Interborehole acoustic measurements are made by transmitting a pulse of sound energy from one borehole to another. Analysis of the recorded signals can provide useful information on ground conditions between and beneath the boreholes. This includes the location of boundaries between different rock and soil types, and an assessment of geotechnical properties in certain cases. A sparker source of energy has been developed for use in boreholes, giving a repetitive pulse shape. The received signal may contain direct, refracted and reflected arrivals, which can be timed. These times are converted to apparent velocities and a vertical scan of two boreholes presented as an apparent velocity versus depth curve.

Two case histories from the Dinorwic hydro-electric scheme in North Wales and from Whatley Quarry near Frome in Somerset, illustrate the nature of the interpretation process, and the quality and distribution of the additional geological data that can be achieved. Interborehole scanning has certain advantages over conventional surface seismic refraction; for example, in the detection of low velocity layers at depth. It does of course have its limitations, in the theoretical, interpretational and the practical senses. With these limitations minimized, interborehole acoustic measurements can be a very useful addition to site investigation.

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