242 



EXPLORATION GEOPHYSICS 



2500 t 



zaoot' 



1500-f 



Fig. 130. — Comparison between magnetic profile made with Askania balance and aerial profile flown 

 150 feet above surface. (Courtesy of Lundberg Explorations, Limited.) 



with floats, it may even be landed in a swamp, on thin ice, on water, or in 

 soft snow. (Figure 121.) It can be kept still in the air or hovering over a 



given point at a given altitude ; it 

 may ascend and descend vertically, 

 flying sideways and even back- 

 wards at the will of the pilot. It 

 flies forward at any speed up to 

 100 m.p.h. It can be kept flying at 

 any useful altitude from a few feet 

 to several thousand feet above the 

 ground and can be flown close to 

 the ground with perfect safety. In 

 clear, bare areas the survey flights 

 may be performed a few feet above 

 the ground, and in forest areas 

 flights are made at a few feet above 

 the tallest trees, or 100 to 150 feet 

 above the ground. With the heli- 

 copter it is possible to mark a point of reference by using a small parachute- 

 like marker which may be draped over a tree or laid out over the ground. 

 This can be accomplished from the helicopter without touching the ground. 



Fig. 121. — Helicopter equipped with floats, for 

 mapping water-covered areas. (Courtesy of Lund- 

 berg Explorations, Limited.) 



