Prediction of DC current flow between the Otjiwarongo and Katima Mulilo regions, Namibia
Date
2013-02-14
Authors
Share, Pieter-Ewald
Journal Title
Journal ISSN
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Abstract
As an additional opportunistic component to the Southern African Mag-
netotelluric Experiment (SAMTEX), audio-magnetotelluric (AMT) data were
acquired during the most recent phase of the experiment (Phase IV) to inves-
tigate the local-scale conductivity substructure in the Otjiwarongo and Ka-
tima Mulilo regions (Namibia), as to aid in the installation of high-voltage
direct current (HVDC) earth electrodes that has since taken place. Both
of the AMT surveys are situated close to the edge of the orogenic Neo-
Proterozoic Damara Mobile Belt (DMB). Previous studies all point to the
existence of a highly conductive mid-crustal zone which correlates well with
the spatial location of the DMB. Two-dimensional (2D) inverse modelling
of the Otjiwarongo AMT data con rms the existence of the high conductive
zone at mid-crustal depths (10-15 km). The high conductivity of the DMB
is explained by the presence of interconnected graphite in the marble units
present. The Katima Mulilo inversion results are characterized by a con-
ductive upper crustal layer that does not form part of the DMB conductive
belt. It is deduced that at the uppermost subsurface Kalahari sediments are
responsible for the high conductivity observed while at greater depth it is
due to ironstone within the Ghanzi Group. In contrast to the conductive
DMB, the lithospheric structure of the neighbouring Archaean cratons, the
Congo and Kalahari, are generally found to be electrically resistive. There-
fore, it is hypothesized that ground return current, if present, will
ow along
a path between the Otjiwarongo and Katima Mulilo regions that lies either
exclusively, or almost entirely within the DMB. The hypothesis is tested
by inputting a three dimensional (3D) conductivity model (calculated using
available magnetotelluric (MT) data and geological information) of the re-
gion into a DC resistivity forward modelling code. Forward modelling shows
that the return current is only con ned to, and follows regional trends char-
acteristic of, the conductive DMB for approximately 200-300 km away from
the injection point, after which there is no preferential flow.