Groundwater exploration with AEM in the Boteti area, Botswana

Abstract

AEM data acquired with the TEMPEST system in the Boteti area, Botswana, were used to map lithologies and structures for potential fresh groundwater. The survey area is characterised by the Makgadikgadi paleolake system bordered by elevated terrain to the south. Lower-lying areas are characterised by a thick layer of Kalahari Beds with saline groundwater generally situated less than 20 m below surface. Elevated areas are generally characterised by Karoo sediments, consisting of sandstones and mudstones, at shallow depths. Within the paleolacustrine terrain, target aquifers are freshwater zones associated with recharge pans above a saline water table. The correlation of AEM-derived conductivity-depth profiles with borehole records shows that the conductivity of Kalahari Beds is primarily a function of clay content, water saturation, and water salinity. The shallow conductivity structure from modelled AEM data outlines several resistive zones, some of them located within Boteti River alluvium beneath the present and past river channels. In the elevated terrain, target aquifers are sandstones lying between dry alluvium or basalt, and mudstone. Basalts are characterised by very low conductivities and a strong magnetic response. The underlying sandstones and mudstones have low and high conductivities, respectively, and are offset by horst and graben structures, which are mapped in both the magnetic data and conductivity-depth sections. Resistivity iso-surfaces generated from the 3D conductivity data facilitate the spatial appreciation of the geometry of modelled structures. Favourable locations for freshwater exploitation interpreted from TEMPEST data include faults, dykes, a paleochannel incised into mudstone, and shallow fractured sandstone especially when overlain by basalt.