future view

Eys borehole solves another piece of the subsurface puzzle

25 March 2019 • 2 min reading time

Last year the Geological Survey of the Netherlands (Geologische Dienst Nederland, GDN), part of TNO, drilled a 180-metre borehole at Eys to gather information on the geology of South Limburg, where mosasaurs and dinosaurs lived during the Late Cretaceous. The results of this subsurface exploration are essential to many decisions made at the surface.

Drilling or digging tunnels, laying out new neighbourhoods, extracting minerals, and sustainably using geothermal heat are just some of the activities that require a detailed knowledge of underlying strata, both deep underground and closer to the surface. As of 1 January 2018, all this information is being combined in the Dutch Key Register of the Subsurface (Basisregistratie Ondergrond, BRO), a national knowledge base managed by the GDN. The Eys borehole is solving another piece of the underground puzzle.

Transiton between rock formations

“We are working on a mapping programme for the Ministry of Economic Affairs and Climate Policy and the Ministry of the Interior and Kingdom Relations,” explains geologist Geert-Jan Vis of the GDN. “Last year we chose to drill a borehole near Eys, principally because the existing geological model shows a transition there between two Cretaceous rock formations but it’s not clear whether this is a sudden or a gradual transition. The answer will give us more insight into groundwater flow patterns.”

A second reason to drill a borehole at Eys lies at the surface, 190 metres above sea level. Vis: “Elsewhere in the Netherlands, the Cretaceous limestone sequence is seldom exposed because it lies kilometres below the surface. Here, however, it is almost at the surface and is quite thick. We wanted to exploit the opportunity to examine this as much as possible. For example, the sandstone layers beneath the limestone sequence form an excellent aquifer, a water-retaining ground layer. If we ever need to extract water from it in the future, then the knowledge we gain now will be very useful.”

Einstein Telescope

The third aim of the Eys borehole is to examine the upper layers of the underlying, much more ancient Carboniferous strata. “Much of the rock that dates from after that era is absent – a so-called ‘hiatus’. Cretaceous rock lies directly on top of the Carboniferous layer. The depth of the Carboniferous strata in this region is roughly known, but the deposits from that era have been there for over 300 million years. The Carboniferous layers are probably inclined. If we can find out whether the upper layers are Upper, Middle or Lower Carboniferous rock, we will better understand whether they were folded, broken, or whatever.”

Michiel van der Meulen, head of mapping for the GDN, emphasizes the importance of the project. “This borehole also contributes towards the national ambition to bring the Einstein Telescope to the South Limburg Euregion: this telescope measures gravity waves, using three 10-kilometre underground tunnels, to tell us more about the origins of the Earth and of the universe. But mapping at much greater depths is also essential – for ultradeep geothermal heat, for instance, that could help to make our energy demands CO2-neutral.”

want to know more, or even add your own bore data?

“We will now take the rock core samples and slice them, describe them and analyze them,” explains Vis. “At the base of the borehole, we have already recognized rock that is 80 million years old – the time of the dinosaurs. I invite any companies or municipalities who have questions about the subsurface, or who perhaps have borehole data of their own to contribute, to get in touch with us. And if you’re interested in the newest geological map of the Netherlands, then leave your contact details on the online registration page.”

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