Helicopter-borne Electromagnetics and Geoelectrics to Investigate the Fresh-water Lenses of the North Sea Island Borkum

Helicopter-borne electromagnetic (HEM) measurements were conducted over the North Sea island of Borkum to map the size of the fresh-water lenses. Additionally, geoelectrical DC measurements were carried out at locations where data of 13-17 years old Schlumberger soundings exist. Based on the previously measured data possible changes of the fresh-saltwater boundary should be determined. The lateral extension as well as the thickness of the fresh-water lenses of Borkum could be mapped successfully with HEM. At several locations the depth of the fresh-saltwater boundary determined by HEM was confirmed by the results of the Schlumberger soundings. No significant changes of the fresh-saltwater boundary could be investigated at the locations where previous Schlumberger soundings exist. Considering direct push and borehole data, the HEM inversion models could be determined more precisely and additional to the fresh-saltwater structure thin clay layers could be revealed. Based on the HEM inversion results the potential volume of the fresh-water resource was estimated.