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

声音在水下传播1000公里;根据其频率，其波长的变化可以探测从毫米到百万米的海洋。在这个项目中，我们利用自然环境声音作为探测器，利用传统海底电缆内的光纤分布式传感作为被动声接收器的巨大阵列。声信号的振幅、相位和传播时间受其传播路径上的水温场和流速场的强烈影响，为了获得这些被测量的空间分辨变化，层析成像技术可以用于联合收割机在连接源和接收机的几个声路径上进行积分。访问更多数量的声学路径改进了海洋结构的估计。已经被层析成像技术捕获的海洋现象的显著例子包括格陵兰海的对流烟囱和热结构的盆地尺度反演。尽管有这些有前途的例子，使用主动声层析成像是有限的，由于i）保持一个强大的声源（与海洋生物的噪声污染后果）的经济，和ii）的横向和时间分辨率的限制与实际约束声路径从主动源。噪声干涉测量（NI）通过用不同的宽带光源代替有源光源来克服这些限制。（10^-3 Hz - 10^-5 Hz）海洋环境噪声，需要在不同位置互相关压力波动，以检索各种波的声学绿色函数的近似值（即，由于点源的确定性波场），然后将其反演以获得海洋结构。这种方法将任何一对离散的声学传感器（例如水听器）转换为虚拟的声学收发器，这使得能够量化路径集成声速（它是温度和压力的函数）和速度。流速从传播时间非互易性中检索，即两个收发器之间相反方向上的传播时间之间的差异。声学非互易性对声速和收发器位置的不确定性的不敏感性使得能够精确地被动测量洋流速度，尽管其绝对幅度小于声速的不确定性。当与离散传感器一起使用时，NI需要在水下系泊系统上保持亚毫秒级的时钟精度长达数月，并且需要大量离散传感器进行有用的时空海洋测量。这项工作克服了这些问题，取代稀疏的点传感器（水听器/地震仪）的数据使用分布式传感光纤在海上传统海底电缆。这使得空间分辨O（10米），动态测量的相对变形的光纤环境噪声场的影响下。虽然这些测量从根本上不同于使用传统水听器测量的声压，但它们的灵敏度相当。在NI环境中，所需的时间同步被大大简化，因为所有信号都来自同一光纤，具有实时数据可用性。此外，大量的可用传感器对和成对传感器分离的多样性产生用于评估噪声互相关函数的更大量的输入数据，这导致声学绿色函数提取，尽管具有成比例地减少的噪声平均时间，例如，从小时-天到秒-分钟。该项目是在越来越多的研究的基础上开展的，这些研究通过将国家海洋研究所的逆估计数与直接测量的全海洋深度速度和温度时间序列进行比较，证明了该方法的基本原理。我们的首要目标是确定实际的限制空间（垂直水平），时间分辨率与被测对象（温度速度）的精度。

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.