A simple approach to 3D 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 results show that there is an increase in the depth of weathering and a decrease in wavespeed in the subweathering 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 wavespeeds 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. The in-line results show that both accurate refractor depths and wavespeeds 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 shot points over the requirements for the normal 2D program would be required in the cross-line direction for measuring azimuthal anisotropy and rock fabric.