A multi-seismic approach to characterize the shallow subsurface for hosting the Einstein-Telescope infrastructure

Within the feasibility study regarding the construction of the Einstein – Telescope (ET), at a depth of 200 – 350 m in the Meuse-Rhine Euroregio, various seismic methods are used for subsurface investigation and imaging. The study includes a 40 km long two-dimensional active seismic-reflection survey using vibroseis. The vibroseis data provide a medium-resolution overview of the subsurface architecture including continuous reflection intervals and major discontinuities. Parts of the vibroseis survey were re-investigated using an electric vibrator to evaluate the potential of this environmental friendly seismic source. Extracted refraction data allowed to calculate a velocity model for the shallow subsurface. Additionally, passive seismic data were collected prior and during times of active surveying in the wider study area; linear data was used to optimise the station spacing for the area set-up and for the acquisition of seismic data perpendicular to the active survey. “Passive” area data were used to gather 3D insights. Geological interpretation of the various seismic reflection and refraction data integrated with subsurface knowledge from boreholes documented a major impedance contrast in the shallow subsurface caused by a prominent unconformity between Palaeozoic basement rocks covered by soft, partly consolidated Upper Cretaceous sediments. The soft-sediment cover effectively dampens surface-induced sounds in the target depth. This hampers the quality of subsurface imaging but is considered as beneficial for the operation of the ET in the Palaeozoic basement. With the collected passive seismic data, the main directions of surface generated noise and its denudation with distance from the respective sources can be identified.