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

The 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 models. Resistivities of the upper resistive layer are similar in the various models, with the ground-based estimates being somewhat higher than those of the HTEM model at one site and somewhat lower at the other site. For the basal resistive layer, it can only be concluded that its resistivity must be substantially higher than that of the overlying conductive layer. An interpretation of the HTEM and ground-based resistivity models in the delta's north-central region, appropriately constrained by the surface geology, high-resolution seismic refraction-reflection models, and borehole logs suggests the following structure: basement overlain at progressively shallower depths by freshwater-saturated sand and gravel that represent the remnants of a Paleo Okavango Megafan, saline-water-saturated sand, and lacustrine clay originally deposited in Paleo Lake Makgadikgadi, and freshwater-saturated megafan and fluvial sediments of the current Okavango Delta.