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

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