MAGNETIC METHODS 231 



Another example of a regional magnetic survey was the work which led to the 

 westward extension of the Randf in South Africa. Vertical component magnetometers 

 were used to map the sub-outcrop of a metamorphosed ferruginous shale formation 

 buried under approximately 2,000 feet of unconformable dolomite. The buried forma- 

 tion was comprised of quartzites, slates, shales, some volcanics, and the gold-bearing 

 conglomerates. The slates are magnetic, and their stratigraphic relation to the main 

 gold-bearing reef is well established. Mapping the magnetic anomaly associated with 

 the slate, therefore, served to locate the desired reef which carries the gold values. As 

 a result of this work, it is estimated that the potential gold reserves of the Rand have 

 been increased about 15 per cent. 



A further example of regional magnetic work was that conducted by the Inter- 

 national and the Falconbridge Nickel Companies in the Sudbury District, Ontario, 

 Canada.J (Figure 114.) The ore is massive nickel and copper sulphides, with sufficient 

 pyrrhotite to make it magnetic. The surface covering is chiefly glacial outwash, at 

 depths varying from SO to 300 feet. After completion of the reconnaissance work, 

 electrical methods were used to check the magnetic anomalies, where detailed work was 

 deemed advisable. The subsequent exploration results have established the success of 

 this type of exploration technique as applied to that area. 



Ilmenite, Pyrrhotite and Nickel Ore Deposits. — Ilmenite deposits 

 have been located by magnetic methods by Gillsonff and Keys.§ Also, 

 certain nickel-ore bodies which contain pyrrhotite have been located by 

 magnetic investigations. || 



THE MOBILE MAGNETOMETER 



The airborne or mobile magnetometer is the most recent development 

 in magnetic measuring instruments applicable to prospecting. §§ The term 

 may be applied in a general way to two types of surveying procedures. In 

 one, the measurements are made at an elevation above ground sufficient to 

 minimize the local surface effects and to emphasize the broader regional 

 features. Work of this type is usually done from high-speed airplanes 

 flying at elevations of from 500 to 5,000 feet. In the second type of survey, 



t H. Krahmann, "Magnetometer Observations on the West Rand", Journal, Chemical, Met- 

 allurgical, and Mining Society of South Africa, March, 1936. 



% F. Galbraith, "The Magnetometer as a Geological Instrument at Sudbury", A.I.M.E. Geo- 

 physics Volume 164, 1945, pp. 98-106. 



tt J. L. Gillson, "Genesis of the Ilmenite Deposits of St. Urbain County, Charlevoix, Quebec," 

 Economic Geology, Vol. 27, Sept.-Oct. 1932, pp. 554-577. 



§ F. W. Lee, "Results of Some Magnetic Measurements on Dikes . . . in the Sudbury Dist. 

 Ontario, Canada," U. S. Bureau of Mines, Tech. Paper No. 510, 1932. 

 tX Lee, lac. cit. 



L. B. Slichter, "Certain Aspects of Mag. Surveying," A.I.M.E. Geophysical Prospecting, 

 1929, pp. 238-260. 



§§ M. Grotewahl, "Bericht iiber die Versuchsfahrt des Bidlingsmaier'schen Doppelkompasses mit 

 dem Luftschiff Graf Zeppelin," Terrestrial Magnetism and Atmospheric Electricity, Vol. 35, pp. 

 226-229 (1930). 



C. A. Heiland, "Geophysical mapping from the air: its possibilities and advantages", Engi- 

 neering and Mining Journal, Vol. ISG, pp. 009-610 (\9Z5). 



K. Ramsayer, "Die Anderung magnetischer Storgebiete mit der Hohe und ihr Einfluss auf die 

 Flugnavigation", Beitrdge sur angewandten Geophysik, Vol. 9, pp. 65-97, (1941). 



E. A. Eckhardt, "Airborne Magnetometer", Petroleo Interamericano, Vol. 4, No. 8, pp. 52-58, 

 August, 1946. 



E. A. Eckhardt, "Airborne magnetometer: new geophysical tool overcomes jungle, mountains, 

 swamps, desert, and water", Oil and Gas Journal, Vol. 45, No. 5, pp. 78-79, 91-92, June 8, 1946. 



A. A. Logachev, "The development and applications of airborne magnetometers in the 

 U. S. S. R.", Geophysics, Vol. 11, pp. 135-147 (1946). Also in Petroleum Engineer, Vol. 17, No. 10, 

 July 1, 1946. 



Homer Jensen, "Flying magnetometer: tool of aerial exploration", World Petroleum, Vol. 19, 

 No. 3, pp. 84-87, March, 1948. 



