Fracture induced anisotropy in the Earth's crust causes vertically propagating shear waves to split into two discrete waves. The two split shear waves propagate at different velocities and are polarised into two discrete orientations. The particle motion of the faster split shear wave is generally aligned with the fractures, which are generally aligned with the direction of maximum compressional stress. The time delay between the two split shear waves is dependant upon the density of the fracturing.
Suitable analysis of multicomponent vertical seismic profile (VSP) data can illuminate the orientations of the split shear waves and the time delay between them. Such analysis allows the insitu examination of the subsurface fracture regime. Shear wave splitting (SWS) analysis is preferably performed on VSPs for which two orthogonal shear wave sources have been recorded intro three component geophones. Surveys such as these are extremely rar. VSPs for which a single explosive source at each level is recorded into a three component geophone are more common. In such a VSP shear waves are generated by mode conversion of compressional waves. For this project SWS and the poplular approaches to SWS analysis are reviewed in detail. Special attention is paid to approaches suitable for application to the more common single-source VSP acquisition format.
SWS analysis is performed on a single-source three component zero-offset VSP from the Otway Basin, Victoria. SWS is observed on this VSP. Six sws analysis techniques are applied to this data and the performance is examined and compared.
The SWS analysis results for the six techniques are correlated. The integrated SWS behaviour of the VSP as determined by the six techniques is interpreted in relation to the fracture regime of the borehole. The fracture orientation as determined by the SWS analysis techniques is found to be not inconsistent with the direction of the regional stress field. Fracture density determinations are less easily achieved on this data, however the interpreted anisotropy of approximately 6% is comparable with estimates from other studies.