Imaging seismic velocity changes caused by mining using underground and surface sources

Abstract

The process of mining causes changes in the seismic velocity of the rocks in the vicinity of the openings. Two effects are present: First, increased stress in the rockmass results in an increase in seismic velocity. Second, regions where the rock is highly fractured show lower seismic velocity and higher attenuation. Mapping the seismic velocity distribution and changes in velocity caused by mining could have two immediate benefits: mining-induced seismic events could be more accurately located, allowing their use in assessment of rockburst hazard; and the zones of high stress and fracture could potentially be directly imaged. A geophone array installed on the surface above a longwall coal mine was used to record seismic signals from the shearer as it cut coal. Major differences in velocity were seen for regions in front of the mining face compared to the highly fractured and caved zones behind the face. In order to test the viability of imaging this caved zone behind the face, a sledgehammer source was used in an underground roadway to augment the data from the shearer, and refracted arrivals were generated using surface sources. Images produced from this data repeatedly show, not only a low-velocity caved zone behind the face, but also a highvelocity region in front of the face which could be caused by increased stress in this region. The conclusion is that seismic changes caused by mining are large enough to be detected and imaged.