The impact of seasonality over the sensitivity of Einstein Telescope and the SNR of CBC signals at the Sardinia candidate site

This study demonstrates that seasonal seismic noise variations at the Einstein Telescope's Sardinia candidate site have only a minor impact (a few percent) on detector sensitivity and signal-to-noise ratios for compact binary coalescence events, confirming the site's suitability for achieving low-frequency gravitational wave detection goals.

The Gist

Imagine the Einstein Telescope (ET) as a super-sensitive microphone designed to listen to the faintest whispers of the universe: the collision of black holes and neutron stars. To hear these cosmic whispers, the microphone must be placed in the quietest possible room. If the room is too noisy, the whispers get drowned out.

This paper investigates whether the chosen "room" for this microphone—a deep underground mine in Sardinia, Italy—gets too noisy when the seasons change.

Here is the breakdown of their findings using simple analogies:

1. The Problem: The "Seasonal Hum"

Just as your house might creak more in winter due to cold or buzz with more activity in summer, the ground itself vibrates differently depending on the time of year. These vibrations are called seismic noise.

For a detector like the Einstein Telescope, which is trying to hear frequencies as low as a deep bass note (2 Hz to 10 Hz), even tiny vibrations from the ground can create a "hum" that masks the cosmic signals. The scientists wanted to know: Does the ground in Sardinia get significantly louder in winter compared to summer, and will this ruin the telescope's ability to hear?

2. The Experiment: Listening to the Ground

The researchers acted like detectives, analyzing data from sensors buried deep underground (about 250 meters down) in Sardinia between 2022 and 2025.

• The "Best" Month: They found that July was the quietest month. The ground was almost perfectly still.
• The "Worst" Month: December was the noisiest. The ground vibrated more, likely due to seasonal weather patterns.

They compared these two extremes to see how much the "hum" changed.

3. The Calculation: The "Gravity Ghost"

The paper focuses on a specific type of noise called Newtonian Noise. Imagine the ground shaking not just because of an earthquake, but because the shifting mass of the Earth itself is tugging on the telescope's mirrors with gravity. It's like a ghost pulling on the equipment.

The scientists calculated how much this "gravity ghost" would interfere with the telescope's hearing during the quietest month (July) and the noisiest month (December).

4. The Results: A Very Quiet Room

The findings were surprisingly good news:

• The "Volume" Change is Tiny: Even in the noisiest month (December), the extra noise added to the telescope's "volume" was very small. In the worst-case scenario (the absolute loudest vibrations possible), the telescope's sensitivity dropped by about 20–25% for a brief moment. However, in a typical December, the drop was only a few percent.
• The "Gain" in Summer: In July, the ground was so quiet that the telescope actually performed slightly better than its standard design goals, gaining a tiny bit of extra hearing power.
• The Impact on "Hearing" (SNR): The most important metric is the Signal-to-Noise Ratio (SNR). Think of this as the clarity of a phone call.
  • If the signal is the person speaking and the noise is the background traffic, the SNR is how clearly you can hear them.
  • The study found that even with the seasonal noise, the clarity of the "cosmic phone call" dropped by only a few percent on average.
  • This means the telescope will still hear the vast majority of black hole and neutron star collisions just as well as it was designed to, regardless of the season.

5. The Conclusion: Sardinia is Ready

The paper concludes that the Sardinia site is robust. It is like a library that stays quiet even when the wind blows harder outside in winter. The seasonal changes in the ground's vibration are not strong enough to significantly block the telescope from hearing the universe.

In short: The Einstein Telescope, if built in Sardinia, will be able to hear the universe's deepest secrets clearly, whether it is July or December. The "seasonal hum" of the ground is too quiet to drown out the cosmic whispers.

Technical Summary

Technical Summary: The Impact of Seasonality over the Sensitivity of Einstein Telescope and the SNR of CBC Signals at the Sardinia Candidate Site

Problem Statement
The Einstein Telescope (ET), a proposed third-generation ground-based gravitational wave (GW) detector, aims to extend the accessible bandwidth down to 2 Hz, significantly improving the detection rate of compact binary coalescences (CBC) and enabling early warnings for multimessenger astronomy. A critical challenge for achieving low-frequency (LF) sensitivity is the mitigation of environmental noise, particularly Newtonian noise (NN) arising from seismic ground motion. While the Sardinia candidate site has been characterized as exceptionally quiet, the impact of seasonal variations in seismic noise on the detector's LF performance and the resulting signal-to-noise ratio (SNR) of CBC signals remains to be quantified. This work investigates whether seasonal fluctuations in seismic noise at the Sardinia site could degrade the ET design sensitivity enough to compromise the detection of binary neutron star (BNS) and intermediate-mass black hole (IMBH) signals, specifically within the 2–10 Hz band where ambient noise is dominant.

Methodology
The study employs a simulation framework based on seismic data collected between January 2022 and July 2025 from deep boreholes (approx. 250–264 m depth) at the Sardinia candidate site (locations P2 and P3). The methodology proceeds as follows:

1. Seismic Characterization: Monthly seismic spectra were analyzed to identify representative best-case (July) and worst-case (December) scenarios. The analysis focused on the 5th and 95th percentiles of the amplitude spectral density (ASD) to bound the variability.
2. Newtonian Noise Estimation: Using the measured horizontal seismic displacement ASD ($\tilde{x}$), the NN contribution to the GW strain ($\tilde{h}_{NN}$) in the 2–10 Hz band was estimated using the analytical model for a spherical cavern in a homogeneous medium (Equation 1). This calculation assumed body waves only, with a specific partition between compressional and shear waves, and explicitly excluded any NN mitigation factors to maintain a conservative estimate.
3. Sensitivity Curve Modification: The estimated NN was added to the ET design sensitivity curve to generate modified sensitivity curves for the identified seasonal scenarios.
4. SNR Simulation: Approximately 2,000 simulated CBC events (BNS and IMBH) were injected into simulated ET noise. The ET triangular configuration (three 10 km arms) was modeled with realistic detector geometry and time-dependent antenna patterns. Waveforms were generated using IMRPhenomD (for IMBH) and IMRPhenomPv2 NRTidalv2 (for BNS) approximants.
5. Performance Evaluation: The matched filter SNR was calculated for each event under the modified seasonal sensitivity curves and compared against the benchmark design sensitivity case. The analysis focused on the distribution of SNR losses and the fraction of events falling below the detection threshold (SNR = 12).

Key Contributions

• Seasonal Noise Quantification: The study provides a detailed characterization of seasonal seismic noise variations at the Sardinia site, identifying July as the quietest month and December as the noisiest, with December exhibiting significant outliers in the 2–5 Hz band.
• Conservative Sensitivity Modeling: By deriving modified sensitivity curves without applying NN mitigation factors, the work establishes an upper bound on the potential degradation of ET performance due to environmental fluctuations.
• SNR Impact Assessment: The paper quantifies the direct impact of these seasonal sensitivity variations on the detectability of BNS and IMBH signals, moving beyond theoretical noise curves to practical detection metrics.

Results

• Sensitivity Degradation: At median noise levels, the Sardinia site shows minimal impact on design sensitivity. In the worst median scenario (December), sensitivity is degraded by a factor of up to 1.6 at 2.3 Hz. However, in the extreme worst-case scenario (95th percentile of December data), the degradation peaks at a factor of 6.9 at 3.6 Hz. Conversely, in the best-case scenario (5th percentile of July), sensitivity can be up to 1.12 times better than the design target at 2.3 Hz.
• SNR Impact:
  • Median Conditions: When considering median seasonal noise levels, the impact on SNR is negligible. Both July and December show an average SNR gain of less than 2% relative to the design case. This slight gain occurs because the modified sensitivity curves are often better than the design target in the frequency bands that contribute most significantly to the SNR for these specific sources.
  • Extreme Conditions: In the best-case scenario (July, 5th percentile), the average SNR increases by approximately 3.6%. In the worst-case scenario (December, 95th percentile), the average SNR decreases by approximately 23% for IMBH and 20% for BNS.
• Detection Threshold: Under median conditions, the fraction of events falling below the SNR = 12 threshold remains largely unchanged. Even in the extreme December scenario, while a significant number of events (312 for IMBH, 165 for BNS) drop below the threshold, the majority of the population remains detectable.

Significance and Claims
The paper concludes that the Sardinia candidate site is robust against seasonal environmental variations. The authors claim that the intrinsically low seismic noise of the site ensures that seasonal fluctuations have only a minor effect on the early inspiral detectability of compact binaries. Specifically:

• The site is suitable for achieving the ET low-frequency sensitivity goals.
• Seasonal fluctuations do not significantly impair the ability to issue early warnings for BNS mergers or observe IMBH systems, provided that standard design sensitivities are met.
• The observed SNR losses in extreme scenarios (20–25%) are associated with rare fluctuations and are expected to be partially mitigated by future noise cancellation strategies.

The work serves as a crucial step in the preparation of noise suppression systems, confirming that the Sardinia site's stability supports the scientific potential of the ET, particularly for multimessenger astronomy and detailed observations of intermediate-mass black holes.