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

使用传统海底电缆中的分布式光纤传感进行广域低成本可持续海洋温度和速度结构提取

• 批准号: NE/Y003365/1
• 负责人: Mohammad Belal
• 金额: $ 13.15万
• 依托单位: NATIONAL OCEANOGRAPHY CENTRE
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2024
• 资助国家: 英国
• 起止时间: 2024 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FY003365%2F1
• 关键词: Wide; area; low; cost; sustainable

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.

声音在水下传播数千公里；根据其频率，其多种波长可以探测从毫米到兆米的海洋。在这个项目中，我们将自然环境声音作为探头，对传统海底电缆中的光纤进行分布式传感，作为大量无源声学接收器。声信号的振幅、相位和传播时间受到其路径中的水温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，可以使用断层扫描技术来组合连接声源和接收器的多个声学路径上的积分。获得更多数量的声学路径可以改善对海洋结构的估计。层析成像技术已经捕获的海洋现象的著名例子包括格陵兰海的对流烟囱和盆地尺度的热结构反转。尽管有这些有希望的例子，但主动声层析成像的使用受到限制，因为 i) 维持强大声源的经济性（对海洋生物造成噪声污染后果），以及 ii) 与实际限制相关的横向和时间分辨率的限制。来自有源声源的声学路径。噪声干涉测量 (NI) 通过使用不同的宽带 (10^-3 Hz - 10^-5 Hz) 环境海洋噪声取代有源源，克服了这些限制，需要互相关不同位置的压力波动，以检索近似值各种波的声学格林函数（即由于点源而产生的确定性波场），然后将其反演以获得海洋结构。这种方法将任何一对离散声学传感器（例如，水听器）转换为虚拟声学收发器，从而能够量化路径积分声速（它是温度和压力的函数）和速度。流速是根据传播时间非互易性（即两个收发器之间相反方向的传播时间之差）获取的。声学非互易性对声速和收发器位置的不确定性不敏感，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 需要在水下系泊装置上保持亚毫秒级时钟精度达数月之久，并且需要使用大量离散传感器来进行有用的时空海洋学测量，这不切实际。这项工作通过使用离岸传统海底电缆中光纤的分布式传感获得的数据来取代稀疏点传感器（水听器/地震计），从而克服了这些问题。这使得能够在环境噪声场影响下对光纤的相对变形进行空间分辨率 O(10 m) 的动态测量。虽然这些测量与使用传统水听器测量的声压有本质上的不同，但它们的灵敏度是相当的。在 NI 环境中，所需的时间同步大大简化，因为所有信号都来自同一光纤，并且具有实时数据可用性。此外，大量可用传感器对和各种成对传感器间隔产生大量输入数据，用于评估噪声互相关函数，这导致声学格林函数提取，尽管噪声平均时间成比例减少，例如，从小时-天到秒-分钟。该项目建立在越来越多的研究的基础上，这些研究通过将 NI 的逆估计值与直接测量的全海洋深度速度和温度的时间序列进行比较，证明了该方法的基础知识。我们的首要目标是确定空间（垂直-水平）-时间分辨率与被测量（温度-速度）精度的实际限制。