DETAILED GEOPHYSICAL MAPPING OF BURIED TUNNEL VALLEYS - A PREREQUISITE FOR SUSTAINABLE GROUNDWATER MANAGEMENT IN DENMARK

Extensive geophysical mapping has over the last 15 years revealed intricate patterns of buried tunnel valleys in the Danish subsurface. Several generations of cross-cutting valleys are found making up a highly complicated setting. Furthermore, individual valleys are often composed of several minor incisions filled with different sorts of sediments, and single valleys can therefore contain numerous aquifers at different levels and locations.

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 and thus constituting important elements in hydrogeological models. A comprehensive amount of geological information is needed in each survey area in order to properly model the hydrogeology if used for groundwater management purposes.

Especially three properties of the valleys are necessary 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 providing 3D data are required. The SkyTEM system is widely used for collecting such 3D data in Denmark and almost 15.000 km² have been covered with the result of more than 4000 km mapped buried valleys. With the SkyTEM system full information about the spatial extent of the valleys (1) can normally be achieved, whereas only some information about the internal architecture and the lithology of the infill (2 and 3) can be achieved. Drilling and borehole data are gathered to get information about the infill and 2D seismic reflection surveying is performed in order to get information about the structural composition. Integrated assessments of the data enable interpretation of the complex erosional and depositional history of the valley formation.

The detailed mapping of the buried tunnel valleys allow us to more reliably assess the amount of available groundwater in a given area and to better calculate recharge areas and transportation pathways to abstraction wells. With these details at hand, we will be also able to evaluate the size of current and future threats to the groundwater quality and thereby be able to secure the future water supply.