Airborne Geophysics: Application to a Ground‐Water Study in Botswana

Abstract

SummaryA ground‐water study carried out in the Serowe area of eastern Botswana between 1985–1988 has provided the opportunity to evaluate the role of a multiparameter low‐level airborne geophysical survey in a hydrogeological investigation. The survey included magnetic, VLF (very low frequency), and coaxial EM (electromagnetic) measurements. In total, 7,500 line kilometers were flown over an area of 3,300 km2 with a nominal ground clearance of 20 m and a line spacing of 400 m.The main aquifer, the Ntane Sandstone Formation (Karoo age), is confined between mudstones below and basalt above, and is broken into a series of graben and horst structures by numerous E‐W striking faults. All bedrock, however, is completely masked by a 20–60 m thick overburden of sands, calcretes, silcretes, and sandstones known collectively as the Kalahari beds.Airborne magnetic and VLF geophysical surveys have been used to penetrate this masking cover. Images and stacked profiles obtained from the survey revealed structural and geological features of major hydrogeological significance. This provided the information necessary for the formulation of a conceptual model.The results helped guide the subsequent exploration drilling program in an efficient and effective manner, cutting down the need for extensive ground surveys. The investigation confirmed the availability of a 35,000 m3/day resource, sustainable for a 25‐year period. Highest yields were obtained from fracture zones associated with VLF anomalies. Potential wellfields were identified in confined sections of the aquifer, with production boreholes to be sited, where possible, on fractures associated with VLF anomalies.