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Aviation History
1948
1948 - 0578.PDF
472 FLIGHT APRIL 29TH, AIRBORNE MAGNETOMETER SURVEY Advances in Geological Exploration : Nhsd for British Development ON Monday, April 19th, Wing Cdr. D. N. Kendall,O.B.E., who is managing director of the Photo-graphic Survey Company, Ltd., of Toronto (a mem- ber of the Hunting group), presented a lecture to t3*e iRoyal Geographical Society on the use of airborne magnetometer equipsaent for mineral exploration. Physically, the earth can be regarded as a giant magxtel with its poles at North and South,. A aaagBetk fcJd exists with lines of force that proceed ver- tically from the South Pole, gradually turning until they faster the North Pole. Thus the lines of force are horizontal at the equator and nearly vertical at the Poles, The unit of measurement of the magnetic force is a •gamma, and the earth's field varies from about 25,000 gamma at the equa- tor to abottt 60,000 gamma near the Poles. The magnetic field at any point is a function of the size and length of the magnetic body and the distance from it. Consequently, if the -field can be measured at a given point, and the length of the body is known, its depth .,'..' . . . . can be computed. This is an import- ant fact in geophysical interpretation. It is thus apparent that, in general terms, physical information of great value to a geophysicist could be provided if, in any locality, the earth's magnetic field was taken as a datum and the plus or minus for this was recorded on a map for a large number of points in the area. It would be possible to outline the location below the ground of magnetic highs and lows. This has been done on the ground for a great many years with an instrument called a magnetometer. It has •been rased widely HI exploration, but is fairly slow and expensive. The idea of putting a magnetometer in an aircraft was, the lecturer said, first tried in Russia in 1915, and there was also same development for naval purposes in England at about the same time ; but development was allowed to lapse between the wars. In 1943, however, the German U-boats, which ap till then could be located by- radar when they surfaced, became equipped with a breatti- Ditail ef detector unit showing fluxgates and gimbal rnountm with slip-ring contacts. The "bird" consists of a bomb-typi housing enclosing the fluxgate detector unit (A) and the orienting motor unit (6) in the tail. The towing cable socket is arranged en ....--.-. the centre of gravity. - ~ ;fc , v >: - ing device which enabled them to remain permanently sub- merged at sea. An alternative method for locating them had to be developed, and the magnetic air detector (M.A.D.) was devised and brought into use by the U.S. Naval Ordnance Laboratory. This was, in effect, an air- borne magnetometer. The air problem did, however, introduce the consider- able difficulty that the aircraft itself gave off a large and ^arying magnetic field of its own and, in addition, it is Atremely difficult to maintain any instrument in a truly ' ertical position in an aircraft; gyros have not the neces- sary precision on the whole. The U.S. Navy solved the problem by constructing an instrument to record the total field, and by placing it in a towed container well behind the aircraft where it would not be affected by the aircraft's magnetic field. The in- jment consists basically of three coils (x, y and z) at right angles to each other. If coil x, the recording coil, is aligned exactly into the total field, coils y and z, being at right angles to it, will record zero. As soon as coil x wanders slightly, coils y and/or z take up small compo- nents. The instrument is so constructed that the impulses in coils y and z actuate servo motors which immediately bring coil x back into the total field. The final airborne magnetometer is thus an instrument which records, in the form of a continuous profile, the small local variations in the total field, and which is fully compensated for errors which arise when the recording coil wanders temporarily from the total field. •....-• :.-•.-: j:v Survey Stages A normal airborne magnetometer survey goes through four main stages, i.e., planning, flying, reduction of data and interpretation. The planning stage involves a study of existing geological information to determine the direction of flights and the spacing of flight lines. It also involves securing high-altitude vertical air photographs for use later in flying and plotting the data. The flying stage first con- sists of the establishment of base lines and then of the stripping. The reduction of data introduces the problem that the profile which is obtained from the aircraft varies (a) in horizontal scale due to changes in the ground speed of the aircraft; (b) in vertical scale due to the necessity for the operator to change his vertical scale in order to keep the ink profile on the recordinig paj>er; (c) in datum, again to
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