Abstract

In areas of shallow crystalline bedrock lineaments have been commonly targeted for the siting of water boreholes, although the nature of these extensive geological features is not fully understood. The study employing borehole geophysical techniques attempted to examine the hydro-geological conditions around a typical lineament and to help explain the reasons for dry boreholes drilled close to these features.

Both vertical and inclined drilling were undertaken in the vicinity of the lineament and the rock characteristics were investigated by both borehole geophysics and core inspection. The study showed a distinct rock quality variation across the lineament which marked the contact between a more massive, friable granite and a weathered granite.

Crosshole seismic tomography suggested, in this case, that the lineament represented a displacement fracture with a downthrow of around seven metres. This displacement could account for the considerable disparity in water levels observed across a relatively short distance and confirmed that fracturing controlled the yield of the boreholes.