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

In 2D-multielectrode electrical surveys using the pole-pole array, the dist ance to 'infmite electrodes' is actually finite. As a matter of fact, the a vailable cable length generally imposes 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 di storted. Numerical simulation validated by field test also shows that a par ticular finite array provides results that are as close as possible to the ones of the ideal pole-pole array. This is achieved when two conditions tha t are weaker than an infinite location are fulfilled: (i) the 'infmite elec trodes' are placed symmetrically on both sides of the in-line electrodes wi th a spread angle of 30 degrees and (ii) the length of 'infinite lines' is at least 20 times the greatest distance between in-line electrodes. The ele ctrical 2D image obtained with this enhanced array is the least distorted o ne with respect to the pole-pole image. The apparent resistivities are gene rally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image wi th direct or inverse software designed for pole-pole data provides an accur ate interpretation of the ground geometry. (C) 1999 Elsevier Science B.V. A ll rights reserved.