Locating Gold by Radio

Radio Record, Volume IV, Issue 30, 6 February 1931, Page 17

Locating Gold by Radio

Successful Experiments

CINCH and experimentation have relieved modern prospecting for buried treasure of its ‘former vicissitudes. Nowadays, magnetism, electricity and radio are being harnessed to locate valuable deposits of subterranean ores and oil. The detailed geological knowledge of promising regions now available replaces the tedious pioneering treks of daring prospectors

who roamed and:roved over unexplored country and delved with pick and pan into rock-ribbed terrain. The modern geophysical methods of prospecting are based on definite physical principles and are the products of rational research. They are not related in any way to the fictitious divining rod or "doodle bug" systems of hocuspocus -oil deposit discoveries. . "The divining rod is either a fraud," says no less an authority than "Uncle Sam," or is founded:on some "physiological principles concerning which we know little or nothing-like the sense ° of direction in migrating birds and fishes." Ore bodies, including all the precious metals as well as salt domes and. oii "caches," are now being located by magnetic gravitational, electrical and seismical survey methods, while radio is being tested for its applicability to similar purposes. The dawn days of geophysical pros-

pecting hark back to the World War when technical detectors of submarines were perfected, The propeller sounds of submarines in motion were located by means of special hydrophones invented for that purpose, while magnetic sleuths discovered the whereabouts of the steel ships at rest, by the reflection of sound waves.

Underlying Principles. A CLOSH analogy exists between hunting submarines tinder water and locating ores buried far underground. ‘Detection of either depends on the difference between the properties of the body and its surrounding medium. In ores the difference is sometimes magnetic, as in cases where iron ore occurs, or it may be electrochemical, as in the case where a sulphide ore is present. Various electromagnetic effects result from certain combinations of ores in the earth. Hence some of the new electrified detectors-are direct or continuous current, and others use alternating current with a wide range of frequencies, Important differences in density facilitate gravitational. detection by the use of a special pendulum or torsion balance. Where the materials differ in elasticity as well as in density, the velocity of sound or shock waves varies and can be determined by seismic systems-a quake recorder is used to pick up any reflected or refracted waves caused: by an explosion. The best prospecting methods to apply depend upon the geological structure of the region and the type of ore sought. Recently, experts of the U.S. Bureau of Mines haye been testing the efficiency. of electrical systems of ore detection on Caribou Mountain in Colorado, @ locality which has yielded more than 50,000,000. dollars (410,000,000) in subterranean wealth

recovered as commercial gold, silver, copper, lead’ and tungsten. The geology of that region has been explored thoroughly, while it is ideally adapted to magnetic methods of geophysical prospecting. ooo. Hence there was a background of conclusive data for direct comparison with the results of the recent electrical surveys. This was the first time that such comparisons have been attempted, and the results demonstrated conclusively the reliability of the new methods of electrical prospecting. [HH simple and portable ‘apparatus used in the latest Government experiments harnessed ordinary radio "B" batteries of -45 volts each as sources of direct current,. while a U.S. Army signalling set, employed in communicating through: the ground in wartime, provided alternating current when required. A frequency of 1300 cycles per second was found most satisfactory. i The electrified current was introduced into the ground by two electrodes, each comprising several stakes linked together with’ stranded copper | wire and two parallel copper wires pegged to the ground every hundred feet. Ordinary angie iron stakes obtained from -the army were more durable than the zinc-coated iron pipes with copper wires attached that were first used. The parallel bare copper wires were 600 feet long and were laid out along meridians 50 feet apart, Special reels designed for the purpose , were used in the placing of this wire, while high resistance headphones were employed in keeping tab on the passage. of the electric current through the soil. ; _ The tests of radio reception through the ground which were successful up to distances of 550 feet were ‘made with a home-made superheterodyne 9valve radio set. ‘The radio receiving set was placed in a shaft of the Caribou mine, 550 feet below the surface, and there picked up music broadcast from a neighbouring city. These experiments tend to confirm the theory that radio waves pass through rock wihtout much’ attenuation. Radio signals will penetrate sea water-a good conductor-to a depth of from 50 to 60 feet, which indicates that they would penetrate approximately ten times as far through a poor conductor like dry rock-G. H. Dacy, in "Science and Invention."