Characterization of the seismic field at Virgo and improved estimates of Newtonian-noise suppression by recesses

TL;DR

This study characterizes the seismic field at Virgo, identifies dominant seismic modes, and demonstrates that recesses under test masses can reduce Newtonian noise by tenfold, enhancing gravitational-wave detector sensitivity.

Contribution

The paper provides detailed seismic mode analysis at Virgo and quantifies how recesses under test masses significantly suppress Newtonian noise.

Findings

Rayleigh wave speed in 10-20 Hz band is very low (≤100 m/s)

Seismic modes can be linked to known sources

Recesses reduce seismic Newtonian noise by a factor of 10

Abstract

Fluctuations of gravitational forces cause so-called Newtonian noise (NN) in gravitational-wave (GW) detectors which is expected to limit their low-frequency sensitivity in upcoming observing runs. Seismic NN is produced by seismic waves passing near a detector's suspended test masses. It is predicted to be the strongest contribution to NN. Modeling this contribution accurately is a major challenge. Arrays of seismometers were deployed at the Virgo site to characterize the seismic field near the four test masses. In this paper, we present results of a spectral analysis of the array data from one of Virgo's end buildings to identify dominant modes of the seismic field. Some of the modes can be associated with known seismic sources. Analyzing the modes over a range of frequencies, we provide a dispersion curve of Rayleigh waves. We find that the Rayleigh speed in the NN frequency band 10 Hz - 20 Hz is very low ($\lesssim$100\,m/s), which has important consequences for Virgo's seismic NN. Using the new speed estimate, we find that the recess formed under the suspended test masses by a basement level at the end buildings leads to a 10 fold reduction of seismic NN.