f Results Of Recent Ground-based Geophysical Surveys In The Okavango Delta

Quasi-3D inversions of an extensive helicopter time-domain electromagnetic (HTEM) data set acquired across the Okavango Delta (OD; Figures 1 and 2) yield 3- to 4-layer electrical resistivity models that include (1) a shallow resistive layer of dry and fresh-water-saturated sands, (2) an intermediate-depth conductive layer of intercalated saline-water-saturated sands and clay, and (3) a relatively deep resistive layer of fresh-water-saturated sands/gravels and/or crystalline basement. The upper resistive layer clearly represents unconsolidated sediments in the current alluvial fan-like environment, whereas the intermediate conductive layer likely represents sediments deposited under earlier lacustrine conditions. The top part of the deeper resistive layer has an intriguing fan shape, centred about the entrance to the main part of the delta (Figures 2e and f). If the fan-shaped portion of the deeper resistive layer comprises fresh-water-saturated gravels/sands, it would be evidence for a paleo-alluvial fan, which we refer to as the Paleo-Okavango Delta in the following text.