Electromagnetic Modeling Studies of Lithospheric Propagation

Abstract

The propagation of radio waves through the lithosphere between buried transmitting and receiving sites is a communication technique of great interest at the moment due to the physical hardness or invulnerability of such a communication system to nuclear blasts. Theoretical studies and laboratory experiments were performed to investigate the propagation of electromagnetic waves in a low-conductivity lithospheric duct (granitic basement layer). The real earth situation was approximated by a low conductivity layer with uniform properties located between two regions of high conductivity (the upper crust of the earth and the earth's mantle). Models were also used to investigate the effects of geological discontinuities and inhomogeneities on the electric field strength. Different models were constructed using scaling factors of the order of 105. Aluminum plates were used in the first model to represent the mantle and the upper crust. The low conductivity layer was modeled by a salt water solution. In a second model the upper crust was modeled by a carbon sheet instead of an aluminum sheet. The conductivity of the carbon sheet accurately modeled the value of conductivity assumed for the upper crust of the earth. Excellent agreement was obtained between theoretical and experimental results for CW propagation. This agreement demonstrates the validity of the laboratory modeling techniques and lends confidence to the experimental results obtained for the effects of inhomogeneities in the duct.