Communicating through the earth and sea by conduction and induction

One of my interests is ELF/VLF with my university thesis, now 35 years ago, was on whistlers and related atmospheric phenomena below 10kHz. This interest started in the mid 1960s when experimenting with communications through the ground using frequencies below 10kHz (see my article in RadCom April 1975). This form of communications is sometimes known as Earth Mode or Earth Current. Currents are injected into the earth or rock via 2 electrodes and detected some distance away as a potential difference between 2 further electrodes driven into the earth or rock. Filters are usually needed to reject mains hum as this can be the limiting factor in range in urban areas. Signals attenuate very rapidly with distance so most people are unable to cover more than 0.5-1km with low power amplifiers and electrode spacings of 10-20m. With 500W - 1kW audio amplifiers ranges up to 10km have been achieved at around 6-9kHz. Greater DX is certainly possible using modern weak signal techniques. Articles on this form of communications have appeared in radio magazines many times over the years. My own interest was first awakened by a couple in Practical Wireless in 1964 and 1965. In recent years earth current or earth mode communications has proved effective for cave communications and rescue work with depths of some 800m being covered into cave systems.

History and Military Use

First use of Earth Mode goes back to WW1. This interesting piece was found at the IEEE site

Ferrié, Gustave-Auguste
(19 Nov. 1868 - 16 Feb. 1932)

Engineers had long known that telegraph signals could travel a few hundred yards through the ground, but little use had been made of this form of wireless communication. In 1914 the enterprising engineer Gustave-Auguste Ferrié, who headed the French Radiotélégraphie Militaire before and during World War I, recognized two things: the newly available electron tube could significantly extend the range of this technique; and it might then be of enormous value in the fighting on the Western Front. Thus was born ground telegraphy or Earth-currents signaling.

Ferrié made improvements in the signal generator and in the receiver -- notably by the use of a triode amplifier -- and achieved a usual range of several kilometers. The transmitter was essentially a buzzer (an electromechanical device that interrupts the circuit at a very high rate) powered by a battery. The receiver was an amplifier, employing a triode electron tube. Earth connections were usually made by driving steel pins into the ground; often a short length of insulated wire was laid along the ground and anchored at each end by a spike.

These devices began to be used in large numbers in 1916, and by the end of the war the French had produced almost 10,000 of them for use by the Allies. The Germans also deployed a system of ground telegraphy; it was mainly the work of a young mathematician, Richard Courant, who became famous after the war for his work on quantum mechanics. The famous physicist Arnold Sommerfeld also contributed to the German development of ground telegraphy. In the United States, Lee de Forest patented a system of signaling by Earth currents.

Users of ground telegraphy discovered that their receivers frequently could pick up telegraph and telephone signals from lines buried nearby. They were thus used to tap enemy lines and also to receive one's own telegraph or telephone signals when a line had been severed. These receivers came to play a large role in eavesdropping. Its portability and its freedom from electrical lines made ground telegraphy an important means of communication during the Great War. It was a technique, however, that scarcely outlived the war. Even before war's end it began to be displaced by another wireless communication technique. This, of course, was radio, the technology to which Ferrié devoted most of his efforts.

The military used "earth mode" in the 1960s for secure communications between buried nuclear installations. More recently they have used extensive ELF arrays to communicate with submarines but this is actually using radiated signals rather than conduction currents.

Radio amateurs have tried "earth mode" at 73 and 136kHz with some success, as have cavers. With modern weak signal detection methods, e.g. waterfall displays on PCs, signals far too weak to hear can be "seen" as traces on the screen.

Another mode useful for Earth Mode is PSK31 which is a very narrow band data mode. Most PSK31 stations use PC sound cards to do the digital signal processing making this one of the easiest digital modes to use. The data rate allows up to 50wpm in bandwidths of about 31Hz (hence the name).

Foremost in sub-10kHz through the ground communications was John, G0AKN, who sadly is a silent key. A paper on his work is available as an attachment on this page (see bottom of page).