Delineation of Fault Systems on Langeland, Denmark Based on AEM Data and Boreholes

Abstract

In 1998 Denmark initiated a national groundwater mapping campaign in order to obtain knowledge of the aquifers with respect to their location, distribution, extension, interconnection and to acquire maps detailing groundwater vulnerability. The aim was to establish site-specific groundwater protection zones to prevent groundwater contamination from urban development and agricultural sources in agreement with The EU Water Framework Directive. The mapping campaign involved a dense data acquisition typically comprising boreholes, electromagnetic surveys — both airborne and land based and geoelectrical surveys. The data serve as basis for constructing 3D hydrogeological and groundwater models from which site-specific protection zones are establish. At present time dense geophysical mapping covers approximately 45% of Denmark.

Based on a dense Airborne ElectroMagnetic (AEM) survey in combination with boreholes, three fault systems in the northern part of the island of Langeland, Denmark are mapped. Two of the fault systems were unknown prior to the mapping campaign. The two unknown fault systems are interpreted as a normal fault and graben structures, respectively. The presence of the hanging-wall block in the fault systems can be observed in the AEM data as a low resistivity layer that clearly distinguish from the underlying and surrounding high resistivity fresh water saturated limestone (footwall block) and the overlying glacial clay till. Soil descriptions from a borehole confirm that the low resistivity layer can be correlated to Palaeocene clay deposits. The fault systems were most likely initiated in the early Neogene during the Alpine orogeny. The fault systems are observed to alter the hydrology significantly and are therefore important to map.