ELECTROMAGNETIC AND GEOELECTRIC INVESTIGATION OF THE FRESHWATER-SALTWATER-BOUNDARY IN EASTERN BRANDENBURG, GERMANY

Within the framework of the project ”brine – CO2 storage in eastern Brandenburg” (BMBF 03G0758A/B) funded by the German Federal Ministry of Education and Research, geophysical investigations are conducted by GFZ Potsdam and BTU on different scales in order to monitor and investigate underground situations with regard to CO2 storage. The research of BTU is focussed on the distribution of underground structures up to a maximum depth of 200m. Of prevalent interest are the detection capabilities for the freshwater-saltwater boundary and near surface failing zones which might serve as favoured pathways for brine migration.

Geophysical investigations with the frequency domain electromagnetic (FEM) and direct current (DC) geoelectric methods are suitable for the identification and monitoring of brine displacement as the measuring parameter is the resistivity/conductivity of the subsurface. In sedimentary rocks this parameter depends strongly on the presence of fluids, their temperature and salinity. Therefore, resistivity/conductivity can serve as a key parameter for the identification and monitoring of brine migration. In eastern Brandenburg the Oligocene Rupelian clay represents the barrier horizon separating the freshwater and saline aquifers. Due to postglacial processes this layer might be reduced or locally eroded and enhancing upward brine migration.

The selection of study areas is carried out according to existing stocks of the Geological Survey and industrial archives. The areas of investigation were selected by known high fluid conductivity values (hydrochemical indication) and the potential presence of quaternary erosion channels in the Rupelian clay (geological indication).

Geophysical monitoring results yield a vertical and horizontal resistivity/conductivity distribution. The interpretation is done by lithology profiles of nearby boreholes and correlation with fluid conductivities in groundwater wells. The results of FEM and DC on coincident profiles are generally in accordance. With direct current geoelectrics supplementing a higher resolution close to the surface and the performed electromagnetics adding additional conductivity information of the deeper underground.