The crosshole seismic reflection method in opencast coal exploration

In opencast coal exploration in the UK, a dense grid of boreholes is drilled to provide the necessary information on the quantity and quality of coal reserves and on the geological structure. Typically, borehole spacing is 40-60 m, but it may be reduced around faults or where there are old room-and-pillar mineworkings. Even with this density of boreholes, it is not possible to detect faults with throws less than 2-3 m, and these can pose a hazard to stability during excavation which may be particularly critical at site boundaries. Also, estimates of the quantity of coal reserves could be improved if old mineworkings were located more accurately. The potential of borehole seismic surveys in this application was discussed in more detail in an earlier paper (Kragh el al. 1991). Here we report on the development of the crosshole reflection method, making use of opencast exploration boreholes, to provide seismic cross-sections between boreholes. Small explosive charges are used as sources with hydrophones as receivers. The use of downhole sources and receivers leads to excellent resolution compared with surface seismic and VSP surveys; signal bandwidth is typically 200-500 Hz. Readers interested in the application of cross-well surveys to the definition of hydrocarbon reservoirs will note that the environmental conditions for our surveys were relatively undemanding: low temperatures and hydrostatic pressures in uncased boreholes at relatively close separations. Nevertheless, the results indicate the potential of the method for deeper, larger scale surveys in sedimentary rocks, where even higher frequencies can be successfully transmitted (e.g. Harris 1988).