Wide-area low-cost sustainable ocean temperature and velocity structure extraction using distributed fibre optic sensing within legacy seafloor cables

Sound travels 1000s of kilometres underwater; depending on its frequency, its variety of wavelengths enables probing of the ocean from millimeters to megameters. In this project, we resource the natural ambient sound as the probe with distributed sensing of optical fibres within legacy seafloor cables as vast arrays of passive acoustic receivers. The amplitude, phase and travel time of acoustic signals are strongly affected by the water temperature and flow velocity fields in their path. To obtain spatially resolved variability in these measurands, tomographic techniques can be used to combine integrals over several acoustic paths that connect a source and a receiver. Access to a higher number of acoustic paths improves estimation of ocean structure. Notable examples of oceanic phenomena already captured by tomographic techniques comprise convective chimneys in the Greenland Sea and basin-scale inversions of thermal structure. Despite these promising examples, use of active acoustic tomography is limited due to i) the economics of maintaining a powerful acoustic source (with noise-pollution consequences on marine life), and ii) the limitations on lateral and temporal resolutions associated with practical constraints on acoustic paths from active sources. Noise interferometry (NI) overcomes these limitations by replacing the use of active sources with diverse and broadband (10^-3 Hz - 10^-5 Hz) ambient marine noise, entails cross-correlating pressure fluctuations at different locations to retrieve an approximation to the acoustic Green's functions of various waves (i.e. the deterministic wave field due to a point source), which is then inverted to obtain ocean structure. This approach transforms any pair of discrete acoustic sensors (say, hydrophones) into virtual acoustic transceivers, which enables the quantification of both path-integrated sound speed (which is a function of temperature and pressure) and velocity. Flow velocity is retrieved from travel time nonreciprocity, i.e. the difference between travel times in opposite directions between two transceivers. Insensitivity of acoustic non-reciprocity to uncertainties in sound speed and transceiver positions enables accurate passive measurements of the oceanic current velocity, despite its absolute magnitude being less than the uncertainty in sound speed. When used with discrete sensors, NI requires maintaining sub-millisecond clock accuracy on underwater moorings for months-long periods and impractically large number of discrete sensors for useful spatio-temporal oceanographic measurements. This work overcomes these problems by replacing sparse point sensors (hydrophones/seismometers) with the data obtained using distributed sensing of optical fibres within offshore legacy seafloor cables. This enables spatially resolved O(10 m), dynamic measurements of relative deformation in optical fibre under the influence of ambient noise fields. Whilst these measurements are fundamentally different from acoustic pressure measured using conventional hydrophones, their sensitivity is comparable. In the NI context, the required time synchronization is greatly simplified as all signals come from the same fiber, with real-time data availability. Moreover, the large number of available sensor pairs and variety of pair-wise sensor separations yields a larger volume of input data for evaluating the noise cross-correlation function which results in the acoustic Green's function extraction, albeit with proportionately reduced noise averaging times, e.g., from hours-days to seconds-minutes. This project builds on the growing number of studies that have demonstrated the basics of the method by comparing inverse estimates from NI with directly measured time series of full ocean depth velocity and temperature. Our overarching aim is to determine the practical limits on spatio (vertical-horizontal) - temporal resolutions with measurand (temperature-velocity) precisions.