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

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

Problem Statement
The Einstein Telescope (ET), a proposed third-generation gravitational-wave observatory, aims to extend the observational band to low frequencies (1–10 Hz) to detect massive black hole mergers and early inspiral phases. However, sensitivity in this range is fundamentally limited by environmental noise, specifically Newtonian noise (NN), also known as gravity-gradient noise. NN arises from time-dependent fluctuations in the local gravitational field caused by moving environmental masses, including atmospheric density perturbations. Unlike other noise sources, NN cannot be shielded by conventional isolation systems. While the Sos Enattos site in Sardinia, Italy, is a leading candidate for the ET due to its low seismicity and anthropogenic noise, the seasonal variability of atmospheric infrasound and its depth-dependent contribution to NN at this specific location remained insufficiently characterized.

Methodology
The study presents a comprehensive, long-term analysis of infrasound measurements conducted at the Sos Enattos mine using a network of sensors deployed at three distinct depths:

• SOE0: Surface station (400 m above sea level).
• SOE1: Underground station at −84 m.
• SOE3: Underground station at −160 m.

Data were collected over multiple seasons (spanning from November 2022 to April 2025) using condenser microphones, including reference GRAS 47AC units and custom-developed sensors. The analysis involved:

1. Spectral and Statistical Analysis: Calculation of Amplitude Spectral Density (ASD) and Cumulative Distribution Functions (CDFs) to characterize temporal variability and seasonal modulation.
2. Environmental Correlation: Synchronization of infrasound data with meteorological variables (wind speed, atmospheric pressure, temperature, relative humidity) from a nearby station and marine wave data (Significant Wave Height) from Copernicus Marine Environment Monitoring Service reanalysis.
3. NN Estimation: Conversion of measured acoustic pressure fluctuations into local air-density perturbations using the linear-acoustic approximation. The resulting gravitational acceleration acting on a test mass was numerically integrated over a modeled cavern volume (10 m × 10 m × 15 m, excluding a vacuum tower). The acceleration was then converted to strain-equivalent amplitude spectral density ($S_h^{1/2}$) for a 10 km arm length, allowing direct comparison with the ET-D design sensitivity curve.

Key Contributions and Results

• Seasonal Variability: The infrasound background exhibits robust seasonal modulation. Winter levels consistently exceed summer values by 10 to 15 dB at 1 Hz, primarily driven by increased wind activity and synoptic-scale atmospheric disturbances.
• Environmental Drivers: Wind speed is identified as the dominant factor controlling infrasound variability across all stations. Atmospheric pressure and marine microbaroms (correlated with wave height) show secondary effects, particularly at the surface. Relative humidity and temperature play minor roles, with effects largely confined to near-surface measurements.
• Depth Dependence: A clear attenuation of infrasound with depth is observed. At the surface (SOE0), the median characteristic strain reaches $\sim 10^{-22}$ at 1 Hz. At the deepest station (SOE3, −160 m), this value decreases to $\sim 10^{-27}$, representing a suppression of approximately five orders of magnitude.
• Newtonian Noise Impact:
  • Surface (SOE0): While the median NN contribution remains below the ET-D design sensitivity curve, high-noise periods (95th percentile) can exceed the design curve between approximately 3 Hz and 10 Hz.
  • Underground (SOE3): The estimated NN remains well below the ET-D design sensitivity curve across the entire 1 to 10 Hz frequency band, even under unfavorable environmental conditions.

Significance and Claims
The authors conclude that atmospheric infrasound does not constitute a limiting noise source for underground gravitational-wave detectors at the Sos Enattos site, provided the instrumentation is deployed at sufficient depth. The study demonstrates that the strong attenuation of infrasound-induced NN with depth effectively mitigates the impact of peak surface atmospheric disturbances. These findings support the suitability of Sos Enattos as a candidate site for the Einstein Telescope and reinforce the critical importance of underground deployment and continued environmental monitoring for achieving low-frequency sensitivity goals. The results confirm that the atmospheric contribution to NN is manageable at this site, unlike the surface environment where it poses a potential challenge during high-noise periods.