Low-frequency seismic wave sensing using coherent optical fiber networks for metrology
Authors: Pottie Paul-Eric , Tonnes Mads , Mazouth-Laurol Maxime , Montlavan-Leyva Hendrix , Cantin Etienne , Pointard Benjamin , Alvarez-Martinez Hector , Le Targat Rodolphe , Lopez Olivier , Chardonnet Christian , Amy-Klein Anne 
Published: February 2023

Optical fiber networks are being implemented in several countries aiming at dissemination of ultra-stable time and frequency references. This enables the comparison of optical clocks, which is a key part of the roadmap towards the future redefinition of the International System of Units (SI) second. Furthermore, this enables uses in chronometric geodesy, where the sensitivity of the optical clocks to the gravitation field enables measurements of height differences as low as 1 cm [1].The frequency signals in the optical fibers are sensitive to acoustic vibrations which are present in the ground, which is the main source of noise to the disseminated signals.In recent years, this has enabled studies in the use of optical fiber links for the detection of earthquakes [2]. In such an approach, the measurement is the integrated noise over the fiber path. This typically allows for one to several orders of magnitudes longer range as compare to DAS techniques, but with the loss of localization along the fiber. Such integrated approaches include measurements of the total polarization change of the light along the fiber [3], or the total phase change of a coherent ultra-stable laser signal, potentially including distributed sensing techniques in submarine fibers [2,4].Here, we will present the first quantitative studies on the use of coherent optical fiber links for seismic detection. Using a the fiber network REFIMEVE in France (see Fig. 1), we present studies on the sensitivity of coherent optical fiber links to seismic events. We describe the dependence of the sensitivity to a number of parameters like incident angle, magnitude and distance, and compare the sensitivity of a fiber link with that of conventional seismometers. We show, for a first time to our knowledge, the detection of seismic waves by a coherent optical fiber network, and we study the prospects of using such a network for the localization of earthquakes. Lastly, we discuss the principles and results of a machine learning algorithm, which enables automatic detection of earthquakes in a coherent optical fiber link.Bibliography:1. M. Takamoto et al., Test of general relativity by a pair of transportable optical lattice clocks, Nat. Phot., 14 (7), 411&#8211;415. doi:30210.1038/s41566-020-0619-82. G. Marra et al. , Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables. Science, eaat4458. doi: 10.1126/science.aat44582793. J.C. Castellanos et al. ,Optical polarization-based sensing and localization of submarine earthquakes. In Optical fiber communication conference (OFC) 2022, doi:26210.1364/OFC.2022.M1H.44. G. Marra et al., Optical interferometry&#8211;based array of seafloor environmental sensors using a transoceanic submarine cable. Science, doi: 10.1126/science.abo193Figure 1 : Map of the French REFIMEVE fiber network, shown in red lines. Dotted lines indicates indicate the full scale of the planned network, and continuous red lines indicate links used in these studies. Blue lines indicates the linear approximations of the links. All seismometers of the RESIF network is shown by small green triangles, and seismometers used in theses studies are shown by larger, turquoise triangles.