Using very low frequency (VLF) communication signals generated from several military transmitters around the world. These transmitters are used to provide communication with submarines deep under water.

Due to the nature of the use of these radio transmitters, they are very powerful and induce electric currents in conductive bodies thousands of miles away. The signals from these transmitters cause secondary effects in the subsurface. Localized conductors, such as water-filled fractures, cause angular disturbances in this signal which can be measured. The depth of investigation is dependent on the subsurface conductivity but, survey depths of several hundred feet are common.