Exploration of buried valleys / paleochannels

Exploration-buried-valleysSetting

Denmark is a world leader in mapping groundwater resources. In the 1990s in order to locate and protect the country’s natural resources the Danish Government embanked on the ambitious plan to map the subsurface of the entire country. The Danish Environmental Protection Agency (EPA) set out to determine the extent of the groundwater resources in the country as well as to gain an understanding of where they could be vulnerable and in need of protection.

Solution

The SkyTEM TDEM method is designed to be sensitive to subtle resistivity variations in soil and rock and is the ideal tool for mapping the interface between the quarternary sediments and the more conductive sticky tertiary clay. For this reason the EPA and the hydrogeological team in Denmark quickly identified the SkyTEM method as the method for outlining the lower boundary of the aquifers. They were able to rapidly collect and assess valuable information describing the lower boundary of the quarternary sediments. These were characterized by a large number of distinct paleochannels/buried valleys ranging in depth from 10 to 20 m and down to more than 300 m.

Findings

Depth to a strong conductor (surface of the tertiary clay) was extracted by modeling the results of the TEM data.

The paleohannels can be traced over long distances. Typically the channels can be divided into primary and secondary channels (secondary channels are those drained by the primary channels). It can be seen that few of the boreholdes drilled into the aquifer are deeper than a few tens of metres. It is obvious that a leap of faith and some creativity would be required to arrive at an interpretation similar to that provided by the SkyTEM system.

An area that was going to be developed as a new industrial area was located directly above one of the primary groundwater aquifers. To protect the water, the country re-evaluated these plans and it was decided to leave the land as farmland and forest to avoid the risk of contaminating the aquifers. The farmers subsequently made agreements with the local water works to reduce the usage of fertilizer and pesticides/herbicides.

Conclusion

This new knowledge made it possible to determine the size of groundwater resources as well as to understand where to protect them. These results have had a significant influence on urban planning. This case is evidence that the SkyTEM method is a valuable investment, a major step towards securing clean drinking water for future generations and an essential input for sustainable groundwater management planning.