Lateral Electromagnetic Waves for Ocean Floor Telemetry

Abstract

ABSTRACT In this paper, an ocean floor telemetry system is proposed that employs the lateral electromagnetic wave. The lateral electromagnetic wave's advantage is that it travels along the low conductivity rock sea boundary instead of through the high conductivity seawater. Existing theory, laboratory scale model experimental data, and at-sea measurements form the basis of the design of such a system. INTRODUCTION The problem of obtaining telemetry in the ocean environment will be addressed through the use of a novel communications system that employs the lateral electromagnetic wave. The lateral wave's unique advantage is that it propagates along the low conductivity ocean bottom making long distance reception of telemetric data possible. In the past rf systems for this purpose have not been feasible due to the high conductivity of seawater which causes a signal propagating through it to be rapidly attenuated and therefore useless for long distance communication. The ocean bottom communications system design presented is introduced by building a foundation based upon a detailed explanation of the theory of the lateral electromagnetic wave, and by verifying this theory with data acquired from laboratory and at-sea measurements. This approach establishes confidence in the subsequent design. THEORY OF LATERAL WAVE PROPAGATION Lateral wave propagation has been investigated quite thoroughly in the electromagnetic literature over the past several decades starting with the work of Sommerfeld in 1909. Surprisingly, its use so far for communications has been limited and its application to the ocean floor environment is unknown. The simplest way to visualize lateral wave propagation is through ray optics (see Figure 1). When an electromagnetic wave impinges on the interface between regions 1 and 2 (seawater and rock respectively) three possible propagation paths may take place. The wave may be reflected back in to region 1 if the incident angle is greater than the critical angle; it may propagate into region 2 if the incident angle is less than the critical angle; or it may propagate along the boundary (in region 2) if the incident angle is equal to the critical angle. As the wave propagates along the interface the amplitude decreases continuously due to a diffusion process into the electrically more dense Region 1. This complicated phenomenon had been analyzed by Sommerfeld and his final expression for the radial E-field in Region 1 generated by a short unit current dipole (also in region 1) is expressed in terms of a very complicated integral1. Mathematical equation (Available in full paper) His expression gives the radial electric field at the receiver point Rx generated by the dipole at the transmitter Tx. Unfortunately, this expression appears to be too involved to be useful. The following years witnessed a series of numerical approaches to the Sommerfeld integral. In 1966, Banes approximated Sommerfeld's integral by means of the near field, intermediate field, and far field2. His approximations are:Mathematical equation (Available in full paper)