Offline Electrode(s) Effect on Two-Dimensional Electrical Resistivity Tomography Survey: Qualitative and Quantitative Assessment

The application of electrical resistivity tomography (ERT) for geophysical prospecting is on the increase due to its cost-effectiveness and capabilities of multi-electrode data acquisition. Multi-electrode resistivity systems use protocols that control the transition between the various electrode pairs. Most of these protocols used for a two-dimensional (2-D) electrical resistivity tomography survey assume that the survey lines are straight to ensure collinear electrode pairs. However, due to the presence of surface constraints associated with most of the survey areas, it is seldom possible to conduct resistivity surveys along a straight line. Consequently, uncertainties might creep into the result when such constraint is encountered. This research is aimed to assess the effect of the electrode(s) offline on a 2-D electrical resistivity tomography survey profile. In light of this, Wenner array configuration was adopted for the survey. The data was acquired with all electrodes inline, one offline, two offline, and three offline at stepwise distances, respectively. The qualitative assessment through the resistivity contour plots, revealed slight to distinct variation of anomalies as two or more electrodes are offline at a distance > 1/2 the minimum electrode spacing (a). The quantitative assessment carried out using Pearson's correlation coefficient, mean absolute error, and mean absolute percentage error yielded similar results with those obtained qualitatively. Hence, the study has revealed that there is a significant effect on the 2-D resistivity model when two or more electrodes are offline at a distance < 1/2 a.