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

Author(s)

Di Giovanni, Matteo, Rozza, Davide, Diaferia, Giovanni, Contu, Andrea, De Rosa, Rosario, Giunchi, Carlo, Naticchioni, Luca, Olivieri, Marco, Allocca, Annalisa, Calloni, Enrico, Cardello, Giovanni Luca, Errico, Luciano, Losurdo, Giovanni, Molinari, Irene, Trozzo, Lucia, D'Urso, Domenico

Abstract

This work investigates the impact of seasonal variations in seismic noise on the low-frequency performance of the Einstein Telescope (ET) at the Sardinia candidate site, focusing on implications for compact binary coalescence observations. Using seismic data collected between 2022 and 2025 in deep boreholes, we characterize monthly noise variations and identify representative best and worst case scenarios, corresponding to July and December. The measured seismic spectra are used to estimate the Newtonian noise contribution in the 2-10 Hz band and to derive modified ET sensitivity curves. These are implemented in a simulation framework to evaluate their effect on the signal-to-noise ratio (SNR) of binary neutron star and intermediate mass black hole signals, assuming the triangular ET configuration. We find that the low seismic noise of the Sardinia site results in only minor seasonal variations in detector sensitivity. The corresponding impact on SNR is limited to a few percent, even without including Newtonian noise mitigation. These results indicate that seasonal environmental fluctuation have a minor effect on the early inspired detectability of compact binaries, confirming the suitability of the Sardinia site for achieving ET low-frequency sensitivity goals.

Figures

Map of Europe showing the locations of the official ET candidate sites. The village of Terziet in the Netherlands is usually taken as a reference for the EMR candidate site; the Sos Enattos mine is taken as a reference for the Sardinia site. In this work, we consider the Sardinia site. The maps are taken from \cite{sardiniamap, netherlandsmap, europemap} and modified according to the creative commons license 3.0. The orientation of the triangle icons is not representative of the actual proposed orientations for the ET detector at each site.
Caption Map of Europe showing the locations of the official ET candidate sites. The village of Terziet in the Netherlands is usually taken as a reference for the EMR candidate site; the Sos Enattos mine is taken as a reference for the Sardinia site. In this work, we consider the Sardinia site. The maps are taken from \cite{sardiniamap, netherlandsmap, europemap} and modified according to the creative commons license 3.0. The orientation of the triangle icons is not representative of the actual proposed orientations for the ET detector at each site.
Spectra from borehole sensors at P2 (blue) and P3 (red) for the period from January 2022 until July 2025. The solid lines represent the median of the distribution of 120 s long spectral segments. The data from a borehole sensor installed in the EMR region are also shown (orange). The colored bands show the area between the 5th and 95th percentile of the distributions. Peterson's NLNM is also shown (dashed line).
Caption Spectra from borehole sensors at P2 (blue) and P3 (red) for the period from January 2022 until July 2025. The solid lines represent the median of the distribution of 120 s long spectral segments. The data from a borehole sensor installed in the EMR region are also shown (orange). The colored bands show the area between the 5th and 95th percentile of the distributions. Peterson's NLNM is also shown (dashed line).
Evaluation of the noise level for P2 at global scale. For each frequency bin, on top of each frame, we calculate the mean ASD for each of the stations that compose the Incorporated Research Institutions for Seismology virtual network FDSN+ALL in the period from January 2022 until July 2025. In each frame, the vertical bar indicates the average level of noise as observed at P2 in the same time span.
Caption Evaluation of the noise level for P2 at global scale. For each frequency bin, on top of each frame, we calculate the mean ASD for each of the stations that compose the Incorporated Research Institutions for Seismology virtual network FDSN+ALL in the period from January 2022 until July 2025. In each frame, the vertical bar indicates the average level of noise as observed at P2 in the same time span.
Comparison of the seismic spectra from P2 in July (red) and December (blue). The two represent the best and worst periods, respectively, in terms of the impact of seasonality over the seismic noise levels in Sardinia.
Caption Comparison of the seismic spectra from P2 in July (red) and December (blue). The two represent the best and worst periods, respectively, in terms of the impact of seasonality over the seismic noise levels in Sardinia.
The impact of seasonality on the design sensitivity of ET (black curve). The seasonal changes are represented by the red (July) and black (December) curves. The shaded regions represent the area included between the 5th and 95th percentiles.
Caption The impact of seasonality on the design sensitivity of ET (black curve). The seasonal changes are represented by the red (July) and black (December) curves. The shaded regions represent the area included between the 5th and 95th percentiles.
Ratio between the modified sensitivity curves for July (red) and December (blue) and the design sensitivity curve. The shaded regions represent the area included between the 5th and 95th percentiles. Considering only the median noise levels, up to 6 Hz the sensitivity is degraded by, at worst, a factor 1.6, whereas beyond 6 Hz Sardinia performs better than the design case. The larger departure from the design case happens when considering the worst possible scenario: the 95th percentile in December.
Caption Ratio between the modified sensitivity curves for July (red) and December (blue) and the design sensitivity curve. The shaded regions represent the area included between the 5th and 95th percentiles. Considering only the median noise levels, up to 6 Hz the sensitivity is degraded by, at worst, a factor 1.6, whereas beyond 6 Hz Sardinia performs better than the design case. The larger departure from the design case happens when considering the worst possible scenario: the 95th percentile in December.
Characteristic strain $2\tilde h(f)\sqrt{f}$ of a sample BNS from \cite{cobacatalog} at 16 Gpc with $M_\mathrm{tot} = 3.47 \, M_{\bigodot}$ overlapped with the median sensitivity curves used in this work.
Caption Characteristic strain $2\tilde h(f)\sqrt{f}$ of a sample BNS from \cite{cobacatalog} at 16 Gpc with $M_\mathrm{tot} = 3.47 \, M_{\bigodot}$ overlapped with the median sensitivity curves used in this work.
SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the 50th percentile of the ground motion PPSDs from July and December. The target SNR is shown as a vertical black dashed line.
Caption SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the 50th percentile of the ground motion PPSDs from July and December. The target SNR is shown as a vertical black dashed line.
SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the 50th percentile of the ground motion PPSDs from July and December. The target SNR is shown as a vertical black dashed line.
Caption SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the 50th percentile of the ground motion PPSDs from July and December. The target SNR is shown as a vertical black dashed line.
SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the best and worst possible case scenarios from July and December. The target SNR is shown as a vertical black dashed line.
Caption SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the best and worst possible case scenarios from July and December. The target SNR is shown as a vertical black dashed line.
SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the best and worst possible case scenarios from July and December. The target SNR is shown as a vertical black dashed line.
Caption SNR loss distributions for IMBH (a) and BNS (b) cases obtained using the best and worst possible case scenarios from July and December. The target SNR is shown as a vertical black dashed line.
Monthly spectra evaluated for this work to extract the best and worst case scenario.
Caption Monthly spectra evaluated for this work to extract the best and worst case scenario.
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