Fluid flow near reservoir lakes inferred from the spatial and temporal analysis of the electric potential

Abstract

Electric self-potential (SP) variations have been monitored continuously from 1995 to 1998 at 14 points on a ridge separating the Roselend and La Gittaz reservoir lakes in the French Alps. The lakes have level variations of at least 50 m over yearly cycles. Seasonal variations of SP associated with lake-level variations are observed on five points of the array. For three points located on the banks of the lakes, a positive correlation to the lake-level variations is observed with a maximal amplitude of about 180 mV, corresponding to an average response of 2.4 mV per meter of water. For two points located on the bottom of each lake, the correlation is negative, with a maximal magnitude of about −50 mV, corresponding to an average response of −1.1 mV per meter of water. Two independent temporary electrical arrays located on the banks of each lake confirm these measurements and allow a better spatial characterization of the sources associated with the observed SP variations. In particular, near the Roselend lake, the electrical response to lake-level variations is increasing for decreasing altitude. The measured SP variations are proposed to result from the electrokinetic coupling associated with a vertical groundwater flow connecting a constant pore pressure source to the bottom of the lakes. Numerical modeling indicates that the spatial variation of the response and the nonlinear response observed at one point can be explained by leakage currents in the conductive lake water. The values of the streaming potential coefficient (SPC), measured in the laboratory with crushed rock samples from the site, range from 14 to 50 mV/0.1 MPa for an electrolyte resistivity of 40 Ω m and are compatible, to first order, with the magnitude of the observed seasonal SP variations. A detailed quantitative electrokinetic modeling is currently limited mainly by the poor knowledge on the contribution of electrical leakage currents and the local variability of the SPC. This experiment indicates that spatial and temporal variations of the electric potential are promising tools to characterize and monitor shallow groundwater flow and provide practical data for the investigation of groundwater flow associated with volcanic or tectonic activity.