Geophysical Examination of Coal Deposits

摘要

In recent years, the coal industry has focussed its attention on geophysical techniques in the solution of mining problems and as an aid to mine planning. One important application of geophysics is in the various techniques of geophysical borehole logging. Since there are large contrasts in physical properties between coal and the other rocks normally found in a coal deposit, the responses of the coal seams to logging tools, each measuring a different physical characteristic of the rocks, may be easily recognized on the logging records. Based upon information from the logs, cores for laboratory analysis are only cut from selected sections of the borehole. In addition, information about the density and chemical composition of the coal may be obtained. Another geophysical technique which has been used with great success in coal deposits is seismic surveying. Surface seismic reflection surveys using two-dimensional arrays of seismic sources and detectors have been found in areas of simple geology to be capable of detecting faults with throws as small as 3 m at depths of 800 m. In geologically disturbed areas, good results have been obtained from three-dimensional surveys. Still smaller faults as far as 500 m in advance of the working face may be detected using in-seam seismic surveying conducted from a roadway or working face. Small disturbances have also been detected successfully by pulse radar and continuous wave electromagnetic methods either from within boreholes or from underground. The range of these techniques is at present smaller than that of the seismic techniques. Other geophysical techniques which exploit the electrical, magnetic, gravitational, and geothermal properties of rocks have been found capable of locating abandoned mine shafts and other subsurface cavities at old mining sites and of delineating areas of clinkered rocks. (ERA citation 06:021545)