Time Domain Electromagnetic (TEM) Measurements on a Buried Subglacial Valley in Northern Germany

Buried subglacial valleys are important groundwater reservoirs in Northern Germany. However, relative little is known about their exact locations, their sizes and the internal structure of their sediment fills. Due to the presence of a conductive clay layer at about 40m depth within the Bremerhaven-Cuxhaven valley, no information about the structure below this layer could be obtained by conventional ground TEM, SkyTEM and helicopter electromagnetic measurements. To overcome this problem, the TEM method in the central loop configuration was used with a large transmitter size of 400m by 400m and a high current of 12.7A for early times and 20A for late time measurements. Five soundings could be realized and the data were interpreted by means of OCCAM and Marquardt type 1D inversions. The inversion results were compared with SkyTEM, HEM and seismic sections of the buried valley. A conductive clay layer was found between 30 and 83m within the valley and the bottom of the valley was determined to be at a depth of 285 to 309m