Some science behind the scenes

Industrial electrical equipment

Industrial electrical equipment or installations can produce  a background magnetic field.  There are a large number of industrial – that is non domestic – forms of equipment that emit low frequency magnetic fields.  Examples include:

Mine communication equipment - Ultra Low Frequency (ULF) is used for communications in mines, as it can penetrate the earth

Railway contact lines and electrified railways

Transformer stations -
Transformers increase or decrease voltages.  They are used in power plants, substations, residential areas and industrial zones. Transformer stations in residential areas are fed via the regional electricity distribution network and change the supply voltage to the levels required for use in households. A basic transformer station  comprises a high voltage component, a transformer and a low-voltage distributor. Both the low voltage distributor itself and its connection to the transformer generate the strongest magnetic fields.

Underground power cables - compared with overhead power lines carrying the same current, the magnetic field of an underground cable system has a much smaller spatial extension.  Although the exposure may be just as high directly above an underground cable system as it is immediately beneath an overhead power line, it decreases more quickly on departing laterally than is the case with overhead lines.  Unlike the magnetic field, the electric field is completely shielded by the cable sheath and the soil. This means that no electric field is detectable even if you are standing directly above the underground cables

Underwater power cables - as powerlines above water emit magnetic fields, underwater cables also emit the same radiation

High voltage power lines -
The magnetic field created is equal to the frequency of the power supply, hence for example, if the power supply is operated with alternating current with a frequency of 50 Hz, the magnetic fields are also 50 Hz.  With high-voltage power lines, the higher the current, and the greater the distances between the power-bearing conductors, the greater the spatial bearing of the magnetic field. The walls of buildings cannot screen magnetic fields effectively.

Avalanche beacons

Wireless heart rate monitors

Geophysics instruments

Oil industry communication equipment- Extremely low frequencies (ELFs) are employed to transmit data from underground to the ground surface in ‘measurement-while-drilling’ electromagnetic (MWD-EM) telemetry system.

Oil industry pigs - Transmitters in the 20 Hz range are found in pipeline inspection gauges, also known as "PIGs". The transmitted signal is often used to track the pig should it become stuck in the pipeline.  A pipeline inspection gauge or pig in the pipeline industry is a tool that is sent down a pipeline and propelled by the pressure of the product in the pipeline itself. It is the chief device used in pigging. 

Communication with submarines and other military communication - LF has been used by the military for secure communications through the ground for some time. In fact the extent of use of Low frequency EM radiation by the military is not well known, but is suspected to be far greater than we might imagine .  The US and NATO have a number of installations using low frequencies. Initiatives such as the US Ground Wave Emergency Network (GWEN) designed as an ultra-high powered VLF [150-175 kHz] network intended to survive massive broadband destructive interference produced by nuclear damage used 200 nodes. GWEN antennae included an 8km trailing long-wire and a 750-6000m tether Aerostat-Augmented balloon. Its replacement is a system called SCAMP.  The radio services Saguine (USA) on 76 hertz and ZEVS (Russia) on 82 hertz operate in this range, which both provide communication services for submarines at a certain depth.  The  scale of involvement in low frequency transmissions “leaves great cause for concern as it appears not to be regulated or monitored “

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.

The frequency of the magnetic field created is equal to the frequency of the power supply, hence for example, if the railway power supply is operated with a frequency of 16.7 oscillations per second, the fields it produces also have a frequency of 16.7 hertz (Hz).