wave velocity (SWV). The subjects were 229 patients with 253 normal thyroid lobes (controls) and 150 CAT lobes. The SWV for CAT (2.47 ± 0.57 m/s) was significantly higher than that for controls (1.59 ± 0.41 m/s) (P < 0.001). The area under the receiver operating characteristics (ROC) curve for CAT was 0.899, and the SWV cut-off value was 1.96 m/s. The sensitivity, specificity, and diagnostic accuracy were 87.4%, 78.7%, and 85.1%, respectively. Levels of anti-thyroperoxidase antibodies and thyroid isthmus thickness were correlated with tissue stiffness in CAT. However, there was no correlation between levels of anti-thyroglobulin antibodies and tissue stiffness. Quantitative SWE is useful for diagnosing CAT, and it is possible that SWE can be used to evaluate the degree of fibrosis in patients with CAT. PMID:26257979
Shear Wave Speed Estimation in the Human Uterine Cervix
Carlson, Lindsey C.; Feltovich, Helen; Palmeri, Mark L.; Dahl, Jeremy J.; del Rio, Alejandro Munoz; Hall, Timothy J.
2014-01-01
Objectives Our goals were to explore the spatial variability within the cervix and the sensitivity of shear wave speeds (SWS) to assess softness/stiffness differences in ripened (softened) versus unripened tissue. Methods We obtained SWS estimates from hysterectomy specimens (n=22), a subset of which were ripened (n = 13). Multiple measurements were made longitudinally along the cervical canal on both the anterior and posterior sides of the cervix. Statistical tests of differences in the proximal vs. distal, anterior vs. posterior, and ripened vs. unripened cervix were performed with individual two-sample t-tests and a linear mixed model. Results We discovered that SWS estimates monotonically increase from distal to proximal longitudinally along the cervix, that they also vary in the anterior compared to the posterior cervix, and that they are significantly different in ripened vs. unripened cervical tissue. Specifically, the mid position SWS estimates for the unripened group were 3.45±0.95 m/s (anterior) and 3.56±0.92 m/s (posterior), and 2.11±0.45 m/s (anterior) and 2.68±0.57 m/s (posterior) for the ripened (p<0.001). Conclusions We propose that shear wave speed estimation may be a valuable research and, ultimately, diagnostic tool for objective quantification of cervical stiffness/softness. PMID:23836486
Downhole electro-hydraulic vertical shear wave seismic source
Cole, J.H.
1993-07-20
A downhole electro-hydraulic vertical shear wave seismic source to be lowered into a wellbore is described comprising: a source cylindrical housing; a reaction mass means for generating seismic shear waves, said reaction mass means having an actuator with an actuator piston and actuator cylinder and located internal to said source cylindrical housing to isolate said actuator from wellbore fluid and pressure, said reaction mass including transversely formed holes through which hydraulic cylinders connected to contact pads pass, said holes having a significantly larger diameter than said hydraulic cylinders; a clamping means to clamp said source cylindrical housing to the wellbore, said clamping means including two serrated pads radiused to match an inside diameter of casing located in said wellbore and hydraulic cylinders having internal compact stacks of spring washers for retraction for actuating said serrated pads; a compact and soft urethane spring for suspending said reaction mass; and a threaded guide rod passing vertically through said urethane spring to allow spring compression to be adjusted until said actuator piston is precisely centered with no differential hydraulic pressure across said actuator piston.
Pengfei Song; Heng Zhao; Urban, Matthew W; Manduca, Armando; Pislaru, Sorin V; Kinnick, Randall R; Pislaru, Cristina; Greenleaf, James F; Shigao Chen
2013-12-01
Ultrasound tissue harmonic imaging is widely used to improve ultrasound B-mode imaging quality thanks to its effectiveness in suppressing imaging artifacts associated with ultrasound reverberation, phase aberration, and clutter noise. In ultrasound shear wave elastography (SWE), because the shear wave motion signal is extracted from the ultrasound signal, these noise sources can significantly deteriorate the shear wave motion tracking process and consequently result in noisy and biased shear wave motion detection. This situation is exacerbated in in vivo SWE applications such as heart, liver, and kidney. This paper, therefore, investigated the possibility of implementing harmonic imaging, specifically pulse-inversion harmonic imaging, in shear wave tracking, with the hypothesis that harmonic imaging can improve shear wave motion detection based on the same principles that apply to general harmonic B-mode imaging. We first designed an experiment with a gelatin phantom covered by an excised piece of pork belly and show that harmonic imaging can significantly improve shear wave motion detection by producing less underestimated shear wave motion and more consistent shear wave speed measurements than fundamental imaging. Then, a transthoracic heart experiment on a freshly sacrificed pig showed that harmonic imaging could robustly track the shear wave motion and give consistent shear wave speed measurements of the left ventricular myocardium while fundamental imaging could not. Finally, an in vivo transthoracic study of seven healthy volunteers showed that the proposed harmonic imaging tracking sequence could provide consistent estimates of the left ventricular myocardium stiffness in end-diastole with a general success rate of 80% and a success rate of 93.3% when excluding the subject with Body Mass Index higher than 25. These promising results indicate that pulse-inversion harmonic imaging can significantly improve shear wave motion tracking and thus potentially
A matrix projection method for on line stable estimation of 1D and 3D shear building models
NASA Astrophysics Data System (ADS)
Angel García-Illescas, Miguel; Alvarez-Icaza, Luis
2016-12-01
An estimation method is presented that combines the use of recursive least squares, a matrix parameterized model, Gershgorin circles and tridiagonal matrices properties to allow the identification of stable shear building models in the presence of low excitation or low damping. The resultant scheme yields a significant reduction on the number of calculations involved, when compared with the standard vector parameterization based schemes. As real buildings are always open loop stable, the use of an stable shear building model for vibration control purposes allows the design of more robust control laws. Extensive simulation results are presented for cases of low excitation comparing the results of using or not this matrix projection method with different sets of initial conditions. Results indicate that the use of this projection method does not have an influence in the recovery of natural frequencies, however, it significantly improves the recovery of mode shapes.
Ouared, Abderrahmane; Montagnon, Emmanuel; Cloutier, Guy
2015-10-21
A method based on adaptive torsional shear waves (ATSW) is proposed to overcome the strong attenuation of shear waves generated by a radiation force in dynamic elastography. During the inward propagation of ATSW, the magnitude of displacements is enhanced due to the convergence of shear waves and constructive interferences. The proposed method consists in generating ATSW fields from the combination of quasi-plane shear wavefronts by considering a linear superposition of displacement maps. Adaptive torsional shear waves were experimentally generated in homogeneous and heterogeneous tissue mimicking phantoms, and compared to quasi-plane shear wave propagations. Results demonstrated that displacement magnitudes by ATSW could be up to 3 times higher than those obtained with quasi-plane shear waves, that the variability of shear wave speeds was reduced, and that the signal-to-noise ratio of displacements was improved. It was also observed that ATSW could cause mechanical inclusions to resonate in heterogeneous phantoms, which further increased the displacement contrast between the inclusion and the surrounding medium. This method opens a way for the development of new noninvasive tissue characterization strategies based on ATSW in the framework of our previously reported shear wave induced resonance elastography (SWIRE) method proposed for breast cancer diagnosis.
NASA Astrophysics Data System (ADS)
Ouared, Abderrahmane; Montagnon, Emmanuel; Cloutier, Guy
2015-10-01
A method based on adaptive torsional shear waves (ATSW) is proposed to overcome the strong attenuation of shear waves generated by a radiation force in dynamic elastography. During the inward propagation of ATSW, the magnitude of displacements is enhanced due to the convergence of shear waves and constructive interferences. The proposed method consists in generating ATSW fields from the combination of quasi-plane shear wavefronts by considering a linear superposition of displacement maps. Adaptive torsional shear waves were experimentally generated in homogeneous and heterogeneous tissue mimicking phantoms, and compared to quasi-plane shear wave propagations. Results demonstrated that displacement magnitudes by ATSW could be up to 3 times higher than those obtained with quasi-plane shear waves, that the variability of shear wave speeds was reduced, and that the signal-to-noise ratio of displacements was improved. It was also observed that ATSW could cause mechanical inclusions to resonate in heterogeneous phantoms, which further increased the displacement contrast between the inclusion and the surrounding medium. This method opens a way for the development of new noninvasive tissue characterization strategies based on ATSW in the framework of our previously reported shear wave induced resonance elastography (SWIRE) method proposed for breast cancer diagnosis.
Wang, Michael; Byram, Brett; Palmeri, Mark; Rouze, Ned; Nightingale, Kathryn
2013-04-01
A system capable of tracking radiation-force-induced shear wave propagation in a 3-D volume using ultrasound is presented. In contrast to existing systems, which use 1-D array transducers, a 2-D matrix array is used for tracking shear wave displacements. A separate single-element transducer is used for radiation force excitation. This system allows shear wave propagation in all directions away from the push to be observed. It is shown that for a limit of 64 tracking beams, by placing the beams at the edges of the measurement region of interest (ROI) at multiple directions from the push, time-of- flight (TOF) shear wave speed (SWS) measurement uncertainty can theoretically be reduced by 40% compared with equally spacing the tracking beams within the ROI along a single plane, as is typical when using a 1-D array for tracking. This was verified by simulation, and a reduction of 30% was experimentally observed on a homogeneous phantom. Analytical expressions are presented for the relationship between TOF SWS measurement uncertainty and various shear wave imaging parameters. It is shown that TOF SWS uncertainty is inversely proportional to ROI size, and inversely proportional to the square root of the number of tracking locations for a given distribution of beam locations relative to the push. TOF SWS uncertainty is shown to increase with the square of the SWS, indicating that TOF SWS measurements are intrinsically less precise for stiffer materials.
Shear Wave Generation by Explosions in Anisotropic Crystalline Rock
NASA Astrophysics Data System (ADS)
Rogers-Martinez, M. A.; Sammis, C. G.; Stroujkova, A. F.
2015-12-01
The use of seismic waves to discriminate between earthquakes and underground explosions is complicated by the observation that explosions routinely radiate strong S waves. Whether these S waves are primarily generated by non-linear processes at the source, or by mode conversions and scattering along the path remains an open question. It has been demonstrated that S waves are generated at the source by any mechanism that breaks the spherical symmetry of the explosion. Examples of such mechanisms include tectonic shear stress, spall, and anisotropy in the emplacement medium. Many crystalline rock massifs are transversely isotropic because they contain aligned fractures over a range of scales from microfractures at the grain scale (called the rift) to regional sets of joints. In this study we use a micromechanical damage mechanics to model the fracture damage patterns and seismic radiation generated by explosions in a material in which the initial distribution of fractures has a preferred direction. Our simulations are compared with a set of field experiments in a granite quarry in Barre, VT conducted by New England Research and Weston Geophysical. Barre granite has a strong rift plane of aligned microfractures. Our model captures two important results of these field studies: 1) the spatial extent of rock fracture and generation of S waves depends on the burn-rate of the explosion and 2) the resultant damage is anisotropic with most damage occurring in the preferred direction of the microfractures (the rift plane in the granite). The physical reason damage is enhanced in the rift direction is that the mode I stress intensity factor is large for each fracture in the array of parallel fractures in the rift plane. Tensile opening on the rift plane plus sliding on the preexisting fractures make strong non-spherical contributions to the moment tensor in the far-field.
Shear Wave Splitting Analysis of Aftershocks of the 2013 Mw6.6 Lushan Earthquake, China
NASA Astrophysics Data System (ADS)
Liu, Y.; Zhang, H.
2013-12-01
Shear wave splits into faster and slower shear waves that are nearly perpendicular when it travels through an anisotropic medium. There are two important parameters of shear wave splitting, one is the fast polarization direction of the fast shear wave and the other one is the time delay of the slow shear wave. The mechanisms for anisotropy in the upper crust can be divided into two categories. The first category is stress-induced anisotropy related to alignment of cracks in response to the in situ stress field. The second category is structural anisotropy associated with aligned planar features such as fault zone fabrics, sedimentary bedding planes and aligned minerals. We can characterize anisotropy around fault zone by shear wave splitting analysis. We used cross-correlation method for the shear wave splitting analysis. Since the faster shear wave and the slower shear wave are from the same source, they will correlate well after the time delay correction. We rotated two horizontal seismograms at a 10 increment of azimuth α from 00 to 1800. For each α, the cross-correlation coefficients between the two orthogonal seismograms are calculated for a range of time delays τ. When the absolute value of cross-correlation coefficient reaches a maximum, the corresponding values of α and τ are chosen as the fast polarization direction of the faster shear wave and the time delay of the slower shear wave, respectively. We chose 200 aftershocks observed at a temporary array consisting of 29 stations in the Lushan region. Shear wave arrivals were first picked for setting up the time window for the shear wave splitting analysis using the cross-correlation method. Because these 200 events are shallower than 20km, we can infer that the shear wave splitting is caused by crustal anisotropy. The rose diagram of the fast polarization directions of the fast shear waves showed two major directions. One is nearly parallel to the south-north trending fault system in this region, and
Shear wave velocities of unconsolidated shallow sediments in the Gulf of Mexico
Lee, Myung W.
2013-01-01
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.
Drift-wave transport in the velocity shear layer
NASA Astrophysics Data System (ADS)
Rosalem, K. C.; Roberto, M.; Caldas, I. L.
2016-07-01
Particle drift driven by electrostatic wave fluctuations is numerically computed to describe the transport in a gradient velocity layer at the tokamak plasma edge. We consider an equilibrium plasma in large aspect ratio approximation with E × B flow and specified toroidal plasma velocity, electric field, and magnetic field profiles. A symplectic map, previously derived for infinite coherent time modes, is used to describe the transport dependence on the electric, magnetic, and plasma velocity shears. We also show that resonant perturbations and their correspondent islands in the Poincaré maps are much affected by the toroidal velocity profiles. Moreover, shearless transport barriers, identified by extremum values of the perturbed rotation number profiles of the invariant curves, allow chaotic trajectories trapped into the plasma. We investigate the influence of the toroidal plasma velocity profile on these shearless transport barriers.
Gamma-ray bursts from sheared Alfven waves
NASA Technical Reports Server (NTRS)
Melia, Fulvio; Fatuzzo, Marco
1991-01-01
The physical process by which sheared Alfven waves can accelerate electrons to a Lorentz factor of 10,000 to 100,000 within 5 km of the stellar surface is applied to a study of gamma-ray bursts, taking both resonant and nonresonant scattering into account. Several very encouraging features of the model are discussed. Although the field is oscillatory, virtually all the charges are ejected from the system, resulting in very little backheating of the stellar surface. The particle number density is accounted for naturally in terms of BA0 and m, which in principle are known from the physical manifestation of the agent causing the crustal disturbance. The resulting gamma-ray spectrum compares very favorably with the observation. The model restricts the geometry of the emission region, in the sense that only the Compton upscattering of soft photons from a warm polar cap can produce the correct spectral shape.
[INVITED] Laser generation and detection of ultrafast shear acoustic waves in solids and liquids
NASA Astrophysics Data System (ADS)
Pezeril, Thomas
2016-09-01
The aim of this article is to provide an overview of the up-to-date findings related to ultrafast shear acoustic waves. Recent progress obtained for the laser generation and detection of picosecond shear acoustic waves in solids and liquids is reviewed. Examples in which the transverse isotropic symmetry of the sample structure is broken in order to permit shear acoustic wave generation through sudden laser heating are described in detail. Alternative photo-induced mechanisms for ultrafast shear acoustic generation in metals, semiconductors, insulators, magnetostrictive, piezoelectric and electrostrictive materials are reviewed as well. With reference to key experiments, an all-optical technique employed to probe longitudinal and shear structural dynamics in the GHz frequency range in ultra-thin liquid films is described. This technique, based on specific ultrafast shear acoustic transducers, has opened new perspectives that will be discussed for ultrafast shear acoustic probing of viscoelastic liquids at the nanometer scale.
Analysis shear wave velocity structure obtained from surface wave methods in Bornova, Izmir
NASA Astrophysics Data System (ADS)
Pamuk, Eren; Özdaǧ, Özkan Cevdet; Akgün, Mustafa
2016-04-01
Properties of the soil from the bedrock is necessary to describe accurately and reliably for the reduction of earthquake damage. Because seismic waves change their amplitude and frequency content owing to acoustic impedance difference between soil and bedrock. Firstly, shear wave velocity and depth information of layers on bedrock is needed to detect this changing. Shear wave velocity can be obtained using inversion of Rayleigh wave dispersion curves obtained from surface wave methods (MASW- the Multichannel Analysis of Surface Waves, ReMi-Refraction Microtremor, SPAC-Spatial Autocorrelation). While research depth is limeted in active source study, a passive source methods are utilized for deep depth which is not reached using active source methods. ReMi method is used to determine layer thickness and velocity up to 100 m using seismic refraction measurement systems.The research carried out up to desired depth depending on radius using SPAC which is utilized easily in conditions that district using of seismic studies in the city. Vs profiles which are required to calculate deformations in under static and dynamic loads can be obtained with high resolution using combining rayleigh wave dispersion curve obtained from active and passive source methods. In the this study, Surface waves data were collected using the measurements of MASW, ReMi and SPAC at the İzmir Bornova region. Dispersion curves obtained from surface wave methods were combined in wide frequency band and Vs-depth profiles were obtained using inversion. Reliability of the resulting soil profiles were provided by comparison with theoretical transfer function obtained from soil paremeters and observed soil transfer function from Nakamura technique and by examination of fitting between these functions. Vs values are changed between 200-830 m/s and engineering bedrock (Vs>760 m/s) depth is approximately 150 m.
On the interaction of deep water waves and exponential shear currents
NASA Astrophysics Data System (ADS)
Cheng, Jun; Cang, Jie; Liao, Shi-Jun
2009-05-01
A train of periodic deep-water waves propagating on a steady shear current with a vertical distribution of vorticity is investigated by an analytic method, namely the homotopy analysis method (HAM). The magnitude of the vorticity varies exponentially with the magnitude of the stream function, while remaining constant on a particular streamline. The so-called Dubreil-Jacotin transformation is used to transfer the original exponentially nonlinear boundary-value problem in an unknown domain into an algebraically nonlinear boundary-value problem in a known domain. Convergent series solutions are obtained not only for small amplitude water waves on a weak current but also for large amplitude waves on a strong current. The nonlinear wave-current interaction is studied in detail. It is found that an aiding shear current tends to enlarge the wave phase speed, sharpen the wave crest, but shorten the maximum wave height, while an opposing shear current has the opposite effect. Besides, the amplitude of waves and fluid velocity decay over the depth more quickly on an aiding shear current but more slowly on an opposing shear current than that of waves on still water. Furthermore, it is found that Stokes criteria of wave breaking is still valid for waves on a shear current: a train of propagating waves on a shear current breaks as the fiuid velocity at crest equals the wave phase speed. Especially, it is found that the highest waves on an opposing shear current are even higher and steeper than that of waves on still water. Mathematically, this analytic method is rather general in principle and can be employed to solve many types of nonlinear partial differential equations with variable coefficients in science, finance and engineering.
NASA Astrophysics Data System (ADS)
Xie, J.; Yang, Y.; Ni, S.; Zhao, K.
2015-12-01
In the past decade, ambient noise tomography (ANT) has become an estimated method to construct the earth's interior structures thanks to its advantage in extracting surface waves from cross-correlations of ambient noise without using earthquake data. However, most of previous ambient noise tomography studies concentrate on short and intermediate periods (<50sec) due to the dominant energy of the microseism at these periods. Studies of long period surface waves from cross-correlation of ambient noise are limited. In this study, we verify the accuracy of the long period (50-250sec) surface wave (Rayleigh wave) from ambient noise by comparing both dispersion curves and seismic waveforms from ambient noise with those from earthquake records quantitatively. After that, we calculate vertical-vertical cross-correlation functions among more than1800 USArray Transportable Array stations and extract high quality interstation phase velocity dispersion curves from them at 10-200 sec periods. Then, we adopt a finite frequency ambient noise tomography method based on Born approximation to obtain high resolution phase velocity maps using the obtained dispersion measurements at 10-150 sec periods. Afterward, we extract local dispersion curves from these dispersion maps and invert them for 1D shear wave velocity profiles at individual grids using a Bayesian Monte Carlo method. Finally, a 3D shear velocity model is constructed by assembling all the 1D Vs profiles. Our 3D model is overall similar to other models constructed using earthquake surface waves and body waves. In summary, we demonstrate that the long period surface waves can be extracted from ambient noise, and the long period dispersion measurements from ambient noise are as accurate as those from earthquake data and can be used to construct 3D lithospheric structure from surface down to lithosphere/asthenosphere depths.
Shear wave velocities from noise correlation at local scale
De Nisco, G.; Nunziata, C.; Vaccari, F.; Panza, G. F.
2008-07-08
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 method from the noise cross correlations and then the non linear inversion of them has resulted in Vs profiles with depth. The information of near by stratigraphies and the range of Vs variability for samples of Neapolitan soils and rocks confirms the validity of results obtained with our expeditious procedure. Moreover, the good comparison of noise H/V frequency of the first main peak with 1D and 2D spectral amplifications encourages to continue experiments of noise cross-correlation. If confirmed in other geological settings, the proposed approach could reveal a low cost methodology to obtain reliable and detailed Vs velocity profiles.
Modelling the impulse diffraction field of shear waves in transverse isotropic viscoelastic medium
NASA Astrophysics Data System (ADS)
Chatelin, Simon; Gennisson, Jean-Luc; Bernal, Miguel; Tanter, Mickael; Pernot, Mathieu
2015-05-01
The generation of shear waves from an ultrasound focused beam has been developed as a major concept for remote palpation using shear wave elastography (SWE). For muscular diagnostic applications, characteristics of the shear wave profile will strongly depend on characteristics of the transducer as well as the orientation of muscular fibers and the tissue viscoelastic properties. The numerical simulation of shear waves generated from a specific probe in an anisotropic viscoelastic medium is a key issue for further developments of SWE in fibrous soft tissues. In this study we propose a complete numerical tool allowing 3D simulation of a shear wave front in anisotropic viscoelastic media. From the description of an ultrasonic transducer, the shear wave source is simulated by using Field’s II software and shear wave propagation described by using the Green’s formalism. Finally, the comparison between simulations and experiments are successively performed for both shear wave velocity and dispersion profile in a transverse isotropic hydrogel phantom, in vivo forearm muscle and in vivo biceps brachii.
Modelling the impulse diffraction field of shear waves in transverse isotropic viscoelastic medium.
Chatelin, Simon; Gennisson, Jean-Luc; Bernal, Miguel; Tanter, Mickael; Pernot, Mathieu
2015-05-01
The generation of shear waves from an ultrasound focused beam has been developed as a major concept for remote palpation using shear wave elastography (SWE). For muscular diagnostic applications, characteristics of the shear wave profile will strongly depend on characteristics of the transducer as well as the orientation of muscular fibers and the tissue viscoelastic properties. The numerical simulation of shear waves generated from a specific probe in an anisotropic viscoelastic medium is a key issue for further developments of SWE in fibrous soft tissues. In this study we propose a complete numerical tool allowing 3D simulation of a shear wave front in anisotropic viscoelastic media. From the description of an ultrasonic transducer, the shear wave source is simulated by using Field's II software and shear wave propagation described by using the Green's formalism. Finally, the comparison between simulations and experiments are successively performed for both shear wave velocity and dispersion profile in a transverse isotropic hydrogel phantom, in vivo forearm muscle and in vivo biceps brachii.
A 1D exact treatment of shock waves within spectral methods in plane geometry
NASA Astrophysics Data System (ADS)
Bonazzola, Silvano; Marck, Jean-Alain
1991-12-01
A high-accuracy numerical technique is presented which employs two moving grids, respectively before and after the shock formation, to solve 1D Euler equations that are coupled with the transport equations for the entropy and the chemical abundances in cases with and without shock formation. Chebychev polynomial series are used to expand quantities on both sides of the matching point, and Rankine-Hugoniot conditions are used to ascertain the shock velocity after shock formation. Illustrative results are presented.
Modelling study of challenges in sinkhole detection with shear wave reflection seismics
NASA Astrophysics Data System (ADS)
Burschil, Thomas; Krawczyk, CharLotte M.
2016-04-01
The detection of cavities with reflection seismics is a difficult task even if high impedance contrasts are assumed. Especially the shear wave reflection method with a higher resolution potential trough lower velocities and short wavelength has come into focus of investigation. But shear wave propagation fails if material exists that partially has no shear strength. The shear wave does not propagate into or through those voids. Here, we evaluate the influence of a possible fracture zone above a cavity. We simulate shear wave propagation with finite difference modelling for two reference models, with and without cavity, and various sets of input models with a fracture zone above the cavity. Reflections and multiples of the reference models image the subsidence structure and the cavity. For the fracture input models, we implemented a fracture network, derived from numerical crack propagation modelling (Schneider-Löbens et al., 2015). The cracks possess the minimum possible aperture of one grid point (i.e. 0.1 m) and no shear stiffness. The seismic modelling exhibits that the shear wave does not pass through the fracture zone and shadows the subjacent cavity. Sequences of randomly discontinuous cracks, cf. displacement discontinuity model with zero crack stiffness, approximate partially seismic connected rock on both sides of the crack. The amount of these seismic pathways determines whether a reflection of the cavity can be detected at the surface or not. Cracks with higher aperture, e.g. two or three grid points, need a higher amount of intact rock/defective cracks, since more connected grid points are necessary to create seismic pathways. Furthermore, it turns out that the crack filling is important for shear wave transmission. While a mineralized fracture zone, implemented with high velocity, facilitate shear wave propagation, water or air-filled cracks avoid shear wave transmission. Crack orientation affects the shear wave propagation through the geometry. A
Estimation of viscoelastic parameters in Prony series from shear wave propagation
NASA Astrophysics Data System (ADS)
Jung, Jae-Wook; Hong, Jung-Wuk; Lee, Hyoung-Ki; Choi, Kiwan
2016-06-01
When acquiring accurate ultrasonic images, we must precisely estimate the mechanical properties of the soft tissue. This study investigates and estimates the viscoelastic properties of the tissue by analyzing shear waves generated through an acoustic radiation force. The shear waves are sourced from a localized pushing force acting for a certain duration, and the generated waves travel horizontally. The wave velocities depend on the mechanical properties of the tissue such as the shear modulus and viscoelastic properties; therefore, we can inversely calculate the properties of the tissue through parametric studies.
Fluid Effects on Shear for Seismic Waves in Finely Layered Porous Media
Berryman, J G
2004-07-22
Although there are five effective shear moduli for any layered VTI medium, one and only one effective shear modulus of the layered system (namely the uniaxial shear) contains all the dependence of pore fluids on the elastic or poroelastic constants that can be observed in vertically polarized shear waves. Pore fluids can increase the magnitude the shear energy stored in this modulus by an amount that ranges from the smallest to the largest effective shear moduli of the VTI system. But, since there are five shear moduli in play, the overall increase in shear energy due to fluids is reduced by a factor of about 5 in general. We can therefore give definite bounds on the maximum increase of overall shear modulus, being about 20% of the allowed range as liquid is fully substituted for gas. An attendant increase of density (depending on porosity and fluid density) by approximately 5 to 10% decreases the shear wave speed and, thereby, partially offsets the effect of this shear modulus increase. The final result is an increase of shear wave speed on the order of 5 to 10%. This increase is shown to be possible under most favorable circumstances - i.e. when the shear modulus fluctuations are large (resulting in strong anisotropy) and the medium behaves in an undrained fashion due to fluid trapping. At frequencies higher than seismic (such as sonic and ultrasonic waves for well-logging or laboratory experiments), resulting short response times also produce the requisite undrained behavior and, therefore, fluids also affect shear waves at high frequencies by increasing rigidity.
1D waves in a random poroelastic medium with large fluctuations
NASA Astrophysics Data System (ADS)
Lenoach, B.
1999-10-01
We examine the problem of wave propagation in a random poroelastic medium. The porous medium is modelled as a Biot poroelastic solid whose constitutive parameters fluctuate substantially over finite distances. Our main results are asymptotic analytical expressions for the mean velocity-stress wave; this solution incorporates two distinct length scales. The effect of the fluctuations appears on the regular depth coordinate while the parameters of the effective medium arise on a shorter scale of distance. Thus the method that we apply, the theory of averaging, allows us to give a rigorous derivation of the effective medium parameters. It also provides the correction terms which are caused by the fluctuations in the random medium; we find that the relative effect of the latter increases in proportion to icons/Journals/Common/omega" ALT="omega" ALIGN="TOP"/>1/2 where icons/Journals/Common/omega" ALT="omega" ALIGN="TOP"/> denotes the wave frequency. We also show that the fluctuations introduce significant attenuation of the fast Biot compressional wave and dispersion of the slow Biot wave. These results are illustrated by numerical examples using real oilfield data.
Xia, J.; Miller, R.D.; Park, C.B.; Hunter, J.A.; Harris, J.B.; Ivanov, J.
2002-01-01
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.
Shear wave splitting and upper mantle anisotropy beneath Japan
NASA Astrophysics Data System (ADS)
Long, M. D.; van der Hilst, R. D.
2003-12-01
Shear wave birefringence is a consequence of seismic anisotropy and is often used, with constraints from mineral physics, to characterize the pattern of upper mantle deformation. In the context of a subduction zone, however, the relationship between measured shear wave splitting parameters (φ , δ t) and geodynamical processes is not straightforward. The three-dimensional pattern of anisotropy in a subduction zone may reflect processes such as corner flow in the mantle wedge, flow around the slab edge, back-arc extension, and motion of the overriding plate. This relationship may be further complicated by complex slab morphology, by the presence of frozen anisotropy in the slab itself, and by the presence of volatiles such as water. In this study, we take advantage of dense station coverage in Japan and use seismic phases covering a wide range of incidence angles, incoming polarization angles, and backazimuths. We take advantage of the good data coverage needed to consider complexities in structure such as multiple anisotropic layers, dipping symmetry axes, and small-scale lateral variations in anisotropic properties. We utilize data from the Japanese F-net network, which comprises 65 broadband seismic stations. We have compiled a database of approximately 1500 splitting measurements of S, SKS, and SKKS phases at F-net stations, and investigate the variations of measured splitting parameters with incoming polarization angle and incidence angle. In the southern part of the array, along the Ryukyu arc, we find that fast directions are consistently trench-parallel, with splitting times of 1 second or more. Moving northward along the array, the measured splitting patterns become more complicated, with significant variations in apparent splitting parameters that indicate complex anisotropic structure. Additionally, measured fast directions vary significantly over short length scales, and stations separated by less than 100 km often exhibit very different splitting
Regional variations in shear wave anisotropy beneath western North America
NASA Astrophysics Data System (ADS)
Currie, C.; Cassidy, J.; Hyndman, R.; Bostock, M.
2003-04-01
We have examined shear wave splitting of SKS phases at 25 broadband stations in western North America to constrain regional trends in anisotropy at the Cascadia subduction zone (CSZ) and adjacent regions. At most stations, well-constrained shear wave splitting parameters (delay time and fast direction) were obtained for data from a wide range of azimuths. Delay times of 1.0 to 1.5 s indicate a mantle source for the anisotropy, most likely strain-induced lattice preferred orientation of anisotropic mantle minerals. The fast directions at the CSZ are in good agreement with models for mantle deformation associated with subduction. Within the forearc, fast directions at stations above the Juan de Fuca Plate are parallel to the subduction direction (N70E), suggesting deformation in the mantle beneath the plate due to plate motion. Above the Explorer Plate at the northern end of the CSZ, fast directions are N30E. This may reflect either the more northerly subduction direction of that plate, or a transition from subduction-related deformation to along-margin flow parallel to the transcurrent Queen Charlotte Fault to the north. At four stations in the central backarc, fast directions are parallel to the Juan de Fuca-North America convergence direction, consistent with models of subduction-induced mantle wedge flow. No clear splitting was observed at the two most northern backarc stations, indicating little to no horizontal anisotropy beneath these stations, possibly due to vertical mantle flow around the northern edge of the subducted plate. At a station near the western edge of the North America craton, the splitting parameters show significant azimuthal variations with a 90° periodicity, characteristic of multiple layers of anisotropy. The observations were fit using a two-layer model with an upper anisotropic layer with a fast direction of N12E and delay time of 1.4 s, and a lower layer with a fast direction of N81E and delay time of 2.0 s. The North America craton is
Measurement of sound speed vs. depth in South Pole ice: pressure waves and shear waves
IceCube Collaboration; Klein, Spencer
2009-06-04
We have measured the speed of both pressure waves and shear waves as a function of depth between 80 and 500 m depth in South Pole ice with better than 1% precision. The measurements were made using the South Pole Acoustic Test Setup (SPATS), an array of transmitters and sensors deployed in the ice at the South Pole in order to measure the acoustic properties relevant to acoustic detection of astrophysical neutrinos. The transmitters and sensors use piezoceramics operating at {approx}5-25 kHz. Between 200 m and 500 m depth, the measured profile is consistent with zero variation of the sound speed with depth, resulting in zero refraction, for both pressure and shear waves. We also performed a complementary study featuring an explosive signal propagating vertically from 50 to 2250 m depth, from which we determined a value for the pressure wave speed consistent with that determined for shallower depths, higher frequencies, and horizontal propagation with the SPATS sensors. The sound speed profile presented here can be used to achieve good acoustic source position and emission time reconstruction in general, and neutrino direction and energy reconstruction in particular. The reconstructed quantities could also help separate neutrino signals from background.
High speed all optical shear wave imaging optical coherence elastography (Conference Presentation)
NASA Astrophysics Data System (ADS)
Song, Shaozhen; Hsieh, Bao-Yu; Wei, Wei; Shen, Tueng; O'Donnell, Matthew; Wang, Ruikang K.
2016-03-01
Optical Coherence Elastography (OCE) is a non-invasive testing modality that maps the mechanical property of soft tissues with high sensitivity and spatial resolution using phase-sensitive optical coherence tomography (PhS-OCT). Shear wave OCE (SW-OCE) is a leading technique that relies on the speed of propagating shear waves to provide a quantitative elastography. Previous shear wave imaging OCT techniques are based on repeated M-B scans, which have several drawbacks such as long acquisition time and repeated wave stimulations. Recent developments of Fourier domain mode-locked high-speed swept-source OCT system has enabled enough speed to perform KHz B-scan rate OCT imaging. Here we propose ultra-high speed, single shot shear wave imaging to capture single-shot transient shear wave propagation to perform SW-OCE. The frame rate of shear wave imaging is 16 kHz, at A-line rate of ~1.62 MHz, which allows the detection of high-frequency shear wave of up to 8 kHz. The shear wave is generated photothermal-acoustically, by ultra-violet pulsed laser, which requires no contact to OCE subjects, while launching high frequency shear waves that carries rich localized elasticity information. The image acquisition and processing can be performed at video-rate, which enables real-time 3D elastography. SW-OCE measurements are demonstrated on tissue-mimicking phantoms and porcine ocular tissue. This approach opens up the feasibility to perform real-time 3D SW-OCE in clinical applications, to obtain high-resolution localized quantitative measurement of tissue biomechanical property.
McAleavey, Stephen A
2014-05-01
Shear wave induced phase encoding (SWIPE) imaging generates ultrasound backscatter images of tissue-like elastic materials by using traveling shear waves to encode the lateral position of the scatters in the phase of the received echo. In contrast to conventional ultrasound B-scan imaging, SWIPE offers the potential advantages of image formation without beam focusing or steering from a single transducer element, lateral resolution independent of aperture size, and the potential to achieve relatively high lateral resolution with low frequency ultrasound. Here a Fourier series description of the phase modulated echo signal is developed, demonstrating that echo harmonics at multiples of the shear wave frequency reveal target k-space data at identical multiples of the shear wavenumber. Modulation transfer functions of SWIPE imaging systems are calculated for maximum shear wave acceleration and maximum shear constraints, and compared with a conventionally focused aperture. The relative signal-to-noise ratio of the SWIPE method versus a conventionally focused aperture is found through these calculations. Reconstructions of wire targets in a gelatin phantom using 1 and 3.5 MHz ultrasound and a cylindrical shear wave source are presented, generated from the fundamental and second harmonic of the shear wave modulation frequency, demonstrating weak dependence of lateral resolution with ultrasound frequency.
3-D FDTD simulation of shear waves for evaluation of complex modulus imaging.
Orescanin, Marko; Wang, Yue; Insana, Michael
2011-02-01
The Navier equation describing shear wave propagation in 3-D viscoelastic media is solved numerically with a finite differences time domain (FDTD) method. Solutions are formed in terms of transverse scatterer velocity waves and then verified via comparison to measured wave fields in heterogeneous hydrogel phantoms. The numerical algorithm is used as a tool to study the effects on complex shear modulus estimation from wave propagation in heterogeneous viscoelastic media. We used an algebraic Helmholtz inversion (AHI) technique to solve for the complex shear modulus from simulated and experimental velocity data acquired in 2-D and 3-D. Although 3-D velocity estimates are required in general, there are object geometries for which 2-D inversions provide accurate estimations of the material properties. Through simulations and experiments, we explored artifacts generated in elastic and dynamic-viscous shear modulus images related to the shear wavelength and average viscosity.
3D mapping of elastic modulus using shear wave optical micro-elastography
Zhu, Jiang; Qi, Li; Miao, Yusi; Ma, Teng; Dai, Cuixia; Qu, Yueqiao; He, Youmin; Gao, Yiwei; Zhou, Qifa; Chen, Zhongping
2016-01-01
Elastography provides a powerful tool for histopathological identification and clinical diagnosis based on information from tissue stiffness. Benefiting from high resolution, three-dimensional (3D), and noninvasive optical coherence tomography (OCT), optical micro-elastography has the ability to determine elastic properties with a resolution of ~10 μm in a 3D specimen. The shear wave velocity measurement can be used to quantify the elastic modulus. However, in current methods, shear waves are measured near the surface with an interference of surface waves. In this study, we developed acoustic radiation force (ARF) orthogonal excitation optical coherence elastography (ARFOE-OCE) to visualize shear waves in 3D. This method uses acoustic force perpendicular to the OCT beam to excite shear waves in internal specimens and uses Doppler variance method to visualize shear wave propagation in 3D. The measured propagation of shear waves agrees well with the simulation results obtained from finite element analysis (FEA). Orthogonal acoustic excitation allows this method to measure the shear modulus in a deeper specimen which extends the elasticity measurement range beyond the OCT imaging depth. The results show that the ARFOE-OCE system has the ability to noninvasively determine the 3D elastic map. PMID:27762276
Surface and downhole shear wave seismic methods for thick soil site investigations
Hunter, J.A.; Benjumea, B.; Harris, J.B.; Miller, R.D.; Pullan, S.E.; Burns, R.A.; Good, R.L.
2002-01-01
Shear wave velocity-depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites. ?? 2002 Published by Elsevier Science Ltd.
Shear wave elasticity imaging of cervical lymph nodes.
Bhatia, Kunwar S S; Cho, Carmen C M; Tong, Cina S L; Yuen, Edmund H Y; Ahuja, Anil T
2012-02-01
A pilot study of real-time shear wave ultrasound elastography (SWE) for cervical lymphadenopathy in routine clinical practice was conducted on 55 nodes undergoing conventional ultrasound (US) with US-guided needle aspiration for cytology. Elastic moduli of stiffest regions in nodes were measured on colour-coded elastograms, which were correlated with cytology. Malignant nodes (n = 31, 56.4%) were stiffer (median 25.0 kPa, range 6.9-278.9 kPa) than benign nodes (median 21.4 kPa, range 8.9-30.2 kPa) (p = 0.008, Mann Whitney U test). A cut-off of 30.2 kPa attained highest accuracy of 61.8%, corresponding to 41.9% sensitivity, 100% specificity and 0.77 area under the receiver operating characteristic curve. Qualitatively, elastograms of benign nodes were homogeneously soft; malignant nodes were homogeneously soft or markedly heterogeneous with some including regions lacking elasticity signal. SWE is feasible for neck nodes. It appears unsuitable for cancer screening but may detect a subset of malignant nodes. The cause of spatial heterogeneity of malignant nodes on SWE is yet to be established.
Shear Wave Splitting Across Eastern, Western and Southern Africa
NASA Astrophysics Data System (ADS)
Nyblade, A.; Ramirez, C.; Bagley, B. C.; Mulibo, G. D.; Tugume, F.; Wysession, M. E.; Wiens, D. A.
2014-12-01
The expansion of the AfricaArray network across eastern, western and southern Africa, in conjunction with seismic data from many PASSCAL deployments over the past 20 years, is helping to fill in major gaps in the global coverage of shear wave splitting measurements. New results from stations in Ghana, Nigeria, Mozambique, Botswana, Angola, Namibia and South Africa are presented in this study that when combined with previously published measurements help to map the pattern of seismic anisotropy over much of the African continent. A general pattern of fast polarization directions, characterized by NE orientations, is found, and superimposed on this subcontinental-scale pattern is local and regional variability, most notably around the Archean Tanzania craton in eastern Africa. The subcontinental-scale pattern, as well as local and regional variations in this pattern, are interpreted in terms of large-scale mantle flow from the African Superplume, fossil anisotropy in the lithosphere, and shape anisotropy in magmatic regions of the East African rift system.
Gravitational, shear and matter waves in Kantowski-Sachs cosmologies
Keresztes, Zoltán; Gergely, László Á.; Forsberg, Mats; Bradley, Michael; Dunsby, Peter K.S. E-mail: forsberg.mats.a.b@gmail.com E-mail: peter.dunsby@uct.ac.za
2015-11-01
A general treatment of vorticity-free, perfect fluid perturbations of Kantowski-Sachs models with a positive cosmological constant are considered within the framework of the 1+1+2 covariant decomposition of spacetime. The dynamics is encompassed in six evolution equations for six harmonic coefficients, describing gravito-magnetic, kinematic and matter perturbations, while a set of algebraic expressions determine the rest of the variables. The six equations further decouple into a set of four equations sourced by the perfect fluid, representing forced oscillations and two uncoupled damped oscillator equations. The two gravitational degrees of freedom are represented by pairs of gravito-magnetic perturbations. In contrast with the Friedmann case one of them is coupled to the matter density perturbations, becoming decoupled only in the geometrical optics limit. In this approximation, the even and odd tensorial perturbations of the Weyl tensor evolve as gravitational waves on the anisotropic Kantowski-Sachs background, while the modes describing the shear and the matter density gradient are out of phase dephased by π /2 and share the same speed of sound.
Johnston, J.E.; Christensen, N.I. . Dept. of Earth and Atmospheric Sciences)
1992-01-01
The physical properties of a sequence of Paleozoic sedimentary rocks have been examined in detail, with an emphasis on laboratory measurements of density, shear wave velocity, shear wave splitting, and Vp/Vs ratios. Seismic properties of 147 cores from 49 rock samples collected from the thorn hill sedimentary sequence of eastern Tennessee are examined in terms of implications for future seismic studies in the southern Appalachians. The shear wave velocities of these rocks are strongly influenced by the relatively high shear wave velocity of quartz. Shear wave velocity anisotropy is present in most of the lithologic groups: it is highest in the shales while being almost insignificant in the dolostones. The related phenomenon of shear wave splitting occurs to some degree in all of the lithologies studied and at high pressures originates from mineral orientation. Compressional to shear velocity (Vp/Vs) ratios of approximately 1.82 (dolostones) and 1.95 (limestones) effectively characterize the carbonates while other lithologies display wider ranges of Vp/Vs, primarily due to the influence of accessory minerals such as quartz. Densities of the sample suite range from 2.34 g/cm[sup 3] (shale) to 2.86 g/cm[sup 3] (dolostone). Normal incidence shear and compressional wave synthetic seismograms of the entire Thorn Hill section indicate that three zones of high amplitude reflections would be seen on reflection records obtained over this 3,327 meter thick sequence. differences are seen at some interfaces in the Mississippian-Devonian interval, which are more reflective to shear waves, and in the Ordovician Martinsburg Formation, which appears more reflective to compressional waves.
Coupling of an acoustic wave to shear motion due to viscous heating
NASA Astrophysics Data System (ADS)
Liu, Bin; Goree, J.
2016-07-01
Viscous heating due to shear motion in a plasma can result in the excitation of a longitudinal acoustic wave, if the shear motion is modulated in time. The coupling mechanism is a thermal effect: time-dependent shear motion causes viscous heating, which leads to a rarefaction that can couple into a longitudinal wave, such as an acoustic wave. This coupling mechanism is demonstrated in an electrostatic three-dimensional (3D) simulation of a dusty plasma, in which a localized shear flow is initiated as a pulse, resulting in a delayed outward propagation of a longitudinal acoustic wave. This coupling effect can be profound in plasmas that exhibit localized viscous heating, such as the dusty plasma we simulated using parameters typical of the PK-4 experiment. We expect that a similar phenomenon can occur with other kinds of plasma waves.
The propagation of horizontally polarized shear waves in plates bordered with viscous liquid.
Gitis, Alexander; Sauer, Dirk Uwe
2016-09-01
Requirements for ultrasonic horizontally polarized shear waves based viscosity sensors and their applicability for continuous in-line measurement are presented and discussed. The results reveal, that sensors using non-piezoelectric plates as well as wave guides and sensing surface have application-oriented advantages in corrosive and hot liquids. For such non-piezoelectric plate sensors, the dispersion relations are found and the linking equation among propagation velocity as well as attenuation coefficient and Newtonian liquid parameters are obtained. The findings show that in presence of viscous liquids the propagation parameters of horizontally polarized shear waves (HPSW) in non-piezoelectric plate change and a viscosity depending attenuation occurs. It is shown that the measurement sensitivity, in physical terms, of the investigated device highly depends on plate thickness, shear wave impedance of the plate material, and the shear wave impedance of the ambient liquid. Further, reasonable geometrical optimizations and suited plate materials are discussed. PMID:27423968
NASA Astrophysics Data System (ADS)
Pitarka, Arben; Mellors, Robert; Rodgers, Arthur; Vorobiev, Oleg; Ezzedine, Souheil; Matzel, Eric; Ford, Sean; Walter, Bill; Antoun, Tarabay; Wagoner, Jeffery; Pasyanos, Mike; Petersson, Anders; Sjogreen, Bjorn
2014-05-01
We investigate the excitation and propagation of far-field (epicentral distance larger than 20 m) seismic waves by analyzing and modeling ground motion from an underground chemical explosion recorded during the Source Physics Experiment (SPE), Nevada. The far-field recorded ground motion is characterized by complex features, such as large azimuthal variations in P- and S-wave amplitudes, as well as substantial energy on the tangential component of motion. Shear wave energy is also observed on the tangential component of the near-field motion (epicentral distance smaller than 20 m) suggesting that shear waves were generated at or very near the source. These features become more pronounced as the waves propagate away from the source. We address the shear wave generation during the explosion by modeling ground motion waveforms recorded in the frequency range 0.01-20 Hz, at distances of up to 1 km. We used a physics based approach that combines hydrodynamic modeling of the source with anelastic modeling of wave propagation in order to separate the contributions from the source and near-source wave scattering on shear motion generation. We found that wave propagation scattering caused by the near-source geological environment, including surface topography, contributes to enhancement of shear waves generated from the explosion source. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-06NA25946/ NST11-NCNS-TM-EXP-PD15.
Plasma turbulence driven by transversely large-scale standing shear Alfven waves
Singh, Nagendra; Rao, Sathyanarayan
2012-12-15
Using two-dimensional particle-in-cell simulations, we study generation of turbulence consisting of transversely small-scale dispersive Alfven and electrostatic waves when plasma is driven by a large-scale standing shear Alfven wave (LS-SAW). The standing wave is set up by reflecting a propagating LS-SAW. The ponderomotive force of the standing wave generates transversely large-scale density modifications consisting of density cavities and enhancements. The drifts of the charged particles driven by the ponderomotive force and those directly caused by the fields of the standing LS-SAW generate non-thermal features in the plasma. Parametric instabilities driven by the inherent plasma nonlinearities associated with the LS-SAW in combination with the non-thermal features generate small-scale electromagnetic and electrostatic waves, yielding a broad frequency spectrum ranging from below the source frequency of the LS-SAW to ion cyclotron and lower hybrid frequencies and beyond. The power spectrum of the turbulence has peaks at distinct perpendicular wave numbers (k{sub Up-Tack }) lying in the range d{sub e}{sup -1}-6d{sub e}{sup -1}, d{sub e} being the electron inertial length, suggesting non-local parametric decay from small to large k{sub Up-Tack }. The turbulence spectrum encompassing both electromagnetic and electrostatic fluctuations is also broadband in parallel wave number (k{sub ||}). In a standing-wave supported density cavity, the ratio of the perpendicular electric to magnetic field amplitude is R(k{sub Up-Tack }) = |E{sub Up-Tack }(k{sub Up-Tack })/|B{sub Up-Tack }(k{sub Up-Tack })| Much-Less-Than V{sub A} for k{sub Up-Tack }d{sub e} < 0.5, where V{sub A} is the Alfven velocity. The characteristic features of the broadband plasma turbulence are compared with those available from satellite observations in space plasmas.
NASA Astrophysics Data System (ADS)
Grasland-Mongrain, Pol; Miller-Jolicoeur, Erika; Tang, An; Catheline, Stefan; Cloutier, Guy
2016-03-01
This study presents the first observation of shear waves induced remotely within soft tissues. It was performed through the combination of a transcranial magnetic stimulation device and a permanent magnet. A physical model based on Maxwell and Navier equations was developed. Experiments were performed on a cryogel phantom and a chicken breast sample. Using an ultrafast ultrasound scanner, shear waves of respective amplitudes of 5 and 0.5 μm were observed. Experimental and numerical results were in good agreement. This study constitutes the framework of an alternative shear wave elastography method.
Spatial correlation of shear-wave velocity within San Francisco Bay Sediments
Thompson, E.M.; Baise, L.G.; Kayen, R.E.
2006-01-01
Sediment properties are spatially variable at all scales, and this variability at smaller scales influences high frequency ground motions. We show that surface shear-wave velocity is highly correlated within San Francisco Bay Area sediments using shear-wave velocity measurements from 210 seismic cone penetration tests. We use this correlation to estimate the surface sediment velocity structure using geostatistics. We find that the variance of the estimated shear-wave velocity is reduced using ordinary kriging, and that including this velocity structure in 2D ground motion simulations of a moderate sized earthquake improves the accuracy of the synthetics. Copyright ASCE 2006.
Hashemiyan, Z; Packo, P; Staszewski, W J; Uhl, T
2016-01-01
Properties of soft biological tissues are increasingly used in medical diagnosis to detect various abnormalities, for example, in liver fibrosis or breast tumors. It is well known that mechanical stiffness of human organs can be obtained from organ responses to shear stress waves through Magnetic Resonance Elastography. The Local Interaction Simulation Approach is proposed for effective modelling of shear wave propagation in soft tissues. The results are validated using experimental data from Magnetic Resonance Elastography. These results show the potential of the method for shear wave propagation modelling in soft tissues. The major advantage of the proposed approach is a significant reduction of computational effort. PMID:26884808
Packo, P.; Staszewski, W. J.; Uhl, T.
2016-01-01
Properties of soft biological tissues are increasingly used in medical diagnosis to detect various abnormalities, for example, in liver fibrosis or breast tumors. It is well known that mechanical stiffness of human organs can be obtained from organ responses to shear stress waves through Magnetic Resonance Elastography. The Local Interaction Simulation Approach is proposed for effective modelling of shear wave propagation in soft tissues. The results are validated using experimental data from Magnetic Resonance Elastography. These results show the potential of the method for shear wave propagation modelling in soft tissues. The major advantage of the proposed approach is a significant reduction of computational effort. PMID:26884808
Nonlinear shear wave in a non Newtonian visco-elastic medium
Banerjee, D.; Janaki, M. S.; Chakrabarti, N.
2012-06-15
An analysis of nonlinear transverse shear wave has been carried out on non-Newtonian viscoelastic liquid using generalized hydrodynamic model. The nonlinear viscoelastic behavior is introduced through velocity shear dependence of viscosity coefficient by well known Carreau-Bird model. The dynamical feature of this shear wave leads to the celebrated Fermi-Pasta-Ulam problem. Numerical solution has been obtained which shows that initial periodic solutions reoccur after passing through several patterns of periodic waves. A possible explanation for this periodic solution is given by constructing modified Korteweg de Vries equation. This model has application from laboratory to astrophysical plasmas as well as in biological systems.
Song, Pengfei; Macdonald, Michael; Behler, Russell; Lanning, Justin; Wang, Michael; Urban, Matthew; Manduca, Armando; Zhao, Heng; Callstrom, Matthew; Alizad, Azra; Greenleaf, James; Chen, Shigao
2015-02-01
Two-dimensional shear-wave elastography presents 2-D quantitative shear elasticity maps of tissue, which are clinically useful for both focal lesion detection and diffuse disease diagnosis. Realization of 2-D shear-wave elastography on conventional ultrasound scanners, however, is challenging because of the low tracking pulse-repetition-frequency (PRF) of these systems. Although some clinical and research platforms support software beamforming and plane-wave imaging with high PRF, the majority of current clinical ultrasound systems do not have the software beamforming capability, which presents a critical challenge for translating the 2-D shear-wave elastography technique from laboratory to clinical scanners. To address this challenge, this paper presents a time-aligned sequential tracking (TAST) method for shear-wave tracking on conventional ultrasound scanners. TAST takes advantage of the parallel beamforming capability of conventional systems and realizes high-PRF shear-wave tracking by sequentially firing tracking vectors and aligning shear wave data in the temporal direction. The comb-push ultrasound shear elastography (CUSE) technique was used to simultaneously produce multiple shear wave sources within the field-of-view (FOV) to enhance shear wave SNR and facilitate robust reconstructions of 2-D elasticity maps. TAST and CUSE were realized on a conventional ultrasound scanner. A phantom study showed that the shear-wave speed measurements from the conventional ultrasound scanner were in good agreement with the values measured from other 2-D shear wave imaging technologies. An inclusion phantom study showed that the conventional ultrasound scanner had comparable performance to a state-of-the-art shear-wave imaging system in terms of bias and precision in measuring different sized inclusions. Finally, in vivo case analysis of a breast with a malignant mass, and a liver from a healthy subject demonstrated the feasibility of using the conventional ultrasound
Tweten, Dennis J; Okamoto, Ruth J; Schmidt, John L; Garbow, Joel R; Bayly, Philip V
2015-11-26
This paper describes a method to estimate mechanical properties of soft, anisotropic materials from measurements of shear waves with specific polarization and propagation directions. This method is applicable to data from magnetic resonance elastography (MRE), which is a method for measuring shear waves in live subjects or in vitro samples. Here, we simulate MRE data using finite element analysis. A nearly incompressible, transversely isotropic (ITI) material model with three parameters (shear modulus, shear anisotropy, and tensile anisotropy) is used, which is appropriate for many fibrous, biological tissues. Both slow and fast shear waves travel concurrently through such a material with speeds that depend on the propagation direction relative to fiber orientation. A three-parameter estimation approach based on directional filtering and isolation of slow and fast shear wave components (directional filter inversion, or DFI) is introduced. Wave speeds of each isolated shear wave component are estimated using local frequency estimation (LFE), and material properties are calculated using weighted least squares. Data from multiple finite element simulations are used to assess the accuracy and reliability of DFI for estimation of anisotropic material parameters.
Tweten, Dennis J; Okamoto, Ruth J; Schmidt, John L; Garbow, Joel R; Bayly, Philip V
2015-11-26
This paper describes a method to estimate mechanical properties of soft, anisotropic materials from measurements of shear waves with specific polarization and propagation directions. This method is applicable to data from magnetic resonance elastography (MRE), which is a method for measuring shear waves in live subjects or in vitro samples. Here, we simulate MRE data using finite element analysis. A nearly incompressible, transversely isotropic (ITI) material model with three parameters (shear modulus, shear anisotropy, and tensile anisotropy) is used, which is appropriate for many fibrous, biological tissues. Both slow and fast shear waves travel concurrently through such a material with speeds that depend on the propagation direction relative to fiber orientation. A three-parameter estimation approach based on directional filtering and isolation of slow and fast shear wave components (directional filter inversion, or DFI) is introduced. Wave speeds of each isolated shear wave component are estimated using local frequency estimation (LFE), and material properties are calculated using weighted least squares. Data from multiple finite element simulations are used to assess the accuracy and reliability of DFI for estimation of anisotropic material parameters. PMID:26476762
Spatial correlation of shear-wave velocity in the San Francisco Bay Area sediments
Thompson, E.M.; Baise, L.G.; Kayen, R.E.
2007-01-01
Ground motions recorded within sedimentary basins are variable over short distances. One important cause of the variability is that local soil properties are variable at all scales. Regional hazard maps developed for predicting site effects are generally derived from maps of surficial geology; however, recent studies have shown that mapped geologic units do not correlate well with the average shear-wave velocity of the upper 30 m, Vs(30). We model the horizontal variability of near-surface soil shear-wave velocity in the San Francisco Bay Area to estimate values in unsampled locations in order to account for site effects in a continuous manner. Previous geostatistical studies of soil properties have shown horizontal correlations at the scale of meters to tens of meters while the vertical correlations are on the order of centimeters. In this paper we analyze shear-wave velocity data over regional distances and find that surface shear-wave velocity is correlated at horizontal distances up to 4 km based on data from seismic cone penetration tests and the spectral analysis of surface waves. We propose a method to map site effects by using geostatistical methods based on the shear-wave velocity correlation structure within a sedimentary basin. If used in conjunction with densely spaced shear-wave velocity profiles in regions of high seismic risk, geostatistical methods can produce reliable continuous maps of site effects. ?? 2006 Elsevier Ltd. All rights reserved.
The effect of subducting slabs in global shear wave tomography
NASA Astrophysics Data System (ADS)
Lu, Chang; Grand, Stephen P.
2016-05-01
Subducting slabs create strong short wavelength seismic anomalies in the upper mantle where much of Earth's seismicity is located. As such, they have the potential to bias longer wavelength seismic tomography models. To evaluate the effect of subducting slabs in global tomography, we performed a series of inversions using a global synthetic shear wave traveltime data set for a theoretical slab model based on predicted thermal anomalies within slabs. The spectral element method was applied to predict the traveltime anomalies produced by the 3-D slab model for paths corresponding to our current data used in actual tomography models. Inversion tests have been conducted first using the raw traveltime anomalies to check how well the slabs can be imaged in global tomography without the effect of earthquake mislocation. Our results indicate that most of the slabs can be identified in the inversion result but with smoothed and reduced amplitude. The recovery of the total mass anomaly in slab regions is about 88 per cent. We then performed another inversion test to investigate the effect of mislocation caused by subducting slabs. We found that source mislocation largely removes slab signal and significantly degrades the imaging of subducting slabs-potentially reducing the recovery of mass anomalies in slab regions to only 41 per cent. We tested two source relocation procedures-an iterative relocation inversion and joint relocation inversion. Both methods partially recover the true source locations and improve the inversion results, but the joint inversion method worked significantly better than the iterative method. In all of our inversion tests, the amplitudes of artefact structures in the lower mantle caused by the incorrect imaging of slabs (up to ˜0.5 per cent S velocity anomalies) are comparable to some large-scale lower-mantle heterogeneities seen in global tomography studies. Based on our inversion tests, we suggest including a-priori subducting slabs in the
Effect of Subducting Slabs in Global Shear Wave Tomography
NASA Astrophysics Data System (ADS)
Lu, C.; Grand, S. P.
2015-12-01
Subducting slabs represent strong short wavelength seismic anomalies in the upper mantle where much of Earth's seismicity is located. As such, they have the potential to bias longer wavelength seismic tomography models. To evaluate the effect of subducting slabs in global tomography, we performed a series of inversion tests using a global synthetic shear wave travel time dataset for a theoretical slab model based on predicted thermal anomalies within slabs. The spectral element method (SEM) was applied to predict the travel time anomalies produced by the 3D slab model for paths corresponding to our current data used in actual tomography models. Inversion tests have been conducted first using the raw travel time anomalies to check how well the slabs can be imaged in global tomography without the effect of mislocation. Our results indicate that most of the slabs can be identified in the inversion result but with smoothed and reduced amplitude. The recovery of the total mass anomaly in slab regions is about 84%. We then performed another inversion test to investigate the effect of mislocation caused by subducting slabs. We found that source mislocation significantly degrades the imaging of subducting slabs - potentially reducimg the recovery of mass anomalies in slab regions to only 39%. We tested two source relocation procedures - an iterative relocation inversion and joint relocation inversion. Both methods partially recover the true source locations and improve the inversion results, but the joint inversion method worked significantly better than the iterative method. In all of our inversion tests, the amplitude of artifact structures in the lower mantle caused by the incorrect imaging of slabs (up to ~0.5% S velocity anomalies) are comparable to large scale lower mantle heterogeneities seen in global tomography studies.
NASA Astrophysics Data System (ADS)
Marie Tabaru,; Takashi Azuma,; Kunio Hashiba,
2010-07-01
Acoustic radiation force (ARF) imaging has been developed as a novel elastography technology to diagnose hepatic disease and breast cancer. The accuracy of shear wave speed estimation, which is one of the applications of ARF elastography, is studied. The Young’s moduli of pig liver and foie gras samples estimated from the shear wave speed were compared with those measured the static Young’s modulus measurement. The difference in the two methods was 8%. Distance attenuation characteristics of the shear wave were also studied using finite element method (FEM) analysis. We found that the differences in the axial and lateral beam widths in pressure and ARF are 16 and 9% at F-number=0.9. We studied the relationship between two branch points in distance attenuation characteristics and the shape of ARF. We found that the maximum measurable length to estimate shear wave speed for one ARF excitation was 8 mm.
NASA Astrophysics Data System (ADS)
Tabaru, Marie; Azuma, Takashi; Hashiba, Kunio
2010-07-01
Acoustic radiation force (ARF) imaging has been developed as a novel elastography technology to diagnose hepatic disease and breast cancer. The accuracy of shear wave speed estimation, which is one of the applications of ARF elastography, is studied. The Young's moduli of pig liver and foie gras samples estimated from the shear wave speed were compared with those measured the static Young's modulus measurement. The difference in the two methods was 8%. Distance attenuation characteristics of the shear wave were also studied using finite element method (FEM) analysis. We found that the differences in the axial and lateral beam widths in pressure and ARF are 16 and 9% at F-number=0.9. We studied the relationship between two branch points in distance attenuation characteristics and the shape of ARF. We found that the maximum measurable length to estimate shear wave speed for one ARF excitation was 8 mm.
Improvement of Shear Wave Motion Detection Using Harmonic Imaging in Healthy Human Liver.
Amador, Carolina; Song, Pengfei; Meixner, Duane D; Chen, Shigao; Urban, Matthew W
2016-05-01
Quantification of liver elasticity is a major application of shear wave elasticity imaging (SWEI) to non-invasive assessment of liver fibrosis stages. SWEI measurements can be highly affected by ultrasound image quality. Ultrasound harmonic imaging has exhibited a significant improvement in ultrasound image quality as well as for SWEI measurements. This was previously illustrated in cardiac SWEI. The purpose of this study was to evaluate liver shear wave particle displacement detection and shear wave velocity (SWV) measurements with fundamental and filter-based harmonic ultrasound imaging. In a cohort of 17 patients with no history of liver disease, a 2.9-fold increase in maximum shear wave displacement, a 0.11 m/s decrease in the overall interquartile range and median SWV and a 17.6% increase in the success rate of SWV measurements were obtained when filter-based harmonic imaging was used instead of fundamental imaging.
The dynamics of ion with electrostatic waves in a sheared magnetic field
Yu Limin; Zhang Xianmei; Sheng Zhengmao
2011-12-15
The interaction between an ion and multiple electrostatic waves propagating perpendicularly to an ambient magnetic field with shear is investigated. Based on the Lie transformation method, with the wave amplitude and the magnetic shear both as the perturbation parameters, the analytical formulas for the reduced Hamiltonian is derived and results are compared with numerical calculations of the complete equations of motion for the case of two on-resonance waves and the case of two off-resonance waves, respectively. It is found that the effect of magnetic shear drastically prevents the acceleration of an ion in both cases. This result will help us to understand the behaviors of ions in a magnetic sheared device, such as tokamak.
Improvement of Shear Wave Motion Detection Using Harmonic Imaging in Healthy Human Liver.
Amador, Carolina; Song, Pengfei; Meixner, Duane D; Chen, Shigao; Urban, Matthew W
2016-05-01
Quantification of liver elasticity is a major application of shear wave elasticity imaging (SWEI) to non-invasive assessment of liver fibrosis stages. SWEI measurements can be highly affected by ultrasound image quality. Ultrasound harmonic imaging has exhibited a significant improvement in ultrasound image quality as well as for SWEI measurements. This was previously illustrated in cardiac SWEI. The purpose of this study was to evaluate liver shear wave particle displacement detection and shear wave velocity (SWV) measurements with fundamental and filter-based harmonic ultrasound imaging. In a cohort of 17 patients with no history of liver disease, a 2.9-fold increase in maximum shear wave displacement, a 0.11 m/s decrease in the overall interquartile range and median SWV and a 17.6% increase in the success rate of SWV measurements were obtained when filter-based harmonic imaging was used instead of fundamental imaging. PMID:26803391
Experimental validation of acoustic radiation force induced shear wave interference patterns.
Hoyt, Kenneth; Hah, Zaegyoo; Hazard, Chris; Parker, Kevin J
2012-01-01
A novel elasticity imaging system founded on the use of acoustic radiation forces from a dual beam arrangement to generate shear wave interference patterns is described. Acquired pulse-echo data and correlation-based techniques were used to estimate the resultant deformation and to visualize tissue viscoelastic response. The use of normal versus axicon focal configurations was investigated for effects on shear wave generation. Theoretical models were introduced and shown in simulation to accurately predict shear wave propagation and interference pattern properties. In a tissue-mimicking phantom, experimental results are in congruence with theoretical predictions. Using dynamic acoustic radiation force excitation, results confirm that shear wave interference patterns can be produced remotely in a particular tissue region of interest (ROI). Overall, preliminary results are encouraging and the system described may prove feasible for interrogating the viscoelastic properties of normal and diseased tissue types.
Shear Horizontal Wave Propagation Speed in Mylar Sheet and Coated Paper
NASA Astrophysics Data System (ADS)
Leppänen, M.; Karppinen, T.; Hæggström, E.; Stor-Pellinen, J.
2006-03-01
Soft plate-like membranes find application e.g. as pill or paper coatings, bio-filter membranes, and gas seals in food products. For these applications the integrity and the mechanical properties of the membrane are important. Mechanical properties of these products can be determined by stretching or bending tests, but such methods can damage these fragile products. We propose a rapid nondestructive acoustic method to estimate mechanical film characteristics with shear horizontal (in-plane shear) waves. A 23 kHz, 1-cycle square signal was excited into a thin foil with a piezoceramic pickup and received with an inductive pickup. The SNR (power) was 20 dB in 1 kHz -50 kHz bandwidth. This actuation-detection scheme can be used to excite in-plane longitudinal, shear and even elliptic waves in a thin foil. The method was validated by measuring in-plane shear wave and longitudinal wave time-of-flight TOF at different actuator-receiver separations and calculating the corresponding longitudinal and shear modulus. The samples were Mylar® sheet and coated paper. The anisotropy of MOE for Mylar sheet was close to the manufacturer specifications. For coated paper a maximum shear modulus anisotropy of 5% and a shear modulus dependence on temperature of 0.7 MPa/°C were found. Laser doppler vibrometry showed that the excited waves were confined in-plane.
Research on measurement of bed shear stress under wave-current interaction
NASA Astrophysics Data System (ADS)
Xu, Hua; Xia, Yun-feng; Ma, Bing-he; Hao, Si-yu; Zhang, Shi-zhao; Du, De-jun
2015-06-01
The movement of sediment in estuary and on coast is directly restricted by the bed shear stress. Therefore, the research on the basic problem of sediment movement by the bed shear stress is an important way to research the theory of sediment movement. However, there is not a measuring and computing method to measure the bed shear stress under a complicated dynamic effect like wave and current. This paper describes the measurement and test research on the bed shear stress in a long launder of direct current by the new instrument named thermal shearometer based on micro-nanotechnology. As shown by the research results, the thermal shearometer has a high response frequency and strong stability. The measured results can reflect the basic change of the bed shear stress under wave and wave-current effect, and confirm that the method of measuring bed shear stress under wave-current effect with thermal shearometer is feasible. Meanwhile, a preliminary method to compute the shear stress compounded by wave-current is put forward according to the tested and measured results, and then a reference for further study on the basic theory of sediment movement under a complicated dynamic effect is provided.
A technique for generating shear waves in cylindrical shells under radial impact
NASA Technical Reports Server (NTRS)
Blum, A.; Mortimer, R. W.; Rose, J. L.
1974-01-01
Experimental techniques are developed to study and measure the shear-wave velocity in an aluminum cylindrical shell subjected to a radial impact. The radial impact is obtained by exploding an electrical detonator inserted in plastic plugs mounted on the end of the shell. Strain gages, mounted on the outside surface of the shell at various axial locations, are used to obtain oscilloscope traces from which the shear-wave velocity can be calculated.
Measurement of shear-wave velocity by ultrasound critical-angle reflectometry (UCR)
NASA Technical Reports Server (NTRS)
Mehta, S.; Antich, P.; Blomqvist, C. G. (Principal Investigator)
1997-01-01
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.
NASA Astrophysics Data System (ADS)
Pandey, Vikash; Holm, Sverre
2016-04-01
An analogy is drawn between the diffusion-wave equations derived from the fractional Kelvin-Voigt model and those obtained from Buckingham's grain-shearing (GS) model [J. Acoust. Soc. Am. 108, 2796-2815 (2000)] of wave propagation in saturated, unconsolidated granular materials. The material impulse response function from the GS model is found to be similar to the power-law memory kernel which is inherent in the framework of fractional calculus. The compressional wave equation and shear wave equation derived from the GS model turn out to be the Kelvin-Voigt fractional-derivative wave equation and the fractional diffusion-wave equation respectively. Also, a physical interpretation of the characteristic fractional-order present in the Kelvin-Voigt fractional derivative wave equation and time-fractional diffusion-wave equation is inferred from the GS model. The shear wave equation from the GS model predicts both diffusion and wave propagation in the fractional framework. The overall goal is intended to show that fractional calculus is not just a mathematical framework which can be used to curve-fit the complex behavior of materials, but rather it can be justified from real physical process of grain-shearing as well.
Shear-wave velocity of slope sediments near Hudson Canyon from analysis of ambient noise
NASA Astrophysics Data System (ADS)
Miller, N. C.; Ten Brink, U. S.; Collins, J. A.; McGuire, J. J.; Flores, C. H.
2014-12-01
We present new ambient noise data that help constrain the shear strength of marine sediments on the continental slope north of Hudson Canyon on the U.S. Atlantic margin. Sediment shear strength is a key parameter in models of potentially tsunamigenic, submarine slope failures, but shear strength is difficult to measure in situ and is expected to evolve in time with changes in pore pressure. The ambient noise data were recorded by 11 short-period, ocean-bottom seismometers and hydrophones deployed in a ~1 by 1.5 km array for ~6 months on the continental slope. These high frequency (~0.1 - 50 Hz), narrow-aperture data are expected to record noise propagating as interface waves and/or resonating in the upper ~500 m of sediment. Propagation of interface waves is controlled by the shear-wave velocity of the sediment, which we measure by calculating lag-times in cross-correlations of waveforms recorded by pairs of receivers. These measurements of shear-wave velocity will be used to constrain shear strength. The data also appear to record wind-generated noise resonating in layered sediment. We expect this resonance to also be sensitive to shear-wave velocity, and spectral analysis and modeling of harmonics may provide a second constraint on sediment shear strength. Both the correlogram- and spectral-based measurements can be made using hour- to day-long segments of data, enabling us to constrain temporal evolution of shear-wave velocity and potential forcing mechanisms (e.g., tidal and storm loading and submarine groundwater discharge) through the ~6 month deployment.
Linear mechanism of surface gravity wave generation in horizontally sheared flow
Kalashnik, M. V.
2008-01-15
An analysis is presented of a linear mechanism of surface gravity wave generation in a horizontally sheared flow in a fluid layer with free boundary. A free-surface flow of this type is found to be algebraically unstable. The development of instability leads to the formation of surface gravity waves whose amplitude grows with time according to a power law. Flow stability is analyzed by using a nonmodal approach in which the behavior of a spatial Fourier harmonic of a disturbance is considered in a semi-Lagrangian frame of reference moving with the flow. Shear-flow disturbances are divided into two classes (wave and vortex disturbances) depending on the value of potential vorticity. It is shown that vortex disturbances decay with time while the energy of wave disturbances increases indefinitely. Transformation of vortex disturbances into wave ones under strong shear is described.
Apparatus for checking the direction of polarization of shear-wave ultrasonic transducers
Karplus, H.H.B.; Forster, G.A.
An apparatus for checking the direction of polarization of shear-wave ultrasonic transducers comprises a first planar surface for mounting the shear-wave transducer, a second planar surface inclined at a predetermined angle to the first surface to generate longitudinal waves by mode conversion, and a third planar surface disposed at a second predetermined angle to the first for mounting a longitudinal-wave ultransonic transducer. In an alternate embodiment, two second planar surfaces at the predetermined angle are placed at an angle to each other. The magnitude of the shear wave is a function of the angle between the direction of polarization of the transducer and the mode-conversion surface.
Apparatus for checking the direction of polarization of shear-wave ultrasonic transducers
Karplus, Henry H. B.
1980-01-01
An apparatus for checking the direction of polarization of shear-wave ultrasonic transducers comprises a first planar surface for mounting the shear-wave transducer, a second planar surface inclined at a predetermined angle to the first surface to generate longitudinal waves by mode conversion, and a third planar surface disposed at a second predetermined angle to the first for mounting a longitudinal-wave ultrasonic transducer. In an alternate embodiment, two second planar surfaces at the predetermined angle are placed at an angle to each other. The magnitude of the shear wave is a function of the angle between the direction of polarization of the transducer and the mode-conversion surface.
Over-reflection of slow magnetosonic waves by homogeneous shear flow: Analytical solution
Dimitrov, Z. D.; Maneva, Y. G.; Hristov, T. S.; Mishonov, T. M.
2011-08-15
We have analyzed the amplification of slow magnetosonic (or pseudo-Alfvenic) waves (SMW) in incompressible shear flow. As found here, the amplification depends on the component of the wave-vector perpendicular to the direction of the shear flow. Earlier numerical results are consistent with the general analytic solution for the linearized magnetohydrodynamic equations, derived here for the model case of pure homogeneous shear (without Coriolis force). An asymptotically exact analytical formula for the amplification coefficient is derived for the case when the amplification is sufficiently large.
Kgaswane, E M; Nyblade, A A; Julia, J; Dirks, P H H M; Durrheim, R J; Pasyanos, M E
2008-11-11
Crustal structure in southern Africa has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities for 89 broadband seismic stations spanning much of the Precambrian shield of southern Africa. 1-D shear wave velocity profiles obtained from the inversion yield Moho depths that are similar to those reported in previous studies and show considerable variability in the shear wave velocity structure of the lower part of the crust between some terrains. For many of the Archaean and Proterozoic terrains in the shield, S velocities reach 4.0 km/s or higher over a substantial part of the lower crust. However, for most of the Kimberley terrain and adjacent parts of the Kheis Province and Witwatersrand terrain, as well as for the western part of the Tokwe terrain, mean shear wave velocities of {le} 3.9 km/s characterize the lower part of the crust along with slightly ({approx}5 km) thinner crust. These findings indicate that the lower crust across much of the shield has a predominantly mafic composition, except for the southwest portion of the Kaapvaal Craton and western portion of the Zimbabwe Craton, where the lower crust is intermediate-to-felsic in composition. The parts of the Kaapvaal Craton underlain by intermediate-to-felsic lower crust coincide with regions where Ventersdorp rocks have been preserved, and thus we suggest that the intermediate-to-felsic composition of the lower crust and the shallower Moho may have resulted from crustal melting during the Ventersdorp tectonomagmatic event at c. 2.7 Ga and concomitant crustal thinning caused by rifting.
Source signature and elastic waves in a half-space under a momentary shear line impulse
NASA Astrophysics Data System (ADS)
Ziv, Moche
2003-03-01
The transient deformation of an elastic half-space under a line-concentrated impulsive vector shear load applied momentarily is disclosed in this paper. While in an earlier work, the author gave an analytical-numerical method for the solution to this transient boundary-value problem, here, the resultant response of the half-space is presented and interpreted. In particular, a probe is set up for the kinematics of the source signature and wave fronts, both explicitly revealed in the strained half-space by the solution method. The source signature is the imprint of the spatiotemporal configuration of the excitation source in the resultant response. Fourteen wave fronts exist behind the precursor shear wave S: four concentric cylindrical, eight plane, and two relativistic cylindrical initiated at propagating centres that are located on the stationary boundaries of the solution domain. A snapshot of the stressed half-space reveals that none of the 14 wave fronts fully extend laterally. Instead, each is enclosed within point bounds. These wave arresting points and the two propagating centres of the relativistic waves constitute the source signature. The obtained 14 wave fronts are further combined into 11 disparate wave fronts that are grouped into four categories: an axis of symmetry wave - so named here by reason of being a wave front that is contiguous to the axis of symmetry, three body waves, five surface waves and two inhibitor waves - so named here by reason that beyond them the material motion dies out. Of the three body waves, the first is an unloading shear wave, the second is a diffracted wave and the third is a reflected longitudinal two-branch wave. Of the two inhibitor waves, the first is a two-joint relativistic wave, while the second is a two-branch wave. The wave system, however, is not the same for all the dependent variables; a wave front that appears in the behaviour of one dependent variable may not exist in the behaviour of another. It is evident
Modified ion-acoustic solitary waves in plasmas with field-aligned shear flows
Saleem, H.; Haque, Q.
2015-08-15
The nonlinear dynamics of ion-acoustic waves is investigated in a plasma having field-aligned shear flow. A Korteweg-deVries-type nonlinear equation for a modified ion-acoustic wave is obtained which admits a single pulse soliton solution. The theoretical result has been applied to solar wind plasma at 1 AU for illustration.
Modified ion-acoustic solitary waves in plasmas with field-aligned shear flows
NASA Astrophysics Data System (ADS)
Saleem, H.; Ali, S.; Haque, Q.
2015-08-01
The nonlinear dynamics of ion-acoustic waves is investigated in a plasma having field-aligned shear flow. A Koeteweg-deVries-type nonlinear equation for a modified ion-acoustic wave is obtained which admits a single pulse soliton solution. The theoretical result has been applied to solar wind plasma at 1 AU for illustration.
Experimental observations of transverse shear waves in strongly coupled dusty plasmas.
Pramanik, J; Prasad, G; Sen, A; Kaw, P K
2002-04-29
We report experimental observations of transverse shear waves in a three-dimensional dusty plasma that is in the strongly coupled fluid regime. These spontaneous oscillations occur when the ambient neutral pressure is reduced below a threshold value and the measured dispersion characteristics of these waves are found to be in good agreement with predictions of a viscoelastic theory of dusty plasmas.
NASA Technical Reports Server (NTRS)
Bui, Trong T.; Mankbadi, Reda R.
1995-01-01
Numerical simulation of a very small amplitude acoustic wave interacting with a shock wave in a quasi-1D convergent-divergent nozzle is performed using an unstructured finite volume algorithm with a piece-wise linear, least square reconstruction, Roe flux difference splitting, and second-order MacCormack time marching. First, the spatial accuracy of the algorithm is evaluated for steady flows with and without the normal shock by running the simulation with a sequence of successively finer meshes. Then the accuracy of the Roe flux difference splitting near the sonic transition point is examined for different reconstruction schemes. Finally, the unsteady numerical solutions with the acoustic perturbation are presented and compared with linear theory results.
NASA Astrophysics Data System (ADS)
Chatterjee, Mita; Dhua, Sudarshan; Chattopadhyay, Amares
2015-12-01
The present paper is concerned with the propagation of shear waves in an isotropic, viscoelastic, heterogeneous layer lying over a homogeneous half space under initial stress. For the layer the inhomogeneity associated to rigidity, internal friction and density is assumed to be linear function of depth. The dispersion equation of shear waves has been obtained in closed form. The dimensionless phase velocity and damping velocity have been plotted against dimensionless wave number for different values of inhomogeneity parameter and initial stress. The effects of inhomogeneity and initial stress have been shown in the dispersion curves.
Estimation of pseudo-2D shear-velocity section by inversion of high frequency surface waves
Luo, Y.; Liu, J.; Xia, J.; Xu, Y.; Liu, Q.
2006-01-01
A scheme to generate pseudo-2D shear-velocity sections with high horizontal resolution and low field cost by inversion of high frequency surface waves is presented. It contains six steps. The key step is the joint method of crossed correlation and phase shift scanning. This joint method chooses only two traces to generate image of dispersion curve. For Rayleigh-wave dispersion is most important for estimation of near-surface shear-wave velocity, it can effectively obtain reliable images of dispersion curves with a couple of traces. The result of a synthetic example shows the feasibility of this scheme. ?? 2005 Society of Exploration Geophysicists.
Reduced Imaging Rate in Liver Elastometery Using Shear Wave Interference Patterns.
Soozande, Mehdi; Arabalibeik, Hossein; Alavian, Seyed Moayed
2016-01-01
Inducing interference patterns of shear wave is one of the proposed methods for reducing the frame rate in measuring wave speed during tissue elastography. Previously, the Nyquist rate must be met in order to provide an appropriate image for extracting the patterns with a reasonable accuracy. In this article we propose a technique based on image registration, and apply it to ultrasound images acquired before and after inducing the shear waves to estimate the amplitude of displacement. The displacement of the tissue is then used to form the interference pattern of shear waves. The method does not induce any restrictions on the time interval between images, so the tissue elasticity can be calculated independent of the imaging rate. The average error in measuring the elasticity of the simulated phantom is 13.7%. PMID:27046611
Reduced Imaging Rate in Liver Elastometery Using Shear Wave Interference Patterns.
Soozande, Mehdi; Arabalibeik, Hossein; Alavian, Seyed Moayed
2016-01-01
Inducing interference patterns of shear wave is one of the proposed methods for reducing the frame rate in measuring wave speed during tissue elastography. Previously, the Nyquist rate must be met in order to provide an appropriate image for extracting the patterns with a reasonable accuracy. In this article we propose a technique based on image registration, and apply it to ultrasound images acquired before and after inducing the shear waves to estimate the amplitude of displacement. The displacement of the tissue is then used to form the interference pattern of shear waves. The method does not induce any restrictions on the time interval between images, so the tissue elasticity can be calculated independent of the imaging rate. The average error in measuring the elasticity of the simulated phantom is 13.7%.
Generation of shear Alfven waves by a rotating magnetic field source: Three-dimensional simulations
Karavaev, A. V.; Gumerov, N. A.; Papadopoulos, K.; Shao, Xi; Sharma, A. S.; Gekelman, W.; Wang, Y.; Van Compernolle, B.; Pribyl, P.; Vincena, S.
2011-03-15
The paper discusses the generation of polarized shear Alfven waves radiated from a rotating magnetic field source created via a phased orthogonal two-loop antenna. A semianalytical three-dimensional cold two-fluid magnetohydrodynamics model was developed and compared with recent experiments in the University of California, Los Angeles large plasma device. Comparison of the simulation results with the experimental measurements and the linear shear Alfven wave properties, namely, spatiotemporal wave structure, a dispersion relation with nonzero transverse wave number, the magnitude of the wave dependences on the wave frequency, show good agreement. From the simulations it was found that the energy of the Alfven wave generated by the rotating magnetic field source is distributed between the kinetic energy of ions and electrons and the electromagnetic energy of the wave as: {approx}1/2 is the energy of the electromagnetic field, {approx}1/2 is the kinetic energy of the ion fluid, and {approx}2.5% is the kinetic energy of electron fluid for the experiment. The wave magnetic field power calculated from the experimental data and using a fluid model differ by {approx}1% and is {approx}250 W for the experimental parameters. In both the experiment and the three-dimensional two-fluid magnetohydrodynamics simulations the rotating magnetic field source was found to be very efficient for generating shear Alfven waves.
Song, Shaozhen; Le, Nhan Minh; Huang, Zhihong; Shen, Tueng; Wang, Ruikang K
2015-11-01
The purpose of this study is to implement a beam-steering ultrasound as the wave source for shear-wave optical coherence elastography (SW-OCE) to achieve an extended range of elastic imaging of the tissue sample. We introduce a linear phased array ultrasound transducer (LPAUT) as the remote and programmable wave source and a phase-sensitive optical coherence tomography (OCT) as the sensitive shear-wave detector. The LPAUT is programmed to launch acoustic radiation force impulses (ARFI) focused at desired locations within the range of OCT imaging, upon which the elasticity map of the entire OCT B-scan cross section is recovered by spatial compounding of the elastic maps derived from each launch of AFRIs. We also propose a directional filter to separate the shear-wave propagation at different directions in order to reduce the effect of tissue heterogeneity on the shear-wave propagation within tissue. The feasibility of this proposed approach is then demonstrated by determining the stiffness of tissue-mimicking phantoms with agarose concentrations of 0.5% and 1% and also by imaging the Young's modulus of retinal and choroidal tissues within a porcine eye ball ex vivo. The approach opens up opportunities to combine medical ultrasound imaging and SW-OCE for high-resolution localized quantitative assessment of tissue biomechanical property.
Prediction of shear wave velocity using empirical correlations and artificial intelligence methods
NASA Astrophysics Data System (ADS)
Maleki, Shahoo; Moradzadeh, Ali; Riabi, Reza Ghavami; Gholami, Raoof; Sadeghzadeh, Farhad
2014-06-01
Good understanding of mechanical properties of rock formations is essential during the development and production phases of a hydrocarbon reservoir. Conventionally, these properties are estimated from the petrophysical logs with compression and shear sonic data being the main input to the correlations. This is while in many cases the shear sonic data are not acquired during well logging, which may be for cost saving purposes. In this case, shear wave velocity is estimated using available empirical correlations or artificial intelligent methods proposed during the last few decades. In this paper, petrophysical logs corresponding to a well drilled in southern part of Iran were used to estimate the shear wave velocity using empirical correlations as well as two robust artificial intelligence methods knows as Support Vector Regression (SVR) and Back-Propagation Neural Network (BPNN). Although the results obtained by SVR seem to be reliable, the estimated values are not very precise and considering the importance of shear sonic data as the input into different models, this study suggests acquiring shear sonic data during well logging. It is important to note that the benefits of having reliable shear sonic data for estimation of rock formation mechanical properties will compensate the possible additional costs for acquiring a shear log.
NASA Astrophysics Data System (ADS)
Yoshizawa, K.
2014-10-01
A new radially anisotropic shear wave speed model for the Australasian region is constructed from multi-mode phase dispersion of Love and Rayleigh waves. An automated waveform fitting technique based on a global optimization with the Neighbourhood Algorithm allows the exploitation of large numbers of three-component broad-band seismograms to extract path-specific dispersion curves covering the entire continent. A 3-D shear wave model is constructed including radial anisotropy from a set of multi-mode phase speed maps for both Love and Rayleigh waves. These maps are derived from an iterative inversion scheme incorporating the effects of ray-path bending due to lateral heterogeneity, as well as the finite frequency of the surface waves for each mode. The new S wave speed model exhibits major tectonic features of this region that are in good agreement with earlier shear wave models derived primarily from Rayleigh waves. The lateral variations of depth and thickness of the lithosphere-asthenosphere transition (LAT) are estimated from the isotropic (Voigt average) S wave speed model and its vertical gradient, which reveals correlations between the lateral variations of the LAT and radial anisotropy. The thickness of the LAT is very large beneath the Archean cratons in western Australia, whereas that in south Australia is thinner. The radial anisotropy model shows faster SH wave speed than SV beneath eastern Australia and the Coral Sea at the lithospheric depth. The faster SH anomaly in the lithosphere is also seen in the suture zone between the three cratonic blocks of Australia. One of the most conspicuous features of fast SH anisotropy is found in the asthenosphere beneath the central Australia, suggesting anisotropy induced by shear flow in the asthenosphere beneath the fast drifting Australian continent.
Qiang, Bo; Brigham, John C; Aristizabal, Sara; Greenleaf, James F; Zhang, Xiaoming; Urban, Matthew W
2015-02-01
In this paper, we propose a method to model the shear wave propagation in transversely isotropic, viscoelastic and incompressible media. The targeted application is ultrasound-based shear wave elastography for viscoelasticity measurements in anisotropic tissues such as the kidney and skeletal muscles. The proposed model predicts that if the viscoelastic parameters both across and along fiber directions can be characterized as a Voigt material, then the spatial phase velocity at any angle is also governed by a Voigt material model. Further, with the aid of Taylor expansions, it is shown that the spatial group velocity at any angle is close to a Voigt type for weakly attenuative materials within a certain bandwidth. The model is implemented in a finite element code by a time domain explicit integration scheme and shear wave simulations are conducted. The results of the simulations are analyzed to extract the shear wave elasticity and viscosity for both the spatial phase and group velocities. The estimated values match well with theoretical predictions. The proposed theory is further verified by an ex vivo tissue experiment measured in a porcine skeletal muscle by an ultrasound shear wave elastography method. The applicability of the Taylor expansion to analyze the spatial velocities is also discussed. We demonstrate that the approximations from the Taylor expansions are subject to errors when the viscosities across or along the fiber directions are large or the maximum frequency considered is beyond the bandwidth defined by radii of convergence of the Taylor expansions.
Qiang, Bo; Brigham, John C.; Aristizabal, Sara; Greenleaf, James F.; Zhang, Xiaoming; Urban, Matthew W.
2015-01-01
In this paper, we propose a method to model the shear wave propagation in transversely isotropic, viscoelastic and incompressible media. The targeted application is ultrasound-based shear wave elastography for viscoelasticity measurements in anisotropic tissues such as the kidney and skeletal muscles. The proposed model predicts that if the viscoelastic parameters both across and along fiber directions can be characterized as a Voigt material, then the spatial phase velocity at any angle is also governed by a Voigt material model. Further, with the aid of Taylor expansions, it is shown that the spatial group velocity at any angle is close to a Voigt type for weakly attenuative materials within a certain bandwidth. The model is implemented in a finite element code by a time domain explicit integration scheme and shear wave simulations are conducted. The results of the simulations are analyzed to extract the shear wave elasticity and viscosity for both the spatial phase and group velocities. The estimated values match well with theoretical predictions. The proposed theory is further verified by an ex vivo tissue experiment measured in a porcine skeletal muscle by an ultrasound shear wave elastography method. The applicability of the Taylor expansion to analyze the spatial velocities is also discussed. We demonstrate that the approximations from the Taylor expansions are subject to errors when the viscosities across or along the fiber directions are large or the maximum frequency considered is beyond the bandwidth defined by radii of convergence of the Taylor expansions. PMID:25591921
Song, Shaozhen; Arnal, Bastien; Huang, Zhihong; O’Donnell, Matthew; Wang, Ruikang K.
2015-01-01
We report on the use of phase-sensitive optical coherence tomography (PhS-OCT) to detect and track temporally and spatially shear wave propagation within tissue induced by ultrasound radiation force. Kilohertz-range shear waves are remotely generated in sample using focused ultrasound emission and their propagation is tracked using PhS-OCT. Cross-sectional maps of the local shear modulus are reconstructed from local estimates of shear wave speed in tissue-mimicking phantoms. We demonstrate the feasibility of combining ultrasound radiation force and PhS-OCT to perform high-resolution mapping of the shear modulus. PMID:24562220
NASA Astrophysics Data System (ADS)
Pitarka, A.
2015-12-01
Arben Pitarka, Souheil M. Ezzedine, Oleg Y. Vorobiev, Tarabay H. Antoun, Lew A. Glenn, William R. Walter, Robert J. Mellors, and Evan Hirakawa. We have analyzed effects of wave scattering due to near-source structural complexity and sliding joint motion on generation of shear waves from SPE-4Pprime, a shallow chemical explosion conducted at the Nevada National Security Site. In addition to analyzing far-field ground motion recorded on three-component geophones, we performed high-frequency simulations of the explosion using a finite difference method and heterogeneous media with stochastic variability. The stochastic variations of seismic velocity were modeled using Gaussian correlation functions. Using simulations and recorded waveforms we demonstrate the implication of wave scattering on generation of shear motion, and show the gradual increase of shear motion energy as the waves propagate through media with variable scattering. The amplitude and duration of shear waves resulting from wave scattering are found to be dependent on the model complexity and to a lesser extent to source distance. Analysis of shear-motion generation due to joint motion were conducted using numerical simulations performed with GEODYN-L, a parallelized Lagrangian hydrocode, while a stochastic approach was used in depicting the properties of joints. Separated effects of source and wave scattering on shear motion generation will be shown through simulated motion. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 Release Number: LLNL-ABS-675570
Hedayatrasa, Saeid; Abhary, Kazem; Uddin, Mohammad
2015-03-01
The optimum topology of bimaterial phononic crystal (PhCr) plates with one-dimensional (1D) periodicity to attain maximum relative bandgap width of low order Lamb waves is computationally investigated. The evolution of optimized topology with respect to filling fraction of constituents, alternatively stiff scattering inclusion, is explored. The underlying idea is to develop PhCr plate structures with high specific bandgap efficiency at particular filling fraction, or further with multiscale functionality through gradient of optimized PhCr unitcell all over the lattice array. Multiobjective genetic algorithm (GA) is employed in this research in conjunction with finite element method (FEM) for topology optimization of silicon-tungsten PhCr plate unitcells. A specialized FEM model is developed and verified for dispersion analysis of plate waves and calculation of modal response. Modal band structure of regular PhCr plate unitcells with centric scattering layer is studied as a function of aspect ratio and filling fraction. Topology optimization is then carried out for a few aspect ratios, with and without prescribed symmetry, over various filling fractions. The efficiency of obtained solutions is verified as compared to corresponding regular centric PhCr plate unitcells. Moreover, being inspired by the obtained optimum topologies, definite and easy to produce topologies are proposed with enhanced bandgap efficiency as compared to centric unitcells. Finally a few cases are introduced to evaluate the frequency response of finite PhCr plate structures produced by achieved topologies and also to confirm the reliability of calculated modal band structures. Cases made by consecutive unitcells of different filling fraction are examined in order to attest the bandgap efficiency and multiscale functionality of such graded PhCr plate structures.
Nonlinear electron acoustic waves in presence of shear magnetic field
Dutta, Manjistha; Khan, Manoranjan; Ghosh, Samiran; Chakrabarti, Nikhil
2013-12-15
Nonlinear electron acoustic waves are studied in a quasineutral plasma in the presence of a variable magnetic field. The fluid model is used to describe the dynamics of two temperature electron species in a stationary positively charged ion background. Linear analysis of the governing equations manifests dispersion relation of electron magneto sonic wave. Whereas, nonlinear wave dynamics is being investigated by introducing Lagrangian variable method in long wavelength limit. It is shown from finite amplitude analysis that the nonlinear wave characteristics are well depicted by KdV equation. The wave dispersion arising in quasineutral plasma is induced by transverse magnetic field component. The results are discussed in the context of plasma of Earth's magnetosphere.
NASA Technical Reports Server (NTRS)
Dahl, Milo D.
2000-01-01
An acoustic source inside of a 2-D jet excites an instability wave in the shear layer resulting in sound radiating away from the shear layer. Solve the linearized Euler equations to predict the sound radiation outside of the jet. The jet static pressure is assumed to be constant. The jet flow is parallel and symmetric about the x-axis. Use a symmetry boundary condition along the x-axis.
Large-Scale Magnetic Field Generation by Randomly Forced Shearing Waves
NASA Astrophysics Data System (ADS)
Heinemann, T.; McWilliams, J. C.; Schekochihin, A. A.
2011-12-01
A rigorous theory for the generation of a large-scale magnetic field by random nonhelically forced motions of a conducting fluid combined with a linear shear is presented in the analytically tractable limit of low magnetic Reynolds number (Rm) and weak shear. The dynamo is kinematic and due to fluctuations in the net (volume-averaged) electromotive force. This is a minimal proof-of-concept quasilinear calculation aiming to put the shear dynamo, a new effect recently found in numerical experiments, on a firm theoretical footing. Numerically observed scalings of the wave number and growth rate of the fastest-growing mode, previously not understood, are derived analytically. The simplicity of the model suggests that shear dynamo action may be a generic property of sheared magnetohydrodynamic turbulence.
Observations of intense velocity shear and associated electrostatic waves near an auroral arc
NASA Technical Reports Server (NTRS)
Kelley, M. C.; Carlson, C. W.
1977-01-01
Simultaneous measurements of energetic particles and ac electric fields made by the javelin sounding rocket NASA 8:56 during the late expansion phase of a magnetic storm have revealed an intense shear in plasma flow of magnitude 20 (m/s)/m at the edge of an auroral arc. Structure with two characteristic scales sizes is displayed in the region of shear. Larger structures are of the order of several kilometers in size. Intense irregularities with characteristic wavelengths smaller than the scale size of the shear have also been detected. The large-scale changes in the orientation of the charge sheet at the edge of the arc may be due to the Kelvin-Helmholtz branch; shorter-wavelength modes may be related to the shear driven resistive drift wave. Observations are consistent with the suggestion that velocity shear instabilities may play a role in the formation of high-latitude irregularities.
Helgerud, M.B.; Waite, W.F.; Kirby, S.H.; Nur, A.
2003-01-01
We report on laboratory measurements of compressional- and shear-wave speeds in a compacted, polycrystalline ice-Ih sample. The sample was made from triply distilled water that had been frozen into single crystal ice, ground into small grains, and sieved to extract the 180-250 ??m diameter fraction. Porosity was eliminated from the sample by compacting the granular ice between a hydraulically driven piston and a fixed end plug, both containing shear-wave transducers. Based on simultaneous compressional- and shear-wave-speed measurements, we calculated Poisson's ratio and compressional-wave, bulk, and shear moduli from -20 to -5??C and 22 to 33 MPa.
NASA Astrophysics Data System (ADS)
Zvietcovich, Fernando; Yao, Jianing; Chu, Ying-Ju; Meemon, Panomsak; Rolland, Jannick P.; Parker, Kevin J.
2016-03-01
Optical Coherence Elastography (OCE) is a widely investigated noninvasive technique for estimating the mechanical properties of tissue. In particular, vibrational OCE methods aim to estimate the shear wave velocity generated by an external stimulus in order to calculate the elastic modulus of tissue. In this study, we compare the performance of five acquisition and processing techniques for estimating the shear wave speed in simulations and experiments using tissue-mimicking phantoms. Accuracy, contrast-to-noise ratio, and resolution are measured for all cases. The first two techniques make the use of one piezoelectric actuator for generating a continuous shear wave propagation (SWP) and a tone-burst propagation (TBP) of 400 Hz over the gelatin phantom. The other techniques make use of one additional actuator located on the opposite side of the region of interest in order to create an interference pattern. When both actuators have the same frequency, a standing wave (SW) pattern is generated. Otherwise, when there is a frequency difference df between both actuators, a crawling wave (CrW) pattern is generated and propagates with less speed than a shear wave, which makes it suitable for being detected by the 2D cross-sectional OCE imaging. If df is not small compared to the operational frequency, the CrW travels faster and a sampled version of it (SCrW) is acquired by the system. Preliminary results suggest that TBP (error < 4.1%) and SWP (error < 6%) techniques are more accurate when compared to mechanical measurement test results.
Heating of coronal loops by phase-mixid shear Alfven waves
NASA Technical Reports Server (NTRS)
Abdelatif, Toufik E.
1987-01-01
The dissipation of shear Alfven waves in a coronal loop driven externally by an incident wave in the subcoronal region is investigated. The phase mixing of these incident shear Alfven waves serves as the dissipation mechanism in the corona. The wave solution found by Heyvaerts and Priest (1983) for coronal holes is used to compute the total energy deposited in a loop. The energy deposited is shown to depend upon the magnetic diffusivity nu(m) and viscosity nu(v), contrary to the conclusion of authors who assumed that coronal loops are perfect resonators. The energy deposited in a three-layer model is computed for incident waves with periods of five minutes or five seconds. For a five-minute period, almost no energy is deposited, especially for small loops. For a five-second period, a substantial amount of energy is deposited in the loop, but not enough to account for the heating of small loops.
NASA Astrophysics Data System (ADS)
Papazoglou, S.; Hamhaber, U.; Braun, J.; Sack, I.
2007-02-01
A method based on magnetic resonance elastography is presented that allows measuring the weldedness of interfaces between soft tissue layers. The technique exploits the dependence of shear wave scattering at elastic interfaces on the frequency of vibration. Experiments were performed on gel phantoms including differently welded interfaces. Plane wave excitation parallel to the planar interface with corresponding motion sensitization enabled the observation of only shear-horizontal (SH) wave scattering. Spatio-temporal filtering was applied to calculate scattering coefficients from the amplitudes of the incident, transmitted and reflected SH-waves in the vicinity of the interface. The results illustrate that acoustic wave scattering in soft tissues is largely dependent on the connectivity of interfaces, which is potentially interesting for imaging tissue mechanics in medicine and biology.
Gravity shear waves atop the cirrus layer of intense convective storms
NASA Technical Reports Server (NTRS)
Stobie, J. G.
1975-01-01
Recent visual satellite photographs of certain intense convective storms have revealed concentric wave patterns. A model for the generation and growth of these waves is proposed. The proposed initial generating mechanism is similar to the effect noticed when a pebble is dropped into a calm pond. The penetration of the tropopause by overshooting convection is analogous to the pebble's penetration of the water's surface. The model for wave growth involves instability due to the wind shear resulting from the cirrus outflow. This model is based on an equation for the waves' phase speed which is similar to the Helmholtz equation. It, however, does not assume an incompressible atmosphere, but rather assumes density is a logarithmic function of height. Finally, the model is evaluated on the two mid-latitude and three tropical cases. The data indicate that shearing instability may be a significant factor in the appearance of these waves.
NASA Astrophysics Data System (ADS)
Lontsi, A. M.; Ohrnberger, M.; Krüger, F.
2016-07-01
We present an integrated approach for deriving the 1D shear wave velocity (Vs) information at few tens to hundreds of meters down to the first strong impedance contrast in typical sedimentary environments. We use multiple small aperture seismic arrays in 1D and 2D configuration to record active and passive seismic surface wave data at two selected geotechnical sites in Germany (Horstwalde & Löbnitz). Standard methods for data processing include the Multichannel Analysis of Surface Waves (MASW) method that exploits the high frequency content in the active data and the sliding window frequency-wavenumber (f-k) as well as the spatial autocorrelation (SPAC) methods that exploit the low frequency content in passive seismic data. Applied individually, each of the passive methods might be influenced by any source directivity in the noise wavefield. The advantages of active shot data (known source location) and passive microtremor (low frequency content) recording may be combined using a correlation based approach applied to the passive data in the so called Interferometric Multichannel Analysis of Surface Waves (IMASW). In this study, we apply those methods to jointly determine and interpret the dispersion characteristics of surface waves recorded at Horstwalde and Löbnitz. The reliability of the dispersion curves is controlled by applying strict limits on the interpretable range of wavelengths in the analysis and further avoiding potentially biased phase velocity estimates from the passive f-k method by comparing to those derived from the SPatial AutoCorrelation method (SPAC). From our investigation at these two sites, the joint analysis as proposed allows mode extraction in a wide frequency range (~ 0.6-35 Hz at Horstwalde and ~ 1.5-25 Hz at Löbnitz) and consequently improves the Vs profile inversion. To obtain the shear wave velocity profiles, we make use of a global inversion approach based on the neighborhood algorithm to invert the interpreted branches of the
Imaging of shear waves induced by Lorentz force in soft tissues.
Grasland-Mongrain, P; Souchon, R; Cartellier, F; Zorgani, A; Chapelon, J Y; Lafon, C; Catheline, S
2014-07-18
This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 μm. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample.
Imaging of Shear Waves Induced by Lorentz Force in Soft Tissues
NASA Astrophysics Data System (ADS)
Grasland-Mongrain, P.; Souchon, R.; Cartellier, F.; Zorgani, A.; Chapelon, J. Y.; Lafon, C.; Catheline, S.
2014-07-01
This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 μm. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample.
Tariq, Sabeen; Mirza, Arshad M.; Masood, W.
2010-10-15
The propagation of high and low frequency (in comparison with the cyclotron frequency) electrostatic drift-waves is investigated in a nonuniform, dense magnetoplasma (composed of electrons and ions), in the presence of parallel shear flow, by employing the quantum magnetohydrodynamic (QMHD) model. Using QMHD model, a new set of equations is presented in order to investigate linear properties of electrostatic drift-waves with sheared plasma flows for dense plasmas. In this regard, dispersion relations for coupled electron-thermal and drift-ion acoustic modes are derived and several interesting limiting cases are discussed. For instance, it is found that sheared ion flow parallel to the external magnetic field can drive the quantum drift-ion acoustic wave unstable, etc. The present investigation may have relevance in dense astrophysical environments where quantum effects are significant.
Self-limiting feedback between baroclinic waves and a NAO-like sheared zonal flow
NASA Astrophysics Data System (ADS)
Watanabe, Masahiro
2009-04-01
The eddy-mean flow interaction associated with the North Atlantic Oscillation (NAO) is examined by using the baroclinic wave life cycle experiments. When a sheared zonal flow perturbation akin to the NAO-related dipole wind anomaly is added to the basic state, momentum fluxes due to baroclinic waves tend to reinforce the initial zonal flow dipole in the upper troposphere both for the anticyclonic and cyclonic shears. The eddy feedback is stronger for the anticyclonic shear because the node of the dipole flow is asymmetric about the basic jet, suggesting that the positive NAO is more favored by the eddy feedback. For the zonal wind anomaly with extremely large amplitude, the baroclinic wave breaking cannot efficiently intensify the zonal flow dipole, indicating a self-limitation in the positive eddy feedback to the NAO-like zonal flow anomaly.
NASA Astrophysics Data System (ADS)
Mochizuki, Yuta; Taki, Hirofumi; Kanai, Hiroshi
2016-07-01
An elastic property of biological soft tissue is an important indicator of the tissue status. Therefore, quantitative and noninvasive methods for elasticity evaluation have been proposed. Our group previously proposed a method using acoustic radiation pressure irradiated from two directions for elastic property evaluation, in which by measuring the propagation velocity of the shear wave generated by the acoustic radiation pressure inside the object, the elastic properties of the object were successfully evaluated. In the present study, we visualized the propagation of the shear wave in a three-dimensional space by the synchronization of signals received at various probe positions. The proposed method succeeded in visualizing the shear wave propagation clearly in the three-dimensional space of 35 × 41 × 4 mm3. These results show the high potential of the proposed method to estimate the elastic properties of the object in the three-dimensional space.
Nature and dynamics of overreflection of Alfvén waves in MHD shear flows
NASA Astrophysics Data System (ADS)
Gogichaishvili, D.; Chagelishvili, G.; Chanishvili, R.; Lominadze, J.; Lominadze
2014-10-01
Our goal is to gain new insights into the physics of wave overreflection phenomenon in magnetohydrodynamic (MHD) nonuniform/shear flows changing the existing trend/approach of the phenomenon study. The performed analysis allows to separate from each other different physical processes, grasp their interplay and, by this way, construct the basic physics of the overreflection in incompressible MHD flows with linear shear of mean velocity, U 0=(Sy,0,0), that contain two different types of Alfvén waves. These waves are reduced to pseudo- and shear-Alfvén waves when wavenumber along Z-axis equals zero (i.e. when kz =0). Therefore, for simplicity, we labeled these waves as: P-Alfvén and S-Alfvén waves (P-AWs and S-AWs). We show that: (1) the linear coupling of counter-propagating waves determines the overreflection, (2) counter-propagating P-AWs are coupled with each other, while counter-propagating S-AWs are not coupled with each other, but are asymmetrically coupled with P-AWs; S-AWs do not participate in the linear dynamics of P-AWs, (3) the transient growth of S-AWs is somewhat smaller compared with that of P-AWs, (4) the linear transient processes are highly anisotropic in wave number space, (5) the waves with small streamwise wavenumbers exhibit stronger transient growth and become more balanced, (6) maximal transient growth (and overreflection) of the wave energy occurs in the two-dimensional case - at zero spanwise wavenumber. To the end, we analyze nonlinear consequences of the described anisotropic linear dynamics - they should lead to an anisotropy of nonlinear cascade processes significantly changing their essence, pointing to a need of revisiting the existing concepts of cascade processes in MHD shear flows.
Fracture characterization at the Conoco Borehole Test Facility using shear-wave anisotropy
Horne, S.A.; MacBeth, C.D.; Queen, J.; Rizer, W.D.
1995-12-31
Two multi-component near-offset VSP experiments are used, in conjunction with borehole data, to characterise the subsurface fracture system at the Conoco Borehole Test Facility, Oklahoma. Time delays between the fast and slow split shear-waves are observed to correlate with the heavily fractured sandstone formations. Inversion of the shear-wave splitting estimates is achieved using a Genetic Algorithm which incorporates an anisotropic ray tracing scheme. The inversion results suggest that the fracture orientation is sub-vertical. A method of determining fracture dip using an opposite azimuth VSP method is suggested.
Single Tracking Location Methods Suppress Speckle Noise in Shear Wave Velocity Estimation
Elegbe, Etana C.; McAleavey, Stephen A.
2014-01-01
In ultrasound-based elastography methods, the estimation of shear wave velocity typically involves the tracking of speckle motion due to an applied force. The errors in the estimates of tissue displacement, and thus shear wave velocity, are generally attributed to electronic noise and decorrelation due to physical processes. We present our preliminary findings on another source of error, namely, speckle-induced bias in phase estimation. We find that methods that involve tracking in a single location, as opposed to multiple locations, are less sensitive to this source of error since the measurement is differential in nature and cancels out speckle-induced phase errors. PMID:23493611
Guided torsional wave generation of a linear in-plane shear piezoelectric array in metallic pipes.
Zhou, Wensong; Yuan, Fuh-Gwo; Shi, Tonglu
2016-02-01
Cylindrical guided waves based techniques are effective and promising tools for damage detection in long pipes. The essential operations are generation and reception of guided waves in the structures utilizing transducers. A novel in-plane shear (d36 type) PMNT wafer is proposed to generate and receive the guided wave, especially the torsional waves, in metallic pipes. In contrast to the traditional wafer, this wafer will directly introduce in-plane shear deformation when electrical field is conveniently applied through its thickness direction. A single square d36 PMNT wafer is bonded on the surface of the pipe positioned collinearly with its axis, when actuated can predominantly generate torsional (T) waves along the axial direction, circumferential shear horizontal (C-SH) waves along circumferential direction, and other complex cylindrical Lamb-like wave modes along other helical directions simultaneously. While a linear array of finite square size d36 PMNT wafers was equally spaced circumferentially, when actuated simultaneously can nearly uniform axisymmetric torsional waves generate in pipes and non-symmetric wave modes can be suppressed greatly if the number of the d36 PMNT wafer is sufficiently large. This paper first presents the working mechanism of the linear d36 PMNT array from finite element analysis (FEA) by examining the constructive and destructive displacement wavefield phenomena in metallic pipes. Furthermore, since the amplitude of the received fundamental torsional wave signal strongly depends on frequency, a series of experiments are conducted to determine the frequency tuning curve for the torsional wave mode. All results indicate the linear d36 PMNT array has potential for efficiently generating uniform torsional wavefield of the fundamental torsional wave mode, which is more effective in monitoring structural health in metallic pipes. PMID:26548525
On System-Dependent Sources of Uncertainty and Bias in Ultrasonic Quantitative Shear-Wave Imaging.
Deng, Yufeng; Rouze, Ned C; Palmeri, Mark L; Nightingale, Kathryn R
2016-03-01
Ultrasonic quantitative shear-wave imaging methods have been developed over the last decade to estimate tissue elasticity by measuring the speed of propagating shear waves following acoustic radiation force excitation. This work discusses eight sources of uncertainty and bias arising from ultrasound system-dependent parameters in ultrasound shear-wave speed (SWS) measurements. Each of the eight sources of error is discussed in the context of a linear, isotropic, elastic, homogeneous medium, combining previously reported analyses with Field II simulations, full-wave 2-D acoustic propagation simulations, and experimental studies. Errors arising from both spatial and temporal sources lead to errors in SWS measurements. Arrival time estimation noise, speckle bias, hardware fluctuations, and phase aberration cause uncertainties (variance) in SWS measurements, while pulse repetition frequency (PRF) and beamforming errors, as well as coupling medium sound speed mismatch, cause biases in SWS measurements (accuracy errors). Calibration of the sources of bias is an important step in the development of shear-wave imaging systems. In a well-calibrated system, where the sources of bias are minimized, and averaging over a region of interest (ROI) is employed to reduce the sources of uncertainty, an SWS error can be expected. PMID:26886980
Measurement of mechanical properties of homogeneous tissue with ultrasonically induced shear waves
NASA Astrophysics Data System (ADS)
Greenleaf, James F.; Chen, Shigao
2007-03-01
Fundamental mechanical properties of tissue are altered by many diseases. Regional and systemic diseases can cause changes in tissue properties. Liver stiffness is caused by cirrhosis and fibrosis. Vascular wall stiffness and tone are altered by smoking, diabetes and other diseases. Measurement of tissue mechanical properties has historically been done with palpation. However palpation is subjective, relative, and not quantitative or reproducible. Elastography in which strain is measured due to stress application gives a qualitative estimate of Young's modulus at low frequency. We have developed a method that takes advantage of the fact that the wave equation is local and shear wave propagation depends only on storage and loss moduli in addition to density, which does not vary much in soft tissues. Our method is called shearwave dispersion ultrasonic velocity measurement (SDUV). The method uses ultrasonic radiation force to produce repeated motion in tissue that induces shear waves to propagate. The shear wave propagation speed is measured with pulse echo ultrasound as a function of frequency of the shear wave. The resulting velocity dispersion curve is fit with a Voight model to determine the elastic and viscous moduli of the tissue. Results indicate accurate and precise measurements are possible using this "noninvasive biopsy" method. Measurements in beef along and across the fibers are consistent with the literature values.
Effect of two ion species on the propagation of shear Alfven waves of small transverse scale
Vincena, S. T.; Morales, G. J.; Maggs, J. E.
2010-05-15
The results of a theoretical modeling study and experimental investigation of the propagation properties of shear Alfven waves of small transverse scale in a plasma with two ion species are reported. In the two ion plasma, depending on the mass of the heavier species, ion kinetic effects can become prominent, and significant parallel electric fields result in electron acceleration. The theory predicts the appearance of frequency propagation gaps at the ion-ion hybrid frequency and between harmonics of the lower cyclotron frequency. Within these frequency bands spatial structures arise that mix the cone-propagation characteristics of Alfven waves with radially expanding ion Bernstein modes. The experiments, performed at the Basic Plasma Science Facility (BaPSF) at UCLA, consist of the spatial mapping of shear waves launched by a loop antenna. Although a variety of two ion-species combinations were explored, only results from a helium-neon mix are reported. A clear signature of a shear wave propagation gap, as well as propagation between multiple harmonics, is found for this gas combination. The evanescence of shear waves beyond the reflection point at the ion-ion hybrid frequency in the presence of an axial magnetic field gradient is also documented.
Cortes, Daniel H; Suydam, Stephen M; Silbernagel, Karin Grävare; Buchanan, Thomas S; Elliott, Dawn M
2015-06-01
Viscoelastic mechanical properties are frequently altered after tendon injuries and during recovery. Therefore, non-invasive measurements of shear viscoelastic properties may help evaluate tendon recovery and compare the effectiveness of different therapies. The objectives of this study were to describe an elastography method for measuring localized viscoelastic properties of tendons and to discuss the initial results in healthy and injured human Achilles and semitendinosus tendons. The technique used an external actuator to generate the shear waves in the tendon at different frequencies and plane wave imaging to measure shear wave displacements. For each of the excitation frequencies, maps of direction-specific wave speeds were calculated using local frequency estimation. Maps of viscoelastic properties were obtained using a pixel-wise curve fit of wave speed and frequency. The method was validated by comparing measurements of wave speed in agarose gels with those obtained using magnetic resonance elastography. Measurements in human healthy Achilles tendons revealed a pronounced increase in wave speed as a function of frequency, which highlights the importance of tendon viscoelasticity. Additionally, the viscoelastic properties of the Achilles tendon were larger than those reported for other tissues. Measurements in a tendinopathic Achilles tendon indicated that it is feasible to quantify local viscoelastic properties. Similarly, measurement in the semitendinosus tendon revealed substantial differences in viscoelastic properties between the healthy and contralateral tendons. Consequently, this technique has the potential to evaluate localized changes in tendon viscoelastic properties caused by injury and during recovery in a clinical setting.
NASA Astrophysics Data System (ADS)
de Figueiredo, J. J. S.; Schleicher, J.; Stewart, R. R.; Dayur, N.; Omoboya, B.; Wiley, R.; William, A.
2013-04-01
To understand their influence on elastic wave propagation, anisotropic cracked media have been widely investigated in many theoretical and experimental studies. In this work, we report on laboratory ultrasound measurements carried out to investigate the effect of source frequency on the elastic parameters (wave velocities and the Thomsen parameter γ) and shear wave attenuation) of fractured anisotropic media. Under controlled conditions, we prepared anisotropic model samples containing penny-shaped rubber inclusions in a solid epoxy resin matrix with crack densities ranging from 0 to 6.2 per cent. Two of the three cracked samples have 10 layers and one has 17 layers. The number of uniform rubber inclusions per layer ranges from 0 to 100. S-wave splitting measurements have shown that scattering effects are more prominent in samples where the seismic wavelength to crack aperture ratio ranges from 1.6 to 1.64 than in others where the ratio varied from 2.72 to 2.85. The sample with the largest cracks showed a magnitude of scattering attenuation three times higher compared with another sample that had small inclusions. Our S-wave ultrasound results demonstrate that elastic scattering, scattering and anelastic attenuation, velocity dispersion and crack size interfere directly in shear wave splitting in a source-frequency dependent manner, resulting in an increase of scattering attenuation and a reduction of shear wave anisotropy with increasing frequency.
Turbulence generation by mountain wave breaking in flows with directional wind shear
NASA Astrophysics Data System (ADS)
Vittoria Guarino, Maria; Teixeira, Miguel A. C.
2016-04-01
In this study, wave breaking, and the potential for the generation of turbulence in the atmosphere, is investigated using high-resolution numerical simulations of idealized atmospheric flows with directional wind shear over a three-dimensional isolated mountain. These simulations, which use the WRF-ARW model, differ in degree of flow non-linearity and directional wind shear intensity, quantified through the dimensionless mountain height and the Richardson number of the incoming flow. The aim is to predict wave breaking occurrence based on large-scale variables. The simulation results have been used to produce a regime diagram representing a description of wave breaking behavior in parameter space. By selecting flow overturning occurrence as a discriminating factor, it was possible to split the regime diagram in two sub-regions representing: a non-wave breaking regime and a wave breaking regime. The regime diagram shows that in the presence of directional shear wave breaking may occur over lower mountains that in a constant-wind case. When mountain waves break, the associated convective instability can lead to turbulence generation (known as Clear Air Turbulence or CAT in a non-cloudy atmosphere), thus, regions within the simulation domain where wave breaking and potential development of CAT are expected have been identified. The extent of these regions is variable and increases with the background shear intensity. In contrast with constant-wind flows, where wave breaking occurs in the stream-wise direction aligned with the mountain, for the helical wind profiles considered in this study as prototypes of flows with directional wind shear, flow overturning regions have a more three-dimensional geometry. The analysis of the model outputs, supported by theoretical arguments, suggest the existence of a link between wave breaking and the relative orientation of the incoming wind vector and the horizontal velocity perturbation vector. In particular, in a wave breaking
Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma
Ikezoe, R. Ichimura, M.; Okada, T.; Hirata, M.; Yokoyama, T.; Iwamoto, Y.; Sumida, S.; Jang, S.; Takeyama, K.; Yoshikawa, M.; Kohagura, J.; Shima, Y.; Wang, X.
2015-09-15
A shear Alfvén wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfvén ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency background turbulence is quite distinct at the periphery. Nonlinear coupling associated with the AIC waves may play a significant role in the beta- and anisotropy-limits of a mirror-confined plasma through decay of the ICRF heating power and degradation of the plasma confinement by nonlinearly generated waves.
Second-harmonic generation in shear wave beams with different polarizations
Spratt, Kyle S. Ilinskii, Yurii A.; Zabolotskaya, Evgenia A.; Hamilton, Mark F.
2015-10-28
A coupled pair of nonlinear parabolic equations was derived by Zabolotskaya [1] that model the transverse components of the particle motion in a collimated shear wave beam propagating in an isotropic elastic solid. Like the KZK equation, the parabolic equation for shear wave beams accounts consistently for the leading order effects of diffraction, viscosity and nonlinearity. The nonlinearity includes a cubic nonlinear term that is equivalent to that present in plane shear waves, as well as a quadratic nonlinear term that is unique to diffracting beams. The work by Wochner et al. [2] considered shear wave beams with translational polarizations (linear, circular and elliptical), wherein second-order nonlinear effects vanish and the leading order nonlinear effect is third-harmonic generation by the cubic nonlinearity. The purpose of the current work is to investigate the quadratic nonlinear term present in the parabolic equation for shear wave beams by considering second-harmonic generation in Gaussian beams as a second-order nonlinear effect using standard perturbation theory. In order for second-order nonlinear effects to be present, a broader class of source polarizations must be considered that includes not only the familiar translational polarizations, but also polarizations accounting for stretching, shearing and rotation of the source plane. It is found that the polarization of the second harmonic generated by the quadratic nonlinearity is not necessarily the same as the polarization of the source-frequency beam, and we are able to derive a general analytic solution for second-harmonic generation from a Gaussian source condition that gives explicitly the relationship between the polarization of the source-frequency beam and the polarization of the second harmonic.
Excitation of instability waves in a two-dimensional shear layer by sound
NASA Technical Reports Server (NTRS)
Tam, C. K. W.
1978-01-01
The excitation of instability waves in a plane compressible shear layer by sound waves is studied. The problem is formulated mathematically as an inhomogeneous boundary-value problem. A general solution for abitrary incident sound wave is found by first constructing the Green's function of the problem. Numerical values of the coupling constants between incident sound waves and excited instability waves for a range of flow Mach number are calculated. The effect of the angle of incidence in the case of a beam of acoustic waves is analyzed. It is found that for moderate subsonic Mach numbers a narrow beam aiming at an angle between 50 to 80 deg to the flow direction is most effective in exciting instability waves.
2D instabilities of surface gravity waves on a linear shear current
NASA Astrophysics Data System (ADS)
Francius, Marc; Kharif, Christian
2016-04-01
Periodic 2D surface water waves propagating steadily on a rotational current have been studied by many authors (see [1] and references therein). Although the recent important theoretical developments have confirmed that periodic waves can exist over flows with arbitrary vorticity, their stability and their nonlinear evolution have not been much studied extensively so far. In fact, even in the rather simple case of uniform vorticity (linear shear), few papers have been published on the effect of a vertical shear current on the side-band instability of a uniform wave train over finite depth. In most of these studies [2-5], asymptotic expansions and multiple scales method have been used to obtain envelope evolution equations, which allow eventually to formulate a condition of (linear) instability to long modulational perturbations. It is noted here that this instability is often referred in the literature as the Benjamin-Feir or modulational instability. In the present study, we consider the linear stability of finite amplitude two-dimensional, periodic water waves propagating steadily on the free surface of a fluid with constant vorticity and finite depth. First, the steadily propagating surface waves are computed with steepness up to very close to the highest, using a Fourier series expansions and a collocation method, which constitutes a simple extension of Fenton's method [6] to the cases with a linear shear current. Then, the linear stability of these permanent waves to infinitesimal 2D perturbations is developed from the fully nonlinear equations in the framework of normal modes analysis. This linear stability analysis is an extension of [7] to the case of waves in the presence of a linear shear current and permits the determination of the dominant instability as a function of depth and vorticity for a given steepness. The numerical results are used to assess the accuracy of the vor-NLS equation derived in [5] for the characteristics of modulational
Destabilisation of shear flows by counter-propagating Alfven waves at localised magnetic fields
NASA Astrophysics Data System (ADS)
Griffiths, Stephen
2016-04-01
The instability of shear flows in the presence of magnetic fields is fundamental to understanding a wide range of geophysical and astrophysical phenomena. We investigate the simplest paradigm problem of interest, which is the linear instability of a plane parallel shear flow with aligned field, to two-dimensional disturbances. We focus on cases where the shear flow has no inflexion points and is thus hydrodynamically stable, and show how such flows can be destabilised by the addition of two thin regions of magnetic field. An explicit analytical solution is presented for the case of a flow with uniform shear and where the magnetic fields are of infinitesimal width, showing that there is always instability for some range of along-stream wavenumbers. The strength of the instability is reduced for the more realistic case of magnetic fields of finite width, which can be investigated numerically, or analytically using matched-asymptotic expansions. The instability can be unambiguously attributed to the mutual amplification of a pair of counter-propagating Alfven waves, and should therefore be viewed as an extension to astrophysical fluid dynamics of various classical shear instabilities in geophysical fluid dynamics involving counter-propagating Rossby waves or gravity waves.
Compressional and shear wave velocities in granular materials to 2.5 kilobars
NASA Technical Reports Server (NTRS)
Talwani, P.; Nur, A.; Kovach, R. L.
1973-01-01
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.
Shear-wave velocity variation in jointed rock: an attempt to measure tide-induced variations
Beem, L.I.
1987-08-01
The use of the perturbation of seismic wave velocities by solid earth tides as a possible method of exploration for fractured media is discussed. Velocity of compressional seismic waves in fractured homogeneous rock has been observed to vary through solid earth tide cycles by a significant 0.5-0.9%. This variation of seismic velocities may be attributed to the opening and closing of joints by tidal stresses. In an attempt to see if shear-wave velocities show a similar velocity variation, a pneumatic shear-wave generator was used for the source. The 5 receivers, 3-component, 2.0 Hz, moving-coil geophones, were connected to a GEOS digital recorder. The two receivers located 120 m and 110 m from the source showed large shear-to-compression amplitude ratio and a high signal-to-noise ratio. A glaciated valley was chosen for the experiment site, since topography is flat and the granodiorite is jointed by a set of nearly orthogonal vertical joints, with superimposed horizontal sheeting joints. A slight velocity variation was noted in the first 200 consecutive firings; after which, the amplitude of the shear-wave begun to increase. This increase has been attributed to the compacting of the soil beneath the shear-wave generator (SWG). In the future, the soil will be compacted prior to placing the SWG or the SWG will be coupled directly to the rock to alleviate the amplitude fluctuation problem. This research may have application in exploration for fracture permeability in the rock mass between existing wells, by measuring seismic velocities from well to well through the tidal cycle.
NASA Astrophysics Data System (ADS)
Boivin, Guillaume; Viens, Martin; Belanger, Pierre
2016-02-01
The shear horizontal guided wave fundamental mode (SH0) has the particularity of being the only non-dispersive plate guided wave mode. This characteristic makes this ultrasonic guided wave mode very attractive in non-destructive testing, facilitating signal processing for long range inspections. It is, however, difficult to generate only a single guided wave mode when using piezoelectric transduction. This work aims to develop a piezoelectric transducer capable of generating a virtually pure plane zeroth order shear horizontal wave. The chosen material was the PZT-5H for its dominant d15 piezoelectric constant, which makes it a perfect candidate for SH-wave generation. The transducer dimensions were optimised using an analytical model based on the Huygens' principle of superposition and the dipole pattern of a shear point source. A 3D multiphysics finite element model was then used to validate the analytical model results. Experimental validation was finally conducted with a laser Doppler vibrometer (LDV) system. Excellent agreement between the analytical model, finite element model and experimental validation was seen.
NASA Astrophysics Data System (ADS)
Im, Kwang-Hee; Kim, Hak-Joon; Song, Sung-Jin; Hsu, David K.; Lee, Kil-Sung; Yang, In-Young; Park, Je-Woong
2009-03-01
Composite materials are attractive for a wide range of applications due to the advantages associated with their very large strength-to-weight and stiffness-to-weight ratios. Increasingly, high performance engineering structures are being built with critical structural components made from composite materials. It is very important to detect fiber orientation error in composite laminates because the layup of a CFRP composite laminates affects the properties of the laminate, including stiffness, strength and thermal behavior. An NDE technique for stacking orientation determination would be very beneficial because of layup orientation influence to the laminate stiffness. Usually, it is found that ultrasonic shear wave is pretty sensitive to fiber direction of CFRP composite laminates. An investigation of shear wave ultrasonic technique was carried out in order to detect stacking orientation error for quasi-isotropy composite laminates. Also, a jig is developed for generating a shear wave. A pyramid with an isosceles triangle with two 45° was made of aluminum to generate shear waves using two longitudinal transducers based on ultrasonic-polarized mechanism. Also, the signal splitter was connected to the pulser jack on the pulser/receiver and to the longitudinal transducers. An investigation of shear wave ultrasonic technique was carried out in order to generating shear wave. Therefore, it is found that the experimentally shear wave variation of specially designed jig was consistent with simulated results and shear wave ultrasonic measurement might be very useful to detect the defects in CFRP composites.
Large velocity shears and associated electrostatic waves and turbulence in the auroral F region
NASA Technical Reports Server (NTRS)
Earle, G. D.; Kelley, M. C.; Ganguli, G.
1989-01-01
Broadband electrostatic waves at 10-1000 Hz have been observed with very large shears in the plasma flow velocity transverse to the ambient magnetic field in the auroral F region. The shears were detected through their perpendicular electric field signatures, which changed by as much as 200 mV/m over distances of only a few hundred meters. Transverse shears can be uniquely related to field-aligned currents through the current continuity equation, and the resulting field-aligned drift exceeds the threshold for excitation of current-driven electrostatic ion-cyclotron waves. A numerical simulation of this instability has been used to generate electric-field spectra in the rocket frame of reference, and these spectra are similar to the spectra generated form the actual rocket data.
Cheng, Kai Lun; Choi, Young Jun; Shim, Woo Hyun; Lee, Jeong Hyun; Baek, Jung Hwan
2016-02-01
The goal of this study was to prospectively evaluate the diagnostic performance of Virtual Touch tissue imaging quantification (VTIQ) shear wave elastography in the discrimination of benign and malignant cervical lymph nodes in routine clinical practice. Shear wave velocity was analyzed using VTIQ in 100 patients with 100 histologically proven cervical lymph nodes. Diagnostic performance was evaluated using receiver operating characteristic curve analysis and leave-one-out cross-validation. Agreement between measurements was assessed with intra-class correlation coefficients. The mean shear wave velocity was significantly higher in metastatic lymphadenopathy (4.46 ± 1.46 m/s) than in benign lymphadenopathy (2.71 ± 0.85 m/s) (p < 0.001) at a cutoff level of 3.34 m/s. The cross-validated accuracy, sensitivity and specificity were 77%, 78.9% and 74.4%, respectively. Agreement of measurements with VTIQ was excellent (intra-class correlation coefficient = 0.961). VTIQ shear wave elastography may be a feasible quantitative imaging method for differentiating benign and malignant cervical lymph nodes.
Crosshole shear-wave seismic monitoring of an in situ air stripping waste remediation process
Elbring, G.J.
1992-02-01
Crosshole shear-wave seismic surveys have been used to monitor the distribution of injected air in the subsurface during an in situ air stripping waste remediation project at the Savannah River site in South Carolina. To remove the contaminant, in this case TCE's from a leaking sewer line, two horizontal wells were drilled at depths of 20 m and 52 m. Air was pumped into the lower well and a vacuum was applied to the upper well to extract the injected air. As the air passed through the subsurface, TCE's were dissolved into the gas and brought out the extraction well. Monitoring of the air injection by crosshole shear wave seismics is feasible due to the changes in soil saturation during injection resulting in a corresponding change in seismic velocities. Using a downhole shear-wave source and clamped downhole receiver, two sets of shear-wave data were taken. The first data were taken before the start of air injection, and the second taken during. The difference in travel times between the two data sets were tomographically inverted to obtain velocity differences. Velocity changes ranging up to 3% were mapped corresponding to saturation changes up to 24%. The distribution of these changes shows a desaturation around the position of the injection well with a plume extending in the direction of the extraction well. Layers with higher clay content show distinctively less change in saturation than the regions with higher sand content.
Crosshole shear-wave seismic monitoring of an in situ air stripping waste remediation process
Elbring, G.J.
1992-02-01
Crosshole shear-wave seismic surveys have been used to monitor the distribution of injected air in the subsurface during an in situ air stripping waste remediation project at the Savannah River site in South Carolina. To remove the contaminant, in this case TCE`s from a leaking sewer line, two horizontal wells were drilled at depths of 20 m and 52 m. Air was pumped into the lower well and a vacuum was applied to the upper well to extract the injected air. As the air passed through the subsurface, TCE`s were dissolved into the gas and brought out the extraction well. Monitoring of the air injection by crosshole shear wave seismics is feasible due to the changes in soil saturation during injection resulting in a corresponding change in seismic velocities. Using a downhole shear-wave source and clamped downhole receiver, two sets of shear-wave data were taken. The first data were taken before the start of air injection, and the second taken during. The difference in travel times between the two data sets were tomographically inverted to obtain velocity differences. Velocity changes ranging up to 3% were mapped corresponding to saturation changes up to 24%. The distribution of these changes shows a desaturation around the position of the injection well with a plume extending in the direction of the extraction well. Layers with higher clay content show distinctively less change in saturation than the regions with higher sand content.
An empirical method to estimate shear wave velocity of soils in the New Madrid seismic zone
Wei, B.-Z.; Pezeshk, S.; Chang, T.-S.; Hall, K.H.; Liu, Huaibao P.
1996-01-01
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.
Nguyen, Thu-Mai; Song, Shaozhen; Arnal, Bastien; Wong, Emily Y.; Huang, Zhihong; Wang, Ruikang K.; O’Donnell, Matthew
2014-01-01
Abstract. Assessing the biomechanical properties of soft tissue provides clinically valuable information to supplement conventional structural imaging. In the previous studies, we introduced a dynamic elastography technique based on phase-sensitive optical coherence tomography (PhS-OCT) to characterize submillimetric structures such as skin layers or ocular tissues. Here, we propose to implement a pulse compression technique for shear wave elastography. We performed shear wave pulse compression in tissue-mimicking phantoms. Using a mechanical actuator to generate broadband frequency-modulated vibrations (1 to 5 kHz), induced displacements were detected at an equivalent frame rate of 47 kHz using a PhS-OCT. The recorded signal was digitally compressed to a broadband pulse. Stiffness maps were then reconstructed from spatially localized estimates of the local shear wave speed. We demonstrate that a simple pulse compression scheme can increase shear wave detection signal-to-noise ratio (>12 dB gain) and reduce artifacts in reconstructing stiffness maps of heterogeneous media. PMID:24441876
Acoustically induced tissue displacement for shear wave elasticity imaging using MRI
NASA Astrophysics Data System (ADS)
Haworth, Kevin; Kripfgans, Oliver; Steele, Derek; Swanson, Scott; Sutin, Alexander; Sarvazyan, Armen
2005-09-01
Palpitation detects tissue abnormalities by exploiting the vast range of elastic properties found in vivo. The method is limited by tactile sensitivity and the inability to probe tissues at depth. Recent efforts seek to remove these limitation by developing a medical imaging modality based on radiation force shear wave excitation. Our approach uses an acoustic source to launch a shear wave in a tissue-mimicking phantom and MRI to record microscopic displacements. Gelatin (10% wt/vol) was used for the tissue-mimicking phantom. Results for in situ elasticity were obtained using an air-backed 10-cm-diam piezoelectric crystal. To correct for future in vivo beam aberrations, we also employ a high-pressure 1-bit time-reversal cavity. Frequency and pulse duration were selected to optimize the TRA system for acoustic output pressure. Shear wave displacements were recorded by MRI in 1-ms time increments in a complete basis that allowed for 3-D reconstruction and analysis. The Lamé coefficients are then derived from the shear wave velocity and attenuation.
Shear wave filtering in naturally-occurring Bouligand structures.
Guarín-Zapata, Nicolás; Gomez, Juan; Yaraghi, Nick; Kisailus, David; Zavattieri, Pablo D
2015-09-01
Wave propagation was investigated in the Bouligand-like structure from within the dactyl club of the stomatopod, a crustacean that is known to smash their heavily shelled preys with high accelerations. We incorporate the layered nature in a unitary material cell through the propagator matrix formalism while the periodic nature of the material is considered via Bloch boundary conditions as applied in the theory of solid state physics. Our results show that these materials exhibit bandgaps at frequencies related to the stress pulse generated by the impact of the dactyl club to its prey, and therefore exhibiting wave filtering in addition to the already known mechanisms of macroscopic isotropic behavior and toughness. PMID:25983314
Joint seismic tomography for bulk sound and shear wave speed in the Earth's mantle
NASA Astrophysics Data System (ADS)
Kennett, B. L. N.; Widiyantoro, S.; van der Hilst, R. D.
1998-06-01
High-quality P and S travel times are now available from careful reprocessing of data reported to international agencies. A restricted data set has been extracted for which comparable ray coverage is achieved for P and S, and used for a joint inversion to produce a three-dimensional model for shear and bulk sound velocities represented in terms of 2° × 2° cells and 18 layers in depth through the mantle. About 106 times for each of P and S are combined to produce 312,549 summary rays for each wave type. Linearizing about the ak135 reference model, 583,200 coupled tomographic equations are solved using an iterative partitioned scheme. Clear high-resolution images are obtained for both bulk-sound speed and shear wavespeed. The bulk and shear moduli have differing sensitivity to temperature and mineral composition, and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations. Different heterogeneity regimes can be recognised in the upper mantle, the transition zone, most of the lower mantle, and the lowermost mantle. In the upper mantle, many features can be explained by thermal effects; but in some orogenic zones (e.g. western North America), the opposite sense of the bulk-sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects. In the lower mantle, pronounced narrow structures which may represent remnant subduction are most marked in shear. The level of large-scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km. Below this depth, the variability of both wave speeds increases. Near the core-mantle boundary the variations of the two wave speeds show little concordance, suggesting the presence of widespread chemical heterogeneity.
Quantification of muscle co-contraction using supersonic shear wave imaging.
Raiteri, Brent J; Hug, François; Cresswell, Andrew G; Lichtwark, Glen A
2016-02-01
Muscle stiffness estimated using shear wave elastography can provide an index of individual muscle force during isometric contraction and may therefore be a promising method for quantifying co-contraction. We estimated the shear modulus of the lateral gastrocnemius (LG) muscle using supersonic shear wave imaging and measured its myoelectrical activity using surface electromyography (sEMG) during graded isometric contractions of plantar flexion and dorsiflexion (n=7). During dorsiflexion, the average shear modulus was 26 ± 6 kPa at peak sEMG amplitude, which was significantly less (P=0.02) than that measured at the same sEMG level during plantar flexion (42 ± 10 kPa). The passive tension during contraction was estimated using the passive LG muscle shear modulus during a passive ankle rotation measured at an equivalent ankle angle to that measured during contraction. The passive shear modulus increased significantly (P<0.01) from the plantar flexed position (16 ± 5 kPa) to the dorsiflexed position (26 ± 9 kPa). Once this change in passive tension from joint rotation was accounted for, the average LG muscle shear modulus due to active contraction was significantly greater (P<0.01) during plantar flexion (26 ± 8 kPa) than at sEMG-matched levels of dorsiflexion (0 ± 4 kPa). The negligible shear modulus estimated during isometric dorsiflexion indicates negligible active force contribution by the LG muscle, despite measured sEMG activity of 19% of maximal voluntary plantar flexion contraction. This strongly suggests that the sEMG activity recorded from the LG muscle during isometric dorsiflexion was primarily due to cross-talk. However, it is clear that passive muscle tension changes can contribute to joint torque during isometric dorsiflexion.
NASA Astrophysics Data System (ADS)
Wang, Shang; Lopez, Andrew L.; Morikawa, Yuka; Tao, Ge; Li, Jiasong; Larina, Irina V.; Martin, James F.; Larin, Kirill V.
2015-03-01
Optical coherence elastography (OCE) is an emerging low-coherence imaging technique that provides noninvasive assessment of tissue biomechanics with high spatial resolution. Among various OCE methods, the capability of quantitative measurement of tissue elasticity is of great importance for tissue characterization and pathology detection across different samples. Here we report a quantitative OCE technique, termed quantitative shear wave imaging optical coherence tomography (Q-SWI-OCT), which enables noncontact measurement of tissue Young's modulus based on the ultra-fast imaging of the shear wave propagation inside the sample. A focused air-puff device is used to interrogate the tissue with a low-pressure short-duration air stream that stimulates a localized displacement with the scale at micron level. The propagation of this tissue deformation in the form of shear wave is captured by a phase-sensitive OCT system running with the scan of the M-mode imaging over the path of the wave propagation. The temporal characteristics of the shear wave is quantified based on the cross-correlation of the tissue deformation profiles at all the measurement locations, and linear regression is utilized to fit the data plotted in the domain of time delay versus wave propagation distance. The wave group velocity is thus calculated, which results in the quantitative measurement of the Young's modulus. As the feasibility demonstration, experiments are performed on tissuemimicking phantoms with different agar concentrations and the quantified elasticity values with Q-SWI-OCT agree well with the uniaxial compression tests. For functional characterization of myocardium with this OCE technique, we perform our pilot experiments on ex vivo mouse cardiac muscle tissues with two studies, including 1) elasticity difference of cardiac muscle under relaxation and contract conditions and 2) mechanical heterogeneity of the heart introduced by the muscle fiber orientation. Our results suggest the
Harris, J.B.
1996-01-01
Determining the extent and location of surface/near-surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear-wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near-surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow fourcomponent data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source-receiver orientations, an indicator of shear-wave splitting. Following rotation of the data matrix by 40??, the S1 and S2 sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35??W and N55??E, respectively. A dynamic mis-tie of 8 ms at a two-way traveltime of 375 ms produced an average azimuthal anisotropy of ???2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear-wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near-surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear-wave methods in unconsolidated, water-saturated sediments, measurement of near-surface directional polarizations, produced by shear-wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.
NASA Astrophysics Data System (ADS)
Peureux, Charles; Ardhuin, Fabrice
2016-04-01
The stereo-video reconstuction method [Leckler et al. 2015] allows now for the full reconstruction of 3D frequency-wavenumber spectra of short waves. A new field campaign in 2013 on the Katsiveli platform (Black Sea) provided such spectra in various wind and waves conditions, and particularly a stormy event, after which very mature waves had been generated. The short waves energies are found to be mostly located around a dispersion relation of the form, () ° ----------- ω ⃗k = gktanh(kH)+ ⃗kṡ ⃗Ueff The effective advection velocity [Kirby and Chen 1989] ⃗Ueff(k) integrates contributions from both the Stokes drift and quasi-eulerian current [Groeneweg and Klopman 1998]. We find that the effective drift velocity has a very weak wavenumber dependancy, as a result the eulerian current must be vertically sheared. This shear is relevant to the breaking of small scale waves [Banner and Phillips 1974]. It is possible that in field conditions the wind drift is much less important than in the laboratory. Bibliography Banner, M. L. and Phillips, O. M., On the incipient breaking of small scale waves, J. Fluid Mech., 1974, 65, 647. Groeneweg, J. and Klopman, G., Changes of the mean velocity profiles in the combined wave-current motion described in a GLM formulation, J. Fluid Mech., 1998, 370, 271-296. Kirby, J. T. and Chen, T. M., Surface waves on vertically sheared flows : Approximate dispersion relations, J. Geophys. Res., 1989, 94, 1013. Leckler, F., Ardhuin, F., Peureux, C.,Benetazzo, A., Bergamasco, F. and Dulov, V., Analysis and interpretation of frequency-wavenumber spectra of young wind-waves, J. Phys. Oceanogr., 2015, 45, 2484-2496.
Code package MAG c user tool for numerical modeling of 1D shock wave and dynamic processes in solids
NASA Astrophysics Data System (ADS)
Rudenko, Vladimir; Shaburov, Michail
1999-06-01
Design and theoretical and numerical preparation of shock wave experiments require, as a rule, conduction of a large amount of calculations. Usually preparation of a problem for numerical solution, calculation and processing of the results is done be programmers c mathematicians. The appearance of powerful personal computers and interface tools allows to develop such user-oriented programs that a researcher can handle them without the help of a mathematician, even if he does not have special programming background. Code package MAG for numerical solution of 1D system of equations of hydrodynamics, elastoplastics, heat conduction and magnetic hydrodynamic. A number of modern models of elastoplastics and kinetics of power materials is implemented in it. The package includes libraries of equations of state, thermal physical and electromagnetic properties of substances. The code package is an interactive visual medium providing a user with the following capabilities: ? Input and edit initial data for a problem; ? Calculate separate problems, as well as series of problems with a possibility of automatic variation of parameters; ? View the modeled phenomena dynamically using the means of visualization; ? Control the process of calculation: terminate the calculation, change parameters, make express-processing of the results, continue the calculation etc.; ? Process the numerical results producing final plots and tables; ? Record and store numerical results in databases, including the formats supported by Microsoft Word, Acces, Exel; ? Make dynamic visual comparison of the results of several simultaneous calculations; ? Carry out automatic numerical optimization of a selected experimental scheme. The package is easy in use, allows prompt input and convenient information processing. The validity of numerical results obtained with the package MAG has been proved by numerous hydrodynamic experiments and comparisons with numerical results from similar programs. The package was
Overstability of acoustic waves in strongly magnetized anisotropic magnetohydrodynamic shear flows
Uchava, E. S.; Shergelashvili, B. M.; Tevzadze, A. G.; Poedts, S.
2014-08-15
We present a linear stability analysis of the perturbation modes in anisotropic magnetohydrodynamic (MHD) flows with velocity shear and strong magnetic field. Collisionless or weakly collisional plasma is described within the 16-momentum MHD fluid closure model that takes into account not only the effect of pressure anisotropy but also the effect of anisotropic heat fluxes. In this model, the low frequency acoustic wave is revealed into a standard acoustic mode and higher frequency fast thermo-acoustic and lower frequency slow thermo-acoustic waves. It is shown that thermo-acoustic waves become unstable and grow exponentially when the heat flux parameter exceeds some critical value. It seems that velocity shear makes thermo-acoustic waves overstable even at subcritical heat flux parameters. Thus, when the effect of heat fluxes is not profound acoustic waves will grow due to the velocity shear, while at supercritical heat fluxes the flow reveals compressible thermal instability. Anisotropic thermal instability should be also important in astrophysical environments, where it will limit the maximal value of magnetic field that a low density ionized anisotropic flow can sustain.
NASA Astrophysics Data System (ADS)
Mello, Pier A.; Shi, Zhou; Genack, Azriel Z.
2016-08-01
We study the average energy - or particle - density of waves inside disordered 1D multiply-scattering media. We extend the transfer-matrix technique that was used in the past for the calculation of the intensity beyond the sample to study the intensity in the interior of the sample by considering the transfer matrices of the two segments that form the entire waveguide. The statistical properties of the two disordered segments are found using a maximum-entropy ansatz subject to appropriate constraints. The theoretical expressions are shown to be in excellent agreement with 1D transfer-matrix simulations.
Zeng, C.; Xia, J.; Miller, R.D.; Tsoflias, G.P.
2011-01-01
Conventional surface wave inversion for shallow shear (S)-wave velocity relies on the generation of dispersion curves of Rayleigh waves. This constrains the method to only laterally homogeneous (or very smooth laterally heterogeneous) earth models. Waveform inversion directly fits waveforms on seismograms, hence, does not have such a limitation. Waveforms of Rayleigh waves are highly related to S-wave velocities. By inverting the waveforms of Rayleigh waves on a near-surface seismogram, shallow S-wave velocities can be estimated for earth models with strong lateral heterogeneity. We employ genetic algorithm (GA) to perform waveform inversion of Rayleigh waves for S-wave velocities. The forward problem is solved by finite-difference modeling in the time domain. The model space is updated by generating offspring models using GA. Final solutions can be found through an iterative waveform-fitting scheme. Inversions based on synthetic records show that the S-wave velocities can be recovered successfully with errors no more than 10% for several typical near-surface earth models. For layered earth models, the proposed method can generate one-dimensional S-wave velocity profiles without the knowledge of initial models. For earth models containing lateral heterogeneity in which case conventional dispersion-curve-based inversion methods are challenging, it is feasible to produce high-resolution S-wave velocity sections by GA waveform inversion with appropriate priori information. The synthetic tests indicate that the GA waveform inversion of Rayleigh waves has the great potential for shallow S-wave velocity imaging with the existence of strong lateral heterogeneity. ?? 2011 Elsevier B.V.
Poincaré wave-induced shear instability in Lake Michigan
NASA Astrophysics Data System (ADS)
Hsieh, T.; Troy, C. D.; Hawley, N.
2012-12-01
Cross-thermocline mixing is an important process during the stratified period in the Great Lakes, especially in the lake interiors. Recent evidence suggests that basin-scale mixing in the world's largest lakes may actually occur primarily in the lake interiors. Several years of velocity and temperature data are examined from Lake Michigan in order to examine processes that are potentially responsible for cross-thermocline mixing. Near-inertial internal Poincaré waves are shown to thoroughly dominate surface currents and thermocline shear in Lake Michigan's deeper waters. Empirical orthogonal function decomposition of the observed velocity fields show that a vertical mode 1 Poincaré wave is dominant and responsible for most of the observed thermocline shear. Calculated Richardson numbers, observed microstructure, and stability analysis suggest that these waves do cause cross-thermocline mixing, and that high-resolution epxperiments are necessary to better quantify mixing in the interior of Lake Michigan.
A Kramers-Kronig-based quality factor for shear wave propagation in soft tissue.
Urban, M W; Greenleaf, J F
2009-10-01
Shear wave propagation techniques have been introduced for measuring the viscoelastic material properties of tissue, but assessing the accuracy of these measurements is difficult for in vivo measurements in tissue. We propose using the Kramers-Kronig relationships to assess the consistency and quality of the measurements of shear wave attenuation and phase velocity. In ex vivo skeletal muscle we measured the wave attenuation at different frequencies, and then applied finite bandwidth Kramers-Kronig equations to predict the phase velocities. We compared these predictions with the measured phase velocities and assessed the mean square error (MSE) as a quality factor. An algorithm was derived for computing a quality factor using the Kramers-Kronig relationships.
Analytical and numerical modeling of non-collinear shear wave mixing at an imperfect interface.
Zhang, Ziyin; Nagy, Peter B; Hassan, Waled
2016-02-01
Non-collinear shear wave mixing at an imperfect interface between two solids can be exploited for nonlinear ultrasonic assessment of bond quality. In this study we developed two analytical models for nonlinear imperfect interfaces. The first model uses a finite nonlinear interfacial stiffness representation of an imperfect interface of vanishing thickness, while the second model relies on a thin nonlinear interphase layer to represent an imperfect interface region. The second model is actually a derivative of the first model obtained by calculating the equivalent interfacial stiffness of a thin isotropic nonlinear interphase layer in the quasi-static approximation. The predictions of both analytical models were numerically verified by comparison to COMSOL finite element simulations. These models can accurately predict the additional nonlinearity caused by interface imperfections based on the strength of the reflected and transmitted mixed longitudinal waves produced by them under non-collinear shear wave interrogation. PMID:26482394
Bias of shear wave elasticity measurements in thin layer samples and a simple correction strategy.
Mo, Jianqiang; Xu, Hao; Qiang, Bo; Giambini, Hugo; Kinnick, Randall; An, Kai-Nan; Chen, Shigao; Luo, Zongping
2016-01-01
Shear wave elastography (SWE) is an emerging technique for measuring biological tissue stiffness. However, the application of SWE in thin layer tissues is limited by bias due to the influence of geometry on measured shear wave speed. In this study, we investigated the bias of Young's modulus measured by SWE in thin layer gelatin-agar phantoms, and compared the result with finite element method and Lamb wave model simulation. The result indicated that the Young's modulus measured by SWE decreased continuously when the sample thickness decreased, and this effect was more significant for smaller thickness. We proposed a new empirical formula which can conveniently correct the bias without the need of using complicated mathematical modeling. In summary, we confirmed the nonlinear relation between thickness and Young's modulus measured by SWE in thin layer samples, and offered a simple and practical correction strategy which is convenient for clinicians to use. PMID:27588234
Kim, Hoe Woong; Lee, Joo Kyung; Kim, Yoon Young
2013-02-01
Several investigations report effective uses of magnetostrictive patch transducers to generate and measure longitudinal and torsional guided waves in a pipe. They can be used to form a phased array for the circumferential inspection of pipes. Although there are circumferential phased arrays employing piezoelectric transducers or EMAT's, no magnetostrictive patch transducer based array system has been attempted. In this investigation, we aim to develop a circumferential phased magnetostrictive patch transducer (PMPT) array that can focus shear-horizontal waves at any target point on a cylindrical surface of a pipe. For the development, a specific configuration of a PMPT array employing six magnetostrictive patch transducers is proposed. A wave simulation model is also developed to determine time delays and amplitudes of signals generated by the transducers of the array. This model should be able to predict accurately the angular profiles of shear-horizontal waves generated by the transducers. For wave focusing, the time reversal idea will be utilized. The wave focusing ability of the developed PMPT array is tested with multiple-crack detection experiments. Imaging of localized surface inspection regions is also attempted by using wave signals measured by the developed PMPT array system.
Stability of steady rotational water-waves of finite amplitude on arbitrary shear currents
NASA Astrophysics Data System (ADS)
Seez, William; Abid, Malek; Kharif, Christian
2016-04-01
A versatile solver for the two-dimensional Euler equations with an unknown free-surface has been developed. This code offers the possibility to calculate two-dimensional, steady rotational water-waves of finite amplitude on an arbitrary shear current. Written in PYTHON the code incorporates both pseudo-spectral and finite-difference methods in the discretisation of the equations and thus allows the user to capture waves with large steepnesses. As such it has been possible to establish that, in a counter-flowing situation, the existence of wave solutions is not guaranteed and depends on a pair of parameters representing mass flux and vorticity. This result was predicted, for linear solutions, by Constantin. Furthermore, experimental comparisons, both with and without vorticity, have proven the precision of this code. Finally, waves propagating on top of highly realistic shear currents (exponential profiles under the surface) have been calculated following current profiles such as those used by Nwogu. In addition, a stability analysis routine has been developed to study the stability regimes of base waves calculated with the two-dimensional code. This linear stability analysis is based on three dimensional perturbations of the steady situation which lead to a generalised eigenvalue problem. Common instabilities of the first and second class have been detected, while a third class of wave-instability appears due to the presence of strong vorticity. {1} Adrian Constantin and Walter Strauss. {Exact steady periodic water waves with vorticity}. Communications on Pure and Applied Mathematics, 57(4):481-527, April 2004. Okey G. Nwogu. {Interaction of finite-amplitude waves with vertically sheared current fields}. Journal of Fluid Mechanics, 627:179, May 2009.
Correlation of densities with shear wave velocities and SPT N values
NASA Astrophysics Data System (ADS)
Anbazhagan, P.; Uday, Anjali; Moustafa, Sayed S. R.; Al-Arifi, Nassir S. N.
2016-06-01
Site effects primarily depend on the shear modulus of subsurface layers, and this is generally estimated from the measured shear wave velocity (V s) and assumed density. Very rarely, densities are measured for amplification estimation because drilling and sampling processes are time consuming and expensive. In this study, an attempt has been made to derive the correlation between the density (dry and wet density) and V s/SPT (standard penetration test) N values using measured data. A total of 354 measured V s and density data sets and 364 SPT N value and density data sets from 23 boreholes have been used in the study. Separate relations have been developed for all soil types as well as fine-grained and coarse-grained soil types. The correlations developed for bulk density were compared with the available data and it was found that the proposed relation matched well with the existing data. A graphical comparison and validation based on the consistency ratio and cumulative frequency curves was performed and the newly developed relations were found to demonstrate good prediction performance. An attempt has also been made to propose a relation between the bulk density and shear wave velocity applicable for a wide range of soil and rock by considering data from this study as well as that of previous studies. These correlations will be useful for predicting the density (bulk and dry) of sites having measured the shear wave velocity and SPT N values.
Generation and Radiation of Acoustic Waves from a 2D Shear Layer
NASA Technical Reports Server (NTRS)
Dahl, Milo D.
2000-01-01
A thin free shear layer containing an inflection point in the mean velocity profile is inherently unstable. Disturbances in the flow field can excite the unstable behavior of a shear layer, if the appropriate combination of frequencies and shear layer thicknesses exists, causing instability waves to grow. For other combinations of frequencies and thicknesses, these instability waves remain neutral in amplitude or decay in the downstream direction. A growing instability wave radiates noise when its phase velocity becomes supersonic relative to the ambient speed of sound. This occurs primarily when the mean jet flow velocity is supersonic. Thus, the small disturbances in the flow, which themselves may generate noise, have generated an additional noise source. It is the purpose of this problem to test the ability of CAA to compute this additional source of noise. The problem is idealized such that the exciting disturbance is a fixed known acoustic source pulsating at a single frequency. The source is placed inside of a 2D jet with parallel flow; hence, the shear layer thickness is constant. With the source amplitude small enough, the problem is governed by the following set of linear equations given in dimensional form.
The radiation of sound by the instability waves of a compressible plane turbulent shear layer
NASA Technical Reports Server (NTRS)
Tam, C. K. W.; Morris, P. J.
1980-01-01
The problem of acoustic radiation generated by instability waves of a compressible plane turbulent shear layer is solved. The solution provided is valid up to the acoustic far-field region. It represents a significant improvement over the solution obtained by classical hydrodynamic-stability theory which is essentially a local solution with the acoustic radiation suppressed. The basic instability-wave solution which is valid in the shear layer and the near-field region is constructed in terms of an asymptotic expansion using the method of multiple scales. This solution accounts for the effects of the slightly divergent mean flow. It is shown that the multiple-scales asymptotic expansion is not uniformly valid far from the shear layer. Continuation of this solution into the entire upper half-plane is described. The extended solution enables the near- and far-field pressure fluctuations associated with the instability wave to be determined. Numerical results show that the directivity pattern of acoustic radiation into the stationary medium peaks at 20 degrees to the axis of the shear layer in the downstream direction for supersonic flows. This agrees qualitatively with the observed noise-directivity patterns of supersonic jets.
Kinetic theory for electrostatic waves due to transverse velocity shears
NASA Technical Reports Server (NTRS)
Ganguli, G.; Lee, Y. C.; Palmadesso, P. J.
1988-01-01
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.
NASA Astrophysics Data System (ADS)
Kesden, Michael; Cooray, Asantha; Kamionkowski, Marc
2002-07-01
Inflationary gravitational waves (GW) contribute to the curl component in the polarization of the cosmic microwave background (CMB). Cosmic shear--gravitational lensing of the CMB--converts a fraction of the dominant gradient polarization to the curl component. Higher-order correlations can be used to map the cosmic shear and subtract this contribution to the curl. Arcminute resolution will be required to pursue GW amplitudes smaller than those accessible by the Planck surveyor mission. The blurring by lensing of small-scale CMB power leads with this reconstruction technique to a minimum detectable GW amplitude corresponding to an inflation energy near 10(sup 15) GeV .
Godin, O A; Chapman, D M
2001-10-01
In the upper tens of meters of ocean bottom, unconsolidated marine sediments consisting of clay, silt, or fine sand with high porosity are "almost incompressible" in the sense that the shear wave velocity is much smaller than the compressional wave velocity. The shear velocity has very large gradients close to the ocean floor leading to strong coupling of compressional and shear waves in such "soft" sediments. The weak compressibility opens an avenue for developing a theory of elastic wave propagation in continuously stratified soft sediments that fully accounts for the coupling. Elastic waves in soft sediments consist of "fast" waves propagating with velocities close to the compressional velocity and "slow" waves propagating with velocities on the order of the shear velocity. For the slow waves, the theory predicts the existence of surface waves at the ocean-sediment boundary. In the important special case of the power-law depth-dependence of shear rigidity, phase and group velocities of the interface waves are shown to scale as a certain power of frequency. An explicit, exact solution was obtained for the surface waves in sediments characterized by constant density and a linear increase of shear rigidity with depth, that is, for the case of shear speed proportional to the square root of the depth below the sediment-water interface. Asymptotic and perturbation techniques were used to extend the result to more general environments. Theoretical dispersion relations agreed well with numerical simulations and available experimental data and, as demonstrated in a companion paper [D. M. F. Chapman and O. A. Godin, J. Acoust. Soc. Am 110, 1908 (2001)] led to a simple and robust inversion of interface wave travel times for shear velocity profiles in the sediment.
NASA Astrophysics Data System (ADS)
Lorenzo, J. M.; Goff, D.; Hayashi, K.
2015-12-01
Unconsolidated Holocene deltaic sediments comprise levee foundation soils in New Orleans, USA. Whereas geotechnical tests at point locations are indispensable for evaluating soil stability, the highly variable sedimentary facies of the Mississippi delta create difficulties to predict soil conditions between test locations. Combined electrical resistivity and seismic shear wave studies, calibrated to geotechnical data, may provide an efficient methodology to predict soil types between geotechnical sites at shallow depths (0- 10 m). The London Avenue Canal levee flank of New Orleans, which failed in the aftermath of Hurricane Katrina, 2005, presents a suitable site in which to pioneer these geophysical relationships. Preliminary cross-plots show electrically resistive, high-shear-wave velocity areas interpreted as low-permeability, resistive silt. In brackish coastal environments, low-resistivity and low-shear-wave-velocity areas may indicate both saturated, unconsolidated sands and low-rigidity clays. Via a polynomial approximation, soil sub-types of sand, silt and clay can be estimated by a cross-plot of S-wave velocity and resistivity. We confirm that existent boring log data fit reasonably well with the polynomial approximation where 2/3 of soil samples fall within their respective bounds—this approach represents a new classification system that could be used for other mid-latitude, fine-grained deltas.
Transmission, attenuation and reflection of shear waves in the human brain
Clayton, Erik H.; Genin, Guy M.; Bayly, Philip V.
2012-01-01
Traumatic brain injuries (TBIs) are caused by acceleration of the skull or exposure to explosive blast, but the processes by which mechanical loads lead to neurological injury remain poorly understood. We adapted motion-sensitive magnetic resonance imaging methods to measure the motion of the human brain in vivo as the skull was exposed to harmonic pressure excitation (45, 60 and 80 Hz). We analysed displacement fields to quantify the transmission, attenuation and reflection of distortional (shear) waves as well as viscoelastic material properties. Results suggest that internal membranes, such as the falx cerebri and the tentorium cerebelli, play a key role in reflecting and focusing shear waves within the brain. The skull acts as a low-pass filter over the range of frequencies studied. Transmissibility of pressure waves through the skull decreases and shear wave attenuation increases with increasing frequency. The skull and brain function mechanically as an integral structure that insulates internal anatomic features; these results are valuable for building and validating mathematical models of this complex and important structural system. PMID:22675163
Transmission, attenuation and reflection of shear waves in the human brain.
Clayton, Erik H; Genin, Guy M; Bayly, Philip V
2012-11-01
Traumatic brain injuries (TBIs) are caused by acceleration of the skull or exposure to explosive blast, but the processes by which mechanical loads lead to neurological injury remain poorly understood. We adapted motion-sensitive magnetic resonance imaging methods to measure the motion of the human brain in vivo as the skull was exposed to harmonic pressure excitation (45, 60 and 80 Hz). We analysed displacement fields to quantify the transmission, attenuation and reflection of distortional (shear) waves as well as viscoelastic material properties. Results suggest that internal membranes, such as the falx cerebri and the tentorium cerebelli, play a key role in reflecting and focusing shear waves within the brain. The skull acts as a low-pass filter over the range of frequencies studied. Transmissibility of pressure waves through the skull decreases and shear wave attenuation increases with increasing frequency. The skull and brain function mechanically as an integral structure that insulates internal anatomic features; these results are valuable for building and validating mathematical models of this complex and important structural system. PMID:22675163
Transmission, attenuation and reflection of shear waves in the human brain.
Clayton, Erik H; Genin, Guy M; Bayly, Philip V
2012-11-01
Traumatic brain injuries (TBIs) are caused by acceleration of the skull or exposure to explosive blast, but the processes by which mechanical loads lead to neurological injury remain poorly understood. We adapted motion-sensitive magnetic resonance imaging methods to measure the motion of the human brain in vivo as the skull was exposed to harmonic pressure excitation (45, 60 and 80 Hz). We analysed displacement fields to quantify the transmission, attenuation and reflection of distortional (shear) waves as well as viscoelastic material properties. Results suggest that internal membranes, such as the falx cerebri and the tentorium cerebelli, play a key role in reflecting and focusing shear waves within the brain. The skull acts as a low-pass filter over the range of frequencies studied. Transmissibility of pressure waves through the skull decreases and shear wave attenuation increases with increasing frequency. The skull and brain function mechanically as an integral structure that insulates internal anatomic features; these results are valuable for building and validating mathematical models of this complex and important structural system.
Monitoring Spatial and Temporal Variations in Fracture Networks Using Shear-wave Splitting Analysis
NASA Astrophysics Data System (ADS)
Kendall, J. M.; Wuestefeld, A.; Verdon, J.; Rutledge, J. T.; Wookey, J.
2011-12-01
Passive seismic monitoring is used to infer fracturing in a range of settings including hydrocarbon reservoirs, mining environments and volcanological systems. However, most studies concentrate on simply locating microseismic events. Measurements of shear-wave splitting in such events provide unambiguous evidence of seismic anisotropy, which may be caused by the rock fabric and/or aligned fractures, which in turn offers insights into the state of stress in the rock. We present a strategy to automatically process shear-wave splitting in large microseismic datasets. This includes an objective quality control of the shear-wave splitting results, based on characteristic differences between two independent splitting techniques. Reliable measurements can be then used in inversion schemes for anisotropy parameters, including those controlled by rock fracturing. We demonstrate this fully automatic workflow with microseismic events recorded during a hydraulic fracture experiment in a tight gas reservoir in Carthage, East Texas. Events were recorded on two downhole arrays of 3-component sensors, the geometry of which provided good ray coverage for anisotropy analysis. A total of 16633 seismograms from 888 located events yielded 1570 well-constrained shear wave splitting measurements. We consider the anisotropy to be controlled by a sedimentary fabric with superimposed fractures and use the splitting measurements to invert for the orthorhombic anisotropic parameters of the rock mass, including the dominant fracture orientation and facture density. The recovered fracture strike in the rock mass is parallel to directions of regional borehole breakout, but oblique to the hydraulic fracture corridor as mapped by the microseismic events. We relate this to en-echelon fracturing of pre-existing cracks. The magnitude of shear wave splitting shows a clear temporal increase during each pumping stage, indicating the generation of cracks and fissures in a halo around the fracture
Constraints on Shear Velocity in the Cratonic Upper Mantle From Rayleigh Wave Phase Velocity
NASA Astrophysics Data System (ADS)
Hirsch, A. C.; Dalton, C. A.
2014-12-01
In recent years, the prevailing notion of Precambrian continental lithosphere as a thick boundary layer (200-300 km), defined by a depleted composition and a steady-state conductively cooled temperature structure, has been challenged by several lines of seismological evidence. One, profiles of shear velocity with depth beneath cratons exhibit lower wave speed at shallow depths and higher wave speed at greater depths than can be explained by temperature alone. These profiles are also characterized by positive or flat velocity gradients with depth and anomalously high attenuation in the uppermost mantle, both of which are difficult to reconcile with the low temperatures and large thermal gradient expected with a thermal boundary layer. Two, body-wave receiver-function studies have detected a mid-lithospheric discontinuity that requires a large and abrupt velocity decrease with depth in cratonic regions that cannot be achieved by thermal gradients alone. Here, we used forward-modeling to identify the suite of shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves that primarily traversed cratons in North America, South America, Africa, and Australia. We considered two approaches; with the first, depth profiles of shear velocity were predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the crust and upper mantle. With the second approach, depth profiles of shear velocity were randomly generated. In both cases, Rayleigh wave phase velocity was calculated from the Earth models and compared to the observed values. We show that it is very difficult to match the observations with an Earth model containing a low-velocity zone in the upper mantle; instead, the best-fit models contain a flat or positive velocity gradient with depth. We explore the implications of this result for the thermal and
Jugé, Lauriane; Petiet, Anne; Lambert, Simon A; Nicole, Pascal; Chatelin, Simon; Vilgrain, Valerie; Van Beers, Bernard E; Bilston, Lynne E; Sinkus, Ralph
2015-12-01
Magnetic Resonance Elastography (MRE) uses macroscopic shear wave propagation to quantify mechanical properties of soft tissues. Micro-obstacles are capable of affecting the macroscopic dispersion properties of shear waves. Since disease or therapy can change the mechanical integrity and organization of vascular structures, MRE should be able to sense these changes if blood vessels represent a source for wave scattering. To verify this, MRE was performed to quantify alteration of the shear wave speed cs due to the presence of vascular outgrowths using an aortic ring model. Eighteen fragments of rat aorta included in a Matrigel matrix (n=6 without outgrowths, n=6 with a radial outgrowth extent of ~600 µm and n=6 with ~850 µm) were imaged using a 7 Tesla MR scanner (Bruker, PharmaScan). High resolution anatomical images were acquired in addition to multi-frequency MRE (ν = 100, 115, 125, 135 and 150 Hz). Average cs was measured within a ring of ~900 µm thickness encompassing the aorta and were normalized to cs0 of the corresponding Matrigel. The frequency dependence was fit to the power law model cs ~ν(y). After scanning, optical microscopy was performed to visualize outgrowths. Results demonstrated that in presence of vascular outgrowths (1) normalized cs significantly increased for the three highest frequencies (Kruskal-Wallis test, P = 0.0002 at 125 Hz and P = 0.002 at 135 Hz and P = 0.003 at 150 Hz) but not for the two lowest (Kruskal-Wallis test, P = 0.63 at 100 Hz and P = 0.87 at 115 Hz), and (2) normalized cs followed a power law behavior not seen in absence of vascular outgrowths (ANOVA test, P < 0.0001). These results showed that vascular outgrowths acted as micro-obstacles altering the dispersion relationships of propagating shear waves and that MRE could provide valuable information about microvascular changes.
NASA Astrophysics Data System (ADS)
Brum, J.; Bernal, M.; Gennisson, J. L.; Tanter, M.
2014-02-01
Non-invasive evaluation of the Achilles tendon elastic properties may enhance diagnosis of tendon injury and the assessment of recovery treatments. Shear wave elastography has shown to be a powerful tool to estimate tissue mechanical properties. However, its applicability to quantitatively evaluate tendon stiffness is limited by the understanding of the physics on the shear wave propagation in such a complex medium. First, tendon tissue is transverse isotropic. Second, tendons are characterized by a marked stiffness in the 400 to 1300 kPa range (i.e. fast shear waves). Hence, the shear wavelengths are greater than the tendon thickness leading to guided wave propagation. Thus, to better understand shear wave propagation in tendons and consequently to properly estimate its mechanical properties, a dispersion analysis is required. In this study, shear wave velocity dispersion was measured in vivo in ten Achilles tendons parallel and perpendicular to the tendon fibre orientation. By modelling the tendon as a transverse isotropic viscoelastic plate immersed in fluid it was possible to fully describe the experimental data (deviation<1.4%). We show that parallel to fibres the shear wave velocity dispersion is not influenced by viscosity, while it is perpendicularly to fibres. Elasticity (found to be in the range from 473 to 1537 kPa) and viscosity (found to be in the range from 1.7 to 4 Pa.s) values were retrieved from the model in good agreement with reported results.
Observation of fast-ion Doppler-shifted cyclotron resonance with shear Alfven waves
Zhang Yang; Heidbrink, W. W.; Boehmer, H.; McWilliams, R.; Vincena, S.; Carter, T. A.; Gekelman, W.; Leneman, D.; Pribyl, P.
2008-10-15
The Doppler-shifted cyclotron resonance ({omega}-k{sub z}v{sub z}={omega}{sub f}) between fast ions and shear Alfven waves is experimentally investigated ({omega}, wave frequency; k{sub z}, axial wavenumber; v{sub z}, fast-ion axial speed; {omega}{sub f}, fast-ion cyclotron frequency). A test particle beam of fast ions is launched by a Li{sup +} source in the helium plasma of the LArge Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)], with shear Alfven waves (SAW) (amplitude {delta} B/B up to 1%) launched by a loop antenna. A collimated fast-ion energy analyzer measures the nonclassical spreading of the beam, which is proportional to the resonance with the wave. A resonance spectrum is observed by launching SAWs at 0.3-0.8{omega}{sub ci}. Both the magnitude and frequency dependence of the beam-spreading are in agreement with the theoretical prediction using a Monte Carlo Lorentz code that launches fast ions with an initial spread in real/velocity space and random phases relative to the wave. Measured wave magnetic field data are used in the simulation.
NASA Astrophysics Data System (ADS)
Ma, Qingzeng; Jiao, Jingpin; Hu, Ping; Zhong, Xi; Wu, Bin; He, Cunfu
2014-03-01
The fundamental shear horizontal(SH0) wave has several unique features that are attractive for long-range nondestructive testing(NDT). By a careful design of the geometric configuration, electromagnetic acoustic transducers(EMATs) have the capability to generate a wide range of guided wave modes, such as Lamb waves and shear-horizontal(SH) waves in plates. However, the performance of EMATs is influenced by their parameters. To evaluate the performance of periodic permanent magnet(PPM) EMATs, a distributed-line-source model is developed to calculate the angular acoustic field cross-section in the far-field. Numerical analysis is conducted to investigate the performance of such EMATs with different geometric parameters, such as period and number of magnet arrays, and inner and outer coil widths. Such parameters have a great influence on the directivity of the generated SH0 waves that arises mainly in the amplitude and width of both main and side lobes. According to the numerical analysis, these parameters are optimized to obtain better directivity. Optimized PPM EMATs are designed and used for NDT of strip plates. Experimental results show that the lateral boundary of the strip plate has no perceivable influence on SH0-wave propagation, thus validating their used in NDT. The proposed model predicts the radiation pattern of PPM EMATs, and can be used for their parameter optimization.
NASA Astrophysics Data System (ADS)
Kayen, R.; Carkin, B.; Minasian, D.
2006-12-01
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
Experimental verification of nanofluid shear-wave reconversion in ultrasonic fields.
Forrester, Derek Michael; Huang, Jinrui; Pinfield, Valerie J; Luppé, Francine
2016-03-14
Here we present the verification of shear-mediated contributions to multiple scattering of ultrasound in suspensions. Acoustic spectroscopy was carried out with suspensions of silica of differing particle sizes and concentrations in water to find the attenuation at a broad range of frequencies. As the particle sizes approach the nanoscale, commonly used multiple scattering models fail to match experimental results. We develop a new model, taking into account shear mediated contributions, and find excellent agreement with the attenuation spectra obtained using two types of spectrometer. The results determine that shear-wave phenomena must be considered in ultrasound characterisation of nanofluids at even relatively low concentrations of scatterers that are smaller than one micrometre in diameter. PMID:26763173
Ultrasound Shear Wave Simulation of Breast Tumor Using Nonlinear Tissue Elasticity
Park, Dae Woo
2016-01-01
Shear wave elasticity imaging (SWEI) can assess the elasticity of tissues, but the shear modulus estimated in SWEI is often less sensitive to a subtle change of the stiffness that produces only small mechanical contrast to the background tissues. Because most soft tissues exhibit mechanical nonlinearity that differs in tissue types, mechanical contrast can be enhanced if the tissues are compressed. In this study, a finite element- (FE-) based simulation was performed for a breast tissue model, which consists of a circular (D: 10 mm, hard) tumor and surrounding tissue (soft). The SWEI was performed with 0% to 30% compression of the breast tissue model. The shear modulus of the tumor exhibited noticeably high nonlinearity compared to soft background tissue above 10% overall applied compression. As a result, the elastic modulus contrast of the tumor to the surrounding tissue was increased from 0.46 at 0% compression to 1.45 at 30% compression. PMID:27293476
NASA Astrophysics Data System (ADS)
O'Donnell, J. P.; Adams, A.; Nyblade, A. A.; Mulibo, G. D.; Tugume, F.
2013-08-01
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.
The limits of ray theory when measuring shear wave splitting in the lowermost mantle with ScS waves
NASA Astrophysics Data System (ADS)
Nowacki, Andy; Wookey, James
2016-09-01
Observations of shear wave splitting provide unambiguous evidence of the presence of anisotropy in the Earth's lowermost mantle, a region known as D″. Much recent work has attempted to use these observations to place constraints on strain above the core-mantle boundary (CMB), as this may help map flow throughout the mantle. Previously, this interpretation has relied on the assumption that waves can be modelled as infinite-frequency rays, or that the Earth is radially symmetric. Due to computational constraints it has not been possible to test these approximations until now. We use fully three-dimensional, generally-anisotropic simulations of ScS waves at the frequencies of the observations to show that ray methods are sometimes inadequate to interpret the signals seen. We test simple, uniform models, and for a D″ layer as thin as 50 km, significant splitting may be produced, and we find that recovered fast orientations usually reflect the imposed fast orientation above the CMB. Ray theory in these cases provides useful results, though there are occasional, notable differences between forward methods. Isotropic models do not generate apparent splitting. We also test more complex models, including ones based on our current understanding of mineral plasticity and elasticity in D″. The results show that variations of anisotropy over even several hundred kilometres cause the ray-theoretical and finite-frequency calculations to differ greatly. Importantly, models with extreme mineral alignment in D″ yield splitting times not dissimilar to observations (δt ≤ 3 s), suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought-potentially ˜10 % shear wave anisotropy or more. We show that if the base of the mantle is as complicated as we believe, future studies of lowermost mantle anisotropy will have to incorporate finite-frequency effects to fully interpret observations of shear wave splitting.
Chao, Pei-Yu; Li, Pai-Chi
2016-08-22
The high imaging resolution and motion sensitivity of optical-based shear wave detection has made it an attractive technique in biomechanics studies with potential for improving the capabilities of shear wave elasticity imaging. In this study we implemented laser speckle contrast imaging for two-dimensional (X-Z) tracking of transient shear wave propagation in agarose phantoms. The mechanical disturbances induced by the propagation of the shear wave caused temporal and spatial fluctuations in the local speckle pattern, which manifested as local blurring. By mechanically moving the sample in the third dimension (Y), and performing two-dimensional shear wave imaging at every scan position, the three-dimensional shear wave velocity distribution of the phantom could be reconstructed. Based on comparisons with the reference shear wave velocity measurements obtained using a commercial ultrasound shear wave imaging system, the developed system can estimate the shear wave velocity with an error of less than 6% for homogeneous phantoms with shear moduli ranging from 1.52 kPa to 7.99 kPa. The imaging sensitivity of our system makes it capable of measuring small variations in shear modulus; the estimated standard deviation of the shear modulus was found to be less than 0.07 kPa. A submillimeter spatial resolution for three-dimensional shear wave imaging has been achieved, as demonstrated by the ability to detect a 1-mm-thick stiff plate embedded inside heterogeneous agarose phantoms. PMID:27557169
NASA Astrophysics Data System (ADS)
Yazdanpanah Moghadam, Peyman; Quaegebeur, Nicolas; Masson, Patrice
2015-01-01
Piezoelectric transducers are commonly used in structural health monitoring systems to generate and measure ultrasonic guided waves (GWs) by applying interfacial shear and normal stresses to the host structure. In most cases, in order to perform damage detection, advanced signal processing techniques are required, since a minimum of two dispersive modes are propagating in the host structure. In this paper, a systematic approach for mode selection is proposed by optimizing the interfacial shear stress profile applied to the host structure, representing the first step of a global optimization of selective mode actuator design. This approach has the potential of reducing the complexity of signal processing tools as the number of propagating modes could be reduced. Using the superposition principle, an analytical method is first developed for GWs excitation by a finite number of uniform segments, each contributing with a given elementary shear stress profile. Based on this, cost functions are defined in order to minimize the undesired modes and amplify the selected mode and the optimization problem is solved with a parallel genetic algorithm optimization framework. Advantages of this method over more conventional transducers tuning approaches are that (1) the shear stress can be explicitly optimized to both excite one mode and suppress other undesired modes, (2) the size of the excitation area is not constrained and mode-selective excitation is still possible even if excitation width is smaller than all excited wavelengths, and (3) the selectivity is increased and the bandwidth extended. The complexity of the optimal shear stress profile obtained is shown considering two cost functions with various optimal excitation widths and number of segments. Results illustrate that the desired mode (A0 or S0) can be excited dominantly over other modes up to a wave power ratio of 1010 using an optimal shear stress profile.
Hartzell, S.; Carver, D.; Williams, R.A.; Harmsen, S.; Zerva, A.
2003-01-01
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
Primary Analysis of Shear-Wave Splitting Aroundbohai Sea Area in North China
NASA Astrophysics Data System (ADS)
Tai, L.; Gao, Y.; Shi, Y.; Sun, J.
2012-12-01
Bohai Sea area locates at eastern China, including 3 provinces ( Hebei, Liaoning and Shandong ) and 2 metropolitan cities ( Beijing and Tianjin ). There are 5 tectonic units (Yanshan uplift, Taihang uplift, Liaodong uplift, Luxi uplift and Jizhong depression), where is within North China (in short, NC). Zhangjiakou-Bohai seismic belt, an important seismotectonic zone in eastern China and Tanlu fault, the largest fault in eastern China, intersect in this zone, so that the tectonics are complicated. Earthquake activity is very strong in this zone, which is famous of earthquakes with large magnitude and high frequency. This study used a system analysis method of shear-wave splitting, namely SAM method (GAO et al, 2004). It includes mainly three aspects, i.e., calculation of cross-correlation function, elimination of time delay and check of polarization analysis. Preliminarily, we obtained the 378 seismic data within shear-wave window recorded by 27 stations around Bohai Sea area. The polarizations of fast shear-waves show different local features so that we divide the studied zone into 3 different areas, west of Bohai Sea gulf (including Beijing, Tianjin and Hebei province), north of Bohai Sea gulf (Liaoning province) and south of Bohai Sea gulf (Shandong province). In the west of Bohai Sea gulf, we divided the area into 3 parts. In south of Zhangjiakou-Bohai seismic belt, the predominant polarizations of fast shear-waves are a little scattered, lots of stations strike to nearly E-W, consistent with the direction of the in situ principal stress, consistent with the direction of regional tectonic stress in north part of NC, which strikes to nearly WNW or E-W, while some stations strike to NNE or NW, different from the direction of the regional principal stress field. It may be influenced by local tectonics or shallow crust structure. Within Zhangjiakou-Bohai seismic belt, the predominant polarizations of fast shear-waves are in direction of nearly E-W, consistent with
Blanloeuil, Philippe; Croxford, Anthony J; Meziane, Anissa
2014-04-01
The nonlinear interaction of shear waves with a frictional interface are presented and modeled using simple Coulomb friction. Analytical and finite difference implementations are proposed with both in agreement and showing a unique trend in terms of the generated nonlinearity. A dimensionless parameter ξ is proposed to uniquely quantify the nonlinearity produced. The trends produced in the numerical study are then validated with good agreement experimentally. This is carried out loading an interface between two steel blocks and exciting this interface with different amplitude normal incidence shear waves. The experimental results are in good agreement with the numerical results, suggesting the simple friction model does a reasonable job of capturing the fundamental physics. The resulting approach offers a potential way to characterize a contacting interface; however, the difficulty in activating that interface may ultimately limit its applicability. PMID:25234971
Development of a low frequency omnidirectional piezoelectric shear horizontal wave transducer
NASA Astrophysics Data System (ADS)
Belanger, Pierre; Boivin, Guillaume
2016-04-01
Structural health monitoring (SHM) may offer an alternative to time based maintenance of safety critical components. Ultrasonic guided waves have recently emerged as a prominent option because their propagation carries information regarding the location, severity and types of damage. The fundamental shear horizontal ultrasonic guided wave mode has recently attracted interest in SHM because of its unique properties. This mode is not dispersive and has no attenuation due to fluid loading. In order to cover large areas using an SHM system, omnidirectional transduction is desired. Omnidirectional transduction of SH0 is challenging because of the required torsional surface stress. This paper presents a concept based on the discretisation of a torsional surface stress source using shear piezoelectric trapezoidal elements. Finite element simulation and experimental results are used to demonstrate the performance of this concept. The experimental modal selectivity is 17 dB and the transducer has a true omnidirectional behaviour.
NASA Astrophysics Data System (ADS)
Grevemeyer, Ingo; Lange, Dietrich; Schippkus, Sven
2016-04-01
The lithosphere is the outermost solid layer of the Earth and includes the brittle curst and brittle uppermost mantle. It is underlain by the asthenosphere, the weaker and hotter portion of the mantle. The boundary between the brittle lithosphere and the asthenosphere is call the lithosphere-asthenosphere boundary, or LAB. The oceanic lithosphere is created at spreading ridges and cools and thickens with age. Seismologists define the LAB by the presence of a low shear wave velocity zone beneath a high velocity lid. Surface waves from earthquakes occurring in young oceanic lithosphere should sample lithospheric structure when being recorded in the vicinity of a mid-ocean ridge. Here, we study group velocity and dispersion of Rayleigh waves caused by earthquakes occurring at transform faults in the Central Atlantic Ocean. Earthquakes were recorded either by a network of wide-band (up to 60 s) ocean-bottom seismometers (OBS) deployed at the Mid-Atlantic Ridge near 15°N or at the Global Seismic Network (GSN) Station ASCN on Ascension Island. Surface waves sampling young Atlantic lithosphere indicate systematic age-dependent changes of group velocities and dispersion of Rayleigh waves. With increasing plate age maximum group velocity increases (as a function of period), indicating cooling and thickening of the lithosphere. Shear wave velocity is derived inverting the observed dispersion of Rayleigh waves. Further, models derived from the OBS records were refined using waveform modelling of vertical component broadband data at periods of 15 to 40 seconds, constraining the velocity structure of the uppermost 100 km and hence in the depth interval of the mantle where lithospheric cooling is most evident. Waveform modelling supports that the thickness of lithosphere increases with age and that velocities in the lithosphere increase, too.
Effects of neutral interactions on velocity-shear-driven plasma waves
Enloe, C. L.; Tejero, E. M.; Amatucci, W. E.; Crabtree, C.; Ganguli, G.; Sotnikov, V.
2014-06-15
In a laboratory experiment, we demonstrate the substantial effects that collisions between charged and neutral particles have on low-frequency (Ω{sub i} ≪ ω ≪ Ω{sub e}) shear-driven electrostatic lower hybrid waves in a plasma. We establish a strong (up to 2.5 kV/m) highly localized electric field with a length scale shorter than the ion gyroradius, so that the ions in the plasma, unlike the electrons, do not develop the full E × B drift velocity. The resulting shear in the particle velocities initiates the electron-ion hybrid (EIH) instability, and we observe the formation of strong waves in the vicinity of the shear with variations in plasma densities of 10% or greater. Our experimental configuration allows us to vary the neutral background density by more than a factor of two while holding the charged particle density effectively constant. Not surprisingly, increasing the neutral density decreases the growth rate/saturation amplitude of the waves and increases the threshold electric field necessary for wave formation, but the presence of neutrals affects the dominant wave frequency as well. We show that a 50% increase in the neutral density decreases the wave frequency by 20% while also suppressing the electric field dependence of the frequency that is observed when fewer neutrals are present. The majority of these effects, as well as the values of the frequencies we observe, closely match the predictions of previously developed linear EIH instability theory, for which we present the results of a numerical solution.
Shear wave induced resonance elastography of venous thrombi: a proof-of-concept.
Schmitt, Cédric; Montagnon, Emmanuel; Henni, Anis Hadj; Qi, Shijie; Cloutier, Guy
2013-03-01
Shear wave induced resonance elastography (SWIRE) is proposed for deep venous thrombosis (DVT) elasticity assessment. This new imaging technique takes advantage of properly polarized shear waves to induce resonance of a confined mechanical heterogeneity. Realistic phantoms (n = 9) of DVT total and partial clot occlusions with elasticities from 406 to 3561 Pa were built for in vitro experiments. An ex vivo study was also performed to evaluate the elasticity of two fresh porcine venous thrombi in a pig model. Transient shear waves at 45-205 Hz were generated by the vibration of a rigid plate (plane wavefront) or by a needle to simulate a radiation pressure on a line segment (cylindrical wavefront). Induced propagation of shear waves was imaged with an ultrafast ultrasound scanner and a finite element method was developed to simulate tested experimental conditions. An inverse problem was then formulated considering the first resonance frequency of the DVT inclusion. Elasticity agreements between SWIRE and a reference spectroscopy instrument (RheoSpectris) were found in vitro for total clots either in plane (r(2) = 0.989) or cylindrical (r(2) = 0.986) wavefront configurations. For total and partial clots, elasticity estimation errors were 9.0 ±4.6% and 9.3 ±11.3%, respectively. Ex vivo, the blood clot elasticity was 498 ±58 Pa within the inferior vena cava and 436 ±45 Pa in the right common iliac vein (p = 0.22). To conclude, the SWIRE technique seems feasible to quantitatively assess blood clot elasticity in the context of DVT ultrasound imaging.
Khan, S. A.
2011-11-29
Low frequency electrostatic and electromagnetic waves in a dense magnetoplasma are studied. The dispersive contribution of electron quantum effects in an electron-ion plasma in the presence of positively or negatively charged dust particles in the background is emphasized. By employing the quantum hydrodynamic model, a linear dispersion relation is derived which shows coupling of electrostatic and shear Alfven modes which shows influence of electron quantum effects and dust density.
NASA Astrophysics Data System (ADS)
Zeng, Lei; Parvasi, Seyed Mohammad; Kong, Qingzhao; Huo, Linsheng; Lim, Ing; Li, Mo; Song, Gangbing
2015-12-01
Concrete-encased composite structure exhibits improved strength, ductility and fire resistance compared to traditional reinforced concrete, by incorporating the advantages of both steel and concrete materials. A major drawback of this type of structure is the bond slip introduced between steel and concrete, which directly reduces the load capacity of the structure. In this paper, an active sensing approach using shear waves to provide monitoring and early warning of the development of bond slip in the concrete-encased composite structure is proposed. A specimen of concrete-encased composite structure was investigated. In this active sensing approach, shear mode smart aggregates (SAs) embedded in the concrete act as actuators and generate desired shear stress waves. Distributed piezoceramic transducers installed in the cavities of steel plates act as sensors and detect the wave response from shear mode SAs. Bond slip acts as a form of stress relief and attenuates the wave propagation energy. Experimental results from the time domain analysis clearly indicate that the amplitudes of received signal by lead zirconate titanate sensors decreased when bond slip occurred. In addition, a wavelet packet-based analysis was developed to compute the received signal energy values, which can be used to determine the initiation and development of bond slip in concrete-encased composite structure. In order to establish the validity of the proposed method, a 3D finite element analysis of the concrete-steel bond model is further performed with the aid of the commercial finite element package, Abaqus, and the numerical results are compared with the results obtained in experimental study.
Changes in shear wave splitting at Anza near the time of the North Palm Springs earthquake
Crampin, S.; Booth, D.C.; Evans, R. ); Peacock, S. ); Fletcher, J.B. )
1990-07-10
Changes in shear wave splitting are observed at KNW before and after the M = 6 North Palm Springs earthquake of July 8, 1986. KNW is a station of the Anza seismic network monitoring the Anza seismic gap on the San Jacinto fault, southern California. The gradual increase in the delays between the split shear waves over 3 years at KNW, reported by Peacock et al. (1988), ended in June 1986. The further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the North Palm Springs earthquake, 33 km north of KNW. Peacock et al. demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress-aligned fluid-filled inclusions, and speculated that this increase might be the result of a buildup of stress before an impending earthquake. The new data appear to confirm this speculation, but the temporal variations require a more complex interpretation, although they still suggest that the changes in shear wave splitting are caused by earthquake-induced stress changes to the fluid-filled inclusions throughout the rockmass. Central to interpretation of temporal changes in shear wave splitting is the well established existence, throughout at least the uppermost 10 to 20 km of the crust, of small fluid-filled cracks, microcracks, and pores. The existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock. The authors term these distributions of inclusions extensive dilatancy anisotropy or EDA, and the individual inclusions EDA cracks because, although they may include a wide range of shapes, many of the seismic properties can be simulated by distributions of thin parallel cracks.
NASA Astrophysics Data System (ADS)
Khan, S. A.
2011-11-01
Low frequency electrostatic and electromagnetic waves in a dense magnetoplasma are studied. The dispersive contribution of electron quantum effects in an electron-ion plasma in the presence of positively or negatively charged dust particles in the background is emphasized. By employing the quantum hydrodynamic model, a linear dispersion relation is derived which shows coupling of electrostatic and shear Alfven modes which shows influence of electron quantum effects and dust density.
Guo, Min; Abbott, Derek; Lu, Minhua; Liu, Huafeng
2016-03-01
Shear wave propagation speed has been regarded as an attractive indicator for quantitatively measuring the intrinsic mechanical properties of soft tissues. While most existing techniques use acoustic radiation force (ARF) excitation with focal spot region based on linear array transducers, we try to employ a special ARF with a focal line region and apply it to viscoelastic materials to create shear waves. First, a two-dimensional capacitive micromachined ultrasonic transducer with 64 × 128 fully controllable elements is realised and simulated to generate this special ARF. Then three-dimensional finite element models are developed to simulate the resulting shear wave propagation through tissue phantom materials. Three different phantoms are explored in our simulation study using: (a) an isotropic viscoelastic medium, (b) within a cylindrical inclusion, and (c) a transverse isotropic viscoelastic medium. For each phantom, the ARF creates a quasi-plane shear wave which has a preferential propagation direction perpendicular to the focal line excitation. The propagation of the quasi-plane shear wave is investigated and then used to reconstruct shear moduli sequentially after the estimation of shear wave speed. In the phantom with a transverse isotropic viscoelastic medium, the anisotropy results in maximum speed parallel to the fiber direction and minimum speed perpendicular to the fiber direction. The simulation results show that the line excitation extends the displacement field to obtain a large imaging field in comparison with spot excitation, and demonstrate its potential usage in measuring the mechanical properties of anisotropic tissues. PMID:26768475
Nonlinear evolution of interacting oblique waves on two-dimensional shear layers
NASA Technical Reports Server (NTRS)
Goldstein, M. E.; Choi, S.-W.
1989-01-01
The effects of critical layer nonlinearity are considered on spatially growing oblique instability waves on nominally two-dimensional shear layers between parallel streams. The analysis shows that three-dimensional effects cause nonlinearity to occur at much smaller amplitudes than it does in two-dimensional flows. The nonlinear instability wave amplitude is determined by an integro-differential equation with cubic type nonlinearity. The numerical solutions to this equation are worked out and discussed in some detail. The numerical solutions always end in a singularity at a finite downstream distance.
Shi, P; Chen, C Q; Zou, W N
2015-01-01
Coupled shear (SH) elastic and electromagnetic (EM) waves propagating oblique to a one dimensional periodic piezoelectric and piezomagnetic composite are investigated using the transfer matrix method. Closed-form expression of the dispersion relations is derived. We find that the band structures of the periodic composite show simultaneously the features of phononic and photonic crystals. Strong interaction between the elastic and EM waves near the center of the Brillouin zone (i.e., phonon-polariton) is revealed. It is shown the elastic branch of the band structures is more sensitive to the piezoelectric effect while the phonon-polariton is more sensitive to the piezomagnetic effect of the composite.
Recent developments in IR metrology using quadri wave lateral shearing interferometry
NASA Astrophysics Data System (ADS)
Boucher, W.; Bourgeois, G.; Deprez, M.; Homassel, E.; Wattellier, B.
2015-10-01
We present various applications of Quadri Wave Lateral Shearing Interferometry to infrared optical metrology. Phasics has developed wave front sensors compatible with wavelengths ranging from 193nm to 14 μm. Several camera technologies are integrated with patented diffractive optics adapted to the various spectral ranges. In the infrared range, InGaAs, MCT and microbolometers are used as light sensors. We developed a bench to qualify lenses with diameter from 2mm to 100mm in the LWIR spectral range. Thanks to an innovative ray-trace algorithm, we are able to compare the wave front measurement to the optical design enabling a feedback between lens manufacture and design. We also developed a VSWIR wave front sensor with a constant accuracy from 0.4μm to 1.7 μm. It also includes the Extended Tilt feature that enables sub-mrad accuracy on tilt measurement over a range of more than +-30°.
Shear wave velocity structure of the Anatolian Plate: anomalously slow crust in southwestern Turkey
NASA Astrophysics Data System (ADS)
Delph, Jonathan R.; Biryol, C. Berk; Beck, Susan L.; Zandt, George; Ward, Kevin M.
2015-07-01
The Anatolian Plate is composed of different lithospheric blocks and ribbon continents amalgamated during the closure of the Paleotethys Ocean and Neotethys Ocean along a subduction margin. Using ambient noise tomography, we investigate the crustal and uppermost mantle shear wave velocity structure of the Anatolian Plate. A total of 215 broad-band seismic stations were used spanning 7 yr of recording to compute 13 778 cross-correlations and obtain Rayleigh wave dispersion measurements for periods between 8 and 40 s. We then perform a shear wave inversion to calculate the seismic velocity structure of the crust and uppermost mantle. Our results show that the overall crustal shear wave velocities of the Anatolian crust are low (˜3.4 km s-1), indicative of a felsic overall composition. We find that prominent lateral seismic velocity gradients correlate with Tethyan suture zones, supporting the idea that the neotectonic structures of Turkey are exploiting the lithospheric weaknesses associated with the amalgamation of Anatolia. Anomalously slow shear wave velocities (˜3.15 km s-1 at 25 km) are located in the western limb of the Isparta Angle in southwestern Turkey. In the upper crust, we find that these low shear wave velocities correlate well with the projected location of a carbonate platform unit (Bey Dağlari) beneath the Lycian Nappe complex. In the lower crust and upper mantle of this region, we propose that the anomalously slow velocities are due to the introduction of aqueous fluids related to the underplating of accretionary material from the underthrusting of a buoyant, attenuated continental fragment similar to the Eratosthenes seamount. We suggest that this fragment controlled the location of the formation of the Subduction-Transform Edge Propagator fault in the eastern Aegean Sea during rapid slab rollback of the Aegean Arc in early Miocene times. Lastly, we observe that the uppermost mantle beneath continental Anatolia is generally slow (˜4.2 km s-1
Correia, Mafalda; Provost, Jean; Chatelin, Simon; Villemain, Olivier; Tanter, Mickael; Pernot, Mathieu
2016-01-01
Transthoracic shear wave elastography of the myocardium remains very challenging due to the poor quality of transthoracic ultrafast imaging and the presence of clutter noise, jitter, phase aberration, and ultrasound reverberation. Several approaches, such as, e.g., diverging-wave coherent compounding or focused harmonic imaging have been proposed to improve the imaging quality. In this study, we introduce ultrafast harmonic coherent compounding (UHCC), in which pulse-inverted diverging-waves are emitted and coherently compounded, and show that such an approach can be used to enhance both Shear Wave Elastography (SWE) and high frame rate B-mode Imaging. UHCC SWE was first tested in phantoms containing an aberrating layer and was compared against pulse-inversion harmonic imaging and against ultrafast coherent compounding (UCC) imaging at the fundamental frequency. In-vivo feasibility of the technique was then evaluated in six healthy volunteers by measuring myocardial stiffness during diastole in transthoracic imaging. We also demonstrated that improvements in imaging quality could be achieved using UHCC B-mode imaging in healthy volunteers. The quality of transthoracic images of the heart was found to be improved with the number of pulse-inverted diverging waves with reduction of the imaging mean clutter level up to 13.8-dB when compared against UCC at the fundamental frequency. These results demonstrated that UHCC B-mode imaging is promising for imaging deep tissues exposed to aberration sources with a high frame-rate. PMID:26890730
Shear-horizontal wave-based pipe damage inspection by arrays of segmented magnetostrictive patches.
Kim, Hoe Woong; Kwon, Young Eui; Cho, Seung Hyun; Kim, Yoon Young
2011-12-01
The lowest-branch torsional guided wave is very effective in pipe damage inspection because of its non-dispersive characteristics, but it cannot be used for the simultaneous identification of axial and circumferential locations of a defect in a pipe. Motivated by recent developments in magnetostrictive transducer technology, which is especially efficient in torsional and shear wave generation, the goal of this investigation is to extend this technology for simultaneous identification of the axial and circumferential locations of cracks by using shear horizontal (SH) waves. Unlike the conventional magnetostrictive patch method using a single complete patch wound around the pipe's circumference, the proposed method segments the patch into several pieces to generate SH waves propagating over the pipe surface. Accordingly, SH waves in a pipe are generated and sensed individually by a meander coil placed separately on each segment. By using two sets of segmented-patch arrays separated by some distance, the cylindrical surface of a pipe can be inspected both axially and circumferentially. After the underlying angular profile of the patch segment is investigated, experiments identifying the axial and circumferential locations of multiple cracks in a pipe are carried out to demonstrate the potential of the proposed methodology.
Reduced Patellar Tendon Elasticity with Aging: In Vivo Assessment by Shear Wave Elastography.
Hsiao, Ming-Yen; Chen, Yi-Ching; Lin, Che-Yu; Chen, Wen-Shian; Wang, Tyng-Guey
2015-11-01
How aging affects the elasticity of tendons has long been debated, partly because of the limited methods for in vivo evaluation, which differ vastly from those for in vitro animal studies. In this study, we tested the reliability of shear wave elastography (SWE) in the evaluation of patellar tendons and their change in elasticity with age. We recruited 62 healthy participants in three age groups: 20-30 years (group 1), 40-50 years (group 2) and 60-70 years (group 3). Shear wave velocity and elastic modulus were measured at the proximal, middle and distal areas of the patellar tendon. Reliability was excellent at the middle area and fair to good at both ends. Compared with the other groups, group 3 had significantly decreased elastic modulus and shear wave velocity values (p ≤ 0.001 vs. group 1 or 2), with significant increased side-to-side differences. SWE may be valuable in detecting aging tendons before visible abnormalities are observed on B-mode ultrasonography.
NASA Astrophysics Data System (ADS)
Richter, Tom; Wegler, Ulrich
2015-04-01
Modeling of peak ground velocity caused by induced earthquakes requires detailed knowledge about seismic attenuation properties of the subsurface. Especially shear wave attenuation is important, because shear waves usually show the largest amplitude in high frequency seismograms. We report intrinsic and scattering attenuation coefficients of shear waves near three geothermal reservoirs in Germany for frequencies between 2 Hz and 50 Hz. The geothermal plants are located in the sedimentary basins of the upper Rhine graben (Insheim and Landau) and the Molasse basin (Unterhaching). The method optimizes the fit between Green's functions for the acoustic, isotropic radiative transfer theory and observed energy densities of induced earthquakes. The inversion allows the determination of scattering and intrinsic attenuation, site corrections, and spectral source energies for the investigated frequency bands. We performed the inversion at the three sites for events with a magnitude between 0.7 and 2. We determined a transport mean free path of 70 km for Unterhaching. For Landau and Insheim the transport mean free path depends on frequency. It ranges from 2 km (at 2 Hz) to 30 km (at 40 Hz) for Landau and from 9 km to 50 km for Insheim. The quality factor for intrinsic attenuation is constant for frequencies smaller than 10 Hz at all three sites. It is around 100 for Unterhaching and 200 for Landau and Insheim with higher values above 10 Hz.
Variation of shear and compressional wave modulus upon saturation for pure pre-compacted sands
NASA Astrophysics Data System (ADS)
Bhuiyan, M. H.; Holt, R. M.
2016-07-01
Gassmann's fluid substitution theory is commonly used to predict seismic velocity change upon change in saturation, and is hence essential for 4-D seismic and AVO studies. This paper addresses the basics assumptions of the Gassmann theory, in order to see how well they are fulfilled in controlled laboratory experiments. Our focus is to investigate the sensitivity of shear modulus to fluid saturation, and the predictability of Gassmann's fluid substitution theory for P-wave modulus. Ultrasonic P- and S-wave velocities in dry and saturated (3.5 wt per cent NaCl) unconsolidated clean sands (Ottawa and Columbia) were measured in an oedometer test system (uniaxial strain conditions) over a range of 0.5-10 MPa external vertical stress. This study shows shear modulus hardening upon brine saturation, which is consistent with previous data found in the literature. Analysis of the data shows that most of the hardening of the ultrasonic shear modulus may be explained by Biot dispersion. Isotropic Gassmann's fluid substitution is found to underestimate the P-wave modulus upon fluid saturation. However, adding the Biot dispersion effect improves the prediction. More work is required to obtain good measurements of parameters influencing dispersion, such as tortuosity, which is very ambiguous and challenging to measure accurately.
Holzer, T.L.; Bennett, M.J.; Noce, T.E.; Tinsley, J. C.
2005-01-01
Shear-wave velocities of shallow surficial geologic units were measured at 210 sites in a 140-km2 area in the greater Oakland, California, area near the margin of San Francisco Bay. Differences between average values of shear-wave velocity for each geologic unit computed by alternative approaches were in general smaller than the observed variability. Averages estimated by arithmetic mean, geometric mean, and slowness differed by 1 to 8%, while coefficients of variation ranged from 14 to 25%. With the exception of the younger Bay mud that underlies San Francisco Bay, velocities of the geologic units are approximately constant with depth. This suggests that shear-wave velocities measured at different depths in these surficial geologic units do not need to be normalized to account for overburden stress in order to compute average values. The depth dependence of the velocity of the younger Bay mud most likely is caused by consolidation. Velocities of each geologic unit are consistent with a normal statistical distribution. Average values increase with geologic age, as has been previously reported. Velocities below the water table are about 7% less than those above it. ?? 2005, Earthquake Engineering Research Institute.
Shear horizontal surface acoustic waves in a magneto-electro-elastic system
NASA Astrophysics Data System (ADS)
Eskandari, Shahin; Shodja, Hossein M.
2016-04-01
Propagation of shear horizontal surface acoustic waves (SHSAWs) within a functionally graded magneto-electro-elastic (FGMEE) half-space was previously presented (Shodja HM, Eskandari S, Eskandari M. J. Eng. Math. 2015, 1-18) In contrast, the current paper considers propagation of SHSAWs in a medium consisting of an FGMEE layer perfectly bonded to a homogeneous MEE substrate. When the FGMEE layer is described by some special inhomogeneity functions - all the MEE properties have the same variation in depth which may or may not be identical to that of the density - we obtain the exact closed-form solution for the MEE fields. Additionally, certain special inhomogeneity functions with monotonically decreasing bulk shear wave velocity in depth are considered, and the associated boundary value problem is solved using power series solution. This problem in the limit as the layer thickness goes to infinity collapses to an FGMEE half-space with decreasing bulk shear wave velocity in depth. It is shown that in such a medium SHSAW does not propagate. Using power series solution we can afford to consider some FGMEE layers of practical importance, where the composition of the MEE obeys a prescribed volume fraction variation. The dispersive behavior of SHSAWs in the presence of such layers is also examined.
Shear Wave Splitting from Local Earthquakes in the New Madrid Seismic Zone
NASA Astrophysics Data System (ADS)
Martin, P.; Arroucau, P.; Vlahovic, G.
2012-12-01
In this study we investigate crustal anisotropy in the New Madrid seismic zone (NMSZ), by analyzing shear wave splitting from local earthquake data. The NMSZ is centrally located in the United States, spanning portions of western Tennessee, northeastern Arkansas, and southeastern Missouri. The NMSZ is also the location in which three of the largest known earthquakes took place in North America, occurring in 1811-1812. Although many seismic studies have been performed in this region, there is no consensus about which driving mechanism could satisfy both the current observations, as well as the historically observed seismicity. Therefore, it is important to continue investigating the NMSZ, to gain a better understanding of its seismicity, and the possible mechanisms that drive it. The automated technique developed by Savage et al. (2010) is used to perform the shear wave splitting measurements at 120 seismic stations within the NMSZ. The Center for Earthquake Research and Information (CERI) at the University of Memphis provided data for 1151 earthquakes spanning the years 2003-2011. The initial event selection was reduced to 245 earthquakes ranging in magnitude from 2.0 to 4.6, which fell within the shear wave window of one or more of the stations. The results of this study provide information about orientation of microcracks in the upper portion of the crust; future work will include analysis for temporal and spatial variations in order to assess the state of stress in the region.
Anatomy of Drift Ridges Revealed by Shallow Seismic Shear Wave Profiling
NASA Astrophysics Data System (ADS)
Phillips, A. C.
2005-12-01
Ridges, up to 30 m high and generally oriented NE-SW across the Illinois Episode drift plain in southern Illinois, USA, have been variously interpreted as eskers, crevasse fills, moraines, and kames. The ice contact diamictons and sorted sediments that occur in these ridges are typically Illinois Episode in age and likely record the final melting of the Laurentide Ice Sheet near its southernmost extent in the continental U.S. Shallow shear wave seismic profiles across several of these ridges help reveal their complex origins. Borehole control includes sediment cores with shear wave and natural gamma logs. The shear wave profiles reveal m-scale features of drift and bedrock over a depth range of 1 up to 100 m. Terrapin Ridge overlies a bedrock valley with drift up to 70 m thick. Dipping seismic reflectors on the stoss side are interpreted as imbricated till sheets, whereas horizontal reflectors on the lee side are interpreted as mainly outwash sediments over basal till and glacilacustrine sediment. Although most ridges were probably formed during the Illinois Episode, based on current data, the core of this particular ridge may be a remnant moraine from a pre-Illinois Episode glaciation.
NASA Astrophysics Data System (ADS)
Iwasaki, Ryosuke; Takagi, Ryo; Nagaoka, Ryo; Jimbo, Hayato; Yoshizawa, Shin; Saijo, Yoshifumi; Umemura, Shin-ichiro
2016-07-01
Shear wave elastography (SWE) is expected to be a noninvasive monitoring method of high-intensity focused ultrasound (HIFU) treatment. However, conventional SWE techniques encounter difficulty in inducing shear waves with adequate displacements in deep tissue. To observe tissue coagulation at the HIFU focal depth via SWE, in this study, we propose using a two-dimensional-array therapeutic transducer for not only HIFU exposure but also creating shear sources. The results show that the reconstructed shear wave velocity maps detected the coagulated regions as the area of increased propagation velocity even in deep tissue. This suggests that “HIFU-push” shear elastography is a promising solution for the purpose of coagulation monitoring in deep tissue, because push beams irradiated by the HIFU transducer can naturally reach as deep as the tissue to be coagulated by the same transducer.
An EMAT-based shear horizontal (SH) wave technique for adhesive bond inspection
NASA Astrophysics Data System (ADS)
Arun, K.; Dhayalan, R.; Balasubramaniam, Krishnan; Maxfield, Bruce; Peres, Patrick; Barnoncel, David
2012-05-01
The evaluation of adhesively bonded structures has been a challenge over the several decades that these structures have been used. Applications within the aerospace industry often call for particularly high performance adhesive bonds. Several techniques have been proposed for the detection of disbonds and cohesive weakness but a reliable NDE method for detecting interfacial weakness (also sometimes called a kissing bond) has been elusive. Different techniques, including ultrasonic, thermal imaging and shearographic methods, have been proposed; all have had some degree of success. In particular, ultrasonic methods, including those based upon shear and guided waves, have been explored for the assessment of interfacial bond quality. Since 3-D guided shear horizontal (SH) waves in plates have predominantly shear displacement at the plate surfaces, we conjectured that SH guided waves should be influenced by interfacial conditions when they propagate between adhesively bonded plates of comparable thickness. This paper describes a new technique based on SH guided waves that propagate within and through a lap joint. Through mechanisms we have yet to fully understand, the propagation of an SH wave through a lap joint gives rise to a reverberation signal that is due to one or more reflections of an SH guided wave mode within that lap joint. Based upon a combination of numerical simulations and measurements, this method shows promise for detecting and classifying interfacial bonds. It is also apparent from our measurements that the SH wave modes can discriminate between adhesive and cohesive bond weakness in both Aluminum-Epoxy-Aluminum and Composite-Epoxy-Composite lap joints. All measurements reported here used periodic permanent magnet (PPM) Electro-Magnetic Acoustic Transducers (EMATs) to generate either or both of the two lowest order SH modes in the plates that comprise the lap joint. This exact configuration has been simulated using finite element (FE) models to
NASA Astrophysics Data System (ADS)
Song, Shaozhen; Le, Nhan Minh; Wang, Ruikang K.; Huang, Zhihong
2015-03-01
Shear Wave Optical Coherence Elastography (SW-OCE) uses the speed of propagating shear waves to provide a quantitative measurement of localized shear modulus, making it a valuable technique for the elasticity characterization of tissues such as skin and ocular tissue. One of the main challenges in shear wave elastography is to induce a reliable source of shear wave; most of nowadays techniques use external vibrators which have several drawbacks such as limited wave propagation range and/or difficulties in non-invasive scans requiring precisions, accuracy. Thus, we propose linear phase array ultrasound transducer as a remote wave source, combined with the high-speed, 47,000-frame-per-second Shear-wave visualization provided by phase-sensitive OCT. In this study, we observed for the first time shear waves induced by a 128 element linear array ultrasound imaging transducer, while the ultrasound and OCT images (within the OCE detection range) were triggered simultaneously. Acoustic radiation force impulses are induced by emitting 10 MHz tone-bursts of sub-millisecond durations (between 50 μm - 100 μm). Ultrasound beam steering is achieved by programming appropriate phase delay, covering a lateral range of 10 mm and full OCT axial (depth) range in the imaging sample. Tissue-mimicking phantoms with agarose concentration of 0.5% and 1% was used in the SW-OCE measurements as the only imaging samples. The results show extensive improvements over the range of SW-OCE elasticity map; such improvements can also be seen over shear wave velocities in softer and stiffer phantoms, as well as determining the boundary of multiple inclusions with different stiffness. This approach opens up the feasibility to combine medical ultrasound imaging and SW-OCE for high-resolution localized quantitative measurement of tissue biomechanical property.
NASA Astrophysics Data System (ADS)
Grutas, Rhommel; Yamanaka, Hiroaki
2012-07-01
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.
NASA Astrophysics Data System (ADS)
Delph, J. R.; Beck, S. L.; Zandt, G.; Biryol, C. B.; Ward, K. M.
2013-12-01
The Anatolian Plate consists of various lithospheric terranes amalgamated during the closure of the Tethys Ocean, and is currently extruding to the west in response to a combination of the collision of the Arabian plate in the east and the roll back of the Aegean subduction zone in the west. We used Ambient Noise Tomography (ANT) at periods <= 40s to investigate the crust and uppermost mantle structure of the Anatolian Plate. We computed a total of 13,779 unique cross-correlations using one sample-per-second vertical component broadband seismic data from 215 stations from 8 different networks over a period of 7 years to compute fundamental-mode Rayleigh wave dispersion curves following the method of Benson et al. (2007). We then inverted the dispersion data to calculate phase velocity maps for 11 periods from 8 s - 40 s throughout Anatolia and the Aegean regions (Barmin et al. 2001). Using smoothed Moho values derived from Vanacore et al. (2013) in our starting models, we inverted our dispersion curves using a linear least-squares iterative inversion scheme (Herrmann & Ammon 2004) to produce a 3-D shear-wave velocity model of the crust and uppermost mantle throughout Anatolia and the Aegean. We find a good correlation between our seismic shear wave velocities and paleostructures (suture zones) and modern deformation (basin formation and fault deformation). The most prominent crustal velocity contrasts occur across intercontinental sutures zones, resulting from the juxtaposition of the compositionally different basements of the amalgamated terranes. At shallow depths, seismic velocity contrasts correspond closely with surficial features. The Thrace, Cankiri and Tuz Golu basins, and accretionary complexes related to the closure of the Neotethys are characterized by slow shear wave velocities, while the Menderes and Kirsehir Massifs, Pontides, and Istanbul Zone are characterized by fast velocities. We find that the East Anatolia Plateau has slow shear-wave velocities
Generation of shear Alfvén waves by repetitive electron heating
NASA Astrophysics Data System (ADS)
Wang, Y.; Gekelman, W.; Pribyl, P.; Van Compernolle, B.; Papadopoulos, K.
2016-01-01
ELF/ULF waves are powerful tools for submarine communication, geophysical mapping, and radiation belt remediation. However, due to their large wavelength (on the order of 102-104 km or 0.1-10 RE) it is difficult to launch them using ground-based antennas. Alternatively, these waves can be generated by modulating the temperature of the ionosphere using ground-based HF transmitters. The paper reports a detailed laboratory study on the generation of shear Alfvén waves by repetitive electron heating. The experiments were conducted on the large plasma device at University of California, Los Angeles. In the experiment, 10 pulses of high-power microwaves (250 kW, 1 µs each) near the plasma frequency modulated at a variable fraction between 0.1 and 1.0 of fci are launched transverse to the background field. In addition to bulk electron heating the interaction generates a population of fast electrons in the tail of the distribution function. The field-aligned current carried by the fast electrons acts as an antenna that radiates shear Alfvén waves. It is demonstrated that a shear Alfvén wave at a controllable, arbitrary frequency (f < fci) can be coherently driven by the repetitive microwave pulses. The radiation pattern and power dependence of the virtual antenna are also presented. The experiments provide a novel virtual antenna concept relevant to the equatorial region where the Earth's magnetic field is horizontal and the field-aligned plasma density gradient is small. The results are important to design of new mobile ionospheric heaters for equatorial and middle latitude locations.
Nenadic, Ivan Z; Urban, Matthew W; Bernal, Miguel; Greenleaf, James F
2011-12-01
In the past several decades, the fields of ultrasound and magnetic resonance elastography have shown promising results in noninvasive estimates of mechanical properties of soft tissues. These techniques often rely on measuring shear wave velocity due to an external or internal source of force and relating the velocity to viscoelasticity of the tissue. The mathematical relationship between the measured velocity and material properties of the myocardial wall, arteries, and other organs with non-negligible boundary conditions is often complicated and computationally expensive. A simple relationship between the Lamb-Rayleigh dispersion and the shear wave dispersion is derived for both the velocity and attenuation. The relationship shows that the shear wave velocity is around 20% higher than the Lamb-Rayleigh velocity and that the shear wave attenuation is about 20% lower than the Lamb-Rayleigh attenuation. Results of numerical simulations in the frequency range 0-500 Hz are presented. PMID:22225009
NASA Astrophysics Data System (ADS)
Sekiguchi, Tomohiro; Tanabe, Kazuho; Muto, Tetsuji
2015-04-01
Cyclic steps, a kind of upper regime bedforms associated with spatially-periodic hydraulic jumps, autogenically produce distinct cyclicity in delta-foreset bedding. This notion was obtained in our previous series of experiments where discrete packages of sediment accumulation on the foreset slope is synchronized with upstream migration of hydraulic jumps that depart from shoreline, i.e. the topset-foreset break of the delta. In the present series of experiments, conducted within a flume (50 cm × 50 cm × 20 m) with a piston-type wave generator at one end, waves were imposed on a prograding delta, the topset surface of which is characterized with the development of alluvial cyclic steps. A particular set of upstream water discharge, sediment feed rate, and water depth was adopted in all runs, and the experimental conditions including wave period and height were kept constant during each run. A mixture of two quartz sands with different colors and sizes was used in order to visualize sediment grain distribution. The experimental results suggest that waves do not affect behavior of hydraulic jumps (occurrence period at the river mouth, and rate of upstream migration), alluvial topset slope, rate of delta progradation, and the synchronism of cyclic foreset bedding with hydraulic-jump that is about to leave the river mouth. When waves were imposed, a foreshore/upper shoreface layer was intercalated between foreset and topset deposits. This layer became thicker and the foreset slopes tended to be gentler as wave height increased.
Cluster observations of Shear-mode surface waves diverging from Geomagnetic Tail reconnection
NASA Astrophysics Data System (ADS)
Dai, L.; Wygant, J. R.; Dombeck, J. P.; Cattell, C. A.; Thaller, S. A.; Mouikis, C.; Balogh, A.; Reme, H.
2010-12-01
We present the first Cluster spacecraft study of the intense (δB/B~0.5, δE/VAB~0.5) equatorial plane surface waves diverging from magnetic reconnection in the geomagnetic tail at ~17 Re. Using phase lag analysis with multi-spacecraft measurements, we quantitatively determine the wavelength and phase velocity of the waves with spacecraft frame frequencies from 0.03 Hz to 1 Hz and wavelengths from much larger (4Re) than to comparable to the H+ gyroradius (~300km). The phase velocities track the strong variations in the equatorial plane projection of the reconnection outflow velocity perpendicular to the magnetic field. The propagation direction and wavelength of the observed surface waves resemble those of flapping waves of the magnetotail current sheet, suggesting a same origin shared by both of these waves. The observed waves appear ubiquitous in the outflows near magnetotail reconnection. Evidence is found that the observed waves are associated with velocity shear in reconnection outflows. Analysis shows that observed waves are associated with strong field-aligned Alfvenic Poynting flux directed away from the reconnection region toward Earth. These observations present a scenario in which the observed surface waves are driven and convected through a velocity-shear type instability by high-speed (~1000km) reconnection outflows tending to slow down due to power dissipation through Poynting flux. The mapped Poynting flux (100ergs/cm2s) and longitudinal scales (10-100 km) to 100km altitude suggest that the observed waves and their motions are an important boundary condition for night-side aurora. Figure: a) The BX-GSM in the geomagnetic tail current sheet. b) The phase difference wavelet spectrum between Bz_GSM from SC2 and SC3, used to determine the wave phase velocity, is correlated with the reconnection outflow velocity (represented by H+ VX-GSM) c) The spacecraft trajectory through magnetotail reconnection. d) The observed equatorial plane surface wave
Dalyander, P. Soupy; Butman, Bradford; Sherwood, Christopher R.; Signell, Richard P.; Wilkin, John L.
2013-01-01
Waves and currents create bottom shear stress, a force at the seabed that influences sediment texture distribution, micro-topography, habitat, and anthropogenic use. This paper presents a methodology for assessing the magnitude, variability, and driving mechanisms of bottom stress and resultant sediment mobility on regional scales using numerical model output. The analysis was applied to the Middle Atlantic Bight (MAB), off the U.S. East Coast, and identified a tidally-dominated shallow region with relatively high stress southeast of Massachusetts over Nantucket Shoals, where sediment mobility thresholds are exceeded over 50% of the time; a coastal band extending offshore to about 30 m water depth dominated by waves, where mobility occurs more than 20% of the time; and a quiescent low stress region southeast of Long Island, approximately coincident with an area of fine-grained sediments called the “Mud Patch”. The regional high in stress and mobility over Nantucket Shoals supports the hypothesis that fine grain sediment winnowed away in this region maintains the Mud Patch to the southwest. The analysis identified waves as the driving mechanism for stress throughout most of the MAB, excluding Nantucket Shoals and sheltered coastal bays where tides dominate; however, the relative dominance of low-frequency events varied regionally, and increased southward toward Cape Hatteras. The correlation between wave stress and local wind stress was lowest in the central MAB, indicating a relatively high contribution of swell to bottom stress in this area, rather than locally generated waves. Accurate prediction of the wave energy spectrum was critical to produce good estimates of bottom shear stress, which was sensitive to energy in the long period waves.
Rouze, Ned C; Palmeri, Mark L; Nightingale, Kathryn R
2015-08-01
Recent measurements of shear wave propagation in viscoelastic materials have been analyzed by constructing the two-dimensional Fourier transform (2D-FT) of the spatial-temporal shear wave signal and using an analysis procedure derived under the assumption the wave is described as a plane wave, or as the asymptotic form of a wave expanding radially from a cylindrically symmetric source. This study presents an exact, analytic expression for the 2D-FT description of shear wave propagation in viscoelastic materials following asymmetric Gaussian excitations and uses this expression to evaluate the bias in 2D-FT measurements obtained using the plane or cylindrical wave assumptions. A wide range of biases are observed depending on specific values of frequency, aspect ratio R of the source asymmetry, and material properties. These biases can be reduced significantly by weighting the shear wave signal in the spatial domain to correct for the geometric spreading of the shear wavefront using a factor of x(p). The optimal weighting power p is found to be near the theoretical value of 0.5 for the case of a cylindrical source with R = 1, and decreases for asymmetric sources with R > 1.
Laboratory study of spiky potential structures associated with multi-harmonic shear-driven EIC waves
NASA Astrophysics Data System (ADS)
Merlino, Robert; Ganguli, Guru; Kim, Su-Hyun
2015-11-01
A ubiquitous feature of electric fields observed in the Earth's auroral region is their spiky, repetitive nature, and appearance as either unipolar or bipolar pulses. They have been observed on a number of satellites including S3-3, Polar, Viking, and FAST. These spiky structures have been associated with regions of upward ion flows with transverse shear, and multi-harmonic electrostatic ion cyclotron (EIC) waves. The occurrence of these spiky structures has been attributed to various nonlinear processes, e.g., solitary waves. We will present results of a laboratory experiment performed in a double-ended Q machine, in which spiky potential waveforms were observed in association with coherent multi-harmonic EIC waves in a current-less plasma having a region of parallel ion flow with transverse shear. The spiky waveforms are shown to result from the linear superposition of phase-coherent EIC waves. Work at UI supported by NSF and DOE, work at NRL supported by ONR.
Detecting π-phase superfluids with p-wave symmetry in a quasi-1D optical lattice
NASA Astrophysics Data System (ADS)
Liu, Bo; Li, Xiaopeng; Hulet, Randall G.; Liu, W. Vincent
2016-05-01
We propose an experimental protocol to create a p-wave superfluid in a spin-polarized cold Fermi gas tuned by an s-wave Feshbach resonance. A crucial ingredient is to add an anisotropic 3D optical lattice and tune the fillings of two spins to the s and p band, respectively. The pairing order parameter is confirmed to inherit p-wave symmetry in its center-of-mass motion. We find that it can further develop into a state of unexpected π-phase modulation in a broad parameter regime. Experimental signatures are predicted in the momentum distributions, density of states and spatial densities for a realistic experimental setup. The π-phase p-wave superfluid is reminiscent of the π-state in superconductor-ferromagnet heterostructures but differs in symmetry and physical origin. The spatially-varying phases of the superfluid gap provide a novel approach to synthetic magnetic fields for neutral atoms. It would represent another example of p-wave pairing, first discovered in He-3 liquids. Work supported in part by U.S. ARO, AFOSR, NSF, ONR, Charles E. Kaufman Foundation, and The Pittsburgh Foundation, LPS-MPO-CMTC, JQI-NSF-PFC, ARO-Atomtronics-MURI, the Welch Foundation, ARO-MURI and NSF of China.
The Fermi-Pasta-Ulam recurrence and related phenomena for 1D shallow-water waves in a finite basin
Ruban, V. P.
2012-02-15
Different regimes of the Fermi-Pasta-Ulam (FPU) recurrence are simulated numerically for fully nonlinear 'one-dimensional' potential water waves in a finite-depth flume between two vertical walls. In such systems, the FPU recurrence is closely related to the dynamics of coherent structures approximately corresponding to solitons of the integrable Boussinesq system. A simplest periodic solution of the Boussinesq model, describing a single soliton between the walls, is presented in analytic form in terms of the elliptic Jacobi functions. In the numerical experiments, it is observed that depending on the number of solitons in the flume and their parameters, the FPU recurrence can occur in a simple or complicated manner, or be practically absent. For comparison, the nonlinear dynamics of potential water waves over nonuniform beds is simulated, with initial states taken in the form of several pairs of colliding solitons. With a mild-slope bed profile, a typical phenomenon in the course of evolution is the appearance of relatively high (rogue) waves, while for random, relatively short-correlated bed profiles it is either the appearance of tall waves or the formation of sharp crests at moderate-height waves.
Seismic anisotropy indicators in Western Tibet: Shear wave splitting and receiver function analysis
NASA Astrophysics Data System (ADS)
Levin, Vadim; Roecker, Steven; Graham, Peter; Hosseini, Afsaneh
2008-12-01
Using recently collected data from western Tibet we find significant variation in the strength, vertical distribution and attributes of seismic wave speed anisotropy, constrained through a joint application of teleseismic shear wave splitting techniques and a study of P-S mode-converted waves (receiver functions). We find that the crust of Tibet is characterized by anisotropy on the order of 5%-15% concentrated in layers 10-20 km in thickness, and with relatively steep (30°-45° from the vertical) slow symmetry axes of anisotropy. These layers contribute no more than 0.3 s to the birefringence in teleseismic shear waves, significantly smaller than splitting in many of the observations, and much smaller than birefringence predicted by models developed through group inversions of shear-wave recordings. Consequently, we interpret models constrained with shear-wave observations in terms of structures in the upper mantle. Near the Altyn-Tagh fault our data favor a two-layer model, with the upper layer fast polarization approximately aligned with the strike of the fault. Near the Karakorum fault our data are well fit with a single layer of relatively modest (~ 0.5 s delay) anisotropy. Fast polarization in this layer is ~ 60°NE, similar to that of the lower layer in the model for the Altyn Tagh fault site. Assuming that layers of similar anisotropic properties at these two sites reflect a common cause, our finding favors a scenario where Indian lithosphere under-thrusts a significant fraction of the plateau. Data from a site at the southern edge of the Tarim basin appear to be inconsistent with a common model of seismic anisotropy distribution. We suspect that thick sediments underlying the site significantly distort observed waveforms. Our ability to resolve features of anisotropic structure in the crust and the upper mantle of western Tibet is limited by the small amount of data collected in a 6 month observing period. We stress the importance of future teleseismic
Razani, Marjan; Luk, Timothy W.H.; Mariampillai, Adrian; Siegler, Peter; Kiehl, Tim-Rasmus; Kolios, Michael C.; Yang, Victor X.D.
2014-01-01
In this work, we explored the potential of measuring shear wave propagation using optical coherence elastography (OCE) in an inhomogeneous phantom and carotid artery samples based on a swept-source optical coherence tomography (OCT) system. Shear waves were generated using a piezoelectric transducer transmitting sine-wave bursts of 400 μs duration, applying acoustic radiation force (ARF) to inhomogeneous phantoms and carotid artery samples, synchronized with a swept-source OCT (SS-OCT) imaging system. The phantoms were composed of gelatin and titanium dioxide whereas the carotid artery samples were embedded in gel. Differential OCT phase maps, measured with and without the ARF, detected the microscopic displacement generated by shear wave propagation in these phantoms and samples of different stiffness. We present the technique for calculating tissue mechanical properties by propagating shear waves in inhomogeneous tissue equivalent phantoms and carotid artery samples using the ARF of an ultrasound transducer, and measuring the shear wave speed and its associated properties in the different layers with OCT phase maps. This method lays the foundation for future in-vitro and in-vivo studies of mechanical property measurements of biological tissues such as vascular tissues, where normal and pathological structures may exhibit significant contrast in the shear modulus. PMID:24688822
Monitoring the lesion formation during histotripsy treatment using shear wave imaging
NASA Astrophysics Data System (ADS)
Arnal, Bastien; Lee, Wei-Ning; Pernot, Mathieu; Fink, Mathias; Tanter, Mickael
2012-11-01
Monitoring the lesion formation induced by histotripsy has mainly relied on the quantitative change in backscatter intensity using ultrasound B-mode imaging. However, how the mechanical properties of the histotripsy-treated tissue region alter during the procedure is yet to be fully investigated. We thus proposed here to monitor such a therapeutic process based on shear modulus estimated by shear wave imaging (SWI). In the therapeutic procedure, a single-element piezo-composite focused transducer (Imasonic, Besançon, France) with a center frequency of 660 kHz, a focal length of 45 mm, and an fnumber of 1 was driven by a function generator (AFG 3101, Tektronix, Beaverton, OR) and a gated RF power amplifier (GA-2500A, RITEC Inc., USA) to generate ultrasound histotripsy pulses. Histotripsy pulses were delivered for 20 seconds and then followed by a 30-second pause and a rapid monitoring step. Such a treatment and monitoring scheme was repeated for 10 mins. Both the reference measurement and monitoring were realized by SWI, where plane shear waves were generated by an 8 MHz linear array probe connected to a prototype ultrasound scanner, and acquired at a frame rate of 10000 Hz. Shear modulus was estimated and mapped in 2D through a time-of-flight algorithm. Gelatin (8%)-agar (2%) phantoms and ex-vivo porcine liver samples were tested. Regions of interests (ROI's) of 2 mm-by-2 mm in both untreated and treated regions were selected to compute the contrast-to-noise ratio (CNR). In all three scenarios where different PD's and PRF's were implemented, during the first 100 seconds of the treatment, 50% decrease in the shear modulus within the histotripsy-targeted zone was already observed, and the CNR of the shear modulus increased by 18 dB. In contrast, the backscatter intensity began to reduce and the corresponding CNR was found to increase by 6 dB only after 120 seconds of treatment. The results demonstrated that SWI can map quantitatively the change of mechanical
NASA Astrophysics Data System (ADS)
Li, Faxin; Miao, Hongchen
2016-10-01
The non-dispersive fundamental shear horizontal (SH0) wave is extremely useful in guided-wave-based inspection techniques. However, the generation or reception of the SH0 wave by using a piezoelectric transducer is always a challenge. In this work, first, we realized the apparent face-shear (d36) mode in PbZr1-xTixO3 (PZT) ceramics via two-dimensional antiparallel poling. Then, we demonstrated via finite element simulations that the apparent d36 mode PZT wafer can behave as both a SH0 wave actuator and a SH0 wave sensor. Next, by using the apparent d36 PZT wafer as an actuator and a face-shear d36 0.72[Pb(Mg1/3Nb2/3)O3]-0.28[PbTiO3] crystal as the sensor, almost a pure SH0 wave with a high signal-to-noise ratio was successfully excited in an aluminum plate from 180 kHz to 200 kHz. Later, experiments showed that the proposed apparent d36 PZT wafer can also serve as a sensor to detect the SH0 wave over a wide frequency range (160 kHz to 230 kHz). Finally, the amplitude directivity of the SH0 wave generated by the apparent d36 PZT wafer was also investigated. The wave amplitude reaches its maxima at the main direction (0° and 90°) and then decreases monotonically when the propagation direction deviates from the main directions, with the symmetric axis along the 45° direction. The proposed apparent d36 PZT wafer is very suitable for severing as SH0 wave actuators and sensors in structural health monitoring systems.
An omnidirectional shear-horizontal guided wave EMAT for a metallic plate.
Seung, Hong Min; Park, Chung Il; Kim, Yoon Young
2016-07-01
We propose a new electromagnetic acoustic transducer (EMAT) for generation and measurement of omnidirectional shear-horizontal (SH) guided waves in metallic plates. The proposed EMAT requires a magnetic circuit configuration that allows omnidirectional SH wave transduction. It consists of a pair of ring-type permanent magnets that supply static magnetic fluxes and a specially wound coil that induces eddy currents. The Lorentz force acting along the circumferential direction is induced by the vertical static magnetic flux and the radial eddy current in a plate, resulting in omnidirectional SH wave generation. To maximize the transducer output at given excitation frequencies, optimal EMAT configurations are determined by numerical simulations and validated by experiments. The omnidirectivity of the proposed EMAT is also confirmed by the simulations and experiments. PMID:27058629
Bottom shear stress and pressure perturbations under an internal solitary wave
NASA Astrophysics Data System (ADS)
Rivera, Gustavo; Diamessis, Peter
2014-11-01
The bottom boundary layer (BBL) under a mode-1 internal solitary wave (ISW) of depression propagating against an oncoming model barotropic current is examined using 2-D direct numerical simulation based on a spectral multidomain penalty method model. Use of a postprocessing projection onto a modified set of divergence-free basis functions enables investigation of wave-based Reynolds numbers within the range [105 ,106 ] . At sufficiently high ISW amplitude, the BBL undergoes a global instability which produces intermittent vortex shedding from within the separation bubble in the lee of the wave. The interplay between the bottom shear stress field and pressure perturbations during vortex ejection events and the subsequent evolution of the vortices is the focus of this presentation. Implications for resuspension of bottom particulate matter are discussed in the context of specific sediment transport models. Support from the Cornell Sloan Diversity Fellowship program is gratefully acknowledged.
Spectral gap of shear Alfven waves in a periodic array of magnetic mirrors
Zhang Yang; Heidbrink, W. W.; Boehmer, H.; McWilliams, R.; Chen, Guangye; Breizman, B. N.; Vincena, S.; Carter, T.; Leneman, D.; Gekelman, W.; Pribyl, P.; Brugman, B.
2008-01-15
A multiple magnetic mirror array is formed at the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] to study axial periodicity-influenced Alfven spectra. Shear Alfven waves (SAW) are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. Alfven wave spectral gaps and continua are formed similar to wave propagation in other periodic media due to the Bragg effect. The measured width of the propagation gap increases with the modulation amplitude as predicted by the solutions to Mathieu's equation. A two-dimensional finite-difference code modeling SAW in a mirror array configuration shows similar spectral features. Machine end-reflection conditions and damping mechanisms including electron-ion Coulomb collision and electron Landau damping are important for simulation.
NASA Astrophysics Data System (ADS)
Rosa, Angelika D.; Sanchez-Valle, Carmen; Nisr, Carole; Evans, Shaun R.; Debord, Regis; Merkel, Sébastien
2013-09-01
Regions of low seismic velocity and high shear anisotropies in cold subducted slabs have often been related to anisotropic fabrics in hydrous phases mainly induced by slab deformation. The interpretation of these seismic anomalies in terms of hydration thus relies on a better knowledge of the elasticity and plastic deformation mechanisms of candidate hydrous phases. Here we investigate the development of lattice preferred orientations (LPO) in phase D [MgSi2H2O6, 10-18 wt% H2O], the ultimate water carrier in hydrous subducted peridotite. The samples were deformed non-hydrostatically up to 48 GPa in a diamond anvil cell and the texture and strength were obtained from analysis of the X-ray diffraction patterns collected in radial diffraction geometry. We find that at low strains the layered structure of phase D displays strong 0001 texture, where the stacking fault axis (c-axis) preferentially align parallel to the compression axis. A subsidiary 101¯0 texture develops at higher strains. Plasticity simulations in polycrystalline aggregates using a viscoplastic self-consistent model suggest that these LPO patterns are consistent with shape preferred orientation mechanism during the first compaction steps and, with dominant easy glide on basal planes and harder first order pyramidal slip, respectively, upon further compression. We find that phase D displays the lowest strength and the highest anisotropy among phases in hydrous peridotite in the uppermost lower mantle and might thus control the shear wave anisotropy generated in subducted slabs below the transition zone. We further evaluate the effect of textured phase D on the seismic velocity structure and shear wave anisotropy of deformed hydrous peridotite and compare the results to seismic observations in Tonga subduction. We show that 16 vol% of phase D in hydrous subducted peridotite is required to explain the negative velocity anomalies of 3%, the extent of shear wave splitting (0.9±0.3%) and the shear wave ray
Elkateb Hachemi, M; Callé, S; Remenieras, J P
2006-12-22
It is now accepted that an effective way to investigate the elastic properties of soft tissues is to generate a localized transient acoustic radiation force and to follow the associated displacements in the time/space domain. Shear waves induced by this stress field are particularly interesting in this kind of medium because they are governed by the shear elastic modulus mu, which is directly linked to the Young modulus, and spatial distribution and temporal evolution of the transient motion induced must therefore be obtained in detail. We report here a model based on the elastodynamic Green's function formalism to describe these displacements. 3D simulation of radiation force in homogenous elastic media was performed and the displacement curves computed at different radial distances for different temporal force profiles. Amplitude and duration of displacement were found to be reliable parameters to characterize the elastic properties of the medium. Experimental measurements were performed in a homogeneous agar-gelatin tissue-mimicking phantom, and two transducers were used to generate the radiation force and follow the induced displacements. Displacements obtained from different lateral locations around the applied force axis were then used to reconstruct the shear-wave propagation in a scan plane as a function of time. The experimental displacements/curves agreed with the theoretical profiles obtained by the elastodynamic Green's function formalism.
NASA Astrophysics Data System (ADS)
Rotemberg, V.; Palmeri, M.; Nightingale, R.; Rouze, N.; Nightingale, K.
2012-01-01
Increased hepatic venous pressure can be observed in patients with advanced liver disease and congestive heart failure. This elevated portal pressure also leads to variation in acoustic radiation-force-derived shear wave-based liver stiffness estimates. These changes in stiffness metrics with hepatic interstitial pressure may confound stiffness-based predictions of liver fibrosis stage. The underlying mechanism for this observed stiffening behavior with pressurization is not well understood and is not explained with commonly used linear elastic mechanical models. An experiment was designed to determine whether the stiffness increase exhibited with hepatic pressurization results from a strain-dependent hyperelastic behavior. Six excised canine livers were subjected to variations in interstitial pressure through cannulation of the portal vein and closure of the hepatic artery and hepatic vein under constrained conditions (in which the liver was not free to expand) and unconstrained conditions. Radiation-force-derived shear wave speed estimates were obtained and correlated with pressure. Estimates of hepatic shear stiffness increased with changes in interstitial pressure over a physiologically relevant range of pressures (0-35 mmHg) from 1.5 to 3.5 m s-1. These increases were observed only under conditions in which the liver was free to expand while pressurized. This behavior is consistent with hyperelastic nonlinear material models that could be used in the future to explore methods for estimating hepatic interstitial pressure noninvasively.
Heffernan, Kevin S; Lefferts, Wesley K; Kasprowicz, Ari G; Tarzia, Brendan J; Thijssen, Dick H; Brutsaert, Tom D
2013-07-01
Exposure of the arterial wall to retrograde shear acutely leads to endothelial dysfunction and chronically contributes to a proatherogenic vascular phenotype. Arterial stiffness and increased pressure from wave reflections are known arbiters of blood flow in the systemic circulation and each related to atherosclerosis. Using distal external compression of the calf to increase upstream retrograde shear in the superficial femoral artery (SFA), we examined the hypothesis that changes in retrograde shear are correlated with changes in SFA stiffness and pressure from wave reflections. For this purpose, a pneumatic cuff was applied to the calf and inflated to 0, 35, and 70 mmHg (5 min compression, randomized order, separated by 5 min) in 16 healthy young men (23 ± 1 years of age). Doppler ultrasound and wave intensity analysis was used to measure SFA retrograde shear rate, reflected pressure wave intensity (negative area [NA]), elastic modulus (Ep), and a single-point pulse wave velocity (PWV) during acute cuff inflation. Cuff inflation resulted in stepwise increases in retrograde shear rate (P < 0.05 for main effect). There were also significant cuff pressure-dependent increases in NA, Ep, and PWV across conditions (P < 0.05 for main effects). Change in NA, but not Ep or PWV, was associated with change in retrograde shear rate across conditions (P < 0.05). In conclusion, external compression of the calf increases retrograde shear, arterial stiffness, and pressure from wave reflection in the upstream SFA in a dose-dependent manner. Wave reflection intensity, but not arterial stiffness, is correlated with changes in peripheral retrograde shear with this hemodynamic manipulation.
Heffernan, Kevin S; Lefferts, Wesley K; Kasprowicz, Ari G; Tarzia, Brendan J; Thijssen, Dick H; Brutsaert, Tom D
2013-01-01
Exposure of the arterial wall to retrograde shear acutely leads to endothelial dysfunction and chronically contributes to a proatherogenic vascular phenotype. Arterial stiffness and increased pressure from wave reflections are known arbiters of blood flow in the systemic circulation and each related to atherosclerosis. Using distal external compression of the calf to increase upstream retrograde shear in the superficial femoral artery (SFA), we examined the hypothesis that changes in retrograde shear are correlated with changes in SFA stiffness and pressure from wave reflections. For this purpose, a pneumatic cuff was applied to the calf and inflated to 0, 35, and 70 mmHg (5 min compression, randomized order, separated by 5 min) in 16 healthy young men (23 ± 1 years of age). Doppler ultrasound and wave intensity analysis was used to measure SFA retrograde shear rate, reflected pressure wave intensity (negative area [NA]), elastic modulus (Ep), and a single-point pulse wave velocity (PWV) during acute cuff inflation. Cuff inflation resulted in stepwise increases in retrograde shear rate (P < 0.05 for main effect). There were also significant cuff pressure-dependent increases in NA, Ep, and PWV across conditions (P < 0.05 for main effects). Change in NA, but not Ep or PWV, was associated with change in retrograde shear rate across conditions (P < 0.05). In conclusion, external compression of the calf increases retrograde shear, arterial stiffness, and pressure from wave reflection in the upstream SFA in a dose-dependent manner. Wave reflection intensity, but not arterial stiffness, is correlated with changes in peripheral retrograde shear with this hemodynamic manipulation. PMID:24303111
Characterizing Wave- and Current-Induced Bottom Shear Stress: U.S. Middle Atlantic Bight
NASA Astrophysics Data System (ADS)
Dalyander, S.; Butman, B.
2011-12-01
The combined action of waves and currents at the seabed creates bottom shear stress, impacting local geology, habitat, and anthropogenic use. In this study, a methodology is developed to characterize the magnitude of benthic disturbance based on spatially and seasonally-resolved statistics (mean, standard deviation, 95th percentile) of wave-current bottom shear stress. The frequency of stress forcing is used to distinguish regions dominated by storms (return interval longer than 33 hours) from those dominated by the tides (periods shorter than 33 hours). In addition, the relative magnitude of the contribution to stress from waves, tides, and storm-driven currents is investigated by comparing wave stress, tidal current stress, and stress from the residual current (currents with tides removed), as well as through cross-correlation of wave and current stress. The methodology is applied to numerical model time-series data for the Middle Atlantic Bight (MAB) off the U.S. East Coast for April 2010 to April 2011; currents are provided from the Integrated Ocean Observing System (IOOS) operational hydrodynamic forecast Experimental System for Predicting Shelf and Slope Optics (ESPreSSO) and waves are provided from a Simulating WAves Nearshore (SWAN) hindcast developed for this project. Spatial resolution of the model is about 5 km and time-series wave and current data are at 1 and 2-hours respectively. Regions of the MAB delineated by stress characteristics include a tidally-dominated shallow region with relative high stress southeast of Massachusetts over Nantucket Shoals; a coastal band extending offshore to about 30 m water depth dominated by waves; a region dominated by waves and wind-driven currents offshore of the Outer Banks of North Carolina; and a low stress region southeast of Long Island, approximately coincident with an area of fine-grained sediments called the "Mud Patch". Comparison of the stress distribution with surface sediment texture data shows that
NASA Astrophysics Data System (ADS)
Mixa, T.; Fritts, D. C.; Laughman, B.; Wang, L.; Kantha, L. H.
2015-12-01
Multiple observations provide compelling evidence that gravity wave dissipation events often occur in multi-scale environments having highly-structured wind and stability profiles extending from the stable boundary layer into the mesosphere and lower thermosphere. Such events tend to be highly localized and thus yield local energy and momentum deposition and efficient secondary gravity wave generation expected to have strong influences at higher altitudes [e.g., Fritts et al., 2013; Baumgarten and Fritts, 2014]. Lidars, radars, and airglow imagers typically cannot achieve the spatial resolution needed to fully quantify these small-scale instability dynamics. Hence, we employ high-resolution modeling to explore these dynamics in representative environments. Specifically, we describe numerical studies of gravity wave packets impinging on a sheet of high stratification and shear and the resulting instabilities and impacts on the gravity wave amplitude and momentum flux for various flow and gravity wave parameters. References: Baumgarten, Gerd, and David C. Fritts (2014). Quantifying Kelvin-Helmholtz instability dynamics observed in noctilucent clouds: 1. Methods and observations. Journal of Geophysical Research: Atmospheres, 119.15, 9324-9337. Fritts, D. C., Wang, L., & Werne, J. A. (2013). Gravity wave-fine structure interactions. Part I: Influences of fine structure form and orientation on flow evolution and instability. Journal of the Atmospheric Sciences, 70(12), 3710-3734.
Near-Surface Shear-Wave Velocity Measurements in Unlithified Sediment
NASA Astrophysics Data System (ADS)
Rickards, Benjamin Thomas
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.
Material Characterization of In Vivo and In Vitro Porcine Brain using Shear Wave Elasticity
Urbanczyk, Caryn A.; Palmeri, Mark L.; Bass, Cameron R.
2015-01-01
Realistic computer simulation of closed head trauma requires accurate mechanical properties of brain tissue, ideally in vivo. A substantive deficiency of most existing experimental brain data is that properties were identified through in vitro mechanical testing. This study develops a novel application of shear wave elasticity imaging (SWEI) to assess porcine brain tissue shear modulus in vivo. SWEI is a quantitative ultrasound technique that has been used here to examine changes in brain tissue shear modulus as a function of several experimental and physiological parameters. Animal studies were performed using two different ultrasound transducers to explore the differences between physical response with closed skull and open skull arrangements. In vivo intracranial pressure (ICP) in four animal subjects was varied over a relevant physiological range (2-40 mmHg), and was correlated with shear wave speed and stiffness estimates in brain tissue. We found that stiffness does not vary with modulation of ICP. Additional in vitro porcine specimens (n=14) were used to investigate variation in brain tissue stiffness with temperature, confinement, spatial location, and transducer orientation. We found a statistically significant decrease in stiffness with increased temperature (23%) and an increase in stiffness with decreasing external confinement (22 - 37%). This study demonstrated the feasibility of using SWEI to characterize porcine brain tissue both in vitro and in vivo. Our results underline the importance of temperature and skull derived boundary conditions on brain stiffness and suggests that physiological ranges of ICP do not significantly affect in situ brain tissue properties. SWEI allowed for brain material properties to be experimentally-characterized in a physiological setting and provides a stronger basis for assessing brain injury in computational models. PMID:25683220
A shear wave ground surface vibration technique for the detection of buried pipes
NASA Astrophysics Data System (ADS)
Muggleton, J. M.; Papandreou, B.
2014-07-01
A major UK initiative, entitled 'Mapping the Underworld' aims to develop and prove the efficacy of a multi-sensor device for accurate remote buried utility service detection, location and, where possible, identification. One of the technologies to be incorporated in the device is low-frequency vibro-acoustics; the application of this technology for detecting buried infrastructure, in particular pipes, is currently being investigated. Here, a shear wave ground vibration technique for detecting buried pipes is described. For this technique, shear waves are generated at the ground surface, and the resulting ground surface vibrations measured. Time-extended signals are employed to generate the illuminating wave. Generalized cross-correlation functions between the measured ground velocities and a reference measurement adjacent to the excitation are calculated and summed using a stacking method to generate a cross-sectional image of the ground. To mitigate the effects of other potential sources of vibration in the vicinity, the excitation signal can be used as an additional reference when calculating the cross-correlation functions. Measurements have been made at two live test sites to detect a range of buried pipes. Successful detection of the pipes was achieved, with the use of the additional reference signal proving beneficial in the noisier of the two environments.
A global horizontal shear velocity model of the upper mantle from multimode Love wave measurements
NASA Astrophysics Data System (ADS)
Ho, Tak; Priestley, Keith; Debayle, Eric
2016-10-01
Surface wave studies in the 1960s provided the first indication that the upper mantle was radially anisotropic. Resolving the anisotropic structure is important because it may yield information on deformation and flow patterns in the upper mantle. The existing radially anisotropic models are in poor agreement. Rayleigh waves have been studied extensively and recent models show general agreement. Less work has focused on Love waves and the models that do exist are less well-constrained than are Rayleigh wave models, suggesting it is the Love wave models that are responsible for the poor agreement in the radially anisotropic structure of the upper mantle. We have adapted the waveform inversion procedure of Debayle & Ricard to extract propagation information for the fundamental mode and up to the fifth overtone from Love waveforms in the 50-250 s period range. We have tomographically inverted these results for a mantle horizontal shear wave-speed model (βh(z)) to transition zone depths. We include azimuthal anisotropy (2θ and 4θ terms) in the tomography, but in this paper we discuss only the isotropic βh(z) structure. The data set is significantly larger, almost 500 000 Love waveforms, than previously published Love wave data sets and provides ˜17 000 000 constraints on the upper-mantle βh(z) structure. Sensitivity and resolution tests show that the horizontal resolution of the model is on the order of 800-1000 km to transition zone depths. The high wave-speed roots beneath the oldest parts of the continents appear to extend deeper for βh(z) than for βv(z) as in previous βh(z) models, but the resolution tests indicate that at least parts of these features could be artefacts. The low wave speeds beneath the mid-ocean ridges fade by ˜150 km depth except for the upper mantle beneath the East Pacific Rise which remains slow to ˜250 km depth. The resolution tests suggest that the low wave speeds at deeper depths beneath the East Pacific Rise are not solely due
Liu, Yu; Fite, Brett Z.; Mahakian, Lisa M.; Johnson, Sarah M.; Larrat, Benoit; Dumont, Erik; Ferrara, Katherine W.
2015-01-01
Manual palpation is a common and very informative diagnostic tool based on estimation of changes in the stiffness of tissues that result from pathology. In the case of a small lesion or a lesion that is located deep within the body, it is difficult for changes in mechanical properties of tissue to be detected or evaluated via palpation. Furthermore, palpation is non-quantitative and cannot be used to localize the lesion. Magnetic Resonance-guided Focused Ultrasound (MRgFUS) can also be used to evaluate the properties of biological tissues non-invasively. In this study, an MRgFUS system combines high field (7T) MR and 3 MHz focused ultrasound to provide high resolution MR imaging and a small ultrasonic interrogation region (~0.5 x 0.5 x 2 mm), as compared with current clinical systems. MR-Acoustic Radiation Force Imaging (MR-ARFI) provides a reliable and efficient method for beam localization by detecting micron-scale displacements induced by ultrasound mechanical forces. The first aim of this study is to develop a sequence that can concurrently quantify acoustic radiation force displacements and image the resulting transient shear wave. Our motivation in combining these two measurements is to develop a technique that can rapidly provide both ARFI and shear wave velocity estimation data, making it suitable for use in interventional radiology. Secondly, we validate this sequence in vivo by estimating the displacement before and after high intensity focused ultrasound (HIFU) ablation, and we validate the shear wave velocity in vitro using tissue-mimicking gelatin and tofu phantoms. Such rapid acquisitions are especially useful in interventional radiology applications where minimizing scan time is highly desirable. PMID:26439259
Lithospheric deformation in the Canadian Appalachians: evidence from shear wave splitting
NASA Astrophysics Data System (ADS)
Gilligan, Amy; Bastow, Ian D.; Watson, Emma; Darbyshire, Fiona A.; Levin, Vadim; Menke, William; Lane, Victoria; Hawthorn, David; Boyce, Alistair; Liddell, Mitchell V.; Petrescu, Laura
2016-08-01
Plate-scale deformation is expected to impart seismic anisotropic fabrics on the lithosphere. Determination of the fast shear wave orientation (ϕ) and the delay time between the fast and slow split shear waves (δt) via SKS splitting can help place spatial and temporal constraints on lithospheric deformation. The Canadian Appalachians experienced multiple episodes of deformation during the Phanerozoic: accretionary collisions during the Palaeozoic prior to the collision between Laurentia and Gondwana, and rifting related to the Mesozoic opening of the North Atlantic. However, the extent to which extensional events have overprinted older orogenic trends is uncertain. We address this issue through measurements of seismic anisotropy beneath the Canadian Appalachians, computing shear wave splitting parameters (ϕ, δt) for new and existing seismic stations in Nova Scotia and New Brunswick. Average δt values of 1.2 s, relatively short length scale (≥100 km) splitting parameter variations, and a lack of correlation with absolute plate motion direction and mantle flow models, demonstrate that fossil lithospheric anisotropic fabrics dominate our results. Most fast directions parallel Appalachian orogenic trends observed at the surface, while δt values point towards coherent deformation of the crust and mantle lithosphere. Mesozoic rifting had minimal impact on our study area, except locally within the Bay of Fundy and in southern Nova Scotia, where fast directions are subparallel to the opening direction of Mesozoic rifting; associated δt values of >1 s require an anisotropic layer that spans both the crust and mantle, meaning the formation of the Bay of Fundy was not merely a thin-skinned tectonic event.
Lithospheric deformation in the Canadian Appalachians: evidence from shear wave splitting
NASA Astrophysics Data System (ADS)
Gilligan, Amy; Bastow, Ian D.; Watson, Emma; Darbyshire, Fiona A.; Levin, Vadim; Menke, William; Lane, Victoria; Hawthorn, David; Boyce, Alistair; Liddell, Mitchell V.; Petrescu, Laura
2016-08-01
Plate-scale deformation is expected to impart seismic anisotropic fabrics on the lithosphere. Determination of the fast shear wave orientation (φ) and the delay time between the fast and slow split shear waves (δt) via SKS splitting can help place spatial and temporal constraints on lithospheric deformation. The Canadian Appalachians experienced multiple episodes of deformation during the Phanerozoic: accretionary collisions during the Palaeozoic prior to the collision between Laurentia and Gondwana, and rifting related to the Mesozoic opening of the North Atlantic. However, the extent to which extensional events have overprinted older orogenic trends is uncertain. We address this issue through measurements of seismic anisotropy beneath the Canadian Appalachians, computing shear wave splitting parameters (φ, δt) for new and existing seismic stations in Nova Scotia and New Brunswick. Average δt values of 1.2 s, relatively short length scale (≥100 km) splitting parameter variations, and a lack of correlation with absolute plate motion direction and mantle flow models, demonstrate that fossil lithospheric anisotropic fabrics dominate our results. Most fast directions parallel Appalachian orogenic trends observed at the surface, while δt values point towards coherent deformation of the crust and mantle lithosphere. Mesozoic rifting had minimal impact on our study area, except locally within the Bay of Fundy and in southern Nova Scotia, where fast directions are subparallel to the opening direction of Mesozoic rifting; associated δt values of >1 s require an anisotropic layer that spans both the crust and mantle, meaning the formation of the Bay of Fundy was not merely a thin-skinned tectonic event.
Variations in Shear Wave Splitting Beneath Southern Arabia and the Gulf of Aden
NASA Astrophysics Data System (ADS)
Gallacher, R. J.; Eakin, C. M.; Keir, D.; Leroy, S. D.; Stuart, G. W.; Harmon, N.; Ahmed, A.
2015-12-01
Mantle flow beneath Southern Arabia and the Gulf of Aden remains enigmatic due to a paucity of seismic measurements in the region. Potential processes contributing to mantle flow include northward progression of the African Superplume, radial flow from the Afar plume and vertical flow from small-scale convection along the margins of the Gulf of Aden. These would result in characteristic mantle flow directions, creating mantle anisotropy that can be detected by shear wave splitting. We analyse SKS, SKKS & PKS phases for shear wave splitting at 141 stations deployed throughout Yemen, Oman and Socotra along the margins of the Gulf of Aden. Large numbers of null measurements from a range of back azimuths are found beneath the entire region. These may indicate that vertical anisotropy is present in the upper mantle beneath the region, consistent with models of small-scale convection. The null measurements may also be due to complicated layering of crustal anisotropy interfering destructively and precluding measurement of shear wave splitting. Splitting measurements bordering the Red Sea show North-South orientations that may result from shallow aligned melt along the Red Sea or from variations in lower mantle flow. Fast polarization directions of splitting measurements along the Northern margin of the Gulf of Aden are rift parallel suggesting a shallow source such as rift related faulting might be responsible. These results show that anisotropy beneath the region is not controlled by the northward progression of the African Superplume or radial flow from the Afar plume. Upper mantle flow is likely vertical with splitting occurring either in the crust or the lower mantle.
Anisotropic Shear-wave Velocity Structure of East Asian Upper Mantle from Waveform Tomography
NASA Astrophysics Data System (ADS)
Chong, J.; Yuan, H.; French, S. W.; Romanowicz, B. A.; Ni, S.
2012-12-01
East Asia is a seismically active region featuring active tectonic belts, such as the Himalaya collision zone, western Pacific subduction zones and the Tianshan- Baikal tectonic belt. In this study, we applied full waveform time domain tomography to image 3D isotropic, radially and azimuthally anisotropic upper mantle shear velocity structure of East Asia. High quality teleseismic waveforms were collected for both permanent and temporary stations in the target and its adjacent regions, providing good ray path coverage of the study region. Fundamental and overtone wave packets, filtered down to 60 sec, were inverted for isotropic and radially anisotropic shear wave structure using normal mode asymptotic coupling theory (NACT: Li and Romanowicz, 1995). Joint inversion of SKS measurements and seismic waveforms was then carried out following the methodology described in (Marone and Romanowicz, 2007). The 3D velocity model shows strong lateral heterogeneities in the target region, which correlate well with the surface geology in East Asia. Our model shows that Indian lithosphere has subducted beneath Tibet with a different northern reach from western to eastern Tibet,. We also find variations of the slab geometry in Western Pacific subduction zones. Old and stable regions, such as, Indian shield, Siberia platform, Tarim and Yangtze blocks are found to have higher shear wave velocity in the upper mantle. Lower velocity anomalies are found in regions like Baikal rift, Tienshan, Indochina block, and the regions along Japan island-Ryukyu Trench and Izu-bonin Trench. The dominant fast and slow velocity boundaries in the study region are well correlated with tectonic belts, such as the central Asian orogenic belt and Alty/Qilian-Qinling/Dabie orogenic belt. Our radially anisotropic model shows Vsh> Vsv in oceanic regions and at larger depths(>300km), and Vsv > Vsh in some orogenic zones.. We'll show preliminary results of azimuthally anisotropic joint inversion of SKS
Jiang, Yi; Li, Guoyang; Qian, Lin-Xue; Liang, Si; Destrade, Michel; Cao, Yanping
2015-10-01
We use supersonic shear wave imaging (SSI) technique to measure not only the linear but also the nonlinear elastic properties of brain matter. Here, we tested six porcine brains ex vivo and measured the velocities of the plane shear waves induced by acoustic radiation force at different states of pre-deformation when the ultrasonic probe is pushed into the soft tissue. We relied on an inverse method based on the theory governing the propagation of small-amplitude acoustic waves in deformed solids to interpret the experimental data. We found that, depending on the subjects, the resulting initial shear modulus [Formula: see text] varies from 1.8 to 3.2 kPa, the stiffening parameter [Formula: see text] of the hyperelastic Demiray-Fung model from 0.13 to 0.73, and the third- [Formula: see text] and fourth-order [Formula: see text] constants of weakly nonlinear elasticity from [Formula: see text]1.3 to [Formula: see text]20.6 kPa and from 3.1 to 8.7 kPa, respectively. Paired [Formula: see text] test performed on the experimental results of the left and right lobes of the brain shows no significant difference. These values are in line with those reported in the literature on brain tissue, indicating that the SSI method, combined to the inverse analysis, is an efficient and powerful tool for the mechanical characterization of brain tissue, which is of great importance for computer simulation of traumatic brain injury and virtual neurosurgery.
Lithospheric deformation in the Canadian Appalachians: evidence from shear wave splitting
NASA Astrophysics Data System (ADS)
Gilligan, Amy; Bastow, Ian D.; Watson, Emma; Darbyshire, Fiona A.; Levin, Vadim; Menke, William; Lane, Victoria; Hawthorn, David; Boyce, Alistair; Liddell, Mitchell V.; Petrescu, Laura
2016-06-01