Shear-wave splitting analysis of multi-offset coal VSPs in the Bowen Basin

Abstract

A shear-wave (S-wave) splitting analysis has been carried out on multioffset, shallow VSP data from the Gordonstone coal prospect in the Bowen Basin, Australia. In comparison to previous petroleum-scale investigations, the analysis is complicated by the short time window available for shear-wave analysis, and interference from preceding compressional-wave (P-wave) energy. A simple preprocessing scheme incorporates highcut filtering, as well as a two-stage coordinate orientation procedure, designed to enhance the shear wavefield for the subsequent splitting analysis. Several single-source algorithms for shear-wave splitting analysis are considered, with the primary aim being determination of the polarisation azimuth of the fast split shear wave. The results from two boreholes, whose shots were at large offsets, show good consistency between the different algorithms, and from shot to shot. Results from the third borehole studied are considered less reliable, especially for those shots having smallest offsets. The preferred fast shear-wave polarisation azimuth is 060°. This compares well with independent estimates of the direction of maximum horizontal compressive stress in the Bowen Basin. In general, the slow shear wavefield is poorly determined. However, hodogram analysis on some traces permits estimation of the time delay between fast and slow waves. These observations are consistent with a variation of approximately 2% between the fast and slow shear-wave velocities, indicating moderate development of azimuthal anisotropy.