408 MAGNETIC METHOD [Chap. 8 



If the ore body is extended in the direction of strike, the relations are (see 

 theory of magnetized line, formulas [8-55]) : 



(above) AZ2 = -p-; — — , and (below) AZi = -— , 

 a + Act a 



so that 



d = — M__ . (8-646) 



AZi_ 



AZ2 



To attain a fair accuracy, it is necessary that Ad be comparable in magni- 

 tude with d (about O.ld). Haanel^°^ and Eve and Keys^"^ have given 

 examples of such depth determinations. In HaaneFs case, the actual 

 depth of the ore body was about 30 meters; the plane of observation was 

 raised 3 meters and apphcation of eq. (8-64a) gave 30.13 meters. Eve 

 and Keys used a platform 10 feet high on the Falconbridge ore body, whose 

 depth was between 112 and 123 feet; the depth calculated from (8-64b) 

 was 118 feet. Eve and Keys also attempted to determine the depth to the 

 lower pole but were unsuccessful because of the obvious limitations of this 

 method. Where platforms fail to give sufficient change in distance or 

 depth, the geophysicist is forced to "take to the air." Measurements in 

 captive balloons have been used above the Kiiruna ore body. Magnetic 

 measurements by automatic recording devices in airplanes have the ad- 

 vantage not only of direct depth determination but of great speed and 

 applicability to inaccessible country.^ 



VI. MAGNETIC SURVEYS 



The following discussion of the results of magnetic exploration is of 

 necessity limited to a number of the more typical cases. In the United 

 States alone the area covered by magnetic surveys is estimated to be close 

 to f million square miles and only a fraction of the results has been made 

 public. Nevertheless, the international literature on magnetic surveys is 

 fairly extensive owing to the fact that they are not difficult, that equipment 

 is usually available, that geologic objects for the study of magnetic effects 

 are readily found, and that magnetic exploration has applications in min- 

 ing, oil exploration, and engineering geology. 



i'6 Haanel, op. cit., 86. 



"6 Eve and Keys, op. cit., 201. 



1" H. Lundberg and K. Sundberg, A.I.M.E. Geophysical Prospecting, 209 (1932). 



»»8 Heiland,. Eng. and Min. J., 136(12), 609-610 (Dec, 1935). 



