Numerical modelling of self-potential signals associated with a pumping test experiment

Abstract

SUMMARY The study of electrical signals associated with groundwater flow is a powerful method to determine, in a non-invasive way, the distribution of hydraulic heads in aquifers, hence the distribution of the hydraulic conductivity and storativity. Experimental hydraulic heads and electrical (self-potential, SP) signals associated with a pumping test in an unconfined aquifer were reproduced with a numerical model based on the finite-difference method. For simplicity, we assumed axial symmetry around the pumping well. We assessed the equivalent hydraulic conductivity and specific storage of the aquifer by fitting the piezometric levels obtained in the course of the pumping against synthetic hydraulic heads produced by the model. The current coupling coefficient is obtained from the best fit between the experimental and modelled SP signals at the end of the pumping phase. Then, keeping the previously determined hydraulic parameters, the relaxation of both the hydraulic heads and the SP signals were modelled and found to be consistent with the measured values. The data show almost linear relationships between the hydraulic heads in the aquifer and the electrical signals recorded at the ground surface. These results show that SP signals allow monitoring subsurface flow in the course of pumping experiments.