AIRBORNE GEOPHYSICS - APPLICATION TO A GROUNDWATER STUDY IN BOTSWANA

A ground-water study carried out in the Serowe area of eastern Botswan a between 1985-1988 has provided the opportunity to evaluate the role of a multiparameter low-level airborne geophysical survey in a hydroge ological investigation. The survey included magnetic, VLF (very low fr equency), and coaxial EM (electromagnetic) measurements. In total, 7,5 00 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 aquife r, the Ntane Sandstone Formation (Karoo age), is confined between muds tones below and basalt above, and is broken into a series of graben an d horst structures by numerous E-W striking faults. All bedrock, howev er, is completely masked by a 20-60 m thick overburden of sands, calcr etes, silcretes, and sandstones known collectively as the Kalahari bed s. Airborne magnetic and VLF geophysical surveys have been used to pen etrate this masking cover. Images and stacked profiles obtained from t he survey revealed structural and geological features of major hydroge ological significance. This provided the information necessary for the formulation of a conceptual model.The results helped guide the subseq uent exploration drilling program in an efficient and effective manner , cutting down the need for extensive ground surveys. The investigatio n 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 i n confined sections of the aquifer, with production boreholes to be si ted, where possible, on fractures associated with VLF anomalies.