VLF

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.