Comment on “Shear wave anisotropy of active tectonic regions via automated S-wave polarization analysis” by M.K. Savage, X.R. Shih, R.P. Meyer and R.C. Aster, and “Azimuthal variations in P-wave travel times and shear-wave splitting in the Charlevoix seismic zone” by G.G.R. Buchbinder

1990 ◽  
Vol 172 (1-2) ◽  
pp. 195-196 ◽  
Author(s):  
Amos Nur
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
P Wave ◽  
Seismic Zone ◽  
Wave Polarization ◽  
Polarization Analysis ◽  
S Wave ◽  
Active Tectonic ◽  
Wave Splitting ◽  
Wave Travel

Shear-wave anisotropy of active tectonic regions via automated S-wave polarization analysis

1989 ◽  
Vol 165 (1-4) ◽  
pp. 279-292 ◽  
Author(s):  
M.K. Savage ◽  
X.R. Shih ◽  
R.P. Meyer ◽  
R.C. Aster
Keyword(s):  
Shear Wave ◽  
Wave Polarization ◽  
Polarization Analysis ◽  
S Wave ◽  
Active Tectonic

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.

Synthesis of multicomponent quasi-P and converted quasi-P-S seismograms for intersecting fracture systems

Geophysics ◽  
2000 ◽  
Vol 65 (4) ◽  
pp. 1261-1271 ◽  
Author(s):  
Andrey A. Ortega ◽  
George A. McMechan
Keyword(s):  
Shear Wave ◽  
Transverse Isotropy ◽  
Normal Incidence ◽  
Shear Wave Splitting ◽  
P Wave ◽  
Orthorhombic Symmetry ◽  
S Wave ◽  
Stiffness Constant ◽  
Wave Splitting ◽  
Vertical Fractures

Dynamic ray shooting with interpolation is an economical way of computing approximate Green’s functions in 3-D heterogeneous anisotropic media. The amplitudes, traveltimes, and polarizations of the reflected rays arriving at the surface are interpolated to synthesize three‐component seismograms at the desired recording points. The algorithm is applied to investigate kinematic quasi-P-wave propagation and converted quasi-P-S-wave splitting variations produced in reflections from the bottom of a layer containing two sets of intersecting dry vertical fractures as a function of the angle between the fracture sets and of the intensity of fracturing. An analytical expression is derived for the stiffness constant C16 that extends Hudson’s second‐order scattering theory to include tetragonal-2 symmetry systems. At any offset, the amount of splitting in nonorthogonal (orthorhombic symmetry) intersecting fracture sets is larger than in orthogonal (tetragonal-1 symmetry) systems, and it increases nonlinearly as a function of the intensity of fracturing as offset increases. Such effects should be visible in field data, provided that the dominant frequency is sufficiently high and the offset is sufficiently large. The amount of shear‐wave splitting at vertical incidence increases nonlinearly as a function of the intensity of fracturing and increases nonlinearly from zero in the transition from tetragonal-1 anisotropy through orthorhombic to horizontal transverse isotropy; the latter corresponds to the two crack systems degenerating to one. The zero shear‐wave splitting corresponds to a singularity, at which the vertical velocities of the two quasi‐shear waves converge to a single value that is both predicted theoretically and illustrated numerically. For the particular case of vertical fractures, there is no P-to-S conversion of vertically propagating (zero‐offset) waves. If the fractures are not vertical, the normal incidence P-to-S reflection coefficient is not zero and thus is a potential diagnostic of fracture orientation.

Shear-wave splitting and anisotropy in the Charlevoix seismic zone, Quebec, in 1985

1989 ◽  
Vol 26 (12) ◽  
pp. 2691-2696 ◽  
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Active Region ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Crack Density ◽  
Shear Wave Splitting ◽  
Velocity Variation ◽  
Seismic Zone ◽  
Wave Polarization ◽  
Wave Splitting

During the month of October 1985 a second experiment was undertaken in the Charlevoix seismic zone to further test the hypothesis that shear-wave splitting could be observed in a seismically active region. The first experiment had been undertaken in 1984 and yielded only a very limited amount of data. Seismograms recorded by digital three-component seismographs located very close to the epicentres of seven earthquakes showed shear-wave splitting over 15 different paths. The amount of [Formula: see text] wave variation varied from about 24 to 160 ms or from 0.4 to 8.7% of the [Formula: see text] wave velocity. The largest value occurred over the shortest path of about 7 km, for which essentially the whole path may be anisotropic, leading to a crack density (ε) of less than 0.09. For the rest of the data, all with less than 3% shear-wave-velocity variation, ε varies from 0.005 to 0.03, if whole-path anisotropy is assumed. These values of ε are not significantly different from those obtained in 1984. The average azimuth of the initial shear-wave polarization is 37°, also similar to that observed in 1984. All the data in the zone can be explained by the presence of saturated vertical cracks striking 37 °east of north.

Estimation of fracture parameters from reflection seismic data—Part III: Fractured models with monoclinic symmetry

Geophysics ◽  
2000 ◽  
Vol 65 (6) ◽  
pp. 1818-1830 ◽  
Author(s):  
Andrey Bakulin ◽  
Vladimir Grechka ◽  
Ilya Tsvankin
Keyword(s):  
Shear Wave ◽  
Fractured Reservoirs ◽  
Shear Wave Splitting ◽  
Naturally Fractured Reservoirs ◽  
P Wave ◽  
S Wave ◽  
Monoclinic Symmetry ◽  
Weak Anisotropy ◽  
Wave Splitting ◽  
Single Set

Geophysical and geological data acquired over naturally fractured reservoirs often reveal the presence of multiple vertical fracture sets. Here, we discuss modeling and inversion of the effective anisotropic parameters of two types of fractured media with monoclinic symmetry. The first model is formed by two different nonorthogonal sets of rotationally invariant vertical fractures in an isotropic host rock; the other contains a single set of fractures with microcorrugated faces. In monoclinic media with two fracture sets, the shear‐wave polarizations at vertical incidence and the orientation of the NMO ellipses of pure modes in a horizontal layer are controlled by the fracture azimuths as well as by their compliances. While the S-wave polarization directions depend only on the tangential compliances, the axes of the P-wave NMO ellipse are also influenced by the normal compliances and therefore have a different orientation. This yields an apparent discrepancy between the principal anisotropy directions obtained using P and S data that does not exist in orthorhombic media. By first using the weak‐anisotropy approximation for the effective anisotropic parameters and then inverting the exact equations, we devise a complete fracture characterization procedure based on the vertical velocities of the P- and two split S-waves (or converted PS-waves) and their NMO ellipses from a horizontal reflector. Our algorithm yields the azimuths and compliances of both fracture systems as well as the P- and S-wave velocities in the isotropic background medium. In the model with a single set of microcorrugated fractures, monoclinic symmetry stems from the coupling between the normal and tangential (to the fracture faces) slips, or jumps in displacement. We demonstrate that for this model the shear‐wave splitting coefficient at vertical incidence varies with the fluid content of the fractures. Although conventional fracture models that ignore microcorrugation predict no such dependence, our conclusions are supported by experimental observations showing that shear‐wave splitting for dry cracks may be substantially greater than that for fluid‐filled ones.

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.

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