S-wave Polarization Directions Research Articles

Imaging faults and fractures with the difference of fast and slow shear-wave splitting reflectivity, Δγ(S): 3D/9C survey in Midland Basin, West Texas, and 3D/3C survey in Washakie Basin, Wyoming

One of the most important applications of shear-wave (S-wave) seismic exploration has been in reservoir fracture characterization. While many advancements have been made over the past 30 years to compute and correct for the long-wavelength kinematics of S-wave splitting (SWS) (fast S-wave polarization directions and slow S-wave time delays), practically no progress has been made in imaging the short-wavelength reflectivity of fractures directly with Δγ(S). This property is the contrast in the SWS anisotropy parameter, γ(S), and represents the reflection amplitude at vertical incidence for changes in fracture density and orientation across an interface. In this article, we examine the Lupin nine-component survey in Midland Basin, Texas, for the technical reliability of imaging fractures in depth with converted P to S waves (PS waves) guided by pure-mode horizontal shear waves and vertical shear waves. Final Δγ(S) amplitude maps for each mode show sensitivity to fractures, faults, and the maximum horizontal stress direction. These maps are computed from the difference between fast and slow S-wave stacks (after SWS analysis and correction) and P-wave amplitude variation with offset gradient stacks. The S-wave difference maps identify an east–west lineament, possibly a strike-slip fault or fracture corridor, that is not observed by P-wave depth slices. Pure-mode S waves and PS waves are orders of magnitude more sensitive to Δγ(S) than P waves. We also review the development of Δγ(S) and find that it has been relatively unexploited by the exploration industry. In addition, we demonstrate that Δγ(S) can be obtained directly from the objective function of the transverse energy to correct for SWS, and show a four-component Alford rotation example from a previous PS-wave survey in the Washakie Basin, Wyoming.

Reflection and transmission coefficient approximations for P, S1 and S2 waves in triclinic media

SUMMARY The conventional assumptions, in most published approximations of the reflection and transmission (R/T) coefficients of plane waves at a plane interface between two anisotropic half-spaces, confine their applications to weakly anisotropic and/or weak contrast models. We consider the horizontal interface enclosed by two triclinic half-spaces to approximate the R/T coefficients normalized by the vertical energy flux. The homogeneous background medium can be anisotropic with arbitrary symmetry to better simulate the strongly anisotropic media. The second-order approximations are proposed to accommodate the strong contrast interface. We also consider an isotropic background medium under the weak anisotropy assumption. The obtained approximations can be applied to P, S1 and S2 waves, except for the transmission coefficients between the S1 and S2 waves. The S-wave transmission coefficients are insensitive to the model parameter contrasts and predominately rely on the S-wave polarization directions in the half-spaces above and below the interface. The proposed approximations are tested numerically.

Modern Geodynamics and Focal Mechanisms of Earthquakes near the Bushehr Nuclear Power Plant

A method for determining the focal mechanisms of earthquakes using the S-wave polarization direction for weak close seismic events is developed based on the detailed seismological studies carried out by the seismological expedition of the Institute of Physics of the Earth of the Russian Academy of Sciences in 1999–2001 near the Bushehr nuclear power plant (NPP) in Southern Iran. The results of the reconstruction of such determinations are compared with the focal mechanisms of strong earthquakes determined in ISC catalogs using the standard method and with modern concepts of tectonic deformation of the earth’s crust in a wide vicinity of the region under consideration. It has been established that this reconstruction is in good agreement with both the typification of movements in the foci of strong earthquakes and modern concepts about the nature of the deformation of the earth’s crust within the observation network. This allows us to recommend this method for reconstructing focal mechanisms in other regions.

Seismic anisotropies of the Songshugou peridotites (Qinling orogen, central China) and their seismic implications

Though extensively studied, the roles of olivine crystal preferred orientations (CPOs or fabrics) in affecting the seismic anisotropies in the Earth's upper mantle are rather complicated and still not fully known. In this study, we attempted to address this issue by analyzing the seismic anisotropies [e.g., P-wave anisotropy (AVp), S-wave polarization anisotropy (AVs), radial anisotropy (ξ), and Rayleigh wave anisotropy (G)] of the Songshugou peridotites (dunite dominated) in the Qinling orogen in central China, based on our previously reported olivine CPOs. The seismic anisotropy patterns of olivine aggregates in our studied samples are well consistent with the prediction for their olivine CPO types; and the magnitude of seismic anisotropies shows a striking positive correlation with equilibrium pressure and temperature (P–T) conditions. Significant reductions of seismic anisotropies (AVp, max. AVs, and G) are observed in porphyroclastic dunite compared to coarse- and fine-grained dunites, as the results of olivine CPO transition (from A-/D-type in coarse-grained dunite, through AG-type-like in porphyroclastic dunite, to B-type-like in fine-grained dunite) and strength variation (weakening: A-/D-type → AG-type-like; strengthening: AG-type-like → B-type-like) during dynamic recrystallization. The transition of olivine CPOs from A-/D-type to B-/AG-type-like in the forearc mantle may weaken the seismic anisotropies and deviate the fast velocity direction and the fast S-wave polarization direction from trench-perpendicular to trench-oblique direction with the cooling and aging of forearc mantle. Depending on the size and distribution of the peridotite body such as the Songshugou peridotites, B- and AG-type-like olivine CPOs can be an additional (despite minor) local contributor to the orogen-parallel fast velocity direction and fast shear-wave polarization direction in the orogenic crust such as in the Songshugou area in Qinling orogen.

Joint inversion for effective anisotropic velocity model and event locations using S-wave splitting measurements from downhole microseismic data

An accurate velocity model is essential in microseismic imaging. Layered sediments and subvertical cracks from tectonic stress and hydraulic fracturing make velocity models more complicated than isotropy or simple anisotropy, such as vertical transverse isotropy. Downhole microseismic acquisition usually cannot achieve sufficient ray coverage that is required to develop a low-symmetry anisotropic model from traveltimes alone. To solve this problem, we have developed a new type of data, S-wave splitting parameters (the delay time of the slow S-wave and fast S-wave polarization direction), to determine anisotropic models. A genetic algorithm inversion is adopted to solve for the locations of events and the stiffness tensor of an anisotropic medium simultaneously. We applied this method to synthetic waveforms from numerical modeling and successfully recovered the input event locations and the velocity model. The effectiveness of this method is further demonstrated by using real microseismic data acquired in the Bakken shale reservoir. Compared with the inversion with an isotropic velocity model, event locations from an anisotropic model become well-aligned with natural fractures in the Bakken Formation. Our experiments have evidenced that adding full S-wave splitting parameters makes a significant improvement in constraining a low-symmetry anisotropic model from downhole microseismic data.

The influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate-rich rocks

The anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks. Phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust. Crenulation cleavage is one of the most common microstructural fabrics found in multiply-deformed, phyllosilicate-rich, crustal rocks. We calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development: an initial planar foliation, a moderately-developed crenulation cleavage, and a well-developed crenulation cleavage. Mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method. The difficulties involved with sample preparation and data acquisition of phyllosilicate-rich rock samples are also discussed. We compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map, namely Voigt and Reuss averages. These averages do not account for grain-scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization. Our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic, with P-wave anisotropies up to 30.9% and S-wave anisotropies up to 34.2%, whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic, with a P-wave anisotropy of 15.5% and S-wave anisotropy of 10.7%. Progressive development of the fabric also causes the orientations of P- and S-wave velocity maxima and S-wave polarization directions to change markedly. Despite the high anisotropy imparted by a planar schistosity, the variety of folds and fabrics typically found in phyllosilicate-rich rocks within larger-scale crustal volumes will tend to mute the anisotropy, possibly to the point of appearing nearly isotropic.

VP/VS ratio and shear-wave splitting in the Nankai Trough seismogenic zone: Insights into effective stress, pore pressure, and sediment consolidation

To estimate variation of stress state and sediment consolidation in the Nankai plate subduction zone off southwest Japan, we measured the P-wave to S-wave velocity ratio (VP/VS) and S-wave splitting along the seismic line extending from the trench to the seismogenic zone. For this purpose, we used active-source seismic data recorded by multicomponent ocean bottom seismometers (OBS). Because it is difficult to identify the PS-converted reflection waveforms for each of the geological boundaries in this deep offshore region, we focused on the more easily identified PPS-refracted waveforms that register the conversion of the up-going P-waves to S-waves at the igneous crust surface. We estimated the average VP/VS ratio within the sedimentary section by using the time lag between the P-refracted waves and PPS-converted waves. This VP/VS ratio changes abruptly at the trough axis (i.e., the deformation front of the accretionary prism) arguably because of compaction associated with the accretion process. We observed relatively high VP/VS around the seismogenic megasplay fault, which may partially indicate the abnormal pore pressure and intensive fractures associated with the fault. To estimate the stress-induced fracture orientation and stress magnitude, we computed the fast S-wave polarization direction and estimated S-wave velocity anisotropy by applying the crosscorrelation method to the PPS-converted waves. To improve signal-to-noise ratio of the waveform for S-wave splitting analysis, we stacked PPS-converted waveforms on receiver gather. These anisotropic characteristics change at the seismogenic megasplay fault: the fast polarization direction is nearly parallel to the subduction direction seaward of the megasplay fault and is perpendicular to the subduction direction landward of the megasplay fault. This velocity anisotropy is especially strong around the megasplay fault. These results imply that the preferred fracture orientation, as well as the principal stress orientation, is oblique to the direction of plate subduction near the megasplay fault.

Layer stripping of shear-wave splitting in marinePSwaves

SUMMARY The properties of split S waves can be used to infer: (1) the state of stress and strain in the Earth; (2) the directional dependence of hydraulic conductivity and (3) small changes in pore-fluid pressure in the rock mass that occur in response to dynamic processes, such as the earthquake cycle. Measurements of split S waves are particularly useful in shallow (<1000 m subseabed) marine sediments, where S-wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near-vertical aligned cracks. Here we present a method of measuring small amounts of S-wave splitting in marine P-to-S mode-converted waves, and illustrate the technique with data from an ocean-bottom seismometer (OBS) deployed on the west Svalbard continental slope. The analysis applies a modified version of the Alford rotation and layer-stripping technique developed for zero-offset S-wave sources and treats PS waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth. When the seismic record contains coherent signal on both the in-plane and out-of-plane components, the layer-stripping technique is capable of decoupling the Swave splitting from the effects of P-wave velocity anisotropy and reflector dip that influence the downgoing, P wave, part of the ray path. The amount of S-wave splitting in the data is small, however, and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer-stripping theory (such as use of a finite-offset source). For the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal-to-noise levels. These include using a wide range of shot-receiver azimuths to generate data redundancy, methods of identifying and rejecting poor measurements, and a predictive layer-stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer-by-layer results. With the PS waves of the example data, which have a dominant period about 30 ms, we find the technique is capable of measuring S-wave splitting to a precision of about 0.5 ms for 4 or 5 layers. The number of layers successfully treated would increase if the amount of S-wave splitting were larger than in these data, for which the total cumulative S-wave splitting was about 10 ms over a 450 m depth interval. The orientation of the fast split S wave was measured with a precision of about 15 ◦ . Our results give an S-wave velocity anisotropy of 1–2 per cent in the shallowest 25–30 m subseabed that implies the presence of a differential horizontal stress at, or very close to, the seabed. The S-wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel, fluid-filled cracks with crack-density about 0.015. The fast S wave was found to be oriented at 75 ± 15 ◦ for the uppermost 150 m, before drifting clockwise to an azimuth of 190–210 ◦ . The clockwise drift in the fast S-wave polarization direction implies a transfer of dominance between one set of cracks in the near surface (probably related to the slope of the seabed) and a different set in deeper sediments of tectonic origin. A zone of azimuthal isotropy, where the mechanisms that produce the elastic anisotropy may cancel each other out, occupies the interval between the two almost orthogonal sets of cracks. From interpretation

Mantle tectonics beneath New Zealand inferred fromSKSsplitting and petrophysics

SUMMARY We constrain the extent of lithospheric and asthenospheric deformation beneath New Zealand by coupling measurements of shear wave splitting of teleseismic waves and petrophysical analysis of mantle xenoliths. SKS splitting for the central South Island and the eastern part of the North Island confirms earlier observations. The fast S-wave polarization directions are roughly NE/SW, which is parallel to the Alpine Fault in the South Island and to the strike of the Hikurangi subduction zone in the North Island. This suggests that flow parallel to the plate boundary extends up to 160 km away from the boundary. Departures from this pattern are restricted to the southernmost station and the two northernmost stations and indicate changes in the mantle flow or a more complex anisotropic pattern, such as a dipping axis of symmetry or heterogeneous anisotropy. Analysis of mantle xenoliths from the Raglan (North Island), Dunedin (South Island) and Chatham Island regions allows us to constrain the lithospheric contribution to the observed seismic anisotropy. The intrinsic S-wave anisotropy is 1.5 per cent higher for the South Island samples (∼5.0 per cent versus ∼3.5 per cent for the North Island), indicating a strong lithospheric deformation that could explain the major part of the observed splitting. In the North Island, the intrinsic lithospheric anisotropy is too low to explain the SKS splitting times. This indicates that asthenospheric deformation probably plays a major role there. For the Chatham Island, the anisotropy pattern remains unresolved as the xenolith suggests strong lithospheric anisotropy but no present splitting is measured.

Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties

Forward numerical models are used to investigate the effect of deformation regime on the development of olivine lattice-preferred orientations (LPO) and associated seismic anisotropy within continental deformation zones. LPO predicted to form by pure shear, simple shear, transpression, or transtension are compared to a database comprising ca. 200 olivine LPO from naturally deformed upper mantle rocks. This comparison suggests that simple shear or plane combinations of simple and pure shear are probably the dominant deformation regimes in the upper mantle. Seismic properties, calculated using the modeled olivine LPO, suggest that seismic anisotropy data may carry information on the deformation regimes active in the lithospheric mantle, although not all deformation regimes are characterized by a distinct seismic anisotropy signal. Transtensional deformation in continental rift systems should result in fast S-wave polarization and P-wave propagation directions oblique to the rift trend within the extended lithospheric mantle. Simple shear (wrench) or transpression in vertical deformation zones and pure shear (horizontal extension) result in similar seismic anisotropy. Simple shear or widening–thinning shear may, however, induce obliquity between seismic and magnetotelluric electrical conductivity anisotropy data. Similarly, it is not possible to distinguish between simple shear or lengthening–thinning shear (plane transpression) in horizontal deformation zones (thrusts) and pure shear (vertical contraction/horizontal extension). In all cases, the polarization direction of the fast split S-wave and the fast P-wave direction parallels the flow direction, but the anisotropy for both Pn- and S-waves is lower in horizontal structures than in vertical ones. Finally, several deformations show an isotropic response to SKS and/or Pn waves, suggesting that seismic isotropy does not necessarily imply absence (or heterogeneity) of deformation. There is a good agreement between model predictions and seismic anisotropy data in both transtensional and transpressional zones, suggesting coupled deformation of the crust and mantle. Oblique fast S-wave polarization directions in the East African rift, for instance, may result from an early transtensional deformation in the mantle lithosphere below the rift system. In contrast, most thrust belts display fast S-waves polarized parallel to the trend of the belt. One possible interpretation is that the upper mantle is decoupled from the crust in these areas.

A microseismic study in the western part of the Gulf of Corinth (Greece): implications for large-scale normal faulting mechanisms

SUMMARY We present the results of a dense seismological experiment in the western part of the Gulf of Corinth (Psathopyrgos-Aigion area), one of the most active rifts in the Aegean region for which we have precise tectonic information. The network included 51 digital stations that operated during July and August 1991, covering a surface of 40 x 40 km2. Among the 5000 recorded events with ML ranging between 1.0 and 3.0, we precisely located 774 events. We obtained 148 well-constrained focal mechanisms using P-wave first motions. Of these, 60 also have mechanisms obtained by combining the P-wave first motions with the S-wave polarization directions. The observed seismicity is mainly located between 6 and 11 km depth. Most of the fault-plane solutions correspond to E-W-striking normal faulting, in agreement with the geological evidence. Most of the well-determined mechanisms indicate a nodal plane dipping 10-25 due north and a steep south-dipping plane. A similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the Corinth Rift. We discuss this evidence and the inference of a deep detachment zone, a structure where the major faults seen at the surface appear to root. A large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone. This feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes.

Shear-wave polarizations and subsurface stress directions at Lost Hills field : Winterstein, D F; Meadows, M A GeophysicsV56, N9, Sept 1991, P1331–1348

Two closely spaced nine-component VSPs recorded in the Lost Hills oil field in the southern San Joaquin Valley of California show that the polarization of the fast shear (S) wave is aligned with the direction of maximum horizontal compressive stress as determined from analysis of tiltmeter data. Tiltmeters monitored fractures that were hydraulically induced in or near the VSP wells. When fractures were induced in one of the VSP wells, fracture azimuths as determined from tiltmeter data were N53{degree}E and N56{degree}E, respectively, in two different depth zones; in the same zones the fast S-wave polarization directions were N58{degree}E and N59{degree}E. When fractures were induced in a well 537 ft from the second VSP well, tiltmeter data indicated a mean fracture strike of N59{degree}E, while the fast S-wave polarization direction in that case was N40{degree}E. S-wave polarization directions were determined by minimizing energy on off-diagonal components of the 2 {times} 2 S-wave data matrix, accomplished by computationally rotating sources and receivers by the same angle. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization techniquemore » as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks at N55{degree}E in a weakly transversely isotropic matrix with infinite-fold symmetry axis tilted at 10{degree} from the vertical toward N70{degree}E.« less

Shear‐wave polarizations and subsurface stress directions at Lost Hills field

2 × 2 S-wave data matrix, accomplished by computationally rotating sources and receivers. Although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth, considerable improvement in consistency resulted from analytically stripping off a thin near‐surface layer whose fast S-wave polarization direction was about N 6°E. S-wave birefringence for vertical travel averaged 3 percent in two zones, 200–700 ft and 1200–2100 ft (60–210 m and 370–640 m), which had closely similar S-wave polarizations. Between those zones, the polarization direction changed and the birefringence magnitude was not well defined. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization technique as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks striking N 55°E in a weakly transversely isotropic matrix, where the infinite‐fold symmetry axis of the matrix is tilted 10 degrees from the vertical towards N 70°E. Such a model is of monoclinic symmetry.

High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations

Abstract Two borehole seismometer arrays (KNW-BH and PFO-BH) have been established in the Southern California Batholith region of the San Jacinto Fault zone by the U.S. Geological Survey. The sites are within 0.4 km of Anza network surface stations and have three-component seismometers deployed at 300 m depth, at 150 m depth, and at the surface. Downhole horizontal seismometers can be oriented to an accuracy of about 5° using regional and near-regional initial P-wave particle motions. Shear waves recorded downhole at the KNW-BH indicate that the strong alignment of initial S-wave particle motions previously observed at the (surface) KNW Anza site (KNW-AZ) is not generated in the near-surface weathered layer. The KNW-BH surface instrument, which sits atop a highly weathered zone, displays a significantly different (≈ 20°) initial S-wave polarization direction from that observed downhole and at KNW-AZ, which is bolted to an outcrop. Although downhole initial shear-wave particle motion directions are consistent with a shear-wave splitting hypothesis, observations of orthogonally polarized slow shear waves are generally elusive, even in seismograms recorded at 300 m. A cross-correlation measure of the apparent relative velocities of Sfast and Sslow horizontally polarized S waves suggests shallow shear-wave anisotropy, consistent with the observed initial S-wave particle motion direction, of 2.3 ± 1.7 per cent between 300 and 150 m and 7.5 ± 3.5 per cent between 150 and 0 m.

Shear-Wave Polarizations and Subsurface Stress Directions at Lost Hills Field

Two closely spaced nine-component VSPs recorded in the Lost Hills oil field in the southern San Joaquin Valley of California show that the polarization of the fast shear (S) wave is aligned with the direction of maximum horizontal compressive stress as determined from analysis of tiltmeter data. Tiltmeters monitored fractures that were hydraulically induced in or near the VSP wells. When fractures were induced in one of the VSP wells, fracture azimuths as determined from tiltmeter data were N53{degree}E and N56{degree}E, respectively, in two different depth zones; in the same zones the fast S-wave polarization directions were N58{degree}E and N59{degree}E. When fractures were induced in a well 537 ft from the second VSP well, tiltmeter data indicated a mean fracture strike of N59{degree}E, while the fast S-wave polarization direction in that case was N40{degree}E. S-wave polarization directions were determined by minimizing energy on off-diagonal components of the 2 {times} 2 S-wave data matrix, accomplished by computationally rotating sources and receivers by the same angle. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization techniquemore » as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks at N55{degree}E in a weakly transversely isotropic matrix with infinite-fold symmetry axis tilted at 10{degree} from the vertical toward N70{degree}E.« less