Electromagnetic Mapping of Fresh-Water Lenses on Small Oceanic Islands

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Many small oceanic islands have fresh-water lenses that can be used for domestic water supplies or dry season augmentation for rainfall catchment systems. However, these small lenses are often irregular in extent and thickness. Some islands have several small, isolated lenses. By making a few reasonable assumptions, the electromagnetic (EM) profiling method can be used to estimate lens thickness. If it can be assumed that the water table is close to sea level, and that the unsaturated and fresh-water saturated geologic units have low bulk conductivities, the interface depth can be obtained from a three-layer solution. The first layer is the unsaturated zone. Its thickness is assumed to be the land surface elevation above sea level. The third layer is the salt-water saturated zone; it is assumed to be infinitely thick. The layer conductivities are either estimated or obtained from DC resistivity soundings. In resistive environments only the third-layer conductivity has a significant effect on the calculated interface depth. The only remaining unknown is the second-layer thickness, which represents the fresh-water saturated zone. The second-layer thickness is sensitive to the value used for the third-layer conductivity. Calibration of the EM-derived interface depth with other geophysical or water-quality data is recommended, unless relative lens thicknesses are sufficient to meet the survey objectives. Application of this method to field studies of a sandy barrier island and a carbonate key, both in Florida, has provided useful data which correlate well with other geophysical and water-quality data.

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Groundwater, v. 26, issue 2, p. 187-191