Converting undersea optical cables into environmental sensor arrays, without changing infrastructure

Using an approach that converts one long optical cable into many individual segments capable of detecting activity around them, undersea communications cables could create a vast network of seafloor environmental sensors – without the need to change existing infrastructure, researchers report. The new approach, which builds upon previous research demonstrating the utility of optical fiber-based seafloor sensing technology, could be employed to detect and better characterize submarine seismic vibrations and ocean currents with greater precision. “By converting submarine cables into arrays of environmental sensors, a large network of hundreds or thousands of permanent and real-time seafloor sensors could be implemented without modification of the existing subsea infrastructure,” write Giuseppe Marra and colleagues. “This has the potential to transform our understanding of both shallow and deep processes inside the Earth.” A growing body of research demonstrates that existing underwater optical communication cables can be used as seafloor sensors to monitor seismic activity and other underwater disturbances. However, previous techniques are limited in spatial resolution and sensitivity because the entire length of a cable – which can span thousands of kilometers (km) – acts as a single sensor. Using the repeaters that amplify an optical signal as it moves through a cable and a laser, Marra et al. show how a single cable can be divided into many individual segments, effectively converting one cable into an array of interferometry-based environmental sensors. The authors tested the approach using a 5,860-km-long submarine optical fiber link between the United Kingdom and Canada, which contained repeaters approximately every 46 km. Using this approach, Marra et al. detected several earthquakes, weak seismic movements, and ocean currents along the cable. They were also able to determine the epicentral region of a distant quake using signals from different segments along the cable.

For reporters interested in trends, several recent Science publications have presented similar methods for using existing telecommunication cables as geophysical sensors. A February 2021 Science Report demonstrates the use of polarization in regular telecommunication traffic to detect seismic disturbances in a 10,000-km-long fiber-optic submarine cable.

Also, a November 2019 Report in Science demonstrated that ocean and seafloor dynamics can be monitored using Distributed Acoustic Sensing (DAS) in unused optical fiber, or “dark fibers,” in fiber optic cables.