Ultra low frequency

Listening to 500 Hz signal of Ambrose Channel pilot cable in 1920

Ultra low frequency (ULF) is the ITU designation<ref name="1037B">Template:Cite book</ref><ref name="itu-2015-acts">Template:Cite conference</ref> 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,<ref>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, Template:ISSN, Template:Doi.</ref> 1 mHz to 1 Hz,<ref>T. Bösinger and S. L. Shalimov, "On ULF Signatures of Lightning Discharges", Space Science Reviews, Volume 137, Issue 1, pp. 521–532, June 2008, Template:Doi.</ref> and 10 mHz to 10 Hz.<ref>O. Molchanov, A. Schekotov, E. Fedorov, G. Belyaev, and E. Gordeev, "Preseismic ULF electromagnetic effect from observation at Kamchatka", Natural Hazards and Earth System Sciences, Volume 3, pp. 203–209, 2003</ref>

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.<ref>HF and Lower Frequency Radiation - Introduction Template:Webarchive</ref>

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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>

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 M_w 2010 earthquake, and a gradual ebbing of the ULF waves in the month after the quake.<ref>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.</ref><ref>Template:Cite web</ref> 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.<ref>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.</ref><ref>Coggeshall, E. J., E. W. Felegy, and L. H. Harrison. Some Studies on Emergency Mine Communications. BuMines RI 4135, 1948, 44 pp. </ref>

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,<ref>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</ref> 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 10Template:Ndash100Template:NbspW and electrode spacing of around 10Template:Ndash50Template:Nbspm.Template:Citation needed

See also

• Extremely low frequency
• Earth's field NMR
• Valery Troitskaya
• Through-the-earth mine communications
• Voice frequency

References

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External articles

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
• NASA live streaming ELF -> VLF Receiver
• Amateur Radio Below 10 kHz "G3XBM's page on Earth Mode Communication"
• Review of Earth Mode Communications "1966 abstract about Earth Mode Comms by Ames, Frazier and Orange"
• Radio communications within the Earth's crust "Abstract of article by Burrows written in 1963"
• OEAW, "[1]'". ULF Waves -according to geophysical research community Homepage of OEAW, Austrian academy of science, Austria.