Surface electrical resistivity in coastal groundwater exploration

Because of its potential to detect changes in pore-water salinity the surface electrical resistivity method can be a valuable aid in coastal groundwater exploration and investigations. It is essential, however, that the resistivity interpretation be consistent with a hydrogeological model reflecting the fresh-water-salt-water relationship of coastal aquifers. In the electrical resistivity interpretation of phreatic aquifers it should be recognized that the lower boundary of the unsaturated zone corresponds to the top of the capillary zone, not to the water table, and that the lower boundary of the fresh-water layer corresponds only approximately to the top of the fresh-water-salt-water transition zone. The existence of a fresh-water layer can be ascertained qualitatively by visual inspection of electrical sounding curves, provided there is a fresh-water/unsaturated layer thickness ratio of at least four. Good interpretative methodology using an appropriate coastal hydrogeological model can enable the extent of the fresh-water layer to be quantified, but it is not possible to quantify the thickness of the transition zone by geoelectrical interpretation because of suppression effects.