ABSTRACT

Coastal areas such as the German Bight are at risk from storms and rising sea level that may affect the hydrogeologic setting. Because knowledge of the distribution of clayey sediments is important for understanding the current status of this dynamic setting, the German Federal Institute for Geosciences and Natural Resources (BGR) has focused on geophysical research projects on the North Sea coast applying airborne and ground geophysics. The airborne system operated by BGR was used to survey a 20- by 31-km large coastal area in Eastern Friesland, Germany, including the islands of Langeoog and Spiekeroog. Helicopter-borne electromagnetic (HEM) data were collected at six frequencies. In addition, the Leibniz Institute for Applied Geophysics commissioned a SkyTEM survey covering a 2-km-wide north–south strip. On the island of Langeoog, ground geophysical methods, such as transient electromagnetics (TEM) and magnetic resonance soundings (MRS) were used to investigate the hydrogeologic setting in greater detail. Onshore, the airborne electromagnetic results clearly outlined a complex electrically conductive pattern occurring at a shallow depth. Comparison with borehole results provided by the State Authority for Mining, Energy and Geology confirmed that these conductors were caused by clayey material, particularly down to an approximate 20-m depth. This pattern continues offshore, and it is likely that the saltwater is linked to stripes of clayey sediments and the fresh groundwater flows out to the Wadden Sea in between. On the islands, the HEM results revealed the freshwater lenses and showed some indications for clay layers within these freshwater lenses. The application of TEM and MRS helped to distinguish lithology from salinity and confirmed the existence of these clay layers. We demonstrated the usefulness of combining the spatial airborne data with geophysical and borehole data available at sparsely distributed sites on the ground to investigate hydrogeologic settings.

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