Abstract

Self-potential monitoring (SPM) is one of the most promising geophysical methods for hydrologic applications, since any change in subsurface water flow, chemistry, or thermodynamics can induce an electrical response. However, major difficulties may arise because different couplings (e.g., electrokinetics and electrodiffusion) can occur simultaneously. We performed laboratory experiments to isolate the electric response of flow during conditions of constant composition, of ionic diffusion of NaCl in stagnant fluid, and of advective transport of NaCl and KCl. For this purpose, fluid flow and/or salt diffusion were generated in a rectangular sand box, and the resulting electric potential differences were measured between custom-made, small, unpolarizable electrodes. In pure electrokinetic experiments (i.e., flow of water with constant salinity), the electric signal was proportional to the hydraulic gradient and to the salinity, in agreement with previous experimental and theoretical results. The other experiments showed that diffusive and advective transport of salt (i.e., in stagnant and flowing fluid conditions, respectively) can generate significant electric potential differences. Monitoring these potential differences allows determination of the motion of the concentration front in the sand box.

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