Geophysical methods for lineament studies in groundwater exploration. A case history from SE Botswana

Abstract

Exploration studies for village water supply from basement and consolidated sedimentary rocks in Botswana show that groundwater occurrence is often restricted to linear structural features associated with faulting. The fractured aquifer which develops in this situation is characteristically channel-shaped, of narrow width and variable length.

The prevalent cover of Kalahari sands hampers considerably the detection of lineaments from satellite imagery.

By presenting aeromagnetic data as raster images, subtle changes in the magnetic field pattern can be resolved. Breaks or level changes due to structural features, such as faults or fracture zones, are displayed indirectly in airborne magnetic surveys. If satellite imagery and aeromagnetic data are processed in the same format (raster) and geographically referenced by the coordinates of grid points (pixels) they can be inspected simultaneously. The overlay of the two data sets by the use of a Geographic Information System helps to define linear features more accurately than in one data set alone.

The location of fracture zones associated with major lineaments can be mapped successfully on the ground with electromagnetic methods (VLF, HLEM). Multi-frequency horizontal loop systems proved to supply the best guidance to optimal borehole locations if the operating frequencies and the coil separations are properly adjusted to the local geological environment.

Even though an inclined borehole, sited according to the results of this investigation, did not intersect a major fault, the high degree of fracturing in a depth range of 50 to 100 m, together with the circulation losses encountered, correlate with our interpretation.