Infrasound Newtonian Noise Estimation at the Einstein Telescope Candidate Site Sos Enattos

Abstract

We investigate the seasonal variability of atmospheric infrasound and its contribution to Newtonian noise (NN) at the Sos Enattos site, a leading candidate location for the Einstein Telescope (ET). Infrasound data recorded at three stations, SOE0 (surface), SOE1 ($-84$ m) and SOE3 ($-160$ m), are analysed over multiple seasons to characterise both the temporal variability and the depth dependence of the acoustic field. At the surface, the median amplitude spectral density (ASD) at 1 Hz shows no consistent seasonal modulation, and its variability is driven primarily by wind; the apparent dependence on sea state largely reflects its correlation with wind speed. Underground, where local wind noise is suppressed, a winter maximum of 4--7 dB above the quietest season is observed in both years of data. Using the measured sound-pressure spectra, we estimate the corresponding NN contribution for an underground cavern with the dimensions foreseen for ET, assuming an isotropic field of acoustic plane waves. The surface pressure field, acting through the atmosphere above a test mass at 200 m depth, exceeds the ET-D design sensitivity in median conditions between 1.6 and 7.1 Hz, by up to a factor of 4.7. Inside the cavern, the median NN remains a factor of 1.4 below ET-D with the pressure measured at SOE1, but exceeds it between 2.5 and 8.7 Hz, by up to a factor of 3.5, with the pressure measured at SOE3. The attenuation is not monotonic with depth: the deeper station SOE3 shows higher levels than SOE1 above a few hertz, pointing to local, anthropogenic sources in the underground infrastructure. These results show that underground placement is essential but not sufficient on its own, and that the acoustic emission of the underground infrastructure and future laboratory equipment will require careful evaluation and mitigation to keep atmospheric NN below the ET design sensitivity.

arXiv comment17 pages, 11 figures, accepted for publication in Classical and Quantum Gravity