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; 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.

声音在水下行驶1000公里；根据其频率，其各种波长可以探测从毫米到兆面体的海洋。在这个项目中，我们将自然的环境声音作为探测器作为探测器的分布感应传感纤维在传统海底电缆中作为大量被动声音接收器的分布。声学信号的幅度，相位和行进时间受其路径中的水温和流速场的强烈影响。为了在这些测量值中获得空间分辨的可变性，可以使用层析成像技术在连接源和接收器的几种声学路径上结合积分。访问更高数量的声学路径可改善海洋结构的估计。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上的对流烟囱和热结构的盆地规模倒置。尽管有这些有希望的例子，但由于i）维持强大的声学源的经济学（对海洋生物产生噪音污染后果），而ii）ii）II）局限性的局限性是限制的。噪声干涉仪（NI）通过替换使用具有多样和宽带（10^-3-3 Hz-10^-5 Hz）环境海洋噪声的主动来源的使用来克服这些局限性，这需要在不同位置进行交叉压力波动，以将近似值的近似值检索到众所周知的各种浪潮的功能（即确定的海洋）。这种方法将任何一对离散的声传感器（例如氢）转化为虚拟声收发器，这可以量化路径融合的声速（这是温度和压力的函数）和速度。从旅行时间非进取时间，即在两个收发器之间相反的方向上的旅行时间之间的差异。尽管声速和收发器位置在声速和收发器位置上对不确定性的不敏感性对海洋电流速度的准确测量能够准确地被动测量，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需要在长达几个月的时间内维持水下系泊的次毫时钟准确性，并且不切实际的离散传感器数量不切实际，以进行有用的时空海洋学测量。这项工作通过使用在离岸传统海底电缆内的光纤传感获得的数据中替换稀疏点传感器（水力机/地震仪）来克服这些问题。这可以实现空间分辨的O（10 m），在环境噪声场的影响下，光纤中相对变形的动态测量。尽管这些测量与使用常规氢驱动器测量的声压根本不同，但它们的敏感性是可比的。在NI上下文中，所需的时间同步被大大简化，因为所有信号都来自相同的光纤，并具有实时数据可用性。此外，大量可用的传感器对和各种配对传感器的分离产生了大量的输入数据，以评估噪声互相关函数，从而导致声学绿色的功能提取，尽管噪声平均时间是噪声平均时间，例如，从小时为差为秒为秒。该项目建立在越来越多的研究基础上，这些研究通过比较Ni的逆估计与直接测量的全海深度速度和温度的直接测量时间序列进行了比较。我们的总体目的是确定时空的实际限制 - 具有测量和温度（温度 - 速度）精度的时间分辨率。