Ultra low frequency (ULF) is the name given by the ITU to a range of electromagnetic waves with frequencies between 300 hertz and 3 kilohertz. These waves have wavelengths between 1,000 and 100 kilometers. In the study of Earth's magnetic field and earthquake science, different ranges are sometimes used, such as 1 mHz to 100 Hz, 1 mHz to 1 Hz, and 10 mHz to 10 Hz.

ULF waves can be observed in Earth's magnetic field and on the ground. These waves are important for understanding physical processes in the space around Earth. The speed of ULF waves is often linked to a value called the Alfvén velocity, which depends on the strength of Earth's magnetic field and the density of plasma in that area.

This frequency range is used for communication in mines because the waves can go through the Earth's surface.

Earthquakes

Some monitoring stations have found that earthquakes sometimes happen after a sudden increase in ULF activity. An example of this was thought to have happened before the 1989 Loma Prieta earthquake in California, based on data from a nearby sensor array. However, a later study suggested that the 1989 event was likely due to a problem with the sensors, as other sensors in different areas did not detect the same ULF increase.

On December 9, 2010, geoscientists reported that the DEMETER satellite noticed a large increase in ULF radio waves over Haiti in the month before the 2010 earthquake, and a slow decrease in the month after the earthquake. Researchers are trying to learn more about this connection to see if this method can help create an early warning system for earthquakes.

Earth mode communications

Communications through the ground using electric fields are called "Earth-Mode" communications. This method was first used during World War I. Scientists began studying its use for mining operations in the 1920s.

Ultra-Low Frequency (ULF) signals have been used by the military for sending secure messages through the ground. Reports from NATO's AGARD in the 1960s described several systems that used this technology. However, some details about secret defense projects may not have been shared in these reports.

Amateur radio operators and electronics enthusiasts have used Earth-Mode for short-distance communication. They connect amplifiers that boost sound signals to pairs of metal rods driven into the soil. At the receiving end, a weak electric current between another pair of rods detects the signal. By using special computer-based filters that focus on very narrow signal ranges, signals can be received up to a few kilometers away. This works with a transmission power of 10 to 100 watts and rods spaced about 10 to 50 meters apart.