Very-low-frequency propagation below the bottom of the sea

Abstract

In the search for extremely reliable electromagnetic communication to submerged submarines, the question arose, * "What is the effect of the roughness and irregularity of the sea surface on the propagation of electromagnetic waves?" The purpose of this investigation is to obtain an engineering-understanding of the effect of the rough air-sea interface on electromagnetic signals used in communication to submerged submarines. The frequency of the electromagnetic wave is restricted to the3 ELF or VLF range. In the initial part of the investi-gaticin, the sea surface is assumed to .be a two-dimensional (constant in one variable or direction) sinusoidal surface; later a doubly,(three.-dimensional) sinusoidal surface is considered. The source of electromagnetic energy is assumed-to be a plane wave with arbitrary direction of propagation and: polarization. The fields on the air side of the sea surface are com-pited.with the aid of the assumptions that the sea is a p-ect .electric conductor and that the sea surface is only-sg+Eghtly-rough (,i.e., the maximum slope of the sea surface q-tis uch less than 1). The integral equations governing the tangential magnetic fields are formulated and solved. These-solutions show a variation of the tangential magnetic field '(of the otder of 2 db. frn the flat surface case) depending '7 .i I on polarization and direction of propagation of the incident plane wave. The fields in the sea are computed by assuming the tangential magnetic field is continuous through the air-sea interface. The method used in these calculations is a numerical one based on finite differences. Both from the numerical solutions and a heuristic theory of propagation in the sea, it is seen that the perturbation of the fields caused by the roughness of the sea surface decays rapidly with depth if the sea wave wavelength is less than or the order of magnitude of the skin depth of the sea at the frequency considered; if the sea wave wavelength is many orders of magnitude larger than the skin depth, there is little decay (at the depths considered) of the perturbation, so that the phase and amplitude of the fields in the sea vary with the height of the sea vert±cally above them.

On the electromagnetic fields produced by marine frequency domain controlled sources

“…The theory of propagation for ELF/SLF (<300 Hz) electromagnetic (EM) fields through, under and over the ocean from a submerged source has been investigated for more than fifty years as a means for communication, underwater detection and geophysical exploration (e.g. Mott & Biggs 1963; Bannister 1968; Chave et al 1990). Pioneering measurements by Cox et al (1978) showed that the natural background noise in the electric field at frequencies around 1 Hz is extremely low (∼1 pV m –1 ), suggesting that the weak fields induced within Earth by a near‐seafloor artificial source could be detected at large (many kilometres) source–receiver offsets.…”

Computer models for estimating the breakdown voltage of air in uniform and non‐uniform fields at 30 kHz

“…Figure 8 shows the comparison of the uniform field 30 kHz breakdown voltages obtained experimentally with those calculated using Equations (1) and (2) for the original and extrapolated gap Figure 8. Comparison of the experimental uniform field 30 kHz breakdown voltages to the estimates calculated using Equations (1) and (2). Breakdown voltages are plotted as a function of gap length.…”

“…This is because the earth and the D region of the ionosphere work as a waveguide at this frequency range [1]. VLF/LF signals penetrate seawater somewhat and can be used for communication to submerged objects at moderate depths [1,2]. The U.S. Navy operates 10 high-power VLF/LF transmitting stations for strategic purposes.…”

Implementation of pulse position modulated blue-green laser communication system

“…The high-speed timer can ensure the time interval ΔT is shorter so as to have more time slots M in a fixed pulse cycle. Thus, the data bits transmitted by one pulse can be increased according to Equation (1), and higher data rate can be realized for PPM communication. In order to avoid the adverse effects by time slots overlapping, high-stability clock in the range of above 10 -8 is indispensable, too.…”

“…Nowadays, the normal means of underwater communicating is through VLF radio which sends encrypted wireless teletype. Since VLF electromagnetic radiation is attenuated rapidly by seawater, the submerged receiving antenna must be quite near the surface [1][2]. In order to realize greater depth contact, blue-laser communication was developed from the early 1960s.…”