Review of the earth mode communication program at AFCRL

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The program in earth mode communications research at the U.S. Air Force Cambridge Research Laboratories is reviewed. Emphasis is placed on geological and geophysical problems encountered in the course of the program. Communications modes studied include: (1) 'deep-strata', wherein radio frequency electromagnetic waves are propagated between buried antennas over a path entirely within rock strata, and (2) 'conduction current', consisting of ULF conduction field communications between long, grounded dipoles. Deep-strata studies were carried out utilizing both loops and linear antennas immersed in the rock medium. Antenna and transmission path characteristics were studied under a variety of conditions and over paths up to 4.5 km in length. It was found that two of the greatest problems facing the communicator are the complexity of the earth's crust and its low values of electrical resistivity. Therefore, a geological and geophysical study was performed to evaluate possible transmission sites in the United States. Conduction field studies were performed using what was essentially the dipole-dipole geophysical resistivity sounding technique. Field tests were made at frequencies from 0.01 Hz to 10 Hz and at ranges to 32 km. (Author).