MAGNETIC METHODS 241 



therefore not directly usable. However, the modification required is not 

 difficult and can be readily worked out as needed. 



The airborne magnetometer has placed control of the vertical dimension 

 at the disposal of the geophysicist, and suitable exploitation of this advan- 

 tage can be valuable in many ways.f 



It is estimated that to date (1949) well over a half million miles of 

 magnetic profile have been recorded in the air. The speed and low unit 

 cost at which surveys can be made, especially in areas where work on the 

 ground would be slow and difficult, are advantages at present available 

 only with the airborne magnetometer. 



Helicopter Operations 



The helicopter-borne magnetometer has been developed along principles 

 somewhat different from those of the magnetometer carried in fast-flying 

 fixed-wing airplanes.^ It is designed to outline local and detail anomalies 

 of the magnetic field. Detail is advantageous particularly in exploring and 

 prospecting for orebodies. 



As an aircraft ascends from the ground, the distance from the magnetic 

 ore-bearing structures and orebodies increases, and the magnetic intensity 

 therefore decreases rapidly. § The details are lost even faster. As described 

 previously, two sharp magnetic highs may merge into one if the intensity 

 is recorded at an altitude somewhat comparable to the distance between the 

 bodies. In such a case, a drill hole placed on the apparent maximum anomaly 

 would penetrate the ground between the orebodies, never intersecting either 

 of them. 



The helicopter-borne magnetometer was built with the idea of duplicat- 

 ing as much as possible the results of magnetic surveys on the ground. This 

 duplication has been achieved remarkably well, as indicated by Figure 120. 

 The helicopter has proven most satisfactory for the mapping of local areas. 

 As early as 1921 experiments had been made with captive balloons, but 

 these were found to be cumbersome, and completely unmanageable in 

 wooded terrain. The helicopter is an exceedingly maneuverable aircraft ; 

 it may be landed in a small clearing not more than 50 feet square ; equipped 



t A. A. Logatchev, "An experimental application of the aeromagnetic survey to the determina- 

 tion of depths in magnetic masses", materials of the Central Geological and Prospecting Institute, 

 Geophysics, Fascicle 8, pp. 35-38, 1940. In Russian. 



R. E. Gebhardt, "Investigation of height of local magnetic anomaly at Port Snettisham, south- 

 eastern Alaska", Terrestrial Magnetism and Atmospheric Electricity, Vol. 46, pp. 451-454, 1941. 



"Aeromagnetic survey at three levels over Benson mines, St. Lawrence County, New York", 

 U. S. Geological Survey, Geophysical Investigations, Preliminary Map No. 2, 1946. 



Irwin Roman, "The resolving power of magnetic observations", A.I.hl.E. Tech. Pub. 2097, 

 Nov., 1946. 



% "Employ helicopter for first time to explore remote mining regions", Skillings' Mining Review, 

 Vol. 35, No. 12, July 6, 1946. 



"Employ helicopter for geophysical surveying", Eng. and Min. Jnl., Vol. 147, No. 8, August, 

 1946. 



Hans Lundberg, "Magnetic surveys with helicopters", Bull. Inst, of Min. and Met., No. 488, 

 pp. 21-27, July, 1947. 



Hans Lundberg, "Mining geophysics", Min. and Met., Vol. 28, No. 482, Feb., 1947. 



Hans Lundberg, "Results obtained by the helicopter-borne magnetometer", Canadian Institute 

 of Mining and Metallurgy, Transactions, Vol. 50, pp. 392-400, 1947. 



§ Hugo E. Kuehn and Guy E. Dent, "Comparative study of magnetic surveys of Worcester 

 County, Maryland, made on the ground and from airplane observations", U. S. Bureau of Mines 

 Report of Investigations 4070, May, 1947. 



