Tidal influence on self‐potential measurements

Abstract

Long‐term surface and borehole self‐potential (SP) monitoring was conducted in the UK Chalk aquifer at two sites. The coastal site is ~1.7 km from the coast, and the inland site is ~80 km from the coast. At both sites, power spectral density analysis revealed that SP data contain the main ocean tidal periodic components. However, the principal lunar component (M2), the dominant ocean tidal component, was most significant at the coastal site. The M2 signal in surface‐referenced SP data at the inland site was partly due to telluric currents caused by the geomagnetic ocean dynamo. Earth and/or atmospheric tides also contributed, as the SP power spectrum was not typical of a telluric electric field. The M2 component in borehole‐referenced data at the inland site was below the significance level of the analysis method and was 2 orders of magnitude smaller than the M2 signal in borehole‐referenced SP data at the coastal site. The tidal response of the SP data in the coastal borehole is, therefore, primarily driven by ocean tides. These cause changes in fluid pressure and chemical concentration gradients within the coastal aquifer, leading to time varying electrokinetic and exclusion‐diffusion potentials. Borehole‐referenced SP measurements could be used to characterize and monitor tidal processes in coastal aquifers such as the intrusion of seawater.

Self‐Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes

“…In coastal aquifers, V EED and V EK are likely to be the most significant components of SP (MacAllister et al, , ), because of respective variations in pressure induced by tidal fluctuations and in groundwater salinity, due to the presence of seawater at depth below comparatively fresh groundwater near the water table.…”

“…Based on observed drawdown at the Balsdean PS, MWH () inferred a hydraulic conductivity K of 900 m/day for the surrounding aquifer, significantly higher than the median value of 9 m/day for the South Downs as a whole (based on the median transmissivity of 880 m 2 /day reported by MacDonald and Allen () and an assumed aquifer thickness of 50 m). We estimate here a hydraulic conductivity of 250 m/day, based on a mean lag τ of 0.152 days between tidal peaks observed at the coast (in Newhaven, 7 km to the southeast; British Oceanographic Data Centre, ) and using (Jacob, ): where t 0 is the semidiurnal period of tidal fluctuation (0.518 days; MacAllister et al, ), S is storativity (0.0022), x is distance from the coast (1,800 m), and b is aquifer thickness (50 m).…”

“…The monitoring apparatus was connected to a Campbell Scientific CR3000 data logger, which collected data at 5‐min intervals. Further details on the equipment used and data collection methods are given by MacAllister et al ().…”

“…The raw SP data include semidiurnal oscillations consistent with the M2 component of oceanic tides, which dominate the signal (MacAllister et al, ). A first‐order Savitsky‐Golay (SG) moving average filter, with a sampling window of 2.2 days, was applied to the data to assist in identifying longer‐term trends in the SP data set.…”

“…MacAllister et al () was the first study to demonstrate a link between tidal processes and borehole SP in a coastal aquifer. This was done by monitoring borehole SP and fluid electrical conductivity σ f in a coastal borehole in the UK Chalk subject to regular SI (the Saltdean Observation Borehole, OBH).…”