Earth Sciences

Submarine cables as seismic sensors

Print edition : January 17, 2020

Seismic waves generated by a 1.9 magnitude earthquake located north of Fréjus (Var), recorded along the 41-km-long fibre optic cable deployed on the seafloor off Toulon.

French scientists from the National Centre for Scientific Research (Centre national de la recherche scientifique, or CNRS), the Cote d’Azur Observatory (Observatoire de la Cote d’Azur, or OCA), the Research Institute for Development (Institut de Recherche pour le Developpement, France, or IRD) and Universite Cote d’Azur have shown that it is possible to detect the propagation of seismic waves on the seafloor using submarine telecommunications cables.

The researchers worked together in the Geoazur laboratory, in collaboration with the company Febus Optics and the Centre de Physique des Particules de Marseille (CNRS/Aix-Marseille Universite).

According to their observations, the existing infrastructure could be used to detect earthquakes, as well as swell and underwater noise. The results have been published in a recent issue of “Nature Communications”.

The ocean floor is criss-crossed by 1.2 million kilometres of telecommunications cables (three times the distance from the earth to the moon). Made up of optical fibres, they facilitate much of our communication by telephone, SMS and email. And they could soon take on a new role, that of detecting acoustic and seismic waves.

The scientists used a 41-km-long cable deployed off the coast of Toulon in southern France to retrieve data from the sensors of the MEUST-NUMerEnv, an underwater observatory managed by the CNRS that comprises a neutrino telescope and sensors dedicated to earth and environmental sciences at a depth of 2,500 metres.

The method they developed takes advantage of small impurities in the optical fibres, which send part of the light they carry back to the transmitter. By stretching or contracting the fibre, the passage of a seismic or acoustic wave alters the distance between these impurities, and thus the backscattered signal, by a tiny amount, about a nanometre. Yet, they needed to prove that these differences were detectable since, in submarine cables, the optical fibres are surrounded by several insulating layers.

By injecting pulses of light into an optical fibre and analysing the backscattered signal, the team converted the 41 km of optical fibre into more than 6,000 seismic sensors. An earthquake of 1.9 magnitude that occurred during the trial was detected at each of the measuring points with a sensitivity close to that of a coastal seismic station, even though it was located over 100 km from the cable.

The measuring points are also sensitive to waves that travel through the ocean, such as those produced by swell. The authors recorded the impact of waves on the seafloor near the coast, as well as their effect on the abyssal plain, where they generate “seismic background noise”. The sensors made it possible to observe for the first time how these very small vibrations, which constantly interact with the earth’s interior, are produced, enabling geophysicists to probe its structure.

The researchers believe that a telecom cable, rather like a string of microphones, could similarly detect underwater noise produced by ships and cetaceans.

Faced with the logistic and financial challenge of deploying instrumentation on the seafloor, telecom cables could provide a way of improving our understanding of this terra incognita covering two-thirds of the earth’s surface, and address a wide range of scientific and societal issues, such as earthquakes, coastal erosion and interaction between life, the oceans and the solid earth. A certain number of cables currently in operation will be phased out by telecommunications operators over the coming years. Thanks to this research, they could take on a second life.

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