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

2004 ◽  
Vol 109 (B2) ◽  
Author(s):  
Melissa M. Moore ◽  
Edward J. Garnero ◽  
Thorne Lay ◽  
Quentin Williams
Keyword(s):  
Shear Wave ◽  
Transverse Isotropy ◽  
Shear Wave Splitting ◽  
Low Velocity ◽  
Wave Splitting ◽  
Lowermost Mantle

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.

3-D seismic modeling for cracked media: Shear‐wave splitting at zero‐offset

Geophysics ◽  
2000 ◽  
Vol 65 (1) ◽  
pp. 211-221 ◽  
Author(s):  
Jaime Ramos‐Martínez ◽  
Andrey A. Ortega ◽  
George A. McMechan
Keyword(s):  
Shear Wave ◽  
Effective Properties ◽  
Transverse Isotropy ◽  
Shear Waves ◽  
Crack Density ◽  
Shear Wave Splitting ◽  
Carbonate Reservoir ◽  
Vertical Crack ◽  
Wave Splitting ◽  
Zero Offset

Splitting of zero‐offset reflected shear‐waves is measured directly from three‐component finite‐difference synthetic seismograms for media with intersecting vertical crack systems. Splitting is simulated numerically (by finite differencing) as a function of crack density, aspect ratio, fluid content, bulk density, and the angle between the crack systems. The type of anisotropy symmetry in media containing two intersecting vertical crack systems depends on the angular relation between the cracks and their relative crack densities, and it may be horizontal transverse isotropy (HTI), tetragonal, orthorhombic, or monoclinic. The transition from one symmetry to another is visible in the splitting behavior. The polarities of the reflected quasi‐shear waves polarized perpendicular and parallel to the source particle motion distinguish between HTI and orthorhombic media. The dependence of the measured amount of splitting on crack density for HTI symmetry is consistent with that predicted theoretically by the shear‐wave splitting factor. In orthorhombic media (with two orthogonal crack systems), a linear increase is observed in splitting when the difference between crack densities of the two orthogonal crack systems increases. Splitting decreases nonlinearly with the intersection angle between the two crack systems from 0° to 90°. Surface and VSP seismograms are simulated for a model with several flat homogeneous layers, each containing vertical cracks with the same and with different orientations. When the crack orientation varies with depth, previously split shear waves are split again at each interface, leading to complicated records, even for simple models. Isotropic and anisotropic three‐component S-wave zero‐offset sections are synthesized for a zero‐offset survey line over a 2.5-D model of a carbonate reservoir with a complicated geometry and two intersecting, dipping crack sets. The polarization direction of the fast shear wave, propagating obliquely through the cracked reservoir, is predicted by theoretical approximations for effective properties of anisotropic media with two nonorthogonal intersecting crack sets.

scholarly journals A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

2020 ◽  
Vol 221 (3) ◽  
pp. 2075-2090 ◽  
Author(s):  
Joseph Asplet ◽  
James Wookey ◽  
Michael Kendall
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Eastern Pacific ◽  
Core Mantle Boundary ◽  
The Core ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Lowermost Mantle

SUMMARY Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such as SKS and SKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths of SKS and SKKS are nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420 SKS–SKKS event–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30 SKS–SKKS event–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by null SKS splitting, and mean delay time of $1.15 \, \mathrm{ s}$ in SKKS. These measurements corroborate and expand upon previous observations made using SKS–SKKS and S–ScS phases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary.

Sensitivity of SK(K)S and ScS phases to heterogeneous anisotropy in the lowermost mantle from global wavefield simulations

2021 ◽  
Vol 228 (1) ◽  
pp. 366-386
Author(s):  
Jonathan Wolf ◽  
Maureen D Long ◽  
Kuangdai Leng ◽  
Tarje Nissen-Meyer
Keyword(s):  
Wave Propagation ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Forward Modelling ◽  
Finite Frequency ◽  
Specific Flow ◽  
Frequency Wave ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Lowermost Mantle

SUMMARY Observations of seismic anisotropy at the base of the mantle are abundant. Given recent progress in understanding how deformation relates to anisotropy in lowermost mantle minerals at the relevant pressure and temperature conditions, these observations can be used to test specific geodynamic scenarios, and have the potential to reveal patterns of flow at the base of the mantle. For example, several recent studies have sought to reproduce measurements of shear wave splitting due to D″ anisotropy using models that invoke specific flow and texture development geometries. A major limitation in such studies, however, is that the forward modelling is nearly always carried out using a ray theoretical framework, and finite-frequency wave propagation effects are not considered. Here we present a series of numerical wave propagation simulation experiments that explore the finite-frequency sensitivity of SKS, SKKS and ScS phases to laterally varying anisotropy at the base of the mantle. We build on previous work that developed forward modelling capabilities for anisotropic lowermost mantle models using the AxiSEM3D spectral element solver, which can handle arbitrary anisotropic geometries. This approach enables us to compute seismograms for relatively short periods (∼4 s) for models that include fully 3-D anisotropy at moderate computational cost. We generate synthetic waveforms for a suite of anisotropic models with increasing complexity. We first test a variety of candidate elastic tensors in laterally homogeneous models to understand how different lowermost mantle elasticity scenarios express themselves in shear wave splitting measurements. We then consider a series of laterally heterogeneous models of increasing complexity, exploring how splitting behaviour varies across the edges of anisotropic blocks and investigating the minimum sizes of anisotropic heterogeneities that can be reliably detected using SKS, SKKS and ScS splitting. Finally, we apply our modelling strategy to a previously published observational study of anisotropy at the base of the mantle beneath Iceland. Our results show that while ray theory is often a suitable approximation for predicting splitting, particularly for SK(K)S phases, full-wave effects on splitting due to lowermost mantle anisotropy can be considerable in some circumstances. Our simulations illuminate some of the challenges inherent in reliably detecting deep mantle anisotropy using body wave phases, and point to new strategies for interpreting SKS, SKKS and ScS waveforms that take full advantage of newly available computational techniques in seismology.

Reflection moveout and parameter estimation for horizontal transverse isotropy

Geophysics ◽  
1997 ◽  
Vol 62 (2) ◽  
pp. 614-629 ◽  
Author(s):  
Ilya Tsvankin
Keyword(s):  
Shear Wave ◽  
Symmetry Axis ◽  
Transverse Isotropy ◽  
Shear Waves ◽  
Shear Wave Splitting ◽  
P Wave ◽  
Symmetry Direction ◽  
Wave Splitting ◽  
Hti Media ◽  
Splitting Parameter

Transverse isotropy with a horizontal axis of symmetry (HTI) is the simplest azimuthally anisotropic model used to describe fractured reservoirs that contain parallel vertical cracks. Here, I present an exact equation for normal‐moveout (NMO) velocities from horizontal reflectors valid for pure modes in HTI media with any strength of anisotropy. The azimuthally dependent P‐wave NMO velocity, which can be obtained from 3-D surveys, is controlled by the principal direction of the anisotropy (crack orientation), the P‐wave vertical velocity, and an effective anisotropic parameter equivalent to Thomsen's coefficient δ. An important parameter of fracture systems that can be constrained by seismic data is the crack density, which is usually estimated through the shear‐wave splitting coefficient γ. The formalism developed here makes it possible to obtain the shear‐wave splitting parameter using the NMO velocities of P and shear waves from horizontal reflectors. Furthermore, γ can be estimated just from the P‐wave NMO velocity in the special case of the vanishing parameter ε, corresponding to thin cracks and negligible equant porosity. Also, P‐wave moveout alone is sufficient to constrain γ if either dipping events are available or the velocity in the symmetry direction is known. Determination of the splitting parameter from P‐wave data requires, however, an estimate of the ratio of the P‐to‐S vertical velocities (either of the split shear waves can be used). Velocities and polarizations in the vertical symmetry plane of HTI media, that contains the symmetry axis, are described by the known equations for vertical transverse isotropy (VTI). Time‐related 2-D P‐wave processing (NMO, DMO, time migration) in this plane is governed by the same two parameters (the NMO velocity from a horizontal reflector and coefficient ε) as in media with a vertical symmetry axis. The analogy between vertical and horizontal transverse isotropy makes it possible to introduce Thomsen parameters of the “equivalent” VTI model, which not only control the azimuthally dependent NMO velocity, but also can be used to reconstruct phase velocity and carry out seismic processing in off‐symmetry planes.

scholarly journals Insight into NE Tibetan Plateau expansion from crustal and upper mantle anisotropy revealed by shear-wave splitting

2017 ◽  
Vol 478 ◽  
pp. 66-75 ◽  
Author(s):  
Zhouchuan Huang ◽  
Frederik Tilmann ◽  
Mingjie Xu ◽  
Liangshu Wang ◽  
Zhifeng Ding ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Shear Wave ◽  
Upper Mantle ◽  
Shear Wave Splitting ◽  
Upper Mantle Anisotropy ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Insight Into ◽  
Ne Tibetan Plateau
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