Hammer-induced seismic investigations in an area of observed anisotropy

Abstract

A seismic experiment was undertaken in order to investigate the effect of an observed (outcropping) fracture set on shear-wave propagation, and more specifically whether the observed fracture set induced shear-wave birefringence. The source for the experiment was a hammer swung to impact horizontally on a source block coupled to the ground by two large metal pins. Eight three-component geophones recorded arrivals up to 40m offset from the hammer source. Impacts of opposite polarity in the in-line and cross-line direction were differenced in order to maximise S-wave, and minimise P-wave energy. After vector gain, filtering and rotation of the recorded data into in-line and cross-line directions, Alford rotation was undertaken on selected windowed events in order to detect shear-wave birefringence. In-line and cross-line energy is maximised by a co-ordinate rotation to the north?south direction, parallel to the observed fracture direction, suggesting that the observed fracture direction results in an anisotropy that induces shear-wave birefringence.