Evaluation of Electromagnetic Methods for Rapid Mapping of Salt‐Water Interfaces in Coastal Aquifers

Abstract

ABSTRACTIn coastal areas salt‐water intrusion represents a significant threat to water quality in coastal aquifer systems. The electromagnetic method evaluated in this study proved to be rapid, very inexpensive, and it yields results which agree well with geologic and geochemical surveys using direct sampling. The instrument reads directly in conductivity units and the depth of investigation is constant for a particular spacing and orientation of the receiver and transmitter coils. Data can be obtained and interpreted by personnel with little technical training. Data interpretation normally involves producing terrain conductivity contour maps, but quantitative solutions can be obtained where the geoelectric section can be reasonably approximated by a two‐layer section. Some problems were experienced with interference. However, interference sources usually were very localized and could be avoided.This study, conducted in two areas of Florida, demonstrates that the EM conductivity method is very useful for rapid, inexpensive ground‐water surveys where the objective is to locate zones of conductive pore fluids at depths less than 30–40 meters. Although interpretation is indirect and qualitative, neither data aquisition nor interpretation require special training. This characteristic of the EM method makes it very suitable for use by agencies where highly‐trained technical personnel are not available.