Electric and Electromagnetic Measurements on the North Sea Island of Langeoog

Electric and electromagnetic methods were conducted on the North Sea island of Langeoog to investigate its freshwater lens that is managed by the local water works to supply the island with drinking water. Helicopter-borne electromagnetic (HEM), as well as surface geophysical methods were applied, i.e., electrical resistivity tomography (ERT), transient electromagnetic (TEM), and magnetic resonance sounding (MRS). We found that, beside the depth of the fresh-saltwater interface, the combined interpretation of TEM and MRS allows a reliable identification of lithological layering, whereas ERT or TEM alone exhibit a high degree of ambiguity regarding subsurface lithology and groundwater salinity. Future research is focused on the joint inversion of the surface methods, as well as on the combined inversion of surface and HEM data. In this way, 3-D multi-parameter models will be generated as additional input for numerical modeling of groundwater dynamics inside the freshwater lens.