AMT MEASUREMENTS COMPARED WITH GRAVIMETRY AND MAGNETOMETRY FOR STRUCTURAL STUDY OF A SEDIMENTARY BASIN - LETLHAKENG BOTLHAPATLOU GROUNDWATER PROJECT, BOTSWANA

Within the framework of hydrogeological studies for additional water s upply to the main towns of Botswana, the BRGM company has carried out a multimethod geophysical survey of a large sedimentary basin in the K alahari desert, for the purpose of assessing its water resources. On a regional scale, gravimetry, magnetometry and audio-magnetotellurics ( AMT) were used to determine the structure of the basin, which is inten sely faulted into horsts and grabens. Next, a number of favourable are as were selected for detailed profiling with horizontal portable loop EM (HLEM) and DC electrical arrays to locate accurately the fractured zones at the edges of grabens, the targets for exploratory drillholes. The AMT method proved to be the most effective for mapping the baseme nt of the basin, thanks to the good resistivity contrast between the r esistive basement and its conductive cover, and to the regional consta ncy of sedimentary organisation of the cover. The presence of the unif ormly resistive Kalahari Sands at the top of the cover was also very f avourable, protecting AMT data from troublesome ''static'' effects. Th ese good conditions enabled a fast and innovative quantitative interpr etation of AMT soundings, determined by borehole calibration and based on the reading of the apparent resistivity at a single frequency (10 Hz).