TIME LAPSE ELECTRICAL RESISTIVITY TOMOGRAPHY OF TIDAL PUMPING OF NEARSHORE GROUNDWATER FROM GRAHAMS HARBOR, SAN SALVADOR ISLAND, BAHAMAS

Electrical resistivity tomography profiles were collected in January 2014 and June 2015 in Grahams Harbour on San Salvador Island, Bahamas. These data include a suite of replicate profiles taken in intervals ranging from 15 minutes to one hour using an AGI Supersting R8 (2-meter spacing in both data sets; 28 and 14 electrodes in 2014 and 2015, respectively). The goal of the study was to gauge the magnitude of tidal pumping on the nearshore groundwater environment in a strand plain of this carbonate island. Measurements spanned multiple tidal cycles.

Comparing apparent resistivity for each dataset against the initial dataset for each suite of profiles, we demonstrate that total apparent resistivity is sinusoidal with a range of values that is 20% of the maximum value. Changes are three hours out of sync with the predicted tides and consistent with a mixed semi-diurnal tide pattern. Inversion models in R2 illustrate the spatial arrangement of changes to earth resistivity during the tidal cycle. Difference plots between each modeled dataset and the initial modeled data reveal that changes to resistivity are restricted to the upper 10 meters of the transect and expectedly more pronounced closer to the shoreline.