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

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

Policymakers and regulators
We will demonstrate by our feasibility study that it is possible to reliably and cost-effectively measure the soundscape in the marine environment. This is relevant to policy makers concerned about noise levels impacting marine organisms, as well as the potential for the technology in detecting leakages from sequestered carbon dioxide in reservoirs, and in measuring precursor seismicity in zones of active tectonics.

CCS Operators
Operators of CCS systems will benefit from the generation of new methodologies that are able to give confidence in CCS by being able to detect leakages across the seabed.

General Public, NGOs and fisheries organisations
The application of marine Distributed Acoustic Sensing (DAS) is relevant to these groups as it will allow: 1) a full understanding of the noise field in areas sensitive to marine life; 2) a greater acceptance of CCS, especially by local communities, depends on many factors, and confidence in the physical security of stored CO2 is a major issue

We will organise a stakeholder meeting around month nine of the project to ensure that the scenarios that we are modelling, and methods we develop meet the requirements for the widest range of groups possible. We will invite a representatives from: OSPAR (protection of the marine environment of north-east Atlantic); International Ocean Noise Coalition; an offshore CCS operator; and utilise our existing links with Japan and industrial partners of the Photonics Manufacturing Hub that might have interest in the commercial exploitation.