Electrical soundings with combined and symmetrical arrays over geological inhomogeneities: mathematical modelling and experimental examples

Abstract

Introduction: Electric soundings with direct current are widely used for the solution of geological and geotechnical problems by using four-electrode symmetric AMNB arrays, Schlumberger (S) and Wenner (V), which allows reliably determining the thickness and resistivity of layers for horizontally layered media. Geological inhomogeneities that are present in the media create edge effects which are reflected on the sections of apparent resistivity as additional anomalous areas and influence the accuracy of quantitative interpretation. To reduce edge effects, we suggested a method of electric soundings with the AMN+NМА В→∞ combined array that conducts double calculations in relation to the central receiving electrode (М). Methodology: We conducted mathematical modelling of soundings with the AMNB (S) and AMN + NMA arrays over local objects, a sphere and a hemisphere, approximating geoelectrical inhomogeneities. Experimental research was conducted on two sites using a combined and, for comparison, a symmetric array. Results: The calculated theoretical sections of apparent resistivity showed that the subsurface and immersed local inhomogeneities were distinctly contoured by the combined array with minimal display of edge effects. There were edge effects on the sections obtained using the symmetric array, and the lower border was poorly determined. Experimental examples found differences in the structure of the sections of apparent resistivity for the considered arrays, which depended on the geological features of the medium. The sections are similar in structure over the medium with horizontal stratification. The results of inversion mainly differed in the vertical thickness of layers. The sounding curves and the results of inversion may vary considerably over the bedrocks with great resistivity in the near-contact zones. Conclusions: Based on the conducted modelling and experiments, it can be concluded that the combined AMN+NМА array has some advantages as compared to the widely used AMNB array and can be used for electrical sounding of the upper part of the geological section as well as in electrical resistivity tomography of rocks.