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

Ayatri Singha,Alberto Gennai,Federico Paoletti,Enrico Calloni,Stefan Hild,Rosario De Rosa,Bartosz Idzkowski,Alessandro Bertolini,Luciano Errico,Irene Fiori,Tomasz Bulik,Jan Harms,Maria C Tringali

doi:10.1088/1361-6382/ac348a

Abstract

Fluctuations of gravitational forces cause so-called Newtonian noise (NN) in gravitational-wave 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 10–20 Hz is very low (≲100 m s−1), 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.

Highlights

The interferometric gravitational-wave (GW) detectors LIGO and Virgo have brought exciting discoveries in the past years [1,2,3,4,5,6,7]
We presented an analysis of seismic array data from Virgo’s North End Building (NEB)
Based on spatial spectra of the field, we measured the dispersion of Rayleigh waves, which we used to update previous Newtonian-noise (NN) predictions for Virgo

Summary

INTRODUCTION

The interferometric gravitational-wave (GW) detectors LIGO and Virgo have brought exciting discoveries in the past years [1,2,3,4,5,6,7]. We present a spectral analysis (in space and time) of the seismic field at the North End Building (NEB) of the Virgo detector Data used for this analysis were obtained from an array of 38 indoor sensors and 11 outdoor sensors as shown in figure 1. We explicitly discuss such kf maps for three frequencies 10 Hz, 15 Hz, and 20 Hz. In section III, we first show the velocity dispersion curve based on the dominant noise components in the kf maps generated at different frequencies.

CHARACTERISATION OF THE SEISMIC FIELD

VIRGO NEWTONIAN NOISE REASSESSMENT

SUMMARY AND OUTLOOK