WHERE'S THE HEAT? A RE-EVALUATION OF THE COAL FIRE AT CENTRALIA, PA USING AERIAL AND GROUND THERMAL INFRARED IMAGERY SURVEYS

Based on surveys using aerial thermal Infrared imagery (TIR), estimates of the horizontal spreading rate of the coal fire located at Centralia, PA (USA) are less than 2 m per year. Aerial views show there has been little appreciable advance since surveys began in the summer of 2007. Periodic aerial TIR surveys have provided information concerning the history of fire movement since 1971, which has been compiled into a fire history map. The rate of fire movement appears to have slowed considerably over the last two decades; in the early 1980’s, the fire was estimated to move 6 m/yr. In addition, the temperature of gas vents has decreased over the past ten years from over 450° C (summer of 2000) to rarely over 65° C (summer of 2010).

Ground surveys using TIR now provide an intriguing picture of how the landscape and surrounding ecosystem relate to the fire hot spots and aerial surveys. Piles of rock represent the hottest locations in the burn area; these locations tend to lack vegetation. Temperature differences also occur depending on surface regolith. Coal and carbonaceous shale give off more heat than sandstone and conglomerate. As predicted by Ressler and Markel (2006), plant location is temperature dependent. Moss and the succulent weed purslane, which have shallow root structures, colonize hot regions while grasses and larger weeds tend to be found in cooler regions.

Ground surveys also confirm locations of subsidence. Circular to semi-circular patterns observed in aerial surveys have been identified as zones of subsidence. Since 2007, the ground along the leading edge of the fire has fallen approximately 1.0 m. One new hole is 0.4 m deep and 1 m wide, but concentric cracks around the hole suggest that this hole will potentially be up to 3 m in diameter.

The fire is traveling west, along the limbs of the westward plunging Locust Mountain Anticline, presumably burning the Buck Mountain (#5) coal seam 45 m below the surface. It has been burning since 1962. As it moves westward, the coal seam plunges deeper into the subsurface. This structural trend may be preventing abundant oxygen from reaching the fire, stifling it’s present movement and temperature. The change in rate of movement and decreased gas vent temperatures suggest a change in the coal fire and may suggest declining activity as the fire moves deeper into the subsurface.