The location of infinite electrodes in pole-pole electrical surveys : consequences for 2D imaging

In 2D-multielectrode electrical survey using the pole-pole array, the distance to "infinite electrodes" is actually finite. As a matter of fact, the available cable length generally impose a poor approximation of theoretical location of these electrodes at infinity. This study shows that in most of the cases, the resulting apparent resistivity pseudosection is strongly distorted. Numerical simulation validated by field test also shows that a particular finite array provides results that are as close as possible to the one of the ideal pole-pole array. This is achieved when two conditions that are weaker than an infinite location are fulfilled : (i) the "infinite electrodes" are placed symmetrically on both sides of the in-line electrodes with a spread angle of 30° and (ii) the length of "infinite lines" is at least 20 times the greatest distance between in-line electrodes. The electrical 2D image obtained with this enhanced array is the least distorded one with respect to the pole-pole image. Apparent resistivities are generally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image with direct or inverse software designed for pole-pole data provides an accurate interpretation of the ground geometry. (Résumé d'auteur