Investigating the response of distributed fibre optic acoustic sensing for subsea cable motion monitoring

A simultaneous Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) system is presented. In DAS, perturbations across hundreds of metres of fibre are detected by the use of phase-sensitive Optical Time-Domain Reflectometry (OTDR) from Rayleigh backscattering caused by these perturbations. DTS relies on Raman OTDR in a similar fashion. The capabilities of the DAS system are presented, showing detection limits on the order of sub Hz to kHz while achieving spatial resolution on the order of metres. The ability to differentiate between different types of perturbation including tapping, transient signals and smooth, strumming motion is demonstrated.