COMPARISON OF 2D AND 3D ELECTRICAL RESISTIVITY IMAGING METHODS

Abstract

As multi-electrode, multi-channel resistivity imaging systems and commercial 3D resistivity inversion programs are readily available, more and more people are conducting 3D resistivity surveys. We observed that 2D and 3D resistivity surveys at the same location produced very different images on the same cross section. This discrepancy causes much confusion among practitioners about effectiveness of resistivity imaging methods. In addition, many resistivity imaging practitioners are interested in combining multiple 2D data sets collected along parallel survey lines into a pseudo 3D data set for 3D inversion. In this paper, we investigated these two issues with numerical simulations and field data tests. We found that most of the features on an extracted slice image from 3D inversion appeared on the image from 2D inversion, but the 2D inverted resistivity cross-section appeared more complicated with more anomalies and higher resistivity contrast. The objects which did not intersect the imaging plane would be folded onto the 2D inverted cross-section. This is why false anomalies are often seen on the 2D resistivity images. Therefore, 3D resistivity imaging methods are the better technology for subsurface H103 imaging. We also found that the true 3D survey with a large number of cross-line measurements is an ideal approach for a 3D resistivity survey because it offers a better subsurface resolution than a pseudo 3D survey. However, a pseudo 3D survey without any cross-line measurements is an acceptable alternative to a true 3D survey as far as the line spacing is equal to or less than twice the electrode spacing. These findings enable many practitioners with a limited number of electrodes to conduct 3D resistivity surveys efficiently for a reasonable 3D resolution.