The KAGRA underground environment and lessons for the Einstein Telescope

TL;DR

This paper evaluates the KAGRA underground site for gravitational-wave detection, demonstrating its low environmental noise levels and potential suitability for next-generation detectors like the Einstein Telescope.

Contribution

It provides a detailed characterization of KAGRA's underground environment and assesses its impact on low-frequency gravitational-wave detection capabilities.

Findings

Seismic noise below 20 Hz is minor at KAGRA site.

Surface and underground seismic waves contribute minimally to noise.

The site maintains potential for low-frequency gravitational-wave detection.

Abstract

The KAGRA gravitational-wave detector in Japan is the only operating detector hosted in an underground infrastructure. Underground sites promise a greatly reduced contribution of the environment to detector noise thereby opening the possibility to extend the observation band to frequencies well below 10 Hz. For this reason, the proposed next-generation infrastructure Einstein Telescope in Europe would be realized underground aiming for an observation band that extends from 3 Hz to several kHz. However, it is known that ambient noise in the low-frequency band 10 Hz - 20 Hz at current surface sites of the Virgo and LIGO detectors is predominantly produced by the detector infrastructure. It is of utmost importance to avoid spoiling the quality of an underground site with noisy infrastructure, at least at frequencies where this noise can turn into a detector-sensitivity limitation. In this paper, we characterize the KAGRA underground site to determine the impact of its infrastructure on environmental fields. We find that while excess seismic noise is observed, its contribution in the important band below 20 Hz is minor preserving the full potential of this site to realize a low-frequency gravitational-wave detector. Moreover, we estimate the Newtonian-noise spectra of surface and underground seismic waves and of the acoustic field inside the caverns. We find that these will likely remain a minor contribution to KAGRA's instrument noise in the foreseeable future.