Detection of advected concentration and pH fronts from self‐potential measurements

Abstract

In this paper, we report on a series of experiments aimed at testing the possibility of detecting concentration or pH fronts advected through a well‐controlled, laboratory‐scale, artificial aquifer system, using self‐potential monitoring. A rectangular Plexiglas tank was filled up with water‐saturated sand, inside of which an array of unpolarizable electrodes was installed. After a nearly uniform, steady state flow was established by tilting the tank and controlling the water level at both ends, we abruptly changed the salt concentration or pH of the upstream fluid. During the subsequent motion of the concentration or pH front thus generated, the electrical potential differences between each electrode and a reference were recorded. We observed systematic, sharp variations successively occurring at increasing distances in the flow direction. These signals precisely corresponded to the calculated arrival times of the front at the various electrodes. Additional experiments were performed, in which the lateral motion of a transverse concentration or pH front was monitored. In the case of a NaCl front, the self‐potential signal can be modeled as the sum of two terms, (1) a junction potential term produced by the difference in ionic mobility of Na+ and Cl− and (2) an electrokinetic term corresponding to the variations of the electrokinetic coupling coefficient caused by changes in concentration. In the case of a pH front, only the electrokinetic term appears to be present.