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Symposium on Geotechnical Methods for Mine Mapping Verifications Charleston, Nest Virginia October 29, 2002 plore for oil and gas. It is much less commonly applied in environmental and engineering projects because of its relatively high cost. Contrary to most other methods, shallow stud- ies are more expensive than the deep surveys conducted for oil and gas exploration be- cause of the need to make closely spaced measurements. Nevertheless, the method of- fers the possibility to define subsurface structure beyond the ability of other methods. Seismic reflection has been applied to mapping the continuity of coal seams in advance of longwall mining, especially in Europe where mining is commonly at depths greater than 1000 feet and it is difficult and expensive to characterize a coal seam with borings. The method has also been successfully applied to the mapping of mine voids, but the experi- ence base is limited and few practitioners have the skills to properly conduct this type of surrey. Further information on the application of this technique to map underground mines is presented with some of the Case Histories. DC Resistivity - The purpose of DC electrical surveys is to determine the subsurface resis- tivity distribution by making measurements on the ground surface. From these measure- ments, the true resistivity of the subsurface can be estimated. The ground resistivity is re- lated to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations, but the use of this method has been recently increased due to improvements in both data acquisition and processing technologies. Multi -electrode systems have greatly improved the efficiency of data acquisition, as measurements can now be made automatically. Until recently, the DC resistivity method was limited by the need to perform complex calculations to model sub- surface electrical properties. With the development of high-speed PC computer systems and improved 2D and 3D processing software, however, this limitation has been greatly reduced and the technique has seen increased application, including for the detection of subsurface openings. Further information on the application of this technique to map un- derground mines is presented with some of the Case Histories. Electromagnetic (EM) methods— EM methods also offer the possibility to map the distribu- tion of subsurface electrical properties. Unlike the DC resistivity method, however, it is not necessary to insert electrodes in the ground and surveying is more rapid. Disadvantages of the EM methods are that they are generally not as good as the DC resistivity method in resolving variations of electrical properties with depth and they are more subject to cultural interference, such as from electrical lines and metallic objects. For these reasons, EM methods are most commonly used to rapidly measure lateral variations of soil electrical properties, as well as to delineate the distribution of metal. EM measurements could be used to map shallow workings under favorable conditions (uniform cover, absence of cul- tural interference), but there are few situations where the application of this technology is practical. Ground Penetrating Radar (GPR) - GPR offers the highest resolution of any geophysical method. In many cases, the time required for the acquisition of GPR profiles is minimal and subsurface profiles can normally be obtained in real time, making this tool very cost- effective. The main limitation of the GPR technique is depth of penetration under condi- tions commonly encountered in areas with coal workings. GPR works best in non- conductive soils, such as drysand or sand saturated with fresh water. The soils commonly encountered in coal mining area are clays weathered from the claystones associated with M