CHARACTERIZATION OF SUBSURFACE LITHOSTRATIGRAPHY AND HYDROSTRATIGRAPHY USING DC ELECTRICAL RESISTIVITY TOMOGRAPHY, LINE HOLE WELL FIELD, SAN SALVADOR ISLAND, THE BAHAMAS

This study uses DC Electrical Resistivity Tomography to investigate the nature of the freshwater lens and the stratigraphy of the north shore of San Salvador Island, Bahamas, in the Line Hole well field. The goal of the study is a better understanding of the available freshwater resources in an attempt to enhance water management practices on the island.

An AGI Swift R1 Earth Resistivity meter with a 28-electrode array was used to collect three dipole-dipole transects with an electrode spacing of 2.80 m and a fourth, longer transect with an electrode spacing of 6.10 m. These transects span the beach, Holocene strand plain, and the adjacent Late-Pleistocene limestone terrace environments. Two-dimensional cross-sections of measured apparent resistivity (r_ap) and inverted apparent resistivity (r_inv) are created from the measured field resistivities using the AGI Earth Imager 2D program.

In general, the resistivity sections are highly heterogeneous, with zones of r_ap > 1.2*10⁶ W-m and r_inv > 500 W-m in the top four meters of the subsurface, and zones with r_ap < 20 W-m and r_inv < 100 W-m that cluster towards the bottom of the electrical resistivity cross-sections. We interpret these general observations to suggest a vadose zone with high electrical resistivity, and a phreatic zone with lower electrical resistivity. Further interpretation of the four transects is less clear, particularly in the r_inv data. The cross sections of r_ap, however, do provide some insight. In particular, we note the following features in the transects: 1) an irregular and diffuse boundary separating lower r_ap near the beach (r_ap < 10 W-m) and moderately higher r_ap inland (r_ap> 100 W-m) that may represent a mixing zone between freshwater and saltwater, and 2) a sharp boundary between shoreward, and unconsolidated strand plain deposits of r_ap > 10,000 W-m and the inland, partly indurated, Late-Pleistocene strata with r_ap < 1,000 W-m.