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Distributed Fibre Optic Sensing (DFOS) is becoming widespread in submarine geosciences as it enables quasi-continuous sensing of physical changes along fibres in submarine cables. These cables are made with specific architectures and materials chosen to last on the seafloor: consequently, a change in the measurand is transferred from the surface of the cable to the fibre. This transfer needs to be characterised as it impacts the magnitude of the measurement and the spatial resolution. Moreover, fibres could be tightly or loosely buffered in the cable, which could impact the measurement as well. We investigate these questions in a case study where we use the 6 km-long prototype FOCUS cable deployed offshore Catania. This special cable extends the 29 km-long Main Electro Optical Cable of the Italian National Institute for Nuclear Physics, that runs on the seafloor down to depths of 2 km. The FOCUS cable, designed to be used for distributed ground motion sensing, hosts five telecom grade optical fibres all measuring the same path. The uniqueness of the sensor cable is that three fibres are tightly buffered in a steel tube that runs inside the core (referred to as tight); two fibres are loosely buffered (loose) in a steel tube coiled around the core. One loose and two tight fibres are connected in series so that we can sense three differently buffered fibres with a single access from land. We mostly measured static deformation of the fibres with Brillouin Optical Time Domain Reflectometry using the Viavi DTSS system between October 2021 and November 2023. We observed deformation of the cable caused by at least one natural event (causing ~40 microstrain) and different man-made signals: notably around 100 weight drops on the cable (~200 microstrain), and an abrupt pulling of the cable termination (~400 microstrain). The strain measured along tight fibres on-site was up to ten times larger than that measured on the loose fibre. To explain these records and compare the sensitivity of the fibres as cable strain sensors, we conduct controlled loading experiments on the cable at the SMASH laboratories (IFREMER). We measure with high-spatial resolution (< 3mm, using Luna Inc.s ODiSI-B system) how different fibres deform for loads up to 1 kN (the cables working load). We observe that the tight fibre deforms linearly with load and it records 200% the strain of the loose fibre. In Catania, we also measured dynamic strain rate with Distributed Acoustic Sensing using the ASN OptoDAS system during two weeks in November 2023. With it we recorded a series of earthquakes, and ongoing work indicates that tight fibres may be on average 6% more sensitive than loose fibres to dynamic strain rate. Amplitude loss in a tight fibre is -0.26 db/km versus the -0.19 db/km in a loose fibre, anticipating a 30 dB loss by about 40 km. However, the study at hand shows that tight fibres make for sensors that can be two times more sensitive than standard loose fibres for marine geosciences, depending on application.
Cappelli et al. (Mon,) studied this question.