A Comparison Of Electromagnetic Techniques Used In A Reconnaissance Of The Ground-Water Resources Under The Coastal Plain Of Isla De Mona, Puerto Rico

Abstract

An electromagnetic reconnaissance of the ground-water resources of the coastal plain of Isla de Mona, Puerto Rico was conducted with both terrain conductivity (TC) and transient electromagnetic (TEM) geophysical techniques. Quantitative interpretation of the data was done using forward and inverse modeling with various starting geoelectric models. The final geoelectric models produced by the two techniques are similar. Neither method was able to provide useful information about the water table. The freshwater lens under the coastal plain was found to have a maximum thickness of about 10 m near the center of the coastal plain. The lens thins toward the ocean and also thins toward the interior of the island. This finding was not expected and indicates that the flow paths in the aquifer are not radially symmetrical from the center of the island to the ocean. At least some of the ground water flows to the ocean by a more circuitous and longer path. The asymmetry of the flow paths indicate that differences in hydraulic conductivity are controlling the shape of the freshwater lens.