Measurement of rock fabric in shallow refraction seismology

Abstract

A three dimensional (3D) seismic refraction survey carried out across a shear zone shows that there is an increase in the depth of weathering and a decrease in seismic velocity in the sub-weathering associated with the shear zone. Although the shear zone is generally considered to be a two dimensional (2D) feature, the significant lateral variations in both depths to, and seismic velocities within the refractor in the cross-line direction, indicate that it is best treated as a 3D target. These variations are not predictable on the basis of a 2D profile recorded earlier. Qualitative measures of azimuthal anisotropy are obtained from the seismic velocities and the time-depths computed from the traveltime data with the generalised reciprocal method (GRM) algorithms and from the head wave amplitudes. These three methods give similar, consistent results, with the direction of the greater seismic velocity being approximately parallel to the direction of the dominant geological strike over most of the survey area. Conversely, all three methods show that the direction of the greater seismic velocity is approximately orthogonal to the direction of the dominant geological strike in a small region adjacent to the shear zone. The amplitudes of the refracted signals are approximately proportional to the ratio of the specific acoustic impedances between the upper layer and the refractor, and they provide a convenient and detailed qualitative measure of azimuthal anisotropy or rock fabric. The amplitudes also contain additional useful geological information, although some of the cross-line amplitudes could not be completely explained. The in-line results show that both accurate refractor depths and seismic velocities can be computed with moderate cross-line offsets, say less than 20 m, of shot points. These results demonstrate that swath shooting with a number of parallel recording lines would be adequate for 3D surveys over targets such as highways, damsites and pipelines. Only a modest increase in the number of shot points over the requirements for the normal 2D program would be required in the cross-line direction to measure azimuthal anisotropy and rock fabric with amplitudes. The results of this study demonstrate that simple and efficient 3D refraction methods using a GRM approach can provide more useful geological interpretations than would be the case with detailed 2D approaches.