S-wave Velocity Model Estimation using Ambient Seismic Noise at Virgo, Italy

In this study, we present an analysis of seismic noise recorded at the Virgo gravitational wave detector in Italy. The study originates from the need to understand the spatio-spectral characteristics of the seismic noise at the Virgo site. Studying the direction of propagation and Rayleigh wave phase velocities of possible noise sources is of utmost importance for accurate computation of Newtonian noise, which appears to be the ultimate low frequency limit for ground based gravitational wave detectors. For this purpose, a seismic array was designed that would ensure correct estimation of phase velocities and propagation direction of ambient seismic noise in the frequency range 0.4 - 8.0 Hz. Consequently, a passive seismic survey was carried out using a network of 5 Hz vertical component wireless geophones at the site between 13-28 August 2016. The 0.2 - 0.7 Hz frequency band was dominated by the secondary microseismic energy originating from the Mediterranean sea. A strong correlation was observed between the microseismic energy and the significant wave height in the Mediterranean sea during the time. Seismic noise in the frequency band 1.5 - 4.0 Hz was dominated by ground motions induced in the subsurface by road bridges and local traffic activities. In the 4 - 8 Hz band, noise sources were mostly local and transient. The direction of propagation of the ambient noise was estimated using beamforming. To estimate the phase velocities, both beamforming and extended spectral autocorrelation (ESAC) were used. Using the derived Rayleigh wave phase velocity corresponding to the fundamental mode, an inversion was carried out to estimate a 1-D S-wave velocity model for the region. Subsurface properties obtained from the study can be further used for computing the gravity gradient noise associated with seismic motions in the region.