Ground research for Einstein Telescope enters into new phase: “This is a very complex puzzle”

Ground research for Einstein Telescope enters into new phase: “This is a very complex puzzle”

How suitable is the Limburg soil for the construction of a gigantic detector?

07-06-2023 · Background

Will the billion-euro Einstein Telescope project come to Limburg? The decision will be taken in about two years’ time. An important question is: how suitable is the soil for the construction of an underground detector with a length of many kilometres? A large-scale study should provide clarity. “This is a very complex puzzle.”

“It is as if you just put a roll of peppermint into a bottle of cola,” says geologist Bjorn Vink, pointing to a drilling installation several metres high that spews out rocks under great pressure. This colossus has been here since the end of May, in a field near the Walloon village of Aubel, a few kilometres from the Dutch border. The objective is to take a ‘bite’ out of the soil, to determine what it looks like deep down. The counter stands at ten metres at the moment, but ultimately the drill will have to reach at least 250 metres. With a diameter of no more than forty centimetres, it will only be a pinhole in the landscape.

The drilling kicks off a new phase in the soil research for E-TEST, a joint project of the Netherlands, Belgium and Germany in preparation for the possible creation of the Einstein Telescope in the border region. This observatory for minute vibrations in space-time (so-called gravitational waves), which will cost a few billion euros, should throw new light on the universe.

The decision whether the underground detector will be built here or at the competing location in Sardinia, will be taken in 2025 or 2026. Both locations boast favourable geology: ‘quiet’, almost vibration-free soil, in which the sensitive equipment can do its work undisturbed. The South Limburg hills owe this to a combination of a soft upper layer – the marl – with a hard, rocklike layer underneath, which together dampen the noise above ground.

But how suitable is this soil for building a gigantic system of tunnels, for a triangular detector with sides as long as ten kilometres, at a depth of two to three hundred metres? And what is the exact spot where this is supposed to happen? Nobody knows for sure. “It is an area that has hardly been investigated,” says Vink, who works at the Einstein Telescope project office at Maastricht University and is involved with E-TEST on behalf of the scientific institute Nikhef. Around the edges, a certain amount of knowledge has been gathered in the past as a result of mining, but the search area itself – roughly the triangle between Aachen, Liège and Maastricht – is still a blind spot.

Tropical sea

A number of previous studies, including drillings in the villages of Banholt and Epen, have led to some insights in the past few years. “But ultimately, that was mainly about testing the technology. Now we are ready to discover how things really are." Gathering pace is really necessary, because the bid book – and so a definite idea of what is possible in this area – must be ready in two to three years’ time.

Soil drilling, like here in Aubel, plays an important role. The current drilling is commissioned by the University of Liège, but soon the ET project office will direct the drilling of “initially ten, but most likely many more” holes of hundreds of metres deep in different areas in the region. “Of course accompanied by the necessary permits and communication with the surrounding area,” Vink is quick to add. With each ‘pinhole’, the local situation will become clear, and the underground map of the area will become more detailed.

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The drilling site in Aubel

“This is an exceptionally large hole, because this is where special measuring equipment will be lowered,” says Vink, trying to make himself heard over the noise of the machines. The other drill holes will be narrower, less than ten centimetres in diameter. “Like that one there,” Vink points to a hole a few metres further along, which was drilled last week and has now been covered by a pole (“for the safety of the horses that will return to the field after the investigation has been completed”).

That is where they met with an instant surprise: at a depth of 27 metres, there appeared to be a layer of bluestone. “The material used for the famous bluish grey windowsills, but for example also for parts of the Sint Servaasbrug. That is a very old layer, formed approximately 340 million years ago, when this area was located more or less at the equator. There was a shallow sea here, as this type of stone comes from sedimentary deposits at the bottom of such waters, which were bursting with life.” At previous drilling locations a few kilometres from here, they did not find bluestone. “I also suspect that you won’t find it under the hill a hundred metres further along. In those places, there was most likely an island or a cliff. Moreover, this layer appears to dive lower into the ground a little bit further along.”

Crashing cars

It illustrates why the study is so challenging. Not only is the ground here a patchwork quilt of various layers – each with its own characteristics in terms of composition and firmness – but it also varies strongly from one location to the next. A crumple zone, is how Vink describes it, as if Belgium and the Netherlands are two cars that have crashed into one and other. “Because of the collision between the African and European continent, this area contracts slowly, a process that has been going on for many millions of years. Pieces of land are constantly being pushed upwards and downwards. Older stone layers slide over younger ones and vice versa. Layers that were once flat are now chaotically curving throughout the soil. They are even non-existent in some places.”

So, not every layer is suitable for accommodating the Einstein Telescope. “Ultimately, we are looking for certain types of hard stone. It is more difficult to drill into, but it is very strong. Additional support will not be necessary. That is especially important for the three vertices, for which large halls have to be drilled. The challenge is finding three locations where the ‘good’ layers are at the same level of about 250 metres, and then also approximately ten kilometres from each other, so that the tunnels (for the sides of the triangle) fit exactly in between. A complex puzzle.”

Crevices and caves

To make things even more complicated: there is underground water flowing everywhere. “In some places, these streams come to the surface as wells in the landscape, but they can also be really deep. Besides. they are often connected to each other between various layers in the ground. You want to map all that out. Just like crevices and caves, which are often found in the transitional areas between the layers due to collisions and erosion in the past. During the drilling for the tunnels and halls, you want to come across as little as possible. Pumping water away and strengthening or filling in the crevices is expensive and labour-intensive.”

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What Einstein Telescope should look like: a triangle with sides of ten kilometres at a depth of roughly 250 metres. Image: T. Balder/Nikhef

Ultimately, all these elements will come together in a large 3D model of the soil, which the tunnel designers can use to start and put the puzzle together. Vink: “When you combine all the ‘pinholes’, you can simulate how this area has ‘danced’ over the past hundreds of millions of years, and estimate what the soil looks like in other places.”


This information will be supplemented by a series of ‘tools’ that will carry out work above ground. Among them so-called ‘vibrotrucks’: vehicles that use a vibration plate to briefly shake the ground. Every layer in the ground reverberates these vibrations in its own way, allowing sensors on the surface to indirectly determine the composition. A column of such trucks passed through the Heuvelland last year; soon “more economical, smaller and more accurate” versions will follow, said Vink. The use of drones is also being considered. “These will hover above the surface to measure how minerals in the ground react to an electromagnetic field. The strength of the signal indicates the concentrations of these minerals, and therefore the type of stone.”

Even with a 3D model of the soil, not all the pieces of the puzzle have been collected, however. “Vibration sensors at the bottom of the drilled holes will identify sources of ‘noise’. Things like vibrations caused by windmills or motorways, but for example also by a forest when it is very windy. As well as by earthquakes: in this area minimal movements can occur along a fault line. It is not insurmountable if your tunnel is on or in the neighbourhood of such a fault line, but it does require more maintenance and repairs.”

Transporting soil

Then there is the logistics above ground. “During construction, enormous amounts of soil – something in the region of a few million cubic metres – will be brought up to the surface. This will be excavated at one or more of the cervices. We are already thinking about that. Can you transport this here efficiently, for example by the presence of a train tracks in the area, or can it be processed on site, without causing too much of a nuisance to the surrounding area and pollution of the landscape?”

So, puzzles for the experienced. What if the soil turns out to be more complicated than one had hoped? Will the whole project be cancelled then? “No, technically you could almost always build the detector anyway. But in bad layers of soil, it will be much more expensive. In that case, you have to ask yourself: is it worth it? The geology is after all the most determining factor for the total cost of the project.” But we are not that far yet. First, we have to collect a great deal of knowledge.

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Images of the previous 'testing' phase of the soil research. Video: Bjorn Vink/Youtube

“You never know what you will discover”

What if Limburg doesn’t get the Einstein Telescope? Will the whole soil study have been a waste? No, absolutely not, says Vink. The soil study will also provide the necessary ‘incidental discoveries’. “Creviced rocks and underground streams of water are awkward for us, but for others it is very interesting.” For example, for extracting drinking water, or using the heat of this water as a sustainable source of energy – the so-called geothermal heat. In times of scarcity and climate change, this is an added bonus. “The necessary rocks and technologies are relatively easy and cheap to test here. Elsewhere in the country, these layers are located at a depth of several kilometres. Then you need to be sure that it works, before you start drilling.”

Projects concerning conservation could also benefit, says Vink. For example, knowledge about minerals and contamination of the soil and spring water is useful for the protection of vulnerable plant species. “Plus, as always with these types of large projects, this study could produce new and innovative techniques or measuring equipment, for example, in the field of tunnel construction. You never know what you will discover.”

But fundamentally it is also very interesting, Vink concludes enthusiastically. “What makes the area complex, is also what makes it fascinating. The soil is a kind of film of the history of this region.” The first geological discoveries have already taken place. “Near Epen, we found the oldest rock that appears in the Netherlands at surface level. With 328 million years, it turned out to be about four million years older than the previous record.” In the process, Vink also picked up a fossil of a plant, the oldest fossil discovered by hand in the Netherlands.

Photo's: Observant

Tags: einsteintelescope,drilling,ground,soil,study,gravitational waves,geology,science,research,instagram

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