Evaluation of P- and S-wave sources for shallow seismic reflection

Abstract

The theoretical attractiveness of utilising both P- and S-reflection information is offset in the shallow environment by the practical difficulty of isolating high resolution reflections from other non-reflection energy. Source design is a critical component in a range of mandatory acquisition and processing optimisations. Four near-surface P-wave sources (high explosive, shotgun, weight drop, and hammer) have been evaluated in terms of their suitability for shallow reflection at a test site in the Ipswich Basin, southeast Queensland. Cross power spectral analyses of trace segments in the reflection window provide an estimate of the 'coherent bandwidth' of the different sources. Although the high explosive has superior signal levels, consideration of spectral symmetry, autocorrelation shape, and logistic aspects indicates that the shotgun source is a viable alternative. A pneumatic powered S-wave generator has been constructed and tested. Integrated spectral energies have been used to monitor piston velocity limitations and hence define optimum operating parameters. The pneumatic source is more controllable and of higher energy than a sledgehammer-powered equivalent.