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Ultra low frequency

Range 300-3000 Hz of the electromagnetic spectrum

Range 300-3000 Hz of the electromagnetic spectrum

Ultra low frequency (ULF) is the ITU designation for the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz, corresponding to wavelengths between 1,000 and 100 km. In magnetosphere science and seismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, and 10 mHz to 10 Hz.

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 Alfvén 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 phenomena was believed to have occurred before the 1989 Loma Prieta earthquake in California based on data from a nearby sensor array. However, a subsequent study argued this 1989 event was little more than a sensor malfunction, as sensors from other locations did not identify the ULF spike reported from near the quake's epicenter.

On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 Mw 2010 earthquake, and a gradual ebbing of the ULF waves in the month after the quake. 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

Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in World War I. This technology was explored for possible use in the mining industry from the 1920s.

ULF has been used by the military for secure communications through the ground. Publications associated with NATO's AGARD from the 1960s detailed many such systems, although it is possible that the published papers left unmentioned some information about what actually was developed secretly for defense purposes.

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 a further pair 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 kilometers with a transmitting power of 10100W and electrode spacing of around 1050m.

References

External articles

References

  1. (3 June 1991). "US Federal Standard 1037B: Telecommunications, Glossary of Telecommunications Terms". Office of Technology Standards, General Services Administration.
  2. (2015). "Final Acts WRC-15". International Telecommunications Union.
  3. V. A. Pilipenko, "ULF waves on the ground and in space", [[Journal of Atmospheric and Terrestrial Physics]], Volume 52, Issue 12, December 1990, pp. 1193–1209, {{ISSN. 0021-9169, {{doi. 10.1016/0021-9169(90)90087-4.
  4. T. Bösinger and S. L. Shalimov, "On ULF Signatures of Lightning Discharges", [[Space Science Reviews]], Volume 137, Issue 1, pp. 521–532, June 2008, {{doi. 10.1007/s11214-008-9333-4.
  5. O. Molchanov, A. Schekotov, E. Fedorov, G. Belyaev, and E. Gordeev, "[http://www.nat-hazards-earth-syst-sci.net/3/203/2003/nhess-3-203-2003.pdf Preseismic ULF electromagnetic effect from observation at Kamchatka]", ''Natural Hazards and Earth System'' Sciences, Volume 3, pp. 203–209, 2003
  6. [http://www.weather.nps.navy.mil/~psguest/EMEO_online/module3/module_3_1.html HF and Lower Frequency Radiation - Introduction] {{Webarchive. link. (2005-11-09)
  7. Fraser-Smith, Antony C.. (August 1990). "Low-Frequency Magnetic Field Measurements Near the Epicenter of the Ms 7.1 Loma Prieta Earthquake". [[Geophysical Research Letters]].
  8. Thomas, J. N.. (April 2009). "On the reported magnetic precursor of the 1989 Loma Prieta earthquake". [[Physics of the Earth and Planetary Interiors]].
  9. Athanasiou, M. A., Anagnostopoulos, G. C., Iliopoulos, A. C., Pavlos, G. P., and David, C. N.: Enhanced ULF radiation observed by DEMETER two months around the strong 2010 Haiti earthquake, Nat. Hazards Earth Syst. Sci., 11, 1091–1098, https://doi.org/10.5194/nhess-11-1091-2011, 2011.
  10. KentuckyFC . (December 9, 2010). "Spacecraft Saw ULF Radio Emissions over Haiti before January Quake". [[MIT Technology Review]].
  11. C. L. Colburn C. M. Bouton H. B. Freaman RI 2407 Experiments in underground signalling [sic] with radio sets. The National Institute for Occupational Safety and Health (NIOSH), 1922.
  12. Coggeshall, E. J., E. W. Felegy, and L. H. Harrison. Some Studies on Emergency Mine Communications. BuMines RI 4135, 1948, 44 pp.
  13. C. K. H. Tsao and J. T. deBettencourt, "Subsurface radio propagation experiments," in Radio Science, vol. 3, no. 11, pp. 1039-1044, Nov. 1968, doi: 10.1002/rds19683111039
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