Underwater large-area high resolution monitoring by Distributed Optical Fibre Acoustic Sensors

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A variety of problems in environmental science involve determining the location and time of origin of acoustic or seismic signals. Various marine species including whales may be tracked by triangulating their vocalisations. Active faulting and magma intrusion beneath the seabed may be located by tracking the associated seismicity. Similar approaches may be used to track gas escaping through the seabed, which is now important in the context of sub-seabed carbon capture and storage (CCS), where it is important to verify that stored gas is not escaping back to the seabed. Currently in all of these applications, sound is detected by an array comprising a relatively small number (typically a few to a few tens) of point detectors, that may be towed or (more commonly) deployed on or near the seabed. Optical-fibre Distributed Acoustic Sensing (DAS) is a new technology that allows acoustic measurements to be made at an unlimited number of locations along a fibre, with a trade-off between measurement density and sensitivity. The fibre can also be manufactured relatively cheaply and at today's market prices telecom fibres coated with a polymer layer costs less than 1p per metre. Even with fibres are engineered with additional armouring to resist the weight of vehicles passing on them, their cost only increases to a few pounds per metre. Thus, this technology has the potential to locate and quantify sound sources in and beneath the ocean with much greater accuracy, and potentially much lower cost, than hitherto possible. Deployment of this technology in the ocean is limited by poor understanding of the coupling between acoustic waves and a DAS fibre within the water column or resting on the ocean floor. In this feasibility study, we propose to use a DAS system manufactured in Southampton, which can be specifically tailored for the monitoring of underwater acoustic signals and operate at frequencies commonly not used in commercial systems, to reconstruct a 3D map of acoustic fields in the ocean. Our approach will be to firstly determine the relationship between an acoustic signal in the ocean and the signal generated in the DAS fibre laid on the seabed. We will then determine a 3D model of the acoustic sources from the sensing enabled by the seabed fibre. Our next step is to then determine how to adapt and apply DAS technology so that it is suitable for detecting, locating and quantifying acoustic noise sources in the ocean. We will do some simple tests of the new technology in test tanks and in the marine environment (a dock within the port of Southampton). This project will build on research currently funded by NERC, EPSRC, Carbon Trust, the Royal Academy of Engineering and the Royal Society to provide a novel distributed acoustic sensor network capable of high-resolution 3D detection and analysis of underwater acoustic sources