Several radio transmitters operate throughout the world in the VLF range, 15-30 kHz, and are used mainly for marine navigation, communication with submersed submarines, and other purposes. The principle of VLF subsurface exploration is simple. At large distances from the transmitter, the EM field is effectively planar and horizontal. A conductor that strikes in the direction of the transmitter is cut by the magnetic vector, and these primary magnetic fields cause electric currents to flow in subsurface conductors. In another, more recent, theory, the subsurface currents flow along interfaces between bodies of differing conductivity.
The Very Low Frequency (VLF) radio spectrum extends from 3 to 30 kHz. Standard communications receivers do not receive signals that low in frequency. Furthermore, most antennas used with standard communications receivers are optimized for higher frequencies and perform poorly in the VLF spectrum. Because of that, few Radio Amateurs or shortwave listeners have ever heard VLF signals and most know very little about the large number of stations in that part of the spectrum, what those stations are used for, or the nature of VLF radio signal propagation.
However, because VLF receivers and receiving antennas are easy to construct, the VLF spectrum attracts a small number of curious and sometimes very dedicated VLF listeners who want to learn more about this little-known and somewhat secretive part of the radio spectrum.
Signals from powerful VLF stations can be received worldwide. They penetrate deep into the earth and deep into the sea (especially in the lower portion of the VLF spectrum) and therefore can be received by submerged submarines. Though some VLF stations have other purposes, the primary function of most VLF stations is to communicate with submerged military submarines or help them navigate underwater.
http://www.smeter.net/stations/vlf-stations.php
http://sidstation.lionelloudet.homedns.org/stations-list-en.xhtml





