As groundwater reservoirs are crossing national boundaries transnational efforts are required to ensure a sustainable use and an effective protection of these water resources. This is the background of a Danish-German project for the mapping of aquifers and covering layers in the border region Sønderjylland / northern Schleswig-Holstein (Fig. 1). This project was funded by the European Community under the INTERREG-programme. The use of geophysical methods to establish a geological model of the project area is shown in this paper.
ABSTRACT There is an increasing demand for vulnerability maps which show the degree of exposure of aquifers against pollution. Parameters shown in these maps are generally cation exchange capacity or clay content of the near surface layers, or quantities related to the infiltration time of surface water. Instead of determining these parameters by means of drillings and geological investigation, we propose the use of geophysical techniques for vulnerability mapping and for the interpolation between drillings. A geophysical quantity which is closely related to these parameters is the electrical conductivity. In this paper the relation between clay content and electrical conductivity is shown and used for vulnerability mapping. It is demonstrated that ground and airborne electromagnetic (EM) techniques are suited for large scale data acquisition.
Coastal areas such as the German Bight are at risk from storms and rising sea level that may affect the hydrogeologic setting. Because knowledge of the distribution of clayey sediments is important for understanding the current status of this dynamic setting, the German Federal Institute for Geosciences and Natural Resources (BGR) has focused on geophysical research projects on the North Sea coast applying airborne and ground geophysics. The airborne system operated by BGR was used to survey a 20- by 31-km large coastal area in Eastern Friesland, Germany, including the islands of Langeoog and Spiekeroog. Helicopter-borne electromagnetic (HEM) data were collected at six frequencies. In addition, the Leibniz Institute for Applied Geophysics commissioned a SkyTEM survey covering a 2-km-wide north–south strip. On the island of Langeoog, ground geophysical methods, such as transient electromagnetics (TEM) and magnetic resonance soundings (MRS) were used to investigate the hydrogeologic setting in greater detail. Onshore, the airborne electromagnetic results clearly outlined a complex electrically conductive pattern occurring at a shallow depth. Comparison with borehole results provided by the State Authority for Mining, Energy and Geology confirmed that these conductors were caused by clayey material, particularly down to an approximate 20-m depth. This pattern continues offshore, and it is likely that the saltwater is linked to stripes of clayey sediments and the fresh groundwater flows out to the Wadden Sea in between. On the islands, the HEM results revealed the freshwater lenses and showed some indications for clay layers within these freshwater lenses. The application of TEM and MRS helped to distinguish lithology from salinity and confirmed the existence of these clay layers. We demonstrated the usefulness of combining the spatial airborne data with geophysical and borehole data available at sparsely distributed sites on the ground to investigate hydrogeologic settings.
Introduction: Vulnerability Maps Based on Electrical Conductivity Concepts for vulnerability maps The vulnerability of an aquifer can be defined by "How severe are the likely consequences of pollution loading?" (Lobo-Ferreira, 1997). The lateral distribution of the vulnerability, as shown in vulnerability maps, is an important parameter for groundwater management and planning. To ensure the water supply for coming generations, vulnerability mapping should not be restricted to areas with aquifers actually in use. This is the message of the new groundwater regulations of the European Community Commission which require wide mapping of aquifers, water quality, and vulnerability.
Electric and electromagnetic methods were conducted on the North Sea island of Langeoog to investigate its freshwater lens that is managed by the local water works to supply the island with drinking water. Helicopter-borne electromagnetic (HEM), as well as surface geophysical methods were applied, i.e., electrical resistivity tomography (ERT), transient electromagnetic (TEM), and magnetic resonance sounding (MRS). We found that, beside the depth of the fresh-saltwater interface, the combined interpretation of TEM and MRS allows a reliable identification of lithological layering, whereas ERT or TEM alone exhibit a high degree of ambiguity regarding subsurface lithology and groundwater salinity. Future research is focused on the joint inversion of the surface methods, as well as on the combined inversion of surface and HEM data. In this way, 3-D multi-parameter models will be generated as additional input for numerical modeling of groundwater dynamics inside the freshwater lens.
The German Federal Institute for Geosciences and Natural Resources (BGR) carried out six airborne geophysical surveys in Northern Germany close to the estuaries of the Weser and Elbe rivers. Two of them were conducted in coopera-tion with the Leibniz Institute for Applied Geophysics (LIAG). The common aim was the acquisition of a reference data set for monitoring climate or man-made induced changes of the salt-water/fresh-water interface at the German North Sea coast and to build up a data base containing all airborne geophysical data sets. Airborne frequency-domain electromagnetic, magnetic, and radiometric data were collected simultaneously using BGR's helicopter-borne geophysical system. The airborne geophysical results show geological and hydrogeological structures down to about 100 m depth. The elec-tromagnetic data, converted to resistivity, reveal several hydrogeological important features such as the distribution of sandy or clayey sediments, the extension of salt-water intrusion, and buried valleys. The electromagnetic results are supported by magnetic and radiometric data indicating lateral changes of weakly magnetized sediments or mineral compositions of the top soil. The combination of airborne geophysical data sets provides a data base of a huge area serving as base-line data for a variety of applications and particularly for groundwater modelling and monitoring.
