Pumping Tests of Layered Aquifers - A Numerical Analysis of Self-potential Signals

Based on numerical modeling we studied Self-Potential signals associated with pumping test experiment with a layered aquifer containing a main aquifer, a thin aquitard and a shallow aquifer. We show an unusual behaviour of SP signals, which are not linearly correlated with the hydraulic head distributions. However when plotted as a function of time, the SP signals and drawdowns where found to be coherent. In both distributions three time stages of the pumping test were determined: at early times the pumping occurs from the main aquifer and the aquitard, at intermediate times the downward flow from the shallow aquifer occurs, and at late times the layered aquifer acts as a whole. These three stages are clearly seen in the SP distributions. Moreover the SP signals react to the changing of these stages much faster then the drawdowns.