Microseismic monitoring and short term hazard assessments in underground coal mines

Large seismic events sometimes occur in underground coal mines - usually as the roof sandstone strata violently fail. Passive microseismic monitoring is a useful tool for monitoring the stability of these strata as mining progresses. The layered medium results in refracted seismic waves, and thus ray-tracing is used to more accurately locate seismic events once the P-and S-wave arrivals have been identified on a number of associated seismograms. Source parameters such as radiated seismic energy and co-seismic inelastic deformation can be routinely quantified for each seismic event. Often, microseismic event data exhibits precusory patterns before large-scale failures. These patterns range from simple increased seismic activity to simultaneous dropping of stress and accelerating deformation. Some examples from South African and Australian underground coal mines show that accelerating seismic activity correctly indicates large roof strata failures between 5 and 60 minutes in advance, in more than 75% of cases.