Buried valley mapping in Denmark: evaluating mapping method constraints and the importance of data density

A large number of hydrogeological investigations in Denmark comprising TEM (transient electromagnetic) surveys, reflection seismic surveys, drillings and other methods have been performed during the past 10 to 15 years. These data have brought new geological insight into the uppermost 200 to 300 m of the Quaternary and Tertiary sedimentary successions. Buried tunnel valleys occur in intricate cross-cutting networks within this setting and by using the collected data a map showing the occurrence of the valleys has been produced. Local case studies as well as studies of relations between regional valley distribution and data distribution are used to demonstrate the importance of dense data grids and considerable penetration depths provided by the mapping methods. Buried valleys cannot be properly resolved on the basis of scattered borehole data and seismic data and thus dense data grids with a high lateral resolution are needed to resolve the comprehensive structural variations induced by the valleys. Although TEM data do not offer vertical resolutions as detailed as borehole and seismic data, dense TEM data grids and deep penetration make the method very useful for mapping of buried valleys.