ULF-ELF ELECTROMAGNETIC-FIELDS GENERATED ABOVE A SEA OF FINITE DEPTH BY A SUBMERGED VERTICALLY-DIRECTED HARMONIC MAGNETIC DIPOLE

A numerical integration technique is used to compute the amplitudes of the quasi‐static electromagnetic fields produced on and above the surface of a sea of finite depth by a submerged vertically‐directed harmonic magnetic dipole. The purpose of these computations is to determine the conditions under which an electrically conducting sea floor can produce significant changes in the fields, as compared with the fields produced on and above an infinitely deep sea, for frequencies in the ULF/lower‐ELF bands (frequencies less than 300 Hz). It is shown that even a comparatively highly conducting sea floor (conductivity σƒ ∼ 0.4 mho/m) can produce substantial changes in the field amplitudes for certain source‐receiver configurations and that alterations of two orders of magnitude or more can occur in the amplitudes on the sea surface for smaller values of σƒ. The changes generally become smaller as the altitude of the receiver is increased, but even at altitudes equal to ten sea water skin depths (10 δ) the field quantities may be doubled by the presence of a sea floor. As might be expected, the sea floor effects are most marked in seas that are electrically shallow, i.e., for seas that are only a few skin depths deep. The largest changes are produced when the VMD is within 1 δ of the sea floor, and there is essentially no sea floor effect when the VMD is more than 3 δ above the floor. Copyright 1979 by the American Geophysical Union.