Through-the-Earth transmission can overcome these restrictions by using ultra-low frequency (300–3000 Hz) signals, which can travel through several hundred feet of rock strata.
The antenna may be placed in a "loop" formation around the perimeter of the mine site (or wherever coverage is needed) for systems using magnetic fields to carry signals.
This is important in mining applications, particularly after any significant incident, such as fire or explosion, which would destroy much of the fixed communication infrastructure underground.
[1] In a typical setup the transmitting loop, consisting of many turns of copper wire, is oriented horizontally within the cave using a spirit level, and driven at a few kHz.
[7] Systems like Molefone and HeyPhone operate as single-sideband (USB) 87 kHz radio with earthed electrodes of 25–100 metres (80 to 330 feet) length.
2 Coal Mine explosion in 1994 [13] where the need for a communication system to survive major incidents underground was identified in the inquiries into the disasters.
PED is a one-way text paging device, with wide use in Australia, as well as installations in the United States, China, Canada, Mongolia, Chile, Tanzania, and Sweden.
The Rescue Dog is a two-way extended-range portable through-the-Earth solution that was developed in the US in cooperation with The National Institute for Occupational Safety and Health (or NIOSH)[18] which does not rely on large loop surface antennas for signal transmission.