Some notes about three types of inhomogeneity and their effect on the electrical resistivity tomography data

We investigate three types of inhomogeneity and their effect on the measured apparent resistivity and the inverse models of the electrical resistivity tomography (ERT). The inhomogeneity types are produced by the near-surface inhomogeneity (NSI), the very low resistivity (VLR) upper layer, and the High-Low-High (HLH) resistivity upper layer. The effect of these types is tested on the Dipole-dipole and Wenner-Schlumberger arrays. The data are divided into two kinds of sections: Sections extracted from synthetic models and sections extracted from real field data for horizontal layers and underground limited bodies. For the horizontal layers, the VLR upper layer and NSI together influence the resolution of the measured data and the inverse models of both arrays. The VLR upper layer diffuses electrical current and obscures the NSI effect. The resolution is greatly decreased, leading to produce false structures (artifacts) or overestimates resistivity of the deeper layers in the inverse model of the Wenner-Schlumberger array. The Dipole-dipole shows a better resolution and can resolve the subsurface layers. When the VLR upper layer is vanished and NSI exists only, the major effect will be on the Dipole-dipole measured data. The resolution of this array decreases dramatically leading to produce false structures. The Wenner-Schlumberger array can yield a superior resolution of the inverse models if the NSI exists only. For the underground limited structures sections, the existence of the VCR upper layer has a significant influence on the measured data and the inverse model making the detection of most of the underground limited bodies very difficult. The Dipole-dipole array gives reasonable subsurface images with a better resolution even when the VLR upper layer exists, but the underground limited structures do not appear in their actual positions in the inverse models. They are shifted little and present deeper than the actual position by a distance equals to half a-spacing (0.25 m in our case). When the upper layer has HLH resistivity, the two arrays produce false structures in their inverse models. Besides, the resolvability of the Wenner-Schlumberger array becomes much less than the Dipole-dipole array which showed higher resolution and delineated the underground structures accurately. (C) 2021 Elsevier B.V. All rights reserved.