Newtonian-noise characterization at Terziet in Limburg-the Euregio Meuse-Rhine candidate site for Einstein Telescope

Limburg, in the border region between Belgium, Germany and the Netherlands, has been identified as the Euregio Meuse-Rhine candidate site for Einstein Telescope. The site hosting this gravitational-wave observatory must minimize the Newtonian coupling of ground vibrations to the core optics of the low-frequency detectors. Newtonian noise depends on the ambient seismic field which is in turn dependent on the site's geology and the distribution of surface and underground seismic-noise sources. We have characterized the site near Terziet in Limburg in terms of propagation modes, dispersion and angular distribution of seismic noise by employing sensor arrays on the surface. Attenuation of seismic noise with depth was studied with a borehole sensor. Based on the results of these measurements, a realistic seismic-field model has been derived that represents a complete solution of the elastodynamic wave equations for a horizontally-layered soil structure. This seismic-field model allows to estimate the Newtonian-noise contribution to the sensitivity of Einstein Telescope for the characteristic geology and ambient noise conditions in South Limburg. The site's geology features soft-soil layers on hard-rock and is effective in attenuating Newtonian noise from surface waves below the required sensitivity. A random background of body waves with all possible angles of incidence is expected to constitute the dominant source of Newtonian noise.