Time domain electromagnetic (TEM) measurements on a buried subglacial valley in Northern Germany by using a large transmitter size and a high current

摘要

Buried subglacial valleys are important groundwater reservoirs in Northern Germany. However, relatively little is known about their exact location, their size and the internal structure of their sediment fills. Due to the presence of a 20 m thick conductive clay layer at about 40 m depth within the Bremerhaven–Cuxhaven valley in Northern Germany, no information about the structure below this layer could be obtained by inloop ground transient electromagnetic measurements ( TEM) with a 50 m by 50 m transmitter size, as well as by SkyTEM and helicopter electromagnetic measurements (HEM). To overcome this problem, the TEM method in the central loop configuration was used with a large transmitter size of 400 m by 400 m and a high current of 12.7 A for early times and 20 A for late times measurements. Five soundings could be realised and the data were interpreted by means of Occam and Marquardt type 1D inversions. The inversion results were compared with SkyTEM and HEM inversion results of the buried valley. A conductive clay layer was found between 30 and 83 m within the valley and the bottom of the valley was determined to be at a depth of 285 to 309 m.