In situ monitoring of rock fracturing using shear wave splitting analysis: An example from a mining setting

Abstract

Changing stress conditions are well known to cause rupturing of rock. This is well constrained on a small scale from laboratory experiments and inferred on a much larger scale from tectonic earthquakes. Here, we present a study of rock fracturing induced by changes in stress state during block-caving operations in an Australian mine. This intermediate-scale study provides further evidence of the scalability of processes involved in rock fracturing and thus helps to link laboratory and seismological observations. We analyse the temporal evolution of rock fracturing during a production cycle using the analysis of fracture-induced anisotropy. Fracturing of the rock mass is monitored using evidence of seismic anisotropy from estimates of shear wave splitting and their subsequent inversion for fracture parameters. The data set consists of more than 40000 three-component seismograms recorded by an array of sensors, which provides excellent ray coverage. We applied a novel automatic quality assessment technique to handle this large data set and find that anisotropy, and thus fracturing, correlates strongly with the excavation process. We then perform a grid search over a series of synthetic models based on rock physics to invert the splitting parameters for fracture orientation and density. Finally, by applying a sliding window on our results, we are able to identify production related fracture evolution. During production the fracture density increases, with horizontal fracture density being stronger than the vertical fracture density. This can be explained by the removal of the supporting rock during caving. During short intervals of reduced production, the horizontal fracture density decreases, whereas vertical fracture density increases. We relate this to change in stress regime with reducing overburden mass during cavity collapse. This scenario is similar to a collapsing caldera or the inverse of an inflating magma chamber. © 2011 The Authors Geophysical Journal International © 2011 RAS.