Radio waves in rock near overburden-rock interface

An account is given of a theoretical investigation of radio wave propagation in a rock medium situated below a highly conducting plane boundary overburden, due to a vertically polarized electric dipole source immersed in the rock near the boundary. Excluded from this discussion are propagation modes in the rock guided between plane boundaries on top and bottom. Just as with dipoles in air above a plane ground, the rock wave can be considered as the result of a "direct wave," a wave "reflected\´\´ from the overburden and an "interface surface wave." Rock with complex phase constant replaces air with real phase constant . Of interest is the variation with depth below the boundary of the fields in the rock, analogous to "height gain" for fields in air. Of importance is the effect of the relative refractive index of overburden-to-rock. A sample calculation is given for a one-mile path assuming typical electrical constants for the rock and overburden media. At the VLF of interest, loss tangents in the media are large. These results may assist in determining whether or not a received signal is due predominantly to a rock propagated wave or to a possible "up-over-and-down" mode.