Measurement of Rock Fabric in Shallow Refraction Seismology

Abstract

A three dimensional (3D) seismic refraction survey was carried out across a shear zone. The data were processed with the generalized reciprocal method (GRM) rather than with tomographic inversion because of the relatively small volume of data, the occurrence of large variations in depth to and wavespeeds within the main refractor and the presence of azimuthal anisotropy. The amplitude products of the refracted signals are approximately proportional to the square of the ratio of the specific acoustic impedances between the upper layer and the refractor. The ratios of these amplitude products for different azimuths of shot pairs for a given set of geophones provide a convenient and detailed measure of apparent azimuthal anisotropy or rock fabric. Qualitative measures of azimuthal anisotropy are obtained from the wavespeeds and the time-depths computed from the traveltime data with the GRM algorithms and from the amplitudes. These three methods give similar consistent results, with the direction of the greater wavespeed being approximately parallel to the direction of the dominant geological strike. Furthermore, all three methods show that the direction of the greater wavespeed is approximately orthogonal to the direction of the dominant geological strike in one region adjacent to the shear zone.