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Abstract

In order to investigate the possibility of seismic velocity anisotropy within the lower Indus basin of Pakistan an experimental VSP was designed and recorded. Two offset VSPs were acquired utilizing an impulsive source during one common ascent of the sonde in a 7500 ft (2286 m) borehole. A “multi-pathing” phenomenon occurs at some depth levels which has been interpreted as a family of quasi-P waves. Hodogram analysis of the downgoing S-waves reveal similar families of q-S1 and q-S2 arrivals. At least two different mode conversion depths were identified for strong downgoing S-waves. Changes in the azimuth of the q-S1 arrivals versus depth were replicated on both offsets. The q-S1 polarizations are about 30-40 degrees clockwise to the fault azimuths and are interpreted to show the direction of fractures and maximum horizontal permeability. Strike-slip displacement is consistent with the geologic data, the q-S1 polarizations and the current knowledge concerning the intracratonic displacements on the Pak-lndian plate.

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