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.
