Results from a 10 year AEM mapping programme of buried tunnel valleys in Denmark

Extensive mapping with airborne EM has since 2003 revealed intricate patterns of buried tunnel valleys in the Danish subsurface. Several generations of cross-cutting valleys are found making up a highly complicated geological setting. The valleys play a vital role for the Danish water supply because they are assumed to host about 50 % of all extractable groundwater, but they are also important because they impose a significant impact on groundwater flow. Especially three properties of the valleys are essential to map: 1) their spatial extent, 2) their internal architecture, and 3) the lithology of the infill and surrounding environment. Borehole data constitute an insufficient basis for 1) and 2). Instead, techniques like the airborne SkyTEM system providing 3D data are required. Almost 20.000 km² have been mapped with the TEM method resulting in more than 5000 km mapped buried valleys. With the SkyTEM system full information about the spatial extent of the valleys (1) can normally be achieved, but also information about the internal architecture (2) and lithology (3) is obtained to some extent. Collection of seismic data and borehole data provide supplementary information about (2) and (3) at selected sites.