Underground » Geology
Converted-wave seismic reflection is a novel seismic method that takes advantage of the fact that when a compressional (P) wave from a conventional seismic source (e.g. dynamite, mini-SOSIE) strikes a coal seam, a significant fraction of the energy reflected back to the surface will be converted to shear (S) energy. Multi-component (3C) seismic acquisition measures both the vertical and horizontal components of ground motion at the surface to enable exploitation of both the P and converted P-to-S wave types. Over the past five years, Velseis Pty Ltd has been utilising this technology to yield both conventional P-wave and converted-wave (PS-wave) seismic images of coal seams in the Bowen Basin, Australia.
ACARP Project C10020 (completed June 2003) successfully demonstrated the viability of using converted-wave seismic technology to extract complimentary structural information about target coal seams. This project has focused on examining the lithological information that can be extracted from the integrated interpretation of P and PS data. One new 2D-3C seismic dataset has been acquired, to complement the two datasets previously acquired as part of project C10020. The project has also had access to a number of experimental commercial 3C datasets from the Bowen Basin. These data, together with a series of synthetic converted-wave seismic datasets, have undergone extensive analysis and interpretation to help improve our manipulation of PS seismic data, and determine the relevance and resolution of lithological information recoverable from integrated P/PS seismic interpretation.
Acquisition trials have demonstrated that improvements in our PS seismic sections can be achieved if geophones with a low natural frequency (e.g. 10 Hz or 14 Hz) are used for 3C seismic acquisition. Modifications to our S-wave receiver statics method, CCP-binning algorithm and post-stack noise removal techniques have significantly enhanced the outcomes of our converted-wave imaging. Integrated P/PS interpretation synthetic-data trials have demonstrated that, provided the width of a geological anomaly is greater than the lateral resolution limit of the seismic data, and the lithological variation is significant, the Vp/Vs attribute can give an indication of relative changes in lithology away from borehole locations. However, to date, real-data Vp/Vs analysis experiments have produced mixed results, and further validated trials are required to assess the robustness of this new interpretation tool. A preliminary test to quantify the behaviour of PS energy using the two horizontal components of data suggest that fast S-wave polarisations can be interpreted in terms of local stress fields, and may be useful for locating zones of intense fracturing or minor faulting that cannot be detected using standard seismic interpretation methods.
The overall outcomes of this project have been to enhance PS processing, obtain a greater understanding of the influence of geology on PS-wave propagation, and determine to what extent integrated P/PS interpretation can contribute to our geological knowledge of the sub-surface. The results clearly demonstrate that joint use of P and PS sections recovers more geological detail than using P-wave data alone. It remains to be determined how useful this additional lithological information will be to mine-site operations.