Scientists show for the first time that it is possible to detect the propagation of seismic waves at the bottom of the oceans with submarine telecommunication cables. According to their observations, these existing infrastructures could be exploited to detect earthquakes, but also swell or even underwater noise. These results are published in the journal Nature Communications on December 18, 2019, by researchers from CNRS, OCA, IRD and Université Côte d'Azur at the Géoazur laboratory, in collaboration with the company Fébus Optics and the Marseille Particle Physics Center (CNRS / Aix-Marseille University) 1.
1.2 million kilometers of telecommunications cables line the bottom of the oceans (three times the distance from Earth to the Moon). Composed of optical fibers, they make possible a large part of our exchanges by telephone, SMS or email. And they could soon acquire a new function: picking up acoustic and seismic waves.
Scientists here used a 41 km cable installed off the Toulon coast to retrieve data from the sensors of the MEUST-NUMerEnv 2 underwater observatory , 2,500 m deep. The developed method takes advantage of small impurities contained in the optical fibers, which return part of the light they transport to the emitter. By stretching or contracting the fiber, the passage of a seismic or acoustic wave changes minutely 3 the difference between these impurities, and therefore the returned signal. It was still necessary to verify that these differences were perceptible because, in submarine cables, the optical fibers are surrounded by several insulating layers.
By injecting pulses of light into an optical fiber and analyzing the returned signal, the team converted the 41 km of optical fiber into more than 6,000 seismic sensors. An earthquake of magnitude 1.9 occurred during the experiment, yet located more than 100 km from the cable, was detected by each of the measurement points with a sensitivity close to that of an installed seismological station on the coast.
But that's not all: these measurements are also sensitive to waves that propagate within the ocean, like those produced by swells. The authors thus recorded the imprint of the waves on the seabed near the coast, and also their effect on the abyssal plain, where they generate the “seismic background noise”. These sensors have thus made it possible, for the first time, to observe how these very weak vibrations are produced, which permanently agitate the interior of the Earth and allow geophysicists to probe its structure.
Researchers speculate that, like a line of microphones, a telecommunication cable could similarly pick up underwater noise produced by ships or cetaceans.
Faced with the (logistical and financial) challenge represented by the instrumentation of the seabed, telecommunication cables would therefore offer a solution to better understand this terra incognita covering two thirds of the globe, and to respond to a multitude of scientific and societal issues - earthquakes, coastal erosion, interaction between living things, the ocean and "solid Earth", etc.
A number of cables currently in service will be "retired" by telecommunications operators in the coming years. Thanks to this work, they may have a second life.
1. These conclusions are independently confirmed by another team, whose article is published in the same edition (Teleseisms and microseisms on an ocean-bottom distributed acoustic sensing array, E Williams, MR Fernandez-Ruiz, R Magalhaes, R Vanthillo, Z Zhan, M Gonzàlez-Herràez, HF Martins).
2. The MEUST-NUMerEnv underwater observatory , carried by the CNRS, is made up of a neutrino telescope and sensors dedicated to Earth and environmental sciences.
3. On the order of a nanometer (a billionth of a meter, about a thousandth of the diameter of a hair).