Seismic noise characterization at a potential site for the Einstein telescope underground gravitational wave detector

With the first detection of gravitational waves on September 14, 2015 and subsequent detection of four other binary black hole mergers, significant efforts have been made to improve the detector sensitivity especially at low frequencies. Seismic noise and its gravitational counterpart Newtonian noise are the two main factors limiting the sensitivity at frequencies below 20 Hz. Einstein Telescope is a European project for constructing a third generation gravitational wave detector with an effort to increasing its sensitivity at low frequencies. The detector is to be built 300 m underground to reduce the impact of seismic and Newtonian noise. Limburg, Netherlands is one of the sites for the project. Understanding the spatio-spectral properties of seismic noise at the site and the geology of the area is vital for devising schemes for simulation of seismic and Newtonian noise at desired depths. In this paper we report the results of a reconnaissance passive seismic survey, conducted at the site. Array based methods like beamforming, and seismic interferometry is used to understand the propagation characteristics of the seismic noise in the frequency range 2.5-8.0 Hz. The estimated Rayleigh wave phase velocity is further used to invert for a 1D S-wave velocity model.