Impact of Cable-Sediment Coupling for Submarine Strain Sensing: Insights from Catania’s FOCUS Cable

Submarine cables are increasingly used to measure ground deformation using distributed fiber optic sensing (DFOS) thus requiring the characterization of the multi-faceted load transfer mechanism between the seafloor sediment and the sensing optical fiber(s). Relative motion between a fibre-optic cable and marine sediment (commonly soft clays) results in the formation of a mm-thick shear band around the cable where deformation is accommodated plastically, while outside this zone the clay responds elastically. These phenomena can potentially impact DFOS sensing, such as the landslide observed in November 2020 offshore Catania at over 1800 m depth on the prototype FOCUS strain sensing cable, where records display maximum ±20 με strain along a 1 km-long cable segment. To quantify the impact of in-situ cable deployment on environmental strain sensitivity we therefore devised a calibration framework consisting of sediment analyses and laboratory pullout experiments on a segment of the prototype cable. Geotechnical characterization of seafloor sediment samples collected near the cable was conducted to produce a remolded laboratory testbed that recreates seabed conditions. A 90 mm-long segment of the prototype cable was buried at varying depth from lying on the surface to 1, 3 and 5 cm depth (this last depth was simulated using weights) to reproduce different burial scenarios. After 24-72h of settling time, pullout tests were performed at 1 mm/min measuring pullout force, cable displacement, and fiber strain with 1 mm spatial resolution. Load-displacement curves display pullout forces increasing linearly with displacement reaching a peak force before a nonlinear transition, where forces drop to a residual value for the rest of the test. Higher overburden stress increased the maximum pullout force and displacement before the transition: the peak pullout force increased from 0.04 to 3.6 N, and the displacement required to reach these values increased from 0.1 to 1.1 mm as the cable is deployed from the surface to a depth of 5 cm. Strain sensing was most successful with the cable at 5 cm depth, where the average fiber strain increases linearly up to the peak force, at which point the rate of strain accumulation decreases. These results from laboratory tests were expanded upon by developing analytical and numerical schemes that use the empirically derived force-displacement law to further evaluate its impact on DFOS sensitivity at field-scale. In a simple analytical approach, the expected pre-transition pullout force derived from the experiments is balanced with the force required to strain the cable, indicating that the existing burial depth of up to 20 cm should enable 10-20 με of static strain sensitivity over 100 m wavelengths. To improve the consistency of the analysis, we develop a finite difference scheme to model the cable-sediment interaction accepting arbitrary load transfer functions to simulate cable strain in the occurrence of displacement. This model is used to reproduce the laboratory experiment, thus providing a framework to investigate the impact of the surrounding medium on DFOS sensitivity, and can be extended to other on site-specific burial conditions and medium properties.