Over the next two years, a series of provisionally about ten soil drillings combined with seismic surveys should map the exact geological composition of the subsurface within the search area of the Einstein Telescope. Those drillings will take place across the search area in Wallonia, Flanders and (Dutch) South Limburg.
250 metres deep
The soft cushioning upper layer with the hard lower layer make the subsurface in the area suitable for building the underground observatory. In addition, drilling and seismics are needed to map specific information about the soil layers. Therefore, the drilling is not intended to determine exact locations of the three underground vertices.
Drilling will be carried out to an average depth of 250 metres. This is necessary because the Einstein Telescope will be built 200 to 300 metres underground. The average duration of drilling is four to six weeks. Once the locations have been definitively determined, the permit procedure and communication with the surrounding area will start. The intended sites are in the outlying area.
The first drilling in the Walloon town of Aubel commissioned by the University of Liège was recently completed as part of the Interreg EMR project E-TEST. Drilling was carried out to a depth of 250 metres. The next drilling is planned from autumn onwards. There is no exact sequence yet, partly because the results of an earlier drilling will influence the location of a subsequent drilling. Depending on the analysis of the ten wells, some additional drilling may be necessary.
Whereas drilling gives a picture of the subsurface conditions at that location itself, seismic provides insight into the subsurface quality between drilling locations. Seismics involves sending sound waves into the soil. The reflection of those waves teaches how the layers in the subsurface are constructed.
The drilling and seismic programme was developed on behalf of the Einstein Telescope project office. In addition to its own experts working for the project office from Nikhef, TNO is contributing to supervising and testing the implementation.