Cable response for ocean-bottom distributed acoustic sensing

In recent years, widespread application of fiber-optic sensing methods, especially distributed acoustic sensing (DAS), on seafloor telecommunication and power cables has enabled acoustic detection and localization of marine mammals and shipping traffic, inversion of sediment properties from Scholte wave dispersion and shear wave resonance, and quantitative estimation of ocean surface gravity wave statistics in shallow water. However, the effect of cable construction, burial, and sediment characteristics on the measured fiber strain and its relationship to conventional measurands (water-column pressure or seafloor displacement) has not been rigorously addressed. We introduce an idealized model of a fiber-optic cable as a cylindrical, layered rod embedded in a uniform whole space and develop semi-analytical solutions for seismic and acoustic wave forcing. We then discuss several implications for fiber-optic sensing, including Poisson and photoelastic effects. We conclude that the strain measured by DAS on a buried cable is typically on the same order of magnitude as the longitudinal strain in the seafloor, with the difference not exceeding 20% for reasonable elastic parameters. For an unburied cable, the hydroacoustic sensitivity is more strongly dependent on the stiffness of the cladding material. In both cases, the directional response may deviate significantly from conventional theory and warrants further experimental investigation.