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Aviation History
1956
1956 - 1344.PDF
496 FLIGHT, 14 September 1956 Maps and Minerals Survey Techniques by Prince and DCS ON July 13 we published a brief description of the use of electro-magnetic mineral-prospecting equipment in the DC-3 G-AMYW operated by Hunting Geophysics, Ltd. The following article treatsof the subject in greater detail, showing what is involved in aerial mapping as well as in mineral surveying—especially from the point ofview of the pilot working over difficult terrain. AIRBORNE geophysical survey is a particularly interestingbranch of applied flying which has developed greatly over• the last seven or eight years, and has already given spec- tacular results in several parts of the world. Its object is the detection of oil and sub-surface metal ores—an important assignment, because the actual consumption of minerals by world industry is growing at the rate of some fiveper cent every year, and consumption now is vastly higher than it was at the beginning of the last war. Such were the quantitiesof base metals—that is, iron, nickel, copper, lead and zinc—used up during the peak war years 1944/5 that it was generally felt thesame rate could not continue with the return of peace. But this was a mistaken impression, for consumption has steadily risen,as have metal prices; and there is a very real danger of world- wide shortages unless the rate of mineral strikes is maintained.The difficulty is, of course, that the world is still a very big place. This may seem a flippant observation upon a serioussubject, but one can have no idea of the immense size of lonely areas where prospecting has never been carried out, and where theproblems of maintaining communications are sometimes made the more severe by rough climates. The old-time prospectors,with their hammers, shovels and pack-ponies, used to spend years out in the bush searching for surface deposits; but most ofthese easily-found ore bodies have, in fact, by now been found and exploited. Attempting to locate new ones (buried beneath,perhaps, a hundred feet of overburden in the middle of inaccessible, rocky country) on foot or even by Land Rover—assuming it could get through—would be altogether too slow today. What is needed is some method of carrying out a thorough,rapid reconnaissance from which a minerals inventory showing what is likely to be there—and, what is more important, exactlywhere it lies if it is there—can be produced. Airborne geophysi- cal methods can provide that inventory, and that is why thescience has assumed such importance. A Prince at work—showing diagrammatically the method of carrying out vertical air-to-ground photography tor mapping purposes. The camera operator in a Hunting Survey Prince checks the Williamson Eagle IX camera drift/aiming sight before take-off on a sortie. A geophysicist is a man who is concerned with the physicalproperties of rocks—their density, their elasticity, their magnetic permeability, and so on. His knowledge is complementary to thatof the geologist, who studies the earth's structural properties as a whole, and the ways in which minerals occur in association witheach other; the latter has only a limited ability to assess the nature of sub-surface structures and relies upon his colleague's interpre-tations of geophysical measurements to determine what lies beneath the top-soil.Now there are three of these physical properties which can be conveniently measured by sensitive electronic equipmentmounted in an aircraft—magnetic permeability, electrical con- ductivity, and radio-activity. The instruments used to recordthis data are also used on the ground, but for initial "inventory" surveys this is slow, arduous and expensive in forested or moun-tainous terrain—over large areas the cost can be five times as great as the airborne method—and is, in some respects, tech-nically inferior. The task, however, of these instruments, whether from the ground or the air, is to detect the difference inphysical properties between an ore body and its surrounding rock; and to illustrate the way in which a survey is planned andflown, a search for electrically-conducting ores—concentrations of lead, zinc, copper and nickel sulphides—can be taken as anexample. Their probable locations are directly disclosed by the electromagnetic detector—E.M. for short—the latest additionto the flying prospector's arsenal, and by now a highly developed electronic device of great sensitivity. The starting-point of every airborne mineral survey shouldbe a complete vertical photographic coverage of the entire area. If it does not exist, and is specified by the client, the coverage issecured by a Survey Prince or Hudson (depending upon whether U.K. or Canadian Hunting companies, respectively, are engaged)mounting Williamson Eagle IX or Wild RC 5a cameras. The quality of the photography, secured from between 15,000 and25,000ft and giving scales of 1:30,000 to 1:50,000, is very high; and so important are photo-interpretative techniques to thegeologist that photogeology has become a speciality in its own right. Surface rock structures show up beautifully on the mosaics—controlled to the degree of accuracy required—constructed from the 9 x 9in contact prints, and patterns or tones whichmight be associated with valuable ore bodies, are at once apparent to the trained eye. These mosaics are used constantly through-out the survey, and the navigator of the geophysical aircraft may carry a set for low-level contact navigation. Basically, a geophysical survey is carried out by flying the air-craft across the area in a series of parallel lines at a constant height above ground level, while the continuously recordingequipment collects its data. The planning of the flight pattern, or the direction in which the parallel traverses run, is very mucha geologist's job; he first of all studies the general geological situation as revealed by the vertical photographs, considers this inrelation to any definite knowledge of the area which may have been acquired by surface drilling or ground sampling, and thenplans a series of aircraft traverses which should give the best
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