Impact of Cable Structure on Distributed Fiber Optic Sensing

Distributed Fiber Optic Sensing (DFOS) along submarine cables is a promising technology for monitoring geologic hazards and assessing the structural integrity of telecommunication infrastructures, however a key challenge remains in understanding how strain applied to the cable is transferred to the sensing fibers. This study evaluates the strain sensitivity of the FOCUS prototype fiber-optic sensor cable, deployed offshore Mount Etna at 2000 m depth to monitor static strain along the North Alfeo Fault. The cable includes both telecom-grade gel-buffered (loose) and sensing-use acrylate-buffered (tight) fibers. Their respective responses are assessed through theoretical modeling, laboratory experiments, and marine field observations. Results show that compared to the near 1:1 cable strain response of tight fibers, loose fiber sensitivity ranges from 10–50%, deviates from linearity and is more difficult to predict, mainly because of the viscoelastic nature of the buffering gel. Consequently, DFOS on common telecom-use loose fibers can result in significant underestimation of the cable strain magnitude and spatial extent. We demonstrate that fiber buffering impacts the interpretation of static strain recorded with DFOS for both geological and anthropogenic events. We conclude that cable structure affects DFOS sensitivity and offer guidelines for the design of future submarine cables.