Abstract
Three crosshole seismic surveying techniques have been investigated using boreholes on opencast coal exploration sites in northern England. The objective has been to determine their potential for locating old mineworkings and small-scale geological structures. The first method was to propagate seam waves between boreholes, with source and receiver at seam level, to check for coal seam continuity. This method was found not to be viable because of the strong attenuation of high-frequency shear waves in shallow strata. The second and third methods shared a common field technique, with shots at several levels in one borehole recorded on an array of receivers in a neighbouring borehole. Processing the traveltime data for only the seismic first arrivals constitutes a tomography survey. Although careful processing produces a reasonable image of the velocity structure, the tomographic method is not sensitive enough to detect changes in seismic velocity due to subsidence above old workings. However, processing the reflected arrivals from one of the tomographic datasets has produced a seismic section with a vertical resolution of less than 2 m. It is suggested that, in conjunction with hole-to-surface seismic surveys, the crosshole seismic reflection method could complement exploration boreholes in a variety of exploration and site investigation applications.
