Improvement of vertical component location of seismic events in underground mines

Location of seismic events is the first step to study seismicity in mines. Hypocenter location of mining tremors has fundamental meaning for credible evaluation of seismic and rockburst hazard. It is well known, that the distance from tremor source to openings and the size of seismic moment is crucial from various types of underground damage and safety point of view. Dangerous, high dynamic stresses and underground seismic hazards occur in near field wave only (Mutke 2007). Seismic networks in Polish copper and coal mines consist of scismometers located in one horizontal level and in this case correct location of seismic events in coordinates X, Y plane is possible. In such conditions good location in vertical direction Z is not possible. In the paper the way of hypocenter location of mining tremors with good accuracy is presented. The I irst condition to do it, is match receivers up to optimal geometry, it means to create three-dimensional seismic network. The second condition is to use average directional wave velocity for every receiver in location algorithm. The hypocenters location was based on P-wave's first arrival time and modified Powell algorithm. The presented calculations were conducted in field site in Rudna copper mine. The seismic network consisted of twenty seven underground receivers installed in exploitation level, five receivers installed in the shafts on the level between mining openings and surface and two receivers on the surface (three-dimensional seismic network). The maps of error location (standard deviation of Z component) for three-dimensional and two-dimensional networks were performed. Hypocenters of fifty one seismic tremors were calculated using two-dimensional network configuration and three-dimensional network configuration. Z components ofall located seismic events were correlated with location error of Z coordinate for two-dimensional and three-dimensional seismic networks. Application of three-dimensional seismic network and directional velocities decreased of Z component error location 4-5 times, to the values comparable with error location for horizontal components (less than 100 in). Three-dimensional location clearly showed that the most of hypocenters occurred from 100 in to 250 in above level of exploitation. Only a few seismic events were located on the exploitation level. For two-dimensional seismic network all hypocenters were located in vicinity of exploitation level. Better hypocenters location is very important and necessary to improve safety in mines with high seismicity. Method presented in the paper is very useful and can be put into practice in underground mines.