Shear-wave splitting analysis using a single-source, dynamite VSP in the Otway Basin

Abstract

A shear-wave splitting analysis has been carried out on a 3-component dynamite VSP from the Namgib-1 well in the Otway Basin, southeastern Australia. The commonly-used dual-source analysis techniques are not applicable, and hence a number of single-source approaches have been investigated. Energy Maximisation and Aspect Ratio techniques are considered the most robust and efficient. Complex Component and Hodogram approaches provide visual insight, but require more manual effort. Inter-Level and Inter-Component Correlation approaches are uncompetitive in practice. Meaningful estimates of fast shear-wave polarisation azimuth have been obtained for the Otway Basin well. Overall, these suggest that maximum horizontal compressive stress is oriented in the SE-NW quadrants. This result is consistent with independent evidence relating to present-day regional stress regimes. Localised variations in the fast shear-wave azimuth occur within sections of the borehole, and these may relate to inter-formational differences. Over the entire well, a relative time delay of 0.09s is accumulated between the fast and slow shear waves. This implies a shear-wave velocity differential averaging 6%, a value at the upper end of observations reported elsewhere. The successful shear-wave splitting determination reported here has implications for enhanced exploitation of the existing 3-component VSP archive.