Laser interferometry: a tool for seismic monitoring using the telecom fiber network 

In the past decade, the application of Distributed Fiber Optic Sensing (DFOS) in geosciences has grown exponentially, offering improved spatial sampling as compared to point-like sensors and the possibility to reach remote areas of the planet. Among the rich variety of techniques today available, those compatible with data transmission are particularly interesting, opening disruptive opportunities if implemented on a large scale, with implications that extend from fundamental research to population protection and the development of early warning systems. For instance, the possibility to measure strain over transoceanic fiber cables used for global communications would enable us to gather data from the ocean floors, that are today largely unmonitored; on land, sensing in coexistence with data traffic can turn the existing telecommunication network into a pervasive sensing grid, suitable for capillary seismic monitoring, especially in highly anthropized regions. One promising technique for this task is laser interferometry. It is based on an interferometric measurement of the integrated strain of a telecommunication cable, obtained by probing it with a narrow bandwidth, continuous-wavelength laser signal over its entire length. While its localization ability is not yet as good as for other DFOS techniques, its reach extends to thousands of kilometers, in coexistence with other data signals, and it can be implemented over unidirectional fibers where DFOS techniques based on backward scattering phenomena (Rayleigh, Brillouin or Raman) cannot be applied. These aspects are crucial for coexistence with data, as today most telecommunication links are spectrally dense and unidirectional. After earlier demonstration of laser interferometry on subsea [1] and oceanic cables [2], a recent collaboration of the Italian Metrology Institute (INRIM), Open Fiber – a leading telecom provider in Italy-, and the National Institute of Geophysics and Volcanology (INGV) realized a pilot earthquake observatory based on a regional fiber ring used for Internet traffic and showed that this technique can be successfully applied to highly anthropized areas [3]. With systematic, long-term acquisition over year-long timescales and supported by comparison to data from the national seismic monitoring service, we were able to characterize the fiber response to seismic events in a broad range of magnitudes and distances, showing that quantitative information can be extracted from fiber recordings and proving its suitability as a tool for permanent seismic monitoring.   Current topics of investigation include methods to improve the capability to localize events along the cable to the km level, the development of more compact laser interrogators suitable for wide-scale adoption and a deeper understanding of the cable response to different perturbations. As research on these aspects progresses, we envisage a stimulating emerging paradigm for Earth observation where different sensing approaches, including conventional point-like sensors, DFOS and integrated strain measurement by laser interferometry or polarimetry, complement each other offering improved coverage, multi-parameter sensing and extended fields of application, for an improved monitoring of our planet and living habitats. [1] G. Marra et al., Science 361, 486 (2018) [2] G. Marra et al., Science 376, 874 (2022) [3] S. Donadello et al., arXiv:2307.06203 (2023)