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Geophysical Exploration |
Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK
Evaluation of opencast coal reserves in the UK is accomplished by drilling boreholes on a grid spacing of 40–60 m, with even closer spacings across major fault zones and in areas of old mine workings. In spite of this intensive drilling effort, it is difficult to detect small faults with throws of less than about 2 m, which can still present a serious stability hazard at site boundaries. Borehole seismic methods could be used to detect small faults, to give structural detail across major fault zones and to locate old mine workings. Two techniques have been developed for this purpose: cross-hole seismic reflection surveying and hole to surface seismic reflection surveying. Although the cross-hole method produces sections of extremely high resolution, very close source and receiver element spacings are required to image close to the boreholes. Also the borehole separation should be less than half the length of the source and receiver arrays if complete coverage is required between the arrays. The hole to surface method produces high resolution sections and could be extended to three-dimensional surveys, but imaging is limited to a bell-shaped cross-section with its apex just below the water-table. Both methods can be used to yield continuous sections along lines of boreholes and are potentially of value in other applications.
