Ultra low frequency (ULF) is the ITU designation for the

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

range of electromagnetic waves between 300

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...

and 3 kilohertz, corresponding to wavelengths between 1,000 and 100 km. In

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

science and

seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...

, 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

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 An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they c ...

are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the

1989 Loma Prieta earthquake On October 17, 1989, at 5:04 p.m. Pacific Time Zone, PST, the Loma Prieta earthquake occurred at the Central Coast (California), Central Coast of California. The shock was centered in The Forest of Nisene Marks State Park in Santa Cruz Cou ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

, although a subsequent study indicates that this was little more than a sensor malfunction. On December 9, 2010, geoscientists announced that the

DEMETER In ancient Greek religion and Greek mythology, mythology, Demeter (; Attic Greek, Attic: ''Dēmḗtēr'' ; Doric Greek, Doric: ''Dāmā́tēr'') is the Twelve Olympians, Olympian goddess of the harvest and agriculture, presiding over cro ...

satellite observed a dramatic increase in ULF radio waves over

Haiti Haiti, officially the Republic of Haiti, is a country on the island of Hispaniola in the Caribbean Sea, east of Cuba and Jamaica, and south of the Bahamas. It occupies the western three-eighths of the island, which it shares with the Dominican ...

in the month before the magnitude 7.0 M_w 2010 earthquake. 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 Communications security is the discipline of preventing unauthorized interceptors from accessing telecommunications in an intelligible form, while still delivering content to the intended recipients. In the North Atlantic Treaty Organization c ...

through the ground. NATO

AGARD The Advisory Group for Aerospace Research and Development (AGARD) was an agency of NATO that existed from 1952 to 1996. AGARD was founded as an Agency of the NATO Military Committee. It was set up in May 1952 with headquarters in Neuilly sur Sei ...

publications from the 1960s detailed many such systems, although it is possible that 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

World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

Radio amateurs An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators ...

and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

pairs hammered into the soil. At the receiving end, the signal is detected as a weak

electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...

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

* Tomislav Stimac, "
Definition of frequency bands (VLF, ELF... etc.)
'". IK1QFK Home Page (vlf.it).

* Amateur Radio Below 10 kHz

* 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
{{DEFAULTSORT:Ultra Low Frequency Radio spectrum

Earthquakes

Earth mode communications

See also

References

External articles