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Ultra low frequency
The Ultra Low Frequency (ULF) is the frequency range between 300 hertz and 3 kilohertz. Many types of waves in the ULF frequency band can be observed in the magnetosphere and on the ground. These waves represent important physical processes in the near-Earth plasma environment. The speed of the ULF waves is often associated with the Alfven velocity that depends on the ambient magnetic field and plasma mass density.
This band is used for communications in mines, as it can penetrate the earth. 
Additional recommended knowledge
Some monitoring stations have reported that earthquakes are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the Loma Prieta Earthquake in California in 1989. Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes.
Earth Mode Communications
ULF has been used by the military for secure communications through the ground. NATO AGARD publications from the 1960s detailed many such systems, although one suspects the contents of the published papers left a lot unsaid about what actually was developed secretly for defence purposes. Communications through the ground using conduction fields is known as "Earth Mode" communications and was first used in WWI. Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced electrode pairs hammered into the soil. At the receiving end the signal is detected as a weak current between two further pairs of electrodes. Using weak signal reception methods with PC based DSP filtering with extremely narrow bandwidths it is possible to receive signals at a range of a few kilometres with a transmitting power of 10-100W and electrode spacing of around 10-50m.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ultra_low_frequency". A list of authors is available in Wikipedia.|