Ocean electromagnetics

Abstract

Even though acoustic waves can travel long distances in the sea with little attenuation, ocean electromagnetics has important applications in the areas of geophysical surveys and searches of the seafloor and sub-bottom, communications across the sea-air boundary, and high data transfer rate at short ranges. Unlike in-air propagation of electromagnetic fields, the finite conductivity of seawater results in a frequency-dependent phase velocity, attenuation, intrinsic impedance, and reflection and transmission coefficients at the ocean’s surface. After giving a short summary of the electric and magnetic properties of the ocean, this chapter begins with Maxwell’s equations and develops the mathematical descriptions of electromagnetic fields and dipole sources within a conducting media. The differences between plane wave reflection and transmission at the surface of fresh water and seawater are used to highlight how electromagnetic propagation within the electrically conducting ocean is so very different than the more familiar radio frequency transmissions in air. In addition, equations are presented that describe the fields from sub-merged electric and magnetic dipoles that are located both far and near the sea surface. These formulations are valid over the frequency range from 0 Hz to a few MHz. Finally, a brief discussion of ocean electromagnetics at optical wavelengths is given at the end of this chapter.