Hydrogeophysical investigations in the western and north-central Okavango Delta (Botswana) based on helicopter and ground-based transient electromagnetic data and electrical resistance tomography

ABSTRACTThe Okavango Delta is a huge alluvial megafan in northwestern Botswana. Despite numerous geologic, geochemical, geophysical, and hydrologic investigations over the past half-century, the sedimentary units underlying the delta are largely unknown. To address this issue, helicopter transient electromagnetic data (HTEM) have been collected across the entire delta and coincident ground-based electrical resistance tomographic (ERT) and transient electromagnetic (TEM) data have been acquired at two locations, one along the delta’s western margin and one in its north-central region. Inversions of the HTEM data have yielded three-layer resistivity models in which a relatively homogeneous conductive layer is sandwiched between two resistive layers. The three-layer HTEM model is reproduced in models obtained from independently and jointly inverting the ground-based data. The conductive layer’s low resistivities and depths to its upper and lower boundaries are practically equal in the HTEM and ground-based m...