Observing seafloor processes by distributed Fiber Optic Sensing: examples from academic cables offshore East Sicily (Italy) and a commercial telecom network in the Guadeloupe archipelago (Lesser Antilles).

Within the framework of the EU funded FOCUS project, we have established BOTDR (Brillouin Optical Time Domain Reflectometry) time series spanning up to 3 years on academic and commercial fiber-optic cables. Offshore Catania a 6-km-long dedicated fiber-optic cable (connected to a 29-km-long electro-optical cable), recorded natural and man-made strain signals of 40 - 250 microstrain at the seafloor (in 1800 - 2000 m water depths). The strongest natural signal (40 microstrain elongation) developed from 19 - 21 Nov. 2020, where the cable crosses a mapped submarine fault in the first of 4 locations. However, a network of 8 seafloor geodetic stations (acoustic beacons mounted on tripods) recorded no baseline changes, thus ruling out a tectonic movement. A simultaneous positive-negative strain doublet (+40 to -20 microstrain) observed 6 km from shore (in 300 m water depth), suggests a downslope current affected the shallow and deep portions of the fiber-optic cable path length. The 6-km long FOCUS cable includes a redundant, triple-loop, consisting of 1 loose and 2 tight optical fibers. (A second double-loop, consisting of 1 tight and 1 loose fiber is being monitored by a second BOTDR interrogator.) The tightly bound fibers typically record a strain signal twice as high as the loose fibers (40 vs 20 microstrain for the natural signal). Weight bag drops created man-made signals along four 120-m-long segments, with amplitudes of 80 - 250 microstrain (in the tight fibers), which gradually decay in the following months / years. Baseline measurements were repeatedly taken on the 100-km-long Capo Passero (SE Sicily) electro-optical cable in Oct. 2022, March 2023, and Nov. 2023. The main purpose of the monitoring is to test the maximum range attainable with commercially available BOTDR interrogators. While no significant strain development has been measured on the optical path, we report that with an acquisition time of 2+ hours it is possible to extend the measurement range up to 80 km. This represents a benchmark for future BOTDR measurements over long distances. In June 2022, in collaboration with the Conseil Regional of Guadeloupe and Orange, we began long-term monitoring of a network of unrepeated submarine telecom cables that links the islands of the Guadeloupe archipelago. We repeated the measurements of the same fiber segments (30 - 70 km length), in Dec. 2022, June 2023, Nov/ Dec. 2023, and finally May 2024 in order to identify strain or thermal signals along the cable during the 2-year period. We confirm that using the BOTDR technique, we detect significant shifts in the Brillouin frequency (2 - 5 MHz), which could represent substantial strain signals (40 - 100 micro-strain in amplitude) in several locations along the cable network. These signals, which can be positive (elongation) or negative (shortening) occur typically in areas of steep seafloor slopes (e.g. the shelf break) or in submarine valleys/canyons. Our preliminary interpretation is that stretching and shortening of the cable (representing about 1 cm over a few hundred meters) is occurring, most likely due to sea-bottom currents.