Observations of tidal induced responses in Self-Potential data in a UK beach sand aquifer: implications for monitoring seawater intrusion

Affordable and effective early warning of saline intrusion into aquifers is a key challenge affecting coastal water supplies. This paper reports on Self Potential (SP) monitoring close to a mobile saline-fresh water interface over a 3-month period. SP, groundwater level, temperature and fluid electrical conductivity (FEC) were monitored in 12 boreholes, distributed over a variety of depths and distances to the high-water mark (HWM). Power Spectral Density (PSD) analysis of SP data showed a clear semi-diurnal (principal-lunar, M2) tidal component, with an amplitude of around 1mV. The M2 magnitude linearly decreased with depth and distance from the shore. Whilst PSD analysis showed M2 components in borehole water levels, none were detected in groundwater temperature and FEC. The results suggest that SP M2 components were generated remote from the boreholes by the movement of a saline interface, most likely associated with the upper saline plume. Superimposed on these tidal signals were periodic rises and falls in mean SP of around 5-10mV. These responses appear linked to storm surge events, during which seawater advanced beyond the monitoring boreholes, although data interpretation is complex due to salinity changes at the reference electrode. The results provide further support for the use of SP as a precursor to saline intrusion. Critically, they provide the first quantitative spatial characterization of tidal SP responses, demonstrating that the M₂ signal magnitude can be predicted from electrode depth and shoreline distance (R² = 0.96) and suggest the possibility of using multi-point SP monitoring to remotely track saline plume movement.