A comparison of dipole?dipole and gradient arrays

Abstract

In exploration practice in the Western U.S., the two most commonly used IP/Iresistivity electrode configurations are the dipole?dipole and gradient arrays. Differences in the relative responses between the two electrode configurations have been noted and the relative merits debated, but published data are limited. A suite of dipole?dipole and gradient-array profiles have been computed for direct comparison of both conductive and resistive bodies. Profiles for thin vertical and thin horizontal tabular bodies (dykes and slabs respectively), are the primary basis for comparison. The anomalies at a typical resistivity and chargeability contrast are normalized by determining the ratio of amplitude to the background response. Plots of resistivity and chargeability illustrate the differences with depth for the two types of bodies. For thin conductive dykes, dipole data give a strong characteristic response on both resistivity and chargeability parameters; gradient profiles, however, show a weak resistivity and nil chargeability response. Conductive slabs show excellent anomalous response for both arrays. Resistive dykes produce better relative resistivity and chargeability responses on the gradient array than the low amplitude, peculiar characteristics, responses of the dipole array. The resistive slab produces almost no chargeability response on the gradient array. Overburden effects, multiple bodies, dip and several case histories are also examined to show the differences in interpretation potential. An eventual recommendation for use of either array depends upon the interpretation required, geological assumptions of the target size, shape, and physical property contrast, and the cost differential of data acquisition.