Laboratory Scale Demonstration of Self Potential as a Saline Intrusion Early Warning System

Previous studies in a borehole on the Chalk aquifer located on the south coast of UK have indicated that changes in Self-Potential (SP) could be used as a precursor of saline intrusion (SI). Offering the prospect that SP monitoring could provide an early warning of saline ingression into abstraction wells and providing a new geophysical technique for managing coastal aquifers. These studies, however, were in a highly complex system with many uncertainties in the physical, chemical, and electrical characterization of the surrounding aquifer. To overcome these limitations, SP response in advance of a moving saline interface has been investigated at the laboratory scale. A Perspex tank with a 10mm thick, porous plane (of silica beads), bounded by saline and freshwater reservoirs, is subjected to a variety of head differentials to induce the classic wedge-shaped intrusion of the saline interface followed by its retreat. The saline front is mapped using dyed saline water and high-resolution photographic imagery interpreted using machine learning. Silver/silver-chloride electrodes (in specially adapted 3D printed housings) are used with a high impedance logger to map the SP field arising from the movement of the saline interface. The results show a clear and repeatable (electrochemical) SP response (tens of mV in magnitude) at multiple locations ahead of the plume when referenced to an upgradient location. Important differences in electrochemical diffusion coupling coefficients obtained from the tank and a smaller scale test rig have been observed and are providing important insights into the role of SP in SI monitoring. These results are also being used to support a larger-scale field experiment, as part of the SALINA (SALine INtrusion in costal Aquifers) research programme.