Imaging using mining machinery as a source

Abstract

A mine is a difficult geophysical environment to work in because of the presence of large amounts of noise. For seismic techniques in particular, the presence of drilling, blasting, shearers cutting rock, pumping and other activities leads to high levels of background seismic noise. But this seismic noise interacts with the rock in the same way as energy from an active seismic source would do, so it could, in principle, be used to image the rock. Most sources of noise -- drills, shearers, pumps -- are continuous, so there is no well defined 'shot time'? and no well-defined direct or reflected arrivals in measured traces. However, a process of coherence-weighted cross-correlation across an array of sensors can produce a set of relative travel times, which can be used for tomographic imaging. Experiments have been done in a number of coal mines, using a coal shearer as a source of energy, and recording the signal on arrays of geophones, installed either in roadways underground, or on the ground surface above the mine. This data has been processed using various techniques to try to extract velocity and attenuation information. Results indicate that the technique is successful in extracting relative arrival times across an array from continuous noise.