Analog modeling of streaming potential during well-pumping

Electrokinetic potentials, also called streaming potentials (SP), are natural potentials between sets of points on the surface or within the soil. They are produced by the flow of a fluid, generally water, in a porous media. SP being the only surface method able to give indications on the direction of flow, it is used in several applications involving water movement, as in volcanic and geothermic areas, dam and lake leakage, landslides and well-pumping. The electrokinetic phenomenon, the relative motion of ion species producing an opposite conductive current, is well-known, as is the double layer model of Helmholtz-Smoluchowski. Furthermore, from the mathematical point of view, the thermodynamical set of equations allows the distribution of the SP to be modelized (Revil et al. (1999a, 1999b)). Although the overall theory is known, as soon as mineral species are involved, the subject becomes much more complicated. Many laboratory surveys (e.g. Ishido et al. (1981)) had been conducted in order to understand which parameters, in what manner and to which extent, act upon the production of SP. But after more than forty years of research, the authors still do not agree completely on the results. On a larger scale, field measurements, even less is understood. The main reason is that there is no way to know the exact values of the influencing parameters and, moreover, they are likely to change over distance and time. Besides, few field cases are presented in the literature, so it is difficult to know how other research groups cope with these problems. There is a gap between the laboratory and the field measurements that has been rarely studied (e.g. Sato et al. (1998)). We are trying to fill it partially by the analogical study of a scale model of a pumping-well.