TIME LAPSE ELECTRICAL RESISTIVITY TOMOGRAPHY OF NEARSHORE GROUNDWATER FROM A TROPICAL CARBONATE ISLAND

Electrical resistivity measurements were collected over a 2-day period (January 3 & 4, 2014) on the north shoreline in Grahams Harbour on San Salvador Island, Bahamas to determine tidal effects on the nearshore groundwater environment of this carbonate island. These data include 13 sets of measurements taken at approximately 45-minute intervals using a 28 electrode AGI Supersting R1/IP (with a 2 meter spacing). The first two sets of measurements were obtained on the first day using a dipole-dipole configuration, which resulted in a significant error. The eleven sets of measurements taken on the 2^nd day were collected using a modified Schlumberger-Wenner array and yielded much better results. Measurements for the second day spanned one half of a tidal cycle from an hour before high tide until low tide.

Comparing apparent resistivity for each dataset against the initial dataset, we demonstrate that total apparent resistivity decreased sequentially to a minimum of 11.5% below the initial configuration during the measurement period. The timing of the resistivity changes suggests that tidal pumping of seawater into the nearshore groundwater reaches a maximum somewhere between 1.5 and 3 hours after high tide. Inversion models created in R2 and graphed in Surfer illustrate the spatial arrangement of changes to earth resistivity during the tidal cycle. Creating 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.