Seismic Wave Propagation and Imaging in Anisotropic Media: A Review (original) (raw)
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Seismic anisotropy in exploration and reservoir characterization: An overview
GEOPHYSICS, 2010
Recent advances in parameter estimation and seismic processing have allowed incorporation of anisotropic models into a wide range of seismic methods. In particular, vertical and tilted transverse isotropy are currently treated as an integral part of velocity fields employed in prestack depth migration algorithms, especially those based on the wave equation. We briefly review the state of the art in modeling, processing, and inversion of seismic data for anisotropic media. Topics include optimal parameterization, body-wave modeling methods, P-wave velocity analysis and imaging, processing in the [Formula: see text] domain, anisotropy estimation from vertical-seismic-profiling (VSP) surveys, moveout inversion of wide-azimuth data, amplitude-variation-with-offset (AVO) analysis, processing and applications of shear and mode-converted waves, and fracture characterization. When outlining future trends in anisotropy studies, we emphasize that continued progress in data-acquisition technol...
Seismic Wave Features in Anisotropic Modeling and Effects in Imaging Complex Subsurface Structure
IPTC, 2020
Seismic wave propagation in an anisotropic media with complex subsurface geology produces reflected as well as a diffracted wave. Which contains useful information regarding the subsurface geometry and velocity information. We develop vertical transverse isotropy (VTI) wave modeling algorithm based on the low-rank technique which is fast and stable to study the wave propagation behavior. This paper shows the significance of incorporating anisotropy into the seismic exploration data which is proven on different data set. We develop a VTI wave modeling algorithm based on the low-rank technique which is fast and stable to study the wave propagation behavior. The results demonstrate the wave propagation including VTI provides better signal imaging in case of lateral positioning of seismic events which are necessary for prospect evaluation.
Bulletin of the Seismological Society of America, 2008
We investigate the effect of seismic anisotropy on P-wave receiver functions, calculating synthetic seismograms for P-wave incidence on multilayered anisotropic structure with hexagonal symmetry. The main characteristics of the receiver functions affected by the anisotropy are summarized as (1) appearance of seismic energy on radial and transverse receiver functions, (2) systematic change of P-to-S (Ps) converted waveforms on receiver functions as ray back-azimuth increases, and (3) reversal of the Ps-phase polarity on the radial receiver function in a range of the back azimuth. Another important influence is shear-wave splitting of the Ps-converted waves and other later phases reverberated as S wave. By numerical experiments using synthetic receiver functions, we demonstrate that the waveform cross-correlation analysis is applicable to splitting Ps phases on receiver functions to estimate the seismic anisotropy of layer structure. Advantages to utilizing the Ps phases are (1) they appear more clearly on receiver functions than on seismograms and (2) they inform us about what place along the seismic ray path is anisotropic. Real analysis of shear-wave splitting is executed to the Moho-generated Ps phases that are identified on receiver functions at six seismic stations in the Chugoku district, southwest Japan. The time lags between the two arrivals of the split Ps phases are estimated at 0.2-0.7 sec, and the polarization directions of the fast arrival components are from north-south to northeast-southwest. This result is consistent with recent results of shear-wave splitting measurements and the trend of linear epicenter distributions of crustal earthquakes and active fault strikes in the Chugoku district.
Temporal changes in seismic anisotropy can be interpreted as variations in the orientation of cracks in seismogenic zones, and thus as variations in the stress field. Such temporal changes have been observed in seismogenic zones before and after earthquakes, although they are still not well understood. In this study, we investigate the azimuthal polarization of surface waves in anisotropic media with respect to the orientation of anisotropy, from a numerical point of view. This technique is based on the observation of the signature of anisotropy on the nine-component cross-correlation tensor (CCT) computed from seismic ambient noise recorded on pairs of three-component sensors. If noise sources are spatially distributed in a homogeneous medium, the CCT allows the reconstruction of the surface wave Green's tensor between the station pairs. In homogeneous, isotropic medium, four off-diagonal terms of the surface wave Green's tensor are null, but not in anisotropic medium. This technique is applied to three-component synthetic seismograms computed in a transversely isotropic medium with a horizontal symmetry axis, using a spectral element code. The CCT is computed between each pair of stations and then rotated, to approximate the surface wave Green's tensor by minimizing the off-diagonal components. This procedure allows the calculation of the azimuthal variation of quasi-Rayleigh and quasi-Love waves. In an anisotropic medium, in some cases, the azimuth of seismic anisotropy can induce a large variation in the horizontal polarization of surface waves. This variation depends on the relative angle between a pair of stations and the direction of anisotropy, the amplitude of the anisotropy, the frequency band of the signal and the depth of the anisotropic layer.
Global Tomography of Seismic Anisotropy and Interpretations
Seismic anisotropy, in spite of its inherent complexity is becoming an important ingredient for explaining various kinds of seismic data. Global tomographic models have been improved over years not only by an increase in the number of data but more importantly by using more general parameterizations, now including general anisotropy (both radial and azimuthal anisotropies). Different physical processes (lattice preferred orientation of crystals, cracks or fluid inclusions, fine layering...) related to strain field and/or stress field, give rise to observable seismic anisotropy (S-wave splitting, surface wave radial and azimuthal anisotropies), which makes its interpretation sometimes difficult and non-unique. Surface waves are well suited for imaging large scale (>1000km) lateral heterogeneities of velocity and anisotropy in the mantle by using fundamental and higher modes, since they provide an almost uniform lateral and azimuthal coverages, particularly below oceanic areas. The...
Crosshole seismic tomography including the anisotropy effect
Velocity anisotropy caused by fine layering has a strong effect on the travel paths at large vertical offsets in crosshole seismic data. This effect must be considered in seismic waveform tomography, so that the inversion procedure can match the waveforms in real data. In this paper, we propose to properly take into account this anisotropic effect in two stages. First, we invert for the anisotropy parameter simultaneously with the (horizontal) velocity in travel time tomography. Then we incorporate this estimated anisotropy parameter model in the waveform simulation during waveform tomography, to refine the velocity image. When considering this anisotropic effect, far-offset data are properly used in the inversion, then both travel time and waveform tomography may have a better ray coverage, producing a velocity image with a higher resolution.
Journal of Geophysical Research: Solid Earth
P wave tomography has been recently used to study 3-D azimuthal and radial anisotropy of subduction zones and continental regions. However, the fundamental issue about the trade-off between the isotropic and anisotropic structures is still unclear. In this study, we investigate this issue systematically with comprehensive synthetic tests. Our results indicate that good ray coverage in the azimuth (for azimuthal anisotropy) and incidence (for radial anisotropy) is required for determining reliable anisotropic models. The isotropic and anisotropic structures are strongly coupled, and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence. We therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis (i.e., the ratio of the short-axis and long-axis lengths) for estimating the ray coverage quantitatively. Applying our novel approach to a large number of high-quality arrival time data of local shallow-and intermediate-depth earthquakes, we obtained new tomographic images of 3-D P wave azimuthal and radial anisotropy in Northeast Japan. Both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region, whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab. Our results show dominant trench-normal and vertical-fast anisotropy in the mantle wedge while trench-parallel and horizontal-fast anisotropy in the subducting slab, which indicates different dynamics in different domains of the subduction zone.
Seismic Anisotropy Tomography and Mantle Dynamics
Surveys in Geophysics
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D variations of both structural heterogeneity and seismic anisotropy, providing unique constraints on geodynamic processes in the Earth’s crust and mantle. Here we introduce recent advances in the theory and application of seismic anisotropy tomography, thanks to abundant and high-quality data sets recorded by dense seismic networks deployed in many regions in the past decades. Applications of the novel techniques led to new discoveries in the 3-D structure and dynamics of subduction zones and continental regions. The most significant findings are constraints on seismic anisotropy in the subducting slabs. Fast-velocity directions (FVDs) of azimuthal anisotropy in the slabs are generally trench-parallel, reflecting fossil lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation due to transform faults produced at the mid-ocean ridge and intraslab hydra...