240 



EXPLORATION GEOPHYSICS 



general, the determining of position with sufficient accuracy is more difficult 

 than obtaining a satisfactory magnetometer record. f 



When the photographic method of position determination is used, a 

 base map is made from the photo-mosaic to which the position and magnetic 

 data are readily transferred. The map may then be contoured and will 

 show the distribution of the total magnetic intensity at the flight level above 

 ground, projected onto the ground. 



When the radio method of positioning is used, air photographs, though 

 still useful, are not required. The map may then be prepared to a suitable 

 scale and all points will be defined with respect to the positions of the 



O ^^p"-"^ 2 MILES CONTOUR INTERVAL lOT 



' ' ' (Courtesy of Gulf Research and Development Company.) 



Fig. 119. — Portion of an aeromagnetic map, New Mexico. 



Shoran stations on the ground. On the other hand, by first plotting the 

 ground station positions on any available base map of the area, the mag- 

 netic data may be directly transferred to the base map. Figure 1 19 is a small 

 portion of an aeromagnetic map showing total intensity over an area 

 in New Mexico. 



The available literature on the interpretation of magnetic anomalies deals 

 largely with anomalies in the vertical or horizontal intensity. This is because 

 these components have been the most easily, and therefore the most com- 

 monly, measured elements. The usual airborne magnetometer records the 

 variations in the total intensity. The available interpretation literature is 



t Homer Jensen and J. R. Balsley, Jr., "Controlling plane position in aerial magnetic survey- 

 ing", Engineering and Mining Journal, Vol. 147, No. 8, pp. 94-95, 153-154, August, 1946. 



