Governments have established a grid of tall, high powered transmitters, up to 1000 watts, that broadcast a signal field in the 15 to 28 kHz frequency range. These broadcast fields propagate thousands of miles over the Earth’s surface and are essentially uniform in the atmosphere. Due to their power, the signals penetrate into the ground to depths of several hundred feet. Because of the high contrast of material properties on the ground/air interface, the signals are refracted down into the ground at steep angles. Since the Earth is relatively less homogeneous then the atmosphere, the EM flux crowds into zones of higher conductivity and rarefies in zones of higher resistivity.
A VLF receiver tuned to the frequency of a VLF transmitter, traversed across the Earth’s surface, will exhibit high signals over conductive waterbearing fracture zone and a low strength signals over the resistive portions of the crystalline rock mass. Any linear conductive body, in addition to waterbearing fractures, can also be detected by VLF methodology.
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