Surface Geophysical Methods for Detection of Underground Mine Workings

Efforts to delineate underground mine workings in the vicinity of tailings impoundments has relied on available maps and confirmatory boreholes. Often the characterization of regions between boreholes remains suspect or entirely unknown. Surface geophysical measurements can be used to supplement the borehole data and better delineate these intermediate zones. Furthermore, geophysics can be used to optimize the number and locations of the boreholes. There are several surface geophysical methods that are applicable to detect subsurface voids: ● Gravimetry measures variations in the acceleration of gravity. The strength of this acceleration generally depends on the density of the underlying material. Less massive zones, such as cavities, generally constitute relative gravitational lows. ● Seismic measurements delineate reflections and refractions of compressional or shear waves off subsurface layers with differing densities and wave velocities. There is usually a strong reflection on a cavity boundary. ● Electromagnetic and Electrical methods measure changes in resistivity. Cavities alter the electrical resistance. Mine voids in coal mines are usually encountered as resistivity lows because of the presence of acidic mine water. ● Other Methods, such as magnetics and ground penetrating radar have been used for detecting subsurface cavities, but can be subject to precision, interference, or depth limitations which restrict their use in mining applications. Project experience with various surface geophysical methods demonstrates that commercially available technology can effectively detect and delineate mine workings. Two techniques have the greatest potential for application. The DC resistivity method offers the best potential for the rapid mapping mine workings at a depth of 50 – 100 feet or less. For workings at a depth of 50 – 100 feet or greater, the seismic reflection method, especially with the use of S-waves, has the greatest potential for success. Introduction Two recent events have focused national attention on the need for mapping underground mine workings, the failure of the Martin County Coal Corporation tailings impoundment near Inez, Kentucky on October 11, 2000 and the July 24, 2002 Quecreek Mine inundation that trapped nine miners for 77 hours in Somerset County, Pennsylvania. In both cases, unexpected conditions related to abandoned mines were the sources of the accidents. In Symposium on Geotechnical Methods for Mine Mapping Verifications, Charleston, West Virginia, October 29, 2002 2 the case of the Inez, Kentucky tailings impoundment failure, the overburden between an abandoned mine and the base of a slurry impoundment was too thin and the slurry broke into the mine. The consequence was that slurry broke out and flooded two separate watersheds with coal refuse. In the case of the Quecreek Mine flood, miners excavated into the flooded workings of the abandoned Saxman Mine, which they thought was hundreds of feet away. Both of the above conditions require that the location of underground workings be precisely defined. This is not an easy task. Detailed mine maps may be unreliable or missing. Conventional exploration (drilling) can easily miss targets as small as a mine entry. Because of these difficulties, the National Resource Council appointed the Committee on Coal Waste Impoundments whose charge included a task to evaluate alternative technologies to locate mine workings, as recently published in the book Coal Waste Impoundments, Risks, Responses and Alternatives (National Academy of Sciences, 2002). The Governor's Commission on Abandoned Mine Voids and Mine Safety convened in October 2002 as a result of the Quecreek inundation is also in the process of compiling information of the various geophysical technologies that could be applied to the mapping of mine workings. This paper does not repeat geophysical theory of the different technologies that can be found in the NAS book, but focuses on our experience with the practical application of some of these methods. Potentially Applicable Geophysical Technology The starting point of a geophysical investigation must be basic physics. Geophysics will be effective only if a target of interest has a physical contrast with the surrounding ground. For example, a mine entry containing metal tracks could be an easy target for a magnetic survey, but if the tracks are not present, the magnetic contrast of the entry might be too subtle to measure. Another important consideration is if the geophysical contrast of the target can be distinguished from other features with similar contrasts – what geophysicists call the signal to noise ratio. A mine entry might be relatively easy to identify with a gravity survey that can detect a void space as a zone of low density, but if the target is located in an area of rugged topography, the errors associated with the topographic corrections can easily mask the response from the mine workings. Notwithstanding the above difficulties, mine workings can be associated with measurable physical Physical Property Intact Coal Open Void Flooded Void Electrical resistivity high* high** usually low*** Seismic velocity low barrier low Density low very low very low