Abstract
AbstractLong‐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.
Highlights
Coastal aquifers are an important water resource but are threatened by the intrusion of seawater
For the reasons outlined below, the absence of significant periodic tidal components in the borehole-referenced SP at the inland site suggests that Earth, atmospheric or geomagnetic tides, are not primarily responsible for the tidal SP response observed at the coastal site
Because the M2 component was present in surface-referenced SP data at the inland site, caution was required before attributing the M2 response at the coastal site to ocean tides
Summary
Coastal aquifers are an important water resource but are threatened by the intrusion of seawater. The recovery of the aquifer in the area adjacent to a contaminated borehole can take considerable time [Zhou et al, 2005]. Geophysical monitoring may assist in avoiding contamination of coastal water supplies. The most common method for monitoring seawater intrusion is borehole measurements of fluid electrical conductivity (FEC) [Food and Agricultural Organisation, 1997; Werner et al, 2009]. Active geophysical methods, including electrical resistivity tomography (ERT) and time domain electromagnetic methods (TDEM), can offer a means of remotely detecting the saline front. Despite recent advances in such techniques [de Franco et al, 2009; Falgàs et al, 2009; Rosas-Carbajal et al, 2013], ERT and TDEM provide information only about the saturated rock resistivity rather than directly about fluid dynamics within the aquifer
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