Interpretation of Motionally Induced Electric Fields in Oceans of Complex Geometry

海水在地球磁场中的运动产生动感电场、电流和磁场。原则上，任何位置的电磁场取决于整个大洋盆地的水流、海底水深测量和海底电导。幸运的是，根据电压测量确定速度的方法已经表明，海洋感应电磁场只取决于局部速度的垂直分布。水平电场和速度之间的这种简单关系已经被许多仪器用来高质量地测量速度和传输，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，特别是在深水中，但还没有全面地研究大的水深特征、复杂的和时变的速度场或其他此类复杂因素的影响。到目前为止，仪器开发问题受到了极大的关注，并产生了各种基于电磁的仪器，非常适合于许多观测环境。然而，开发工具的一个关键和补充部分是能够解释在它们可能遇到的更复杂环境中不可避免地出现的更高阶项。这个项目将评估那些较高项被认为是重要的运动电场：地形陡峭的地区，泥沙电导率的梯度，以及跨越地形的时变水流。该项目包括三个部分：观测、分析和数值元素。观测将集中在哈特拉斯角附近收集的数据。高阶项的初步估计表明，它们比一阶项小一个数量级，从而允许采取迭代方法。将结合同时开展的一项海流计研究来确定海洋速度结构，以估计跨等深水流、跨等深线和沿等深线的速度剪切以及时间变异性，而对地质结构的分析将确定海底的电学性质。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海底电导梯度的方向或大小或海洋流动的方向有关。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海脊的潮汐流。对于解析分析，将解决底部边界倾斜的简单情况，以确定底部深度变化不小时的地形影响。用现有的EM模型进行数值模拟将扩展分析结果，并允许用准确的海洋和地质信息解决哈特拉斯角附近的全部条件。还将考虑具有代表性的案例，以概括哪些过程会生成更高阶项。该项目将改进对误差源的预测，使基于电磁的测量的用户能够规划或解释观测结果。在误差分析的基础上，可以用这三种方法估计高阶项的贡献，并增加对观测到的各向异性和传导海底作用的物理洞察。将开发一个网站，以支持EM观察，传播实用技术(包括本研究)，并促进调查人员之间的交流。智力价值这项研究将把对运动感应电场的理解扩展到尚未详细考虑的广泛的复杂环境中：包括地形特征和时变流动，在这些环境中，海洋过程除了混杂的影响外，还经常引起人们的极大兴趣。波德影响有许多装置，其中一些正在商业生产中，根据运动感应原理测量海洋速度。必须扩大对这类测量的理论理解，以便进行更准确的分析，改进实验规划和异常影响的预测，并在海洋学界更好地接受和使用这一方法。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。一个在线资源将收集有关进行电磁观测的信息，并将使社区随时可以获得这些信息。人力资源这个项目将构成博士生的博士研究。

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.