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

Abstract

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.

Multiscale geophysical imaging of the critical zone

“…For example, in transitional coastal environments, AEM data have been used to map saltwater intrusion at scale of over tens to hundreds of square kilometers to study processes where fresh and saline surface waters mix with groundwater [Fitterman and Deszcz-Pan, 1998;Viezzoli et al, 2010]. Similar applications of AEM to inland environments revealed the presence of saline sediments in an aquifer system related to a paleo-Okavango Megafan [Meier et al, 2014]. Slater et al [2010] combined waterborne ERT/IP imaging with FO-DTS to investigate groundwater-surface water interaction along~3 km reach of the Columbia River, Washington (Figure 4).…”

Landscape responses to intraplate deformation in the Kalahari constrained by sediment provenance and chronology in the Okavango Basin

“…The structural framework of the Okavango Basin that is characterized by ongoing neo‐tectonic activity (Cooke, 1980; Gumbricht et al., 2001; Haddon, 2005; McCarthy, Green & Franey, 1993; McFarlane & Eckardt, 2007; Shaw, 1985) is well‐defined by satellite and aerial imaging (Hutchins, Hutton, Hutton, Jones, & Loenhert, 1976; Mallick, Habgood, & Skinner, 1981; McCarthy, 2013; McFarlane & Eckardt, 2007), and aeromagnetic and gravity surveys (Campbell, Johnson, Bakaya, Kumar, & Nsatsi, 2006; Kinabo et al., 2008; Meier et al., 2014; Modisi et al., 2000; Podgorski et al., 2013). A time frame of formation and evolution of accommodation space, on the other hand, is poorly constrained (Haddon, 2005; Nash & Eckardt, 2016).…”

Multidimensional Investigation of Bedrock Heterogeneity/Unconformities at a DNAPL‐Impacted Site

“…Gabriel et al () used a combination of seismic, gravity and electrical methods to delineate extent of subglacial valleys and impacts of salt intrusion on the groundwater reservoirs. A number of other studies have shown the strength of multidimensional and multiscale geophysical applications to flow path delineation (e.g., McClymont et al ; Cassidy et al ; Meier et al ), contaminant transport and fate (e.g., Atekwana and Atekwana ; Konstantaki et al ), and karst characterization (e.g., Martínez‐Moreno et al ). While these previous studies demonstrate the capacity of geophysical techniques to understand various hydrogeological and geological problems, few have shown how an integrated, multidisciplinary and multiscaled approach can generate a robust conceptualization of a heterogeneous sedimentary bedrock environment, exhibiting multiple geologic unconformities with contrasting sedimentary rock types and characteristics, with varying hydrological impacts associated with dense nonaqueous phase liquid (DNAPL) migration and source evolution.…”