Prediction of DC Current Flow Between the Otjiwarongo and Katima Mulilo Regions, Using 3D DC Resistivity Forward Modelling and Magnetotelluric and Audio-magnetotelluric Data Recorded During SAMTEX

Abstract

SAMTEX (Southern African Magnetotelluric Experiment) is a multinational project that was initiated in 2003 to study the regional-scale electrical conductivity substructure of southern Africa and to infer from it the tectonic processes involved in the formation of the southern African subcontinent. Audio-magnetotelluric (AMT) data recorded during the most recent phase of the experiment are evaluated to investigate the local-scale conductivity substructure in the Otjiwarongo and Katima Mulilo regions, where in future the installation of high-voltage direct current (HVDC) earth electrodes will commence. Both of the AMT surveys are situated close to the edge of the orogenic Damara Mobile Belt in northern and north-eastern Namibia. Previous studies using magnetotellurics (MT), magnetometer arrays and geomagnetic observatory data all provide evidence of the relatively conductive nature of the Damara Mobile Belt. The Damara Mobile Belt represents in part the collision between the Congo and Kalahari cratons during the amalgamation of South Gondwana and its high conductivity is explained by the presence of interconnected fluids and conductive materials (graphites, sulphides). In contrast, the lithospheric structure of the ancient Archaean cratons, the Congo and Kalahari, are generally found to be electrically resistive and therefore it is hypothesized that the return path of DC current, flowing along the path of least resistance between the two electrodes, is most likely to lie somewhere within or in the vicinity of the Damara Mobile Belt. To obtain a better understanding of the current flow we propose using geological information, previous results of studies of the conductivity of the Damara Mobile Belt and surrounding regions and 2D inversion results from the AMT and MT data recorded during SAMTEX in northern Botswana and Namibia, as input to a 3D DC resistivity forward modelling code, and try to predict the return path that the DC current will follow.