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Aviation History
1956
1956 - 1345.PDF
FLIGHT, 14 September 1956 497 MAPS AND MINERALS ... results. As a general rule, traverses should be at right anglesto the strike of a formation (such as a line of high ground raised by a "folding" of the earth's surface), but in some folded areas aradial flight pattern may be recommended. Then, again, as hap- pened recently in Northern Europe where the rugged nature ofthe terrain prevented sufficiently accurate height-holding, a further coverage down steep-sided valleys and along the edges ofravines can be flown at right-angles to the first one at a different altitude. The electromagnetic detector weighs 1,127 lb and hasthree main components; an external primary coil, mounted between the trailing edge of the aircraft's wing and the tail, orextended by spreaders along the top of the fuselage; a secondary coil, carried in a streamlined "bird" and towed on 500ft of cable;and an internal electronic installation contained in a large console with a graphic recorder in its face. Into the primary coil is fedA.C. at two frequencies—2,300 and 400 cycles per second—and the aircraft thus becomes the centre of an alternating electromag-netic field, which can penetrate 300ft down into the ground under favourable conditions when the aircraft is at 500ft. If a con-ducting ore body—such as a copper, nickel or lead sulphide deposit—is beneath, it will have eddy currents induced in it andre-radiate a secondary field, which is detected by the towed secondary coil. The phase-shift between primary and secondaryfields is measured and continuously recorded by a laterally- travelling pen on a moving belt of graph paper in the consolebefore which the E.M. operator sits. The reason for recording phase-shifts at two frequencies is thatthe response of various conductors varies with the frequency of the energizing field. The low frequency will penetrate moredeeply than the high, and therefore the plot of the latter will show up surface conductors such as lakes, swamps and man-madestructures as well as surface ore bodies. The low-frequency plot will also show these, but it will react more strongly todeeper—and potentially more valuable—areas of conductivity. It is therefore the simple ratio between H.F. and L.F. phase-shifts which will tend to indicate the best conducting ore deposits. Flying Technique. There are interesting points about geo-physical flying. It is essential that a reasonably constant proximity of equipment to ground surface be maintained, and500ft ± 50ft is considered suitable for conductivity surveys. Over rolling country, this can mean a certain strain for the crew, thoughpositive and negative g-loads are not serious and the pilot's chief preoccupation is not to let the towed "bird," which flies 250ftbelow and 200ft behind the aircraft, touch ground. (The "bird" is precisely positioned at a point within primary and re-radiatedfields where electrical "noise" is at its lowest value.) When coverage of the area begins, the nayigator becomes thekey crewman. He uses large scale 1 mile=22in maps, or mosaics, upon which the flight pattern has been ruled, in alternatingnumbered red and blue lines to eliminate confusion. For com- plete E.M. coverage a line-spacing of i mile or 220 yd is calledfor, and maximum permissible error is 70 yd to port or starboard of track. If this is exceeded the whole line must be renown, andthe problems involved can be appreciated when it is considered that over a three-mile traverse in hilly country abrupt wind- changes causing alterations in drift arefrequently encountered. In order to deal with this, the flying and engineering mem-bers of the tJ.K. company quickly devised and installed a drift sight in their recently-equipped E.M. DC-3; mounted on the scuttle before the second pilot/navigator,it projects a point of light at infinity in the manner of a gyro gun-sight and computesdrift alterations very rapidly. Flying geophysical coverage can belikened to a protracted low-level bombing run lasting perhaps eight or nine hours;and a very high degree of skill is necessary to keep within height and track limits.The aircraft is literally "hopped" from one identifiable ground feature to the next, thenavigator pointing out landmarks to the captain and drawing his attention to thenext point on track. The exterior equip- ment reduces a DC-3's cruising speed bysome 15 kt, and stick forces are slightly The Hunting DC-3 equipped for mapping and survey work. On top of the fuselage are masts for E.M. equipment; protruding from the tail is the fixed detector head of a mag- netometer on a 12ft boom; and trailing below the aircraft is the bomb-like sensing coil. Two views of the E.M. "works" aboard a Survey Prince. At right, the recorders of the scintillation counter and radio altimeter; on the left, the magnetometer console. higher; booster pumps are switched on, mixture set in auto-richand convenient engine settings for the low-level run in are 2,100 r.p.m./28in Hg. With one eye on the radio altimeter and bothhands on the column, the pilot lifts the aircraft over rising ground with up to 35 or 36in boost, and throttles back slightly goingdownhill on the other side. With every type of mineral survey flying it is essential to knowexactly which ground-point gave rise to each geophysical measure- ment, and therefore a vertically-mounted 35 mm Vinten position-ing camera is carried. This takes overlapping exposures of track made good, and after a specified number of frames exposed (usuallyten) it emits an electrical pulse which makes a fiducial mark on the edge of the recorder graph paper. Radio-altimeter readingsare also continuously recorded beside the E.M. high and low fre- quency phase-shift measurements, and so when the reduction stageis reached, proximity variations can be compensated for. The E.M. equipment used by Group companies was developedby PSC Applied Research, Ltd., of Hunting Associates in Toronto, and in the Canso aircraft of Aeromagnetic Surveys, Ltd. (HuntingAssociates' geophysical specialists), has uncovered ore deposits worth many millions of dollars in Canada alone. Under favour-able conditions, 1,500 line miles a day have been flown by them (not taking transit or positioning turns into account), thoughfive weeks is reckoned to be the usual period for a 10,000 line- mile survey—allowing for bad weather. So successful were the tests of the recently introduced equipmentin Hunting Geophysics' E.M. DC-3 over the southern half of the Isle of Man, during which all known ore bodies were pin-pointedand others hitherto unknown revealed, that a contract has been placed for the coverage of the remainder of die island. After this,the aircraft is scheduled to go abroad for an extensive period.
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