INTERPRETING DATA MATRIX ASYMMETRY IN NEAR-OFFSET, SHEAR-WAVE 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 degrees E in the overburden to N117 degrees 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 degrees E in the overburden to N125 degrees 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.