Interpretation of Motionally Induced Electric Fields in Oceans of Complex Geometry

海水通过地球磁场的运动产生感生电场、电流和磁场。原则上，任何位置的电磁场取决于整个海洋盆地的水流、海底水深和海底电导率。幸运的是，从电压测量中确定速度的方法已经一级地表明，海洋诱发的电磁场只取决于速度的局部垂直分布。水平电场和速度之间的这种简单关系已被许多仪器利用来进行高质量的速度和传输测量，包括电缆、底部着陆器、垂直剖面仪和拉格朗日浮子。虽然现有的理论已经很好地解释了大多数测量结果，特别是在深水中，但尚未对大的水深特征、复杂的时变速度场或其他此类复杂性的影响进行全面的研究。迄今为止，仪器开发问题受到了广泛关注，并产生了各种基于电磁的仪器，非常适合许多观测环境。然而，开发工具的一个关键和补充组成部分是能够解释在他们可能遇到的更复杂的环境中不可避免地出现的高阶术语。该项目将评估具有显著高项的运动电场：具有陡峭地形、沉积物电导率梯度和穿越地形的时变水流的区域。这个项目有三个组成部分：观察、分析和数值元素。观测将集中在哈特拉斯角收集的数据上。高阶项的初始估计表明它们比一阶项小一个数量级，从而允许采用迭代方法。海洋速度结构的确定将与一项流速计研究相结合，该研究同时进行，以估计跨等深线流、跨等深线和沿等深线的速度切变和时间变化，而地质结构的分析将确定海底的电学特性。从速度和地质结构出发，将研究电场的各向异性，这些各向异性可能与地形或底部电导梯度的方向或大小或洋流的方向有关。结果将与来自不同环境的数据进行比较：即深海平原上的涡流和垂直于山脊的潮汐流。在解析分析中，将求解底部边界倾斜的简单情况，以确定底部深度变化不大时的地形效应。现有电磁模型的数值模拟将扩展分析结果，并允许利用精确的海洋和地质信息解决哈特拉斯角海域的全部情况。还将考虑代表性案例，以概括哪些过程产生高阶项。该项目将改进误差源的预测，使基于电磁测量的用户能够计划或解释观测结果。基于误差分析，可以使用这三种方法来估计高阶项的贡献，并增加对观察到的各向异性和导电海底作用的物理见解。将建立一个网站，以支持电磁观测，传播实用技术（包括目前的研究），并促进研究人员之间的交流。该研究将把对情绪感应电场的理解扩展到尚未详细考虑的复杂环境的广泛范围：包括地形特征和时变流量，其中海洋过程通常是非常有趣的，除了混淆效应。更广泛的影响有许多设备，其中一些在商业生产中，是基于运动感应原理测量海洋速度的。为了更精确的分析，为了改进实验计划和异常效应的预测，为了海洋学界更好地接受和使用这种方法，扩展对这种测量的理论认识是至关重要的。电磁测量及其解释将为海洋观测系统（如IOOS）增加强大的能力。在线资源将收集有关EM观测的信息，并使其随时可供社区使用。本课题将构成博士研究生的博士研究。

The movement of seawater through the earth's magnetic field generates motionally induced electric fields, electric currents and magnetic fields. In principle, the Electro-Magnetic (EM) field at any location depends on the flow, bottom bathymetry, and seabed electrical conductance over the whole ocean basin. Fortunately, the method by which velocity is determined from voltage measurements has shown to first order that the ocean-induced EM field depends only on the local vertical distribution of velocity. This simple relationship between the horizontal electric field and velocity has been exploited to make high quality measurements of velocity and transport by many instruments, including cables, bottom landers, vertical profilers, and Lagrangian floats. Although the existing theory has worked well to interpret most measurements, especially in deep water, comprehensive studies have not been made of the influences of large bathymetric features, complex and time-varying velocity fields, or other such complexities. The issue of instrument development has received much attention to date and has resulted in a variety of EM-based instruments well suited for many observational environments. A crucial and complementary component to developing instruments, however, is being able to interpret higher order terms that will inevitably arise in the more complex environments they may encounter. This project will evaluate motional electric fields where higher terms are seen to be significant: regions with steep topography, gradients in sediment conductance, and time-varying flow that crosses topography.This project has three components: observational, analytical, and numerical elements. Observations will focus on data collected off Cape Hatteras. Initial estimates of higher order terms show that they are an order of magnitude smaller than the first order terms, allowing an iterative approach to be taken. Oceanic velocity structure will be determined in conjunction with a current meter study that was active at the same time to estimate cross-isobath flow, across- and along-isobath velocity shears, and temporal variability, while an analysis of geologic structure will determine the electrical properties of the seafloor. From the velocity and geologic structure the electric fields will be investigated for anisotropies that may be correlated with the direction or magnitude of topographic or bottom conductance gradients or the direction of oceanic flow. The results will be compared with data from different environments: namely an eddy over an abyssal plain and tidal flow perpendicular to a ridge. For the analytic analysis, simple cases with sloping bottom boundaries will be solved to determine topographic effects when changes in bottom depth are not small. Numerical modeling with an existing EM model will extend the analytic results and allow the full conditions off Cape Hatteras to be solved with accurate oceanic and geologic information. Representative cases will also be considered to generalize what processes generate higher order terms. This project will improve the prediction of error sources to allow users of EMbased measurements to plan for or interpret observations. Based upon an error analysis contributions from higher order terms can be estimated with these three approaches, as well as adding physical insight into observed anisotropies and the role of conductive seafloor. A website will be developed to support EM observations that will disseminate practical techniques (including the present research) and will facilitate communication among investigators. Intellectual Merit The research will extend the understanding of motionally induced electric fields to a broad range of complex environments not yet considered in detail: including topographic features and time-varying flow, where oceanographic processes often are of great interest, in addition to confounding effects.Broader Impacts There are many devices, some in commercial production, that measure ocean velocity based on the principles of motional induction. It is essential to extend the theoretical understanding of such measurements for more accurate analysis, for improved experiment planning and prediction of anomalous effects, and for better acceptance and usage of the methodology within the oceanographic community. EM measurements and their interpretation will add powerful capabilities to ocean observing systems (e.g. IOOS). An online resource will collect information about performing EM observations and will make it readily available to the community. Human Resources This project will constitute a Ph.D. candidate's doctoral research.