Waterborne electromagnetics: the Andijk case study

To describe the hydrologic system, it’s mandatory to have a knowledge of the hydrogeological setting below water bodies, which are not easy to survey with classical ground geophysical methods. The waterborne version of the tTEM system, referred to as FloaTEM, is a system to overcome such difficulties. For the survey the tTEM system was adapted to work on water: it’s mounted on a plastic boat (to avoid EM coupling in the data) that pulls other two smaller plastic boats carrying transmitter loop and receiver loop. To adapt the system for the waterborne survey, plastic supports were built to accommodate the instrumentation and to avoid cables to interfere with the propeller. Furthermore, an echo sounder synchronized with a GPS were added in order to record bathymetry. Data were processed using an in-house developed open source and freeware QGIS plug-in, EEMstudio. The data were inverted in the EEMverter inversion suite using a 1D forward mesh interpolated to a 2D model mesh. Bathymetry was incorporated in the inversion breaking the vertical constraints at the bathymetric interface and forcing a narrower resistivity range in the water column. In the Andijk survey the goal was to map the clay layer and understand the interaction between saltwater and freshwater. Thus, ~65 km of FloaTEM data were acquired in the lake, close to PWN (a Dutch water management company) facilities in Andijk. Also, two underwater ERT profiles, 400 m long each, were acquired close to the FloaTEM soundings acquired in the previous days. The N-S profile was acquired with a Wenner array, the E-W with a Wenner-Schlumberger. It was used a 400 m long cable with electrode spacing of 5 meters in the mid 200 m and 10 meters on the sides, 61 electrodes in total. Using this system, it was possible to identify areas where the continuity of the conductive anomaly (clay layer?) was interrupted and it can be seen a more resistive (freshwater?) extending also below the bottom of the lake.