LARGE-AMPLITUDE CHANGES INDUCED BY A SEABED IN THE SUB-LF ELECTROMAGNETIC-FIELDS PRODUCED IN, ON, AND ABOVE THE SEA BY HARMONIC DIPOLE SOURCES

Because its effective electrical conductivity is usually lower than that of seawater, the seabed can influence the amplitude of sub-LF (frequencies less than 30 kHz) electromagnetic fields generated by electric and magnetic dipole sources located in, on, and above the sea surface. The changes produced in the fields depend on the particular source-receiver geometry. In general, they tend to be largest when the seawater is electrically shallow (i. e. , only a few seawater skin depths delta //s deep) and when the dipole sources are located within 1 delta //s of the seafloor. However, even when these general tendencies are taken into account, computations we have made of the fields indicate that there are certain configurations where comparatively large changes in the fields can take place due to the influence of the seabed. The horizontal electric field produced on the sea surface by a vertical electric dipole source located on the seafloor, for example, can have its amplitude increased by 5-6 orders of magnitude at horizontal distances of the order of 10-15 delta //s. This paper describes the form of these comparatively large changes and the conditions under which they can be expected to occur.