Interpreting data matrix asymmetry in near‐offset, shear‐wave VSP data

Geophysics ◽  
1994 ◽  
Vol 59 (2) ◽  
pp. 176-191 ◽  
Author(s):  
Colin MacBeth ◽  
Xinwu Zeng ◽  
Gareth S. Yardley ◽  
Stuart Crampin
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Synthetic Seismograms ◽  
Data Matrix ◽  
Depth Range ◽  
Wave Polarization ◽  
Polarization Azimuth ◽  
Polarization Change ◽  
Wave Splitting ◽  
Vsp Data

Poor experimental control in shear‐wave VSPs may contribute to unreliable estimates of shear‐wave splitting and possible misinterpretation of the medium anisotropy. To avoid this, the acquisition and processing of multicomponent shear‐wave data needs special care and attention. Measurement of asymmetry in the recorded data matrix using singular‐value decomposition (SVD) provides a useful way of examining possible acquisition inaccuracies and may help guide data conditioning and interpretation to ensure more reliable estimates of shear‐wave polarization azimuth. Three examples demonstrate how variations in shear‐wave polarization and acquisition inaccuracies affect the SVD results in different ways. In the first example, analysis of synthetic seismograms with known depth changes in the polarization azimuth show how these may be detected. In the second example, a known source re‐orientation and polarity reversal is detected by applying SVD to near‐offset, shear‐wave VSP data, recorded in the Romashkino field, Tatar Republic. Additional information on a polarization change in the overburden is also obtained by comparing the SVD results with those for full‐wave synthetic seismograms. The polarization azimuth changes from N160°E in the overburden to N117°E within the VSP depth range. Most of the shear‐wave splitting is built up over the VSP depth range. The final example is a near‐offset, shear‐wave VSP data set from Lost Hills, California. Here, most of the shear‐wave splitting is in the shallow layers before the VSP depth range. SVD revealed a known correction for horizontal reorientation of the sources, but also exhibited results with a distinct oscillatory behavior. Stripping the overburden effects reduces but does not eliminate these oscillations. There appears to be a polarization change from N45°E in the overburden to N125°E in the VSP section. The details in these examples would be difficult to detect by visual inspection of the seismograms or polarization diagrams. Results from these preliminary analyses are encouraging and suggest that it may be possible to routinely use this, or a similar technique, to resolve changes in the subsurface anisotropy from multicomponent experiments where acquisition has not been carefully controlled.

Changes in shear‐wave polarization azimuth with depth in Cymric and Railroad Gap oil fields

Geophysics ◽  
1991 ◽  
Vol 56 (9) ◽  
pp. 1349-1364 ◽  
Author(s):  
D. F. Winterstein ◽  
M. A. Meadows
Keyword(s):  
Shear Wave ◽  
Lower Layer ◽  
Oil Fields ◽  
Data Consistency ◽  
Data Matrix ◽  
Wave Polarization ◽  
S Wave ◽  
Polarization Azimuth ◽  
Uppermost Layer ◽  
Vsp Data

Shear‐wave [Formula: see text]-wave) polarization azimuths, although consistent over large depth intervals, changed abruptly and by large amount of various depths in nine-component vertical seismic profiling (VSP) data from the Cymric and Railroad Gap oil fields of the southwest San Joaquin basin. A simple layer‐stripping technique made it possible to follow the polarization changes and determine the [Formula: see text]-wave birefringence over successive depth intervals. Because the birefringence and polarization azimuth are related to in‐situ stresses and fracture, information from such analysis could be important for reservoir development. Near offset VSP data from Cymrix indicated that the subsurface could be appproximated roughly as two anisotropic layers. The upper layer, from the surface to 800 ft (240 m), had vertical [Formula: see text]-wave birefringence as large was about 6 percent down to 1300 ft (400 m). In the upper layer the polarization azimuth of the fast [Formula: see text]-wave was N 60°E, while in the lower layer it was about N 10°E. Refinement of the layer stripping showed that neither layer was anisotropically homogenous, and both could be subdivided into thinner layers. Near offset [Formula: see text]-wave VSP data from the Railroad Gap well also show high birefringence near the surface and less birefringence deeper. In the uppermost layer, which extends down to 1300 ft (400 m), the [Formula: see text]-wave birefringence was 9 percent, and the lag between the fast and slow [Formula: see text]-waves exceeded 60 ms at the bottom of the layer. Seven layers in all were needed to accommodate [Formula: see text]-wave polarization changes. The most reliable azimuth angle determination as judged from the data consistency were those of the uppermost layer, at N 46°E, and those from depths 2900–3700 ft (880–1130 m) and 3900–5300 ft (1190–1610 m), at N 16°E and N 15°W, respectively. Over those intervals the scatter of calculated azimuths about the mean was typically less than 4 degrees. The largest birefringence at both locations occurred in the same formation, the Pliocene Tulare sands and Pebble Conglomerate. In those formations the azimuth of the fast [Formula: see text]-wave polarization was roughly orthogonal to the southwest. In the deeper Antelope shale, [Formula: see text]-wave polarization directions in both areas were close to 45 degrees from the fault. Confidence in the layer stripping procedure was bolstered by major improvement in data quality that resulted from stripping. Before stripping, wavelets of the two [Formula: see text]-waves sometimes had very different waveforms, and it was often impossible to come close to diagonalizing the 2 × 2 S‐wave data matrix by rotating sources and receivers by the same angle. After stripping, wavelets were more similar in shape, and the S‐wave matrix was more nearly diagonalizable by rotating with a single angle.

Shear‐wave splitting in Quaternary sediments: Neotectonic implications in the central New Madrid seismic zone

Geophysics ◽  
1996 ◽  
Vol 61 (6) ◽  
pp. 1871-1882 ◽  
Author(s):  
James B. Harris
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Data Matrix ◽  
New Madrid Seismic Zone ◽  
Seismic Zone ◽  
Earthquake Hazards ◽  
Near Surface ◽  
Neotectonic Deformation ◽  
Wave Splitting

Determining the extent and location of surface/near‐surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear‐wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near‐surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow four‐component data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source‐receiver orientations, an indicator of shear‐wave splitting. Following rotation of the data matrix by 40°, the [Formula: see text] and [Formula: see text] sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35°W and N55°E, respectively. A dynamic mis‐tie of 8 ms at a two‐way traveltime of 375 ms produced an average azimuthal anisotropy of ≈2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear‐wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near‐surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear‐wave methods in unconsolidated, water‐saturated sediments, measurement of near‐surface directional polarizations, produced by shear‐wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.

Estimating interval shear-wave splitting from multicomponent virtual shear check shots

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. A39-A43 ◽  
Author(s):  
Andrey Bakulin ◽  
Albena Mateeva
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Data Set ◽  
Vertical Seismic Profiling ◽  
Wave Splitting ◽  
Vsp Data ◽  
Classic Approach ◽  
A New Technique

Measuring shear-wave splitting from vertical seismic profiling (VSP) data can benefit fracture and stress characterization as well as seismic processing and interpretation. The classic approach to measuring azimuthal anisotropy at depth involves layer stripping. Its inherent weakness is the need to measure and undo overburden effects before arriving at an anisotropy estimate at depth. That task is challenging when the overburden is complex and varies quickly with depth. Moreover, VSP receivers are rarely present all the way from the surface to the target. That necessitates the use of simplistic assumptions about the uninstrumented part of the overburden that limit the quality of the result. We propose a new technique for measuring shear-wave splitting at depth that does not require any knowledge of the overburden. It is based on a multicomponent version of the virtual source method in which each two-component (2-C) VSP receiver is turned into a 2-C shear source and recorded at deeper geophones. The resulting virtual data set is affected only by the properties of the medium between the receivers. A simple Alford rotation transforms the data set into fast and slow shear virtual check shots from which shear-wave splitting can be measured easily and accurately under arbitrarily complex overburden.

Shear‐wave vibrator signals in transversely isotropic shale

Geophysics ◽  
1985 ◽  
Vol 50 (8) ◽  
pp. 1285-1293 ◽  
Author(s):  
Sheila Peacock ◽  
Stuart Crampin
Keyword(s):  
Shear Wave ◽  
Shear Waves ◽  
Transversely Isotropic ◽  
Synthetic Seismograms ◽  
Wave Polarization ◽  
Transversely Isotropic Media ◽  
Wave Splitting ◽  
Isotropic Media ◽  
Component Field ◽  
Theoretical Results

The experiments of Robertson and Corrigan (1983) on shale are among the first three‐component field observations of shear waves in transversely isotropic media to be published. Their data are reprocessed to highlight the effects of the shale’s anisotropy on shear waves. Two results emerge. First, shear‐wave splitting in a transversely isotropic substrate is most easily observed when the vibrator baseplate is oriented so that both SH‐ and SV‐waves reach the geophone. Second, the SV‐wave polarization deviates significantly from perpendicular to the raypath. Both results may significantly affect the interpretation. Both are found to agree with theoretical results and are modeled successfully by synthetic seismograms.

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.

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