2D Electrical Resistivity Tomography (ERT) Survey using the Multi-Electrode Gradient Array at the Bosumtwi Impact Crater, Ghana

Abstract

The 10.5 km diameter Bosuntwi impact crater in Ghana is occupied by a lake of about 8.5 km in diameter. The multi-electrode gradient array has been used to carry out 2D electrical resistivity tomography (ERT) survey at different locations around the crater. The 2 m take-out cable of the ABEM LUND Resistivity Imaging System was modified to function as a 5 m take-out. 2D electrical resistivity survey was carried out along six (6) radial profiles running from the shore of the lake towards the crater rim. The least-square inversion technique was used to invert the topographically corrected data. The area extending from the lake shore towards the crater rim contains essentially three formations: the low resistivity regions from the shore of the lake towards uphill with resistivities < 64 W.m representing the lake sediments; the moderately high resistivity regions with values between 128 and 200 W.m interpreted as impact related breccias such as dikes, allochthonous or parautochthonous depending on their geometries; lastly, the model clearly differentiates the resistive basement metamorphic rocks of resistivities > 128 W.m from the lake sediments and the breccias due to their geometry and lateral extent. The ERT models allowed us to locate faults and fractures and also the thickness of the post impact lake sediments and the breccias. The results showed that the cables take-outs of the multi-core cable can be modified to suit the requirements of a particular survey thus highlighting the utility of this technique in impact cratering studies and geo-electrical imaging studies in general.

Keywords: impact crater, target rock, electrical resistivity tomography (ERT), multi-electrode gradient array, roll-along technique