Pumping test in a layered aquifer: Numerical analysis of self-potential signals

We used numerical modeling to study self-potential (SP) signals associated with a pumping test in a layered aquifer containing the main aquifer, a thin aquitard, and a shallow aquifer. The results revealed an unusual behavior of SP signals, which were not linearly correlated with the hydraulic head distributions. We explained this behavior by a vertical downward groundwater flow, from the shallow aquifer to the main aquifer, in the course of the pumping test. However, when plotted as a function of time, the SP signals and hydraulic heads displayed coherent behavior. In both distributions, three stages of the pumping test were determined: at early times, only the main aquifer and the aquitard responded to the pumping; at intermediate times, the downward flow from the shallow aquifer occurred; and at late times, the layered aquifer responded to the pumping as a whole. The SP signals reacted to the sequence of these stages much faster than the hydraulic head distributions. In principle, this might allow reducing the duration of the pumping tests without losing valuable information.