Subsurface coal-mine fires: Laboratory simulation, numerical modeling, and depth estimation

Subsurface coal-mine fires occur in many mining regions, especially where coal has been previously excavated by “room-and-pillar” mining methods. The surface above these fires heats up to produce a thermal anomaly. The shape of the temperature profile over the fire zone holds clues to the depth of the underground fire. We simulated an underground coal-mine fire in the laboratory by burying a hot glass tube in a sandbox. The thermal anomaly over the tube was recorded using a forward looking infrared radiometer (FLIR TM ) camera. Numerical modeling using finite-element techniques for various combinations of tube depth and tube temperature helped to empirically derive a depth-estimation function, called the linear anomaly surface transect (LAST) function. Comparisons of the results from the LAST function with the half-anomaly-width function for depth estimation developed by Panigrahi et al. (1995) showed that the LAST function gave more accurate results for shallow subsurface coal fires ranging in depth from a few centimeters to ∼10 m. For moderate-depth coal fires, ranging in depth from 10 m to 40 m, the depths estimated by the two functions were comparable.