Very-low-frequency propagation below the bottom of the sea

Radio-wave propagation at very low frequencies (VLF) in the stratified rock below the bottom of the sea is studied. A reasonable assumption of extremely low electrical conductivity in the stratified rock is based upon available geological data. The surface wave traveling along the interface between this region of low conductivity and the highly conducting sea is compared with the vertically polarized ground wave found in VLF radio-wave propagation at the surface of the earth. When extremely low frequencies (ELF) are transmitted, the highly conducting layer found at greater depths below the bottom of the sea forms the lower surface of a spherical waveguide. This waveguide at ELF supports a propagation mode similar to the mode existing at VLF between the surface of the earth and the lower boundary of the ionosphere. The similarity in propagation mechanisms leads to the name "inverted ionosphere" (described by Wheeler [1]) for the underground region. The sea or relatively highly conducting soil at the surface of the earth is an almost impregnable shield against atmospheric noise and effects from sudden ionospheric disturbances or solar flares. In addition to providing a noise-free medium, the sea has the advantage that construction costs are much less than those of a VLF transmitter at the earth's surface. Presumably communication between shore installations and submarines on the floor of the ocean could be achieved with the inverse ionosphere. The power requirement for such communication with existing VLF transmitters at the earth's surface renders such transmission unattainable.