Ground penetrating radar imaging of the freshwater/saltwater interface on a carbonate island Key Largo Florida

摘要

Finite-difference time-domain models of radar wave propagation at freshwater/saltwater transition zones indicate that a reflected event from the electrical conductivity gradient at the top of the transition zone may be detected. Ground penetrating radar surveys were used together with a suite of geophysical techniques in a reconnaissance survey to delineate the freshwater lens on the island of Key Largo, Florida, USA. In the interior of north Key Largo, the water table lies at approximately 3 m depth. There a 1 - 1.5-m thick freshwater lens overlies a 2 to 3-m transition zone to saline water. 50 MHz GPR surveys show a strong water table event followed by a weaker event that correlates with the top of the freshwater/seawater transition. Nearby cores show no lithologic discontinuities in this depth range. In lower-lying areas with minimal freshwater, an enigmatic high-amplitude high-frequency event follows the water table event closely in time. The field data indicate that reflections from the freshwater/saltwater interface may be resolved in environments with fresh water above the transition zone and with a relatively sharp conductivity gradient at the top of the freshwater/saltwater interface.