The study of electromagnetic propagaton deep within the crust of the earth, over paths hundreds to thousands of kilometers long, is significant for the information it may disclose concerning the structure and properties of the crust, as well as for its potentialities in novel systems of communication, detection, and remote control. Factors critical to electromagnetic propagation in the crust are: 1) the gross and intermediate scale structures of the crust; 2) the composition of the constituent

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... ock facies and their compositional variation and discontinuities laterally and in depth; 3) the primary electronic properties of the dominant rock facies, as functions of temperature, pressure, and frequency; 4) the absolute temperatures and pressures within the crust and the temperature-pressure gradients; 5) the frequency and effective wavelengths of the EM wave in the medium; 6) the magnitude and spectrum of the EM noise in the lithosphere; 7) the characteristics of the antenna, of its emplacement in the crust, and the coupling parameters. Laboratory measurements of a typical dry granitic medium of the continental basement are favorable to the transmission of EM energy at low frequencies (103 cycles per second) an unacceptable downward refraction of horizontally launched VLF waves within a few wavelengths. The feasibility of long-distance propagation may, therefore, depend upon the possible influence of a minimum velocity duct or waveguide in which the radiated energy would be conserved either 1) by reflection between an upper and a lower discontinuity; or 2) by refraction along a zone of refractive index inversion; or 3) by a combination of these, that is, a favorable gradient plus a discontinuity. While current knowledge of crustal structure and mineral phase changes at depth do not rule out such gradients and discontinuities, neither do they provide any reliable basis for postulating the existence of such a favorable propagation channel. The possible occurrence of Conrad type discontinuities is relevant here.