Using Seismic Data and Cross-Hole Analysis to Image Fractures

Two stratigraphic bore-holes 335 m apart, drilled to the Bulli coal seam at a mine site in the Southern Sydney Basin, showed an average velocity discrepancy of 10 per cent throughout the entire lithological section. Subsequently, a comprehensive seismic experiment was performed by BHP Steel (AIS) Pty. Ltd. Collieries Division which included the surface seismic reflection, walk-away vertical seismic profile (VSP) and cross-hole recording over the area connecting the two bore-holes.

The surface seismic data indicated possible seam disturbances between the bore-holes. The VSP data were of variable quality, while cross-hole data were of the best quality. Strong transverse isotropy was measured through the sandstone unit. The top 100 m of the Bulgo sandstone showed weak P-wave anisotropy. However, shear wave splitting in the Bulgo sandstone suggested fracture induced anisotropy. Methane accumulations were inferred from the reflection and VSP data and Poisson’s ratio computed at the receiver borehole.

Analysis of these data revealed a high complexity of the zone between and away from the boreholes. The poor quality of reflection and VSP data resulted from the presence of faults, associated fractures and several methane accumulations in the Bulgo sandstone. The overall velocity difference between the boreholes is related to a change in stress field. The results of the experiment showed that where reflection seismic data was poor, the application of three component VSP and cross-hole seismic data can be used to define a structurally complex area which may contain faulting/fracturing and methane. In such areas special care has to be taken in data processing, analysis and time to depth conversion.