f Electromagnetic measurements on the surface of a lake

The contamination of lakes and rivers by industrial activities constitutes a serious problem for human health, especially when lakes are used for swimming an fishing. The radiomagnetotelluric technique (RMT) has been successfully applied for the exploration on waste sites, the detection of contaminative plumes and for archaeological prospection. (Tezkan et al., 1996, Zacher et al., 1996, Recher et al., 1998) In order to test the applicability of the RMT technique for the geophysical investigation of lakes and rivers, RMT measurements were carried out on a profile crossing from one shore to the other shore of a lake close to Cologne in Germany. Fig. 1 demonstrates the field setup of the RMT survey on a lake schematically. A small boat was used for the survey and civilian and military radio stations were used as transmitters broadcasting in frequency range between 10 kHz and 1 MHz. The magnetic field was measured by a coil in the boat and the electric field by an electric antenna placed on the lake’s surface. Apparent resistivities and phase values were derived from the measured magnetic and electric fields so that a sounding information was available for each station. The data were interpreted by a 2D inversion algorithm (Mackie et al., 1997). Thus a 2D conductivity model was derived for the investigated lake and its surrounding host which is demonstrated in Fig. 2. The lake can be seen as a strong conductive anomaly in the 2D inversion model.