Electromagnetic induction effects at an ocean coast

This paper reviews recent progress in model calculations towards understanding electromagnetic induction effects at ocean coasts. Early models consisted of two adjacent quarter-spaces of different conductivity, whereas the newer models simulate the ocean with a very thin sheet of a perfect conductor placed on top of a uniform Earth medium. The inducing field is assumed to arise from a monochromatic plane wave incident vertically from above. With any of these models one succeeds at once in explaining the occurrence of large vertical magnetic fields when the inducing electric field is polarized parallel to the coast (E-polarization), thereby also confirming the highly directional character of the coast effects as discovered a few years before by Parkinson. Another important step was made when, first numerically, then analytically, the behavior of the horizontal component of the magnetic field at the surface was rigorously calculated. For H-polarization (inducing magnetic field parallel to shore) this horizontal surface field is uniform, but is not so for E-polarization. Indeed, it has now been shown that the surface field can vary appreciably close to the coast, particularly on the ocean side of shore. With E-polarization, very large currents flow in the ocean, parallel to shore, with the result that the effect of the coast is felt at very large distances over both land and sea. Under H-polarization induction the range of the coast effect is very much shorter, in fact almost an order of magnitude shorter over the land and even reducing to zero at the surface of the perfectly conducting model ocean. The magnetic fields at the ocean floor have also been calculated, which should be of interest in the rapidly expanding field of marine survey and prospecting.