The U.S. Bureau of Mines has actively conducted microseismic research in coal and hard rock mines for many years. The purpose of this research is to provide the mining industry with reliable techniques to warn of imminent catastrophic ground failures so that miners may implement control measures or evacuate dangerous areas prior to their failure. This report examines the use and benefits of a three tier microseismic monitoring system which includes a surface seismograph, a mine-wide system, and local geophone arrays. The direction that a Bureau research effort is taking in developing a fully digital automatic system is also outlined.
Literally, a microseism is a "small earthquake." The word is derived from the Greek words "micros," meaning small, and "seismos," meaning earthquake. As detected in a mine, a microseism is sometimes an audible, sometimes subaudible, noise resulting from vibrations introduced into the mine structure by rock fracturing or by slippage along planes of weakness in the rock mass. There is no strict magnitude definition of a microseism. On the lower end of the scale, a practical limit is the sensitivity of the actual detection system installed; on the upper end, most all mine related bumps or bursts can be included because they are considered small by earthquake standards. However, some large mining induced rock bursts, which can be classified as earthquakes induced by mining operations, have been reported to have Richter magnitudes greater than 5 (Mendecki et al. 1988). A mountain bump is a sudden, and sometimes catastrophic, ground failure within a coal mine. This type of rapid failure is capable of causing injuries or fatalities to miners in addition to damaging working sections in a mine. A rock burst is a similarly violent failure of an area in a hard rock mine. A coal mine outburst is a sudden release of energy accompanied by a large volume of gas, usually methane, and coal dust. In addition to the potential for worker injury from flying rock, outbursts can leave an explosive mixture of methane, air, and coal dust in the mine air. For example, on April 15, 1981, 15 miners died as a result of an explosion after an outburst in Dutch Creek Mine near Redstone, Colorado (Elam et al. 1981). Microseisms are detected by securely attaching several velocity gages or accelerometers to the rock surrounding the area of interest in a mine. Since these transducers are "listening" for rock vibrations, they are generically called "geophones." Vibrations travel through the mine at the speed of sound in rock, which usually varies along the path of the sound wave. When a rock noise, or microseism, is detected by several geophones, the origin of the noise can be calculated from relative arrival times of the signal at different geophones. Since sources of microseismic activity are caused by rock fracturing or slippage along faults, locating and plotting microseismic sources can point to areas of a mine adjusting to changing stresses induced by mining.
