Float-transient electromagnetic method: in-loop transient electromagnetic measurements on Lake Holzmaar, Germany

摘要

Lake sediments may serve as archives on paleoclimatic fluctuations, geomagnetic field variations and volcanic activities. Lake Holzmaar in Eifel/Germany is a maar lake and its lacustrine sediments provide paleoclimatic proxy data. Therefore, knowledge about the geometry and, especially, about the thickness of the sediments is very important for determining an optimum drilling location for paleoclimatic studies. We have developed a floating in-loop transient electromagnetic method field set up (Float-transient electromagnetic method) with a transmitter and receiver size of 18 x 18 m2 and 6 x 6 m2 respectively. This special set up enables in-loop transient electromagnetic method measurements on the surface of freshwater lakes that define the geometry and the thickness of sediments beneath such lakes thus helping to determine optimum drilling locations. Due to the modular design of the new Float-transient electromagnetic method field set up, this system can be handled by two operators and can easily be transported. Sixteen in-loop soundings were carried out on the surface of Lake Holzmaar. The transient electromagnetic method data could not be interpreted by conventional ID inversions because of the 3D distribution of subsurface conductivity caused by the lake's geometry. Three-dimensional finite element modelling was applied to explain the observed transients and the 3D conductivity distribution beneath the lake was recovered by taking its geometry into account. The 3D interpretation revealed approximately 55 m thick sediments beneath 20 m deep water in the central part of the lake.