Microseismic techniques for monitoring incipient hazardous collapse conditions above abandoned limestone mines

Abstract

Abstract The progressive collapse of abandoned limestone mines over many decades has led to problems of general subsidence and crown-hole formation in parts of the West Midlands. The substantial size and extent of the voids makes the cost of infilling all of the mines prohibitively expensive, at least in the short term. One option is to monitor the workings to identify active areas and assess the rate of collapse. This would provide government and local authorities with additional criteria for establishing priorities for remedial action. Monitoring of the condition of the mines is made difficult by their general inaccessibility, but it was thought that the detection of microseismic energy associated with rock failures and roof-falls might provide a useful method of remote monitoring. This paper summarizes a series of studies designed to develop and field test a method of monitoring microseismic events and discriminating them from the many other seismic sources. Experiments involving simulated roof-falls in an accessible (air-filled) mine indicated that the most detectable source of seismic energy is generated by debris impacting on the mine floor, rather than by stress-induced rock failure. Consequently, the energy is in a lower frequency range (around 10 to 300 Hz), than was at first anticipated. Trials using both continuous and triggered recording, with a variety of transducers, installation techniques and network configurations, provided useful information regarding optimum network design for the discrimination of roof-falls. On the basis of these findings a seismic network was successfully implemented to monitor roof-falls during a mine infill trial. Finally, an experimental network of instruments in boreholes at a water-filled mine demonstrated, by means of artificial seismic sources, that the technique can also be applied to flooded, inaccessible mines.