Abstract

Shear-wave splitting, diagnostic of some form of effective seismic anisotropy, is observed along almost all near-vertical raypaths through the crust. The splitting is caused by propagation through distributions of stress-aligned vertical parallel fluid-filled cracks, microcracks, and preferentially oriented pore space that exist in most crustal rocks. Shear waves have severe interactions with the free surface and may be seriously disturbed by the surface and by near-surface layers.In principle, cross-hole surveys (CHSs) should be free of much of the near-surface interference and could be used for investigating shear waves at higher frequencies and greater resolution along shorter raypaths than is possible with reflection surveys and VSPs. Synthetic seismograms are examined to estimate the effects of vertical cracks on the behavior of shear waves in CHS experiments. The azimuth of the CHS section relative to the strike of the cracks is crucial to the amount of information about seismic anisotropy that can be extracted from such surveys. Interpretation of data from only a few boreholes located at azimuths chosen from other considerations is likely to be difficult and inconclusive. Application to interpreting acoustic events generated by hydraulic pumping is likely to be more successful.

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