The influence of angled survey lines on 2D ERT surveys using the Wenner (α) array with implications for groundwater exploration in Karoo rocks

Abstract Two-dimensional (2D) electrical resistivity tomography (ERT) surveying systems typically use standard protocols that assume straight survey lines and collinear electrodes. However, due to the presence of infrastructures and other surface constraints, it is not always possible to conduct 2D ERT surveys along straight lines. When survey lines are angled, the apparent resistivity data are affected for measurements where electrodes occur on either side of the position of the angle, since the assumed distances between some of the electrodes are larger than the true distances. This study investigates the impact of angled survey lines on 2D ERT data and inverse resistivity models for the Wenner (α) array. Theoretical considerations show that measurements along angled survey line are sensitive to resistivity changes at position laterally displaced from the survey line, while both the calculated apparent resistivities and the depths of investigation are affected when electrodes straddle the angle. The resulting errors are, however, small. Even for a large angle of 45°, the errors are smaller than 6.5% for the affected apparent resistivity measurements, and smaller than 3.8% for the median depths. Since the apparent resistivity data and the pseudo-depths are affected by angles in the survey lines, the inverse resistivity models are also affected. Numerical modelling shows that even small input errors may be propagated (both in magnitude and spatially) during inversion, thereby distorting the resulting resistivity models. The distorted inverse resistivity models could lead to the incorrect interpretation of the locations, geometries and orientations of subsurface structures, which could lead to poorly sited production boreholes. The results of field surveys show that apparent resistivity data sets recorded along angled lines may be partially corrected by calculating the true geometric factors and by adjusting the apparent resistivity data accordingly. Significantly improved inverse resistivity models are found for corrected data sets.