Seismic monitoring and analysis of excessive gas emissions in heterogeneous coal seams

• Over 2000 micro-seismic events were recorded at a longwall top coal caving panel.
• The spatial distribution of microseismic events was evaluated against face advance.
• An episode of increased gas emission was correlated with mining induced microseismic activities.
• A zone with heterogeneous coal structure was successfully detected by seismic monitoring.

Uncontrolled and excessive gas emissions pose a serious threat to safety in underground coal mining. In a recently completed research project, a suite of monitoring techniques were employed to assess the dynamic response of the coal seam being mined to longwall face advance at Coal Mine Velenje in Slovenia. Together with continuous monitoring of gas emissions, two seismic tomography measurement campaigns and a microseismic monitoring programme were implemented at one longwall top coal caving panel. Over 2000 microseismic events were recorded during a period of four months. Over the same period, there also was a recorded episode of relatively high gas emission in the same longwall district. In this paper, a detailed analysis of the processed microseismic data collected during the same monitoring period is presented. Specifically, the analysis includes the spatial distribution of the microseismic events with respect to the longwall face advance, the magnitude of the energy released per week and its temporal evolution. Examination of the spatial distribution of the recorded microseismic events has shown that most of the microseismic activity occurred ahead of the advancing face. Furthermore, the analysis of the gas emission and microseismic monitoring data has suggested that there is a direct correlation between microseismicity and gas emission rate, and that gas emission rate tends to reach a peak when seismic energy increases dramatically. It is believed that localised stress concentration over a relatively strong xylite-rich zone and its eventual failure, which was also identified by the seismic tomography measurements, may have triggered the heightened microseismic activity and the excessive gas emission episode experienced at the longwall panel monitored.