CROSS-HOLE SEISMIC SCANNING AND TOMOGRAPHY

Abstract

Cross-hole seismic scanning involves recording seismograms with a source in one borehole and detectors in another. Detailed coverage of the rock section between the holes is achieved when many depth positions along the boreholes are occupied by the source and detectors. Because measurements are made beneath the strongly dissipative overburden and weathering zone, high-frequency short-wavelength seismic waves capable of resolving small features can be used in most rock types. Although reflections may exist in cross-hole seismograms, the focus is on transmission events, i.e., the direct P or S arrivals which traverse the rock between the holes. Traveltimes and amplitudes of the direct events can be inverted to create maps of seismic properties which may be representative of the geologic cross-section between the boreholes. The inversion is done using computer-assisted tomographic (CAT) imaging techniques similar in concept to those used in diagnostic medicine, but adapted to the seismic problem.