Communications in Caves

by David Gibson

Also see Radiolocation for Cave Surveying

On deep or wet pitches, communicating by means of whistle-blasts is an established practice and, in many situations, this or perhaps a loud voice will be sufficient. However, for cave rescue, expedition management or the execution of projects such as photography in a large chamber, communication by radio or telephone is often essential.

Possibly the first use of a telephone in a cave was during the exploration of Lamb Leer Cavern (Mendip, UK) in 1880 (Williams, 1995). Despite the disadvantage of having to lay a cable, telephones continue to be used because they are simple and robust. A variety of other cable-based systems exist - single-wire telephone, "guide wire" radio, and optical fibre. True "wireless" communication is difficult because rock, being conductive, absorbs radio waves. For line-of-sight work within cave passages, high-frequency (HF) walkie-talkies and CB radios are used but magnetic induction equipment is usually necessary for communication through the rock itself. The technique of "earth current injection" was developed during the First World War (1914-18) but is rarely used now although an enhancement, using a low frequency (LF) carrier, is the basis for the latest high-performance induction radio / earth-current "hybrids" now used in the UK and Europe.

The single-wire telephone (SWT) (also known as an earth-return telephone) uses the conductivity of the ground to provide a return path for the current. The obvious advantage is that only half the weight of cable has to be carried. The devices used by cavers feature electronic amplification and will often operate without any specific earthing other than through the caver's body. SWTs are cheap, rugged, easy to build by amateurs, and are preferred to the traditional army field telephone. They are popular with expeditions and rescue groups throughout the world, and are also convenient for providing flood-warnings to cavers.

If, instead of audio, we pass a LF radio signal along a SWT cable it will work without any earthing, because the signal is able to jump gaps by a capacitive coupling effect, implying that the handset does not need to touch the cable either. Commercial equipment based on this principle is used for mines rescue. HF radio can also be guided along a cable placed in close proximity although the principle of operation is quite different. (For commercial use at VHF, "leaky-feeder" cable presents yet another mode of operation). Confusingly, all these systems are referred to as "guide wire radio".

Rock is electrically conductive, which causes it to absorb radio waves, so HF radio is mostly limited to use within cave passages. Rescue teams use HF walkie-talkies (27 or 49MHz) to communicate on pitches while manhandling an accident victim in a stretcher. Photographer Gavin Newman has used a modified CB radio to co-ordinate photography in Illu River Cave, Irian Jaya. He also found the radios useful to warn of impending flooding in this huge cave system. HF radio will penetrate a few metres into rock and can be used for locating dig points on the surface although this normally requires specialised equipment (See Radiolocation). In 2001 a CB radio was used to verify the surface location of the top of the Titan shaft in Peak Cavern (Derbyshire). Radio waves can travel for some distance along mineralised fault lines or dry well-jointed rock so, occasionally, quite deep penetration of a cave is possible.

Low frequencies are attenuated less than high frequencies - long wave broadcasts such as BBC Radio 4 (198kHz) can be detected at the bottom of deep caves. Unfortunately, detecting a radio signal is easier than transmitting it - a 200kHz signal would require an antenna some 750m long, which is normally only feasible for mining installations. From the early 1960s cavers began to experiment with loop antennas. Because of their small size (typically 1m) these radiate very little "true" radio energy, and mainly generate a magnetic field which couples to the receiver loop by the principle of magnetic induction, and operates over a limited range of just a few hundred metres. Experimental induction systems have covered 27-185kHz, (i.e. most of the long-wave broadcast band), and most have used single-sideband (SSB) operation. The most notable designs have been Bob Mackin's commercially-produced Molefone, dating from 1979, which has seen widespread use in the UK, and Ian Drummond's CB Transverter, used mainly in the US and Canada.

Two of the latest designs are John Hey's HeyPhone and the French "Nicola System". Both operate at 87kHz SSB and are published designs. They are intended to work with induction loops or earth-current antennas, thus reviving the early-1900s trench-communications technique. With earth electrodes the Nicola system has been demonstrated to work through 1000 m of rock. In 2001, working with the Cave Radio & Electronics Group, the British Cave Rescue Council issued over fifty HeyPhones to the UK's cave rescue teams (Bedford, 2001). This is the first time that a national organisation has systematically equipped its rescue teams with such advanced equipment. The South Wales team soon had occasion to test their HeyPhones. A caver dislocated his shoulder some 5km into Daren Cilau - a formidable cave protected by a daunting 520m entrance crawl and requiring an underground camp to explore the far reaches. Whilst the injured caver slowly made his way to the camp, two of his companions made a strenuous exit and raised the alarm. Some 28 hours later, the medical team was able to communicate with the surface to report that the casualty had been attended to, and would not require a stretcher carry (which, it had been estimated, would take up to three days to complete). Use of the HeyPhones (and of a SWT in the entrance crawl) considerably eased the logistics of this rescue, which involved 63 personnel (Welsh News: Daren Cilau. Caves & Caving. 90. p10).

Cave radios have been connected, experimentally, to mobile telephones and HF radios on the surface, and messages relayed automatically over long distances. System development seems likely to continue, with the aim of providing extra functionality, for example medical data logging, text messaging, and digital image transmission.

This article is copyright David Gibson, 2002.

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