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YARDLEY, G. and CRAMPIN, S. 1990. Extensive-dilatancy anisotropy : relative information in VSPs and reflection surveys. Geophysical Prospecting 39,337-355.Shear-waves have complicated interactions with the free surface, particularly in the presence of low-velocity surface layers, topographic irregularities, and the expected near-surface crack and stress anomalies. Consequently, it has been suggested that shear-waves should be recorded subsurface in vertical seismic profiles (VSPs), in order to extract accurate information about the in situ crack and stress geometry contained in shear-wave splitting. This paper compares the information in synthetic shear-waves in reflection gathers and VSPs, in order to assess the relative merits of the two techniques for investigating shear-wave splitting. Synthetic seismograms demonstrate that in the presence of even very simple surface layers, shearwaves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth. In contrast, shear-waves recorded in VSPs are relatively unaffected by surface layers and near-surface stress and crack anomalies, and the behaviour of shear-wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones. Matrix rotations of multicomponent-multisource shear-wave reflection data to extract the information contained in the split shear-waves, are found to be directly meaningful only in situations where crack orientations do not change with depth. known as extensive-dilatancy anisotropy or EDA (Crampin, Evans and Atkinson 1984;Crampin 1985Crampin ,1987a.Since analysis of the detailed waveforms of the shear-waves is important for estimating the internal structure of the rock mass, it is desirable to minimize any disturbance to the behaviour of the split shear-waves as they travel through the reservoir to the recording instrument. Shear-waves have complicated interactions with the free surface (Booth and Crampin 1985), particularly in the presence of low-velocity surface layers, topographic irregularities and the expected near-surface crack-and stress-anomalies (Crampin 1990). Consequently, it has been suggested that shear-waves should be recorded subsurface in VSPs, rather than in surface reflection lines, in order to preserve the valuable information about crack and stress geometries contained in the shear wavetrain (Crampin 1987a;Crampin, Lynn and Booth 1989). VSPs, however, are expensive, especially in the high-cost production environments where information about crack and stress geometries would be particularly valuable.We compare the relative information in synthetic shear-wave seismograms from reflection surveys and VSPs in the presence of near-surface layering. We also examine the viability of matrix rotations of multicomponent-multisource data to extract the anisotropic parameters from recorded shear-wave data.Note that the terminology used for seismic anisotropy is given by Crampin (1989). In general, the faster and slower split shear-waves are re...

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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 reorientation 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.