ky0311 - Geophysical Technologies for Detecting Underground...

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Geophysical Technologies for Detecting Underground Coal Mine Voids, Lexington, KY, July 28-30, 2003 APPLICATIONS OF THE ELECTRICAL RESISTIVITY METHOD FOR DETECTION OF UNDERGROUND MINE WORKINGS by William J. Johnson – D’Appolonia, Monroeville, PA Introduction The need to understand the location of abandoned workings has recently been high- lighted by 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. On a more routine basis, subsidence from abandoned mines represents a significant public cost in many states. Just in Pennsylvania, the State has invested over $100 million to address mine subsidence problems and estimates that 200,000 acres of high priority subsidence prone land still remain to be stabilized. Abandoned mines are often difficult to locate. Detailed mine maps may be unreliable or missing. Conventional exploration (drilling) can easily miss targets as small as a mine entry. Nevertheless, geophysical methods are seldom employed to help map aban- doned mines, possibly because geophysics surveys are perceived to be too expensive or will not help to solve the problem at hand. In actuality, there are situations where a geophysical survey can be expected to be effective and other times when the results may be more problematic. This paper reviews both the theoretical and practical aspects of electrical resistivity methods to define abandoned mine workings with the intent of de- fining conditions where the technique can be expected to produce useful results and the limitations of electrical measurements. DC Electrical Surveys The purpose of a DC electrical survey is to determine the subsurface resistivity distribu- tion of the ground, which can then be related to physical conditions of interest such as lithology, porosity, the degree of water satu- ration, and the presence or absence of voids in the rock. The basic parameter of a DC electrical measurement is resistivity. Resis- tivity is not to be confused with resistance. Resistance (R), measured in ohms, is the result of an electrical measurement, where according to Ohm’s Law: V = I/R or R = V/I where V = voltage in volts and I = current in amps. RESISTANCE (R) IMPRESSED VOLTAGE CURRENT FLOW Sketch of parameters to define resistivity
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Geophysical Technologies for Detecting Underground Coal Mine Voids, Lexington, KY, July 28-30, 2003 2 Resistivity of a material is a fundamental physical property related to the ability of a ma- terial to conduct electricity. If R is the resistance of a block of conductive material hav- ing length L and cross-sectional area A (see sketch), then resistivity is given as: ± = RA/L Resistivity measurements of the ground are normally made by injecting current through two current electrodes and measuring the resulting voltage difference at two potential electrodes.
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ky0311 - Geophysical Technologies for Detecting Underground...

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