Everything about The Ultra Low Frequency totally explained
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.
Earthquakes
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 defense 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
electric current between two further pairs of electrodes. Using weak signal reception methods with PC based
DSP filtering with extremely narrow bandwidths it's possible to receive signals at a range of a few kilometers with a transmitting power of 10-100W and electrode spacing of around 10-50m.
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