Characterization of lake beds with the transient electromagnetic method

Abstract

In the course of this thesis, transient electromagnetic (TEM) measurements were carried out on lakes. The aim of these measurements was to determine the geometry of the water bottom and the electrical resistivity of the water and the underlying geological layers. A buoyant ring was built for the application of TEM on the surface of the lakes. The cable for the TEM antenna was attached to this ring and the antenna was pulled over the water by a motorised boat where the measuring device was located.Another instrument used was a CTD (conductivity temperature depth)-probe. It was used to determine the electrical conductivity of the water in-situ. Based on the CTD data, accurate start models could be created and inversions with a-priori information could be performed. Electrical resistivity tomography (ERT) was also used to validate the results. For the collection of ERT, a cable with electrodes attached to the measuring device was pulled by a motorised boat. The measured value with ERT is the distribution of the electrical resistivity, resolved through the inversion of the field data. The methods used aimed at improving our understanding of the interaction between surface- and groundwater, as well as the geological settings of the lakes. In addition, hydrogeological systems can be examined for the effects of climate change using the methods mentioned above.The results from three case studies show that it is possible to determine the sedimentary layer underneath lakes with TEM. To avoid problems, a non-metal boat should be used. For ERT measurements it is important to make sure that the cable forms a line before starting each measurement. For verification of the TEM results, additional information is very useful. Therefore multi-methodical approaches are recommended.