Coal bumps are brittle, violent failures associated with high stresses and competent host strata. To study bump mechanisms, conditions in the vicinity of a deep longwall mining face in bump-prone strata were monitored using three discrete systems. First, a microseismic monitoring network recorded mininginduced seismic events throughout the mine and surrounding strata. Second, pressure variations and distributions on the longwall shield legs across the face were recorded. Last, tomography surveys imaged seismic transmission properties ahead of the face. Results from the three systems were correlated in an effort to increase understanding of mining-induced stress redistribution and bump potential. An initial, one-week study showed that the correlation between averaged tomogram values and seismically active areas had a coefficient of correlation (R) of 0.89. Further results of these studies demonstrate that the tomography system is capable of imaging heavy shield-leg loading and bump-prone conditions prior to them disrupting the face operations.
The Mine
A modem longwall coal mine is a highly mechanized, capital intensive, system, typically producing more than 10,000 tonnes of coal per day with as few as 10 to 15 people operating the face equipment. Any capital-intensive operation must be continuously utilized to optimize the investment. Further, the profit margin for a longwall face is typically five times that of a room and pillar face. Any loss in production time, therefore, quickly translates into lost profits for the mine. Coal bumps, comparable to rock bursts in non-coal mines, are localized seismic events which can rapidly and forcefully eject many tons of coal into the mine openings. These events not only disrupthe mining process, they can also be extremely dangerous to underground personnel. The longwall coal mine at which the monitoring occurred is located in the western United States. The seam is under approximately 800 m of massive competent overburden with a history of mining-related seismicity. Three systems were used to monitor the ground conditions at this underground longwall mine: a microseismic monitoring network, a seismic tomography system and a longwall shield-leg pressure recording system. Results from the monitoring systems were correlated in an effort to determine whether the diverse information could be used synergistically to increase understanding of mining-induced stress redistribution and bump potential.
