Joint application of surface electrical resistivity- and GPR-measurements for groundwater exploration on the island of Spiekeroog—northern Germany

Abstract

Geophysical surveys have been carried out on the East Frisian island of Spiekeroog at the German North Sea coast in 1996. So-called “freshwater lenses”, lying on top of saltwater-saturated layers because of their lower density, provide most of the freshwater supply on the Frisian Islands. The major goal of the present study is to show the possibilities of a joint application of surface electrical resistivity soundings and ground-penetrating radar (GPR) for the hydrogeological investigation of the island. It is shown that a comprehensive interpretation of large-scale GPR-profiling data in a sandy aquifer provides valuable hydrogeological information, especially if there is not any other information available, such as from boreholes or groundwater measuring points. Applying the techniques of surface electrical resistivity and ground penetrating radar jointly, we were able to construct a groundwater contour map for the entire island of Spiekeroog. In addition, we were able to delineate hydrogeologically important changes in grain size and stratigraphical features and to indicate temporal changes of the spatial extent of the freshwater reservoir in comparison to earlier (less complete) investigations. Finally, we located an area, previously unknown, in the eastern part of the island where precipitation appears to be developing a substantial new freshwater lens.