Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 391, 1991 Sep; :1-2
Objective: Detect hazards associated with abandoned mine openings through the use of integrated geophysical technology. Background: Many abandoned mines in the U.S. present hazards to the public. The first step in abatement of those hazards usually requires locating the abandoned workings. Recent Colorado legislation resulted in a concerted effort aimed at mitigating the hazards associated with abandoned mine workings. The Colorado Department of Natural Resources-Mine Land Reclamation Division (CDNR-MLRD) is responsible for locating and backfilling abandoned mine openings such as shafts, adits and collapsed features. Many shafts and adits can be located by inspection, other openings give little or no surface expression and must be located by drilling, or through the use of geophysical methods. A common method used for locating underground mine workings is to drill throughout the suspected area. However, it is prohibitively expensive to drill holes at the spacing needed to detect backfilled shafts and small voids associated with caving and/or chimneying. It may also be dangerous to move heavy equipment over shallow mined out workings. Geophysical investigations that employ only a single method have had mixed success in detecting hazardous abandoned mine land cavities. In operation with CDNR-MLRD, the Bureau of Mines selected the Canon Imperial abandoned coal mine site, located approximately 3 miles southwest of Florence, CO, to test an integrated geophysical approach to abandoned mine detection. These mine workings were developed during the mid 1800's, with the main sloping entry following the dip of the coal seam. The normal mode of operation was to drift out from the haulage way removing only the "good" coal. If the coal appeared to be of good quality, rooms then were then excavated. The caving and subsidence associated with these abandoned workings prevents reclamation activities using heavy surface equipment. A geophysical approach was needed for locating abandoned mine workings and presented an opportunity for the Bureau to investigate an integrated geophysical approach to the problem. Approach: In an effort aimed at mitigating a broad range of problems common to shallow coal mine workings, the Bureau, in cooperation with the CDNR-MLRD, conducted an integrated geophysical approach using complementary geophysical methods to more accurately detect and delineate abandoned mine openings and associated hazards. The combination of ground penetrating radar (GPR), dipole-dipole and pole-dipole resistivity configurations, and microgravity was used for locating the position of a known open and backfilled haulageway and a chimneyed drift. Since computer modeling of the site indicated that only a relatively poor penetration depth of approximately 2 m could be achieved in the highly conductive coal mine environment, GPR was eliminated as a plausible geophysical field method. Hence, at this site, the integrated geophysical approach included a combination of resistivity and microgravity. How It Works: The integrated geophysical approach is recommended in the initial phase in the detection and delineation of abandoned mine sites so that remedial measures may be safely undertaken to mitigate these hazards. The combination of microgravity, pole-dipole and dipole-dipole resistivity techniques were used to corroborate interpretations of an open and backfilled haulageway and a chimneyed drift. Multiple geophysical methods provide the necessary interpretive constraints for accurate location of abandoned mines and related features. The gravity surveying involved the measurement of variations in the gravitational field of the earth which varies over the surface of the earth depending on topography, elevation latitudinal position, earth tides, and subsurface density. In cavity detection, a negative density contrast can be expected due to the mass deficit associated with an open space. Under ideal conditions, the gravitational field can be measured to a precision of 1 part in one billion. This magnitude of precision is required in effective mine mapping, since the targets are often very small; resulting in subtle gravity anomalies. Measuring the earth electrical resistivity (reciprocal of conductivity) is similar to determining the resistance in a circuit, except that the spacing between electrodes is taken into account. A conventional operation requires four electrodes. Two electrodes introduce direct current into the subsurface, while the other two measure the resulting voltage. The recorded voltage, amperage, and spacing associated with the electrode configuration are then used to calculate an apparent resistivity based on an appropriate mathematical expression. An air-filled cavity produces a positive resistivity contrast, since the resistance of air is infinitely high. Conversely, a water-filled cavity may produce a negative anomaly depending of the amount of dissolved solids in the water as compared to the amount of dissolved solids in the fluids in the host rock. The combination of caving and/or backfilling, i.e. the degree of disruption, results in less pronounced resistivity changes.
Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 391