scholarly journals Time-lapse changes of P- and S-wave velocities and shear wave splitting in the first year after the 2011 Tohoku earthquake, Japan: shallow subsurface

2013 ◽  
Vol 193 (1) ◽  
pp. 238-251 ◽  
Author(s):  
Kaoru Sawazaki ◽  
Roel Snieder
Keyword(s):  
Shear Wave ◽  
Tohoku Earthquake ◽  
Time Lapse ◽  
Shear Wave Splitting ◽  
First Year ◽  
S Wave ◽  
The 2011 Tohoku Earthquake ◽  
Shallow Subsurface ◽  
Wave Velocities ◽  
Wave Splitting

A physical model study of shear‐wave splitting and fracture intensity

Geophysics ◽  
1992 ◽  
Vol 57 (4) ◽  
pp. 647-652 ◽  
Author(s):  
Robert H. Tatham ◽  
Martin D. Matthews ◽  
K. K. Sekharan ◽  
Christopher J. Wade ◽  
Louis M. Liro
Keyword(s):  
Physical Model ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Constant Thickness ◽  
Petroleum Reservoir ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Wave Splitting ◽  
Fracture Intensity ◽  
Degree Of Anisotropy

In a series of physical model experiments, fractured media are simulated by stacks of thin Plexiglas sheets clamped together tightly to form blocks. The plates are assembled underwater, and a very thin water layer between the sheets prevents formation of an effectively welded interface between them. Thus, the stacked material is not a series of welded plates but rather a truly fractured medium simulating a potential petroleum reservoir with only fracture porosity and permeability. Sheets of constant thickness are used, but the intensity of fracturing between the different models is simulated by using different thicknesses of Plexiglas for each model. Observation of direct shear‐wave arrivals through the stack, with propagation parallel to the sheets and polarization of particle motion allowed to be parallel to, normal to, or in any arbitrary angle to the sheets, definitively demonstrate the existence of shear‐wave splitting and hence anisotropy. For Plexiglas sheets 1/16 in thick (0.16 cm) representing a fracture intensity of about 16 fractures per wavelength, shear‐wave splitting and hence anisotropy are clearly observed. For greater fracture intensities (i.e., thinner plates) the degree of anisotropy is greater, and for less intense fracturing (i.e., thicker plates), the degree of anisotropy is less. These experimental data suggest that for fracture intensities of greater than about 10 fractures per wavelength there is a simple relation between fracture intensity and degree of anisotropy in two different polarizations of shear waves. For fracture intensities less than a threshold of about 10 per wavelength, there is no simple observed relation between them. Further, the faster of the split shear‐wave velocities is near the solid material velocity, and the observed variation in velocity for various fracture intensities is in the slower of the split shear‐wave velocities.

Integration of multicomponent time-lapse processing and interpretation: Focus on shear-wave splitting analysis

2013 ◽  
Vol 32 (1) ◽  
pp. 32-38 ◽  
Author(s):  
Jeff P. Grossman ◽  
Gulia Popov ◽  
Chris Steinhoff
Keyword(s):  
Shear Wave ◽  
Time Lapse ◽  
Shear Wave Splitting ◽  
Wave Splitting

scholarly journals Diurnal expansion and contraction of englacial fracture networks revealed by seismic shear wave splitting

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Wojciech Gajek ◽  
Dominik Gräff ◽  
Sebastian Hellmann ◽  
Alan W. Rempel ◽  
Fabian Walter
Keyword(s):  
Shear Wave ◽  
Large Scale ◽  
Elastic Anisotropy ◽  
Shear Wave Splitting ◽  
Subsurface Water ◽  
Water Drainage ◽  
Fracture Networks ◽  
S Wave ◽  
Alpine Glacier ◽  
Wave Splitting

AbstractFractures contribute to bulk elastic anisotropy of many materials in the Earth. This includes glaciers and ice sheets, whose fracture state controls the routing of water to the base and thus large-scale ice flow. Here we use anisotropy-induced shear wave splitting to characterize ice structure and probe subsurface water drainage beneath a seismometer network on an Alpine glacier. Shear wave splitting observations reveal diurnal variations in S-wave anisotropy up to 3%. Our modelling shows that when elevated by surface melt, subglacial water pressures induce englacial hydrofractures whose volume amounts to 1-2 percent of the probed ice mass. While subglacial water pressures decrease, these fractures close and no fracture-induced anisotropy variations are observed in the absence of meltwater. Consequently, fracture networks, which are known to dominate englacial water drainage, are highly dynamic and change their volumes by 90-180 % over subdaily time scales.

Application of seismic interferometry to extract P- and S-wave propagation and observation of shear-wave splitting from noise data at Cold Lake, Alberta, Canada

Geophysics ◽  
2008 ◽  
Vol 73 (4) ◽  
pp. D35-D40 ◽  
Author(s):  
Masatoshi Miyazawa ◽  
Roel Snieder ◽  
Anupama Venkataraman
Keyword(s):  
Wave Propagation ◽  
Shear Wave ◽  
Structural Parameters ◽  
Shear Wave Splitting ◽  
Seismic Interferometry ◽  
Stress Axis ◽  
S Wave ◽  
S Waves ◽  
Wave Splitting ◽  
Cold Lake

We extract downward-propagating P- and S-waves from industrial noise generated by human and/or machine activity at the surface propagating down a borehole at Cold Lake, Alberta, Canada, and measure shear-wave splitting from these data. The continuous seismic data are recorded at eight sensors along a downhole well during steam injection into a 420–470-m-deep oil reservoir. We crosscorrelate the waveforms observed at the top sensor and other sensors to extract estimates of the direct P- and S-wave components of the Green’s function that account for wave propagation between sensors. Fast high-frequency and slow low-frequency signals propagating vertically from the surface to the bottom are found for the vertical and horizontal components of the wave motion, which are identified with P- and S-waves, respectively. The fastest S-wave polarized in the east-northeast–west-southwest direction is about 1.9% faster than the slowest S-wave polarized in the northwest-southeast direction. The direction of polarization of the fast S-wave is rotated clockwise by [Formula: see text] from the maximum principal stress axis as estimated from the regional stress field. This study demonstrates the useful application of seismic interferometry to field data to determine structural parameters, which are P- and S-wave velocities and a shear-wave-splitting coefficient, with high accuracy.

Flin Flon Belt seismic anisotropy: elastic symmetry, heterogeneity, and shear-wave splitting

2005 ◽  
Vol 42 (4) ◽  
pp. 533-554 ◽  
Author(s):  
Pavlo Y Cholach ◽  
Joseph B Molyneux ◽  
Douglas R Schmitt
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Confining Pressure ◽  
Shear Zones ◽  
Shear Wave Splitting ◽  
Metamorphic Rocks ◽  
Wave Velocities ◽  
Wave Splitting ◽  
Flin Flon ◽  
Seismic Reflectors

Laboratory measurements of compressional- and shear-wave velocities, and shear-wave splitting have been carried out on a set of upper greenschist – lower amphibolite facies of metasediments and metavolcanics and plutonic rocks from two ductile shear zones in the Flin Flon Belt (FFB) of the Trans-Hudson Orogen (THO). Selected metamorphic rocks vary in composition from felsic to mafic. Test sites with outcrops of sheared metamorphic rocks were correlated with a series of inclined seismic reflectors possibly extending from the midcrust and intersecting a well-mapped shear zone at the surface. Determination of the lithological and physical properties of highly deformed metamorphic rocks is essential for proper interpretation of the nature of observed seismic reflectors. To investigate the anisotropic properties of the rocks, compressional velocity was measured at confining pressure up to 300 MPa in three mutually orthogonal directions aligned with respect to visible textural features. In addition, on selected samples, shear-wave velocity was measured at two orthogonal polarizations for each of three propagation directions to determine shear-wave splitting. The seismic heterogeneity of hand specimens was also investigated by measuring P- and S-wave velocities on several cores cut in the same direction. Observed compressional-wave anisotropy varied from quasi-isotropic to 24%. Maximum observed shear-wave splitting reaches a value of 0.77 km/s at confining pressure of 300 MPa. The pressure invariance of observed P-wave anisotropy and shear-wave splitting indicates that intrinsic anisotropy due to the lattice-preferred orientation (LPO) of highly anisotropic minerals, such as mica and hornblende, is mainly responsible for the measured seismic anisotropy.

scholarly journals Deriving micro- to macro-scale seismic velocities from ice-core <i>c</i> axis orientations

2018 ◽  
Vol 12 (5) ◽  
pp. 1715-1734 ◽  
Author(s):  
Johanna Kerch ◽  
Anja Diez ◽  
Ilka Weikusat ◽  
Olaf Eisen
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Ice Core ◽  
Shear Wave Splitting ◽  
P Wave ◽  
Seismic Velocities ◽  
Polar Ice ◽  
S Wave ◽  
Crystal Anisotropy ◽  
Wave Splitting

Abstract. One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an ice-core sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135 m s−1 for P-wave and 200 m s−1 for S-wave velocity compared to the earlier framework (estimated 1 % difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shear-wave splitting at the bedrock as 45 m s−1 for the alpine ice core and 59 m s−1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185 m s−1 for P-wave and 280 m s−1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281 m s−1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200 m s−1 for P wave and more than 200 m s−1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystal-orientation fabric (±50 m s−1 per 10 cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding.

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