The Use Of Multiple Geophysical Techniques For Site Assessment In Mine Reclamation

Abstract

Site assessment at a burning coal waste bank in Midvale, Ohio was conducted by the U.S. Bureau of Mines with five different geophysical techniques. The geophysical study was the first phase of a project to test a control method for subsurface fires. The site characterization was performed using terrain conductivity, magnetometry, ground penetrating radar and seismic surveys, as well as infrared imagery. Results from these five geophysical techniques were integrated to obtain information on the subsurface structure of the bank, the probable location of combustion zones, and the presence of underground water. Magnetometry measured magnetic anomalies due to changes in the magnetic properties of materials exposed to elevated temperatures in combustion zones. Electrical terrain conductivity identified changes in ground conductivity of the waste bank; anomalies were believed to be related to the presence of mineral-bearing water in fracture zones beneath the surface of the bank. Both ground penetrating radar and seismic methods determined waste bank structure and interfaces between dissimilar materials. The ground penetrating radar was effective to a maximum depth of approximately 3 m (10 ft); the seismic technique provided information to a depth of 8.3 m (27 ft). Infrared imagery detected areas of elevated temperature near the surface of the waste bank. These combined results indicated that the bank consisted of a surface layer of coal waste that was underlain by a core of another material. Several high temperature areas were identified on the eastern slope of the bank. Drilling and temperature surveys provide supplemental information and verified the results obtained with geophysical techniques. Site characterization that integrated the data from multiple geophysical techniques resolved uncertainties, with respect to subsurface conditions and the locations of heated zones and water saturated areas, that could not have been adequately explained with any single technique.