Geoelectrical modeling of shallow structures using parallel and perpendicular arrays

Abstract

In this article we analyze the sensitivity of a geoelectrical modeling technique to image 2D shallow structures. Firstly, we extend a previously developed 2D method based on Rayleigh-Fourier expansions, in order to allow arbitrary locations for the electrodes and also 3D earth models. This method is an alternative to finite element and finite difference techniques and is especially suitable to model multilayered structures, with smooth irregular boundaries. Then, for simple 2D models we build up two synthetic pseudosections, one for electrode deployments parallel to a profile perpendicular to the strike, and other for deployments perpendicular to it. We analyze the advantages in using both pseudosections to model these structures. We also compare geoelectric results with the corresponding audiomagnetotelluric transverse electric and transverse magnetic responses. Finally, we perform a geoelectrical survey to image a shallow buried structure and show the goodness of the model fit obtained considering both pseudosections. For the examples studied here, we conclude that considering both pseudosections leads to a more accurate description of the structures. When a 2D anomaly is present, its effect on the perpendicular component is more focused, both in width and depth, than in the parallel component. Hence the perpendicular component helps to constrain the localization of the inhomogeneity. In addition, we find similarities between the geoelectric parallel and perpendicular responses and the corresponding audiomagnetotelluric transverse magnetic and transverse electric results, respectively. When inverting audiomagnetotelluric data using 2D codes, better resolution in the electrical imaging is obtained when both modes are considered; then it is expected that 2D imaging of geoelectric data including both arrays should lead to an optimization of the inversion process. Even more, if results of these inversions could be used in correlation with AMT results, it is clear that this kind of joint inversion should contribute to remove uncertainties allowing an improvement in the description of the actual structures.