MAGNETIC METHODS 



243 



A drawback is the limited load capacity of the helicopter, as well as 

 the rather cramped space for mounting instruments. The operating base 

 must be close to the area to be mapped. The helicopter-borne magnetometer 

 must be very light, but since it is generally flown close to the ground the 

 need for great sensitivity becomes of less importance than in magneto- 

 meters flown at higher altitudes. 



Installation in Helicopter. — The detecting unit is suspended for- 

 ward five feet from the helicopter by means of a cantilever type of boom. 

 The length of the boom is adjusted so that mechanical vibrations originating 

 in the helicopter may be reduced to a minimum at the shock-mounted 

 detecting unit. 



The electronic section and the recorder are mounted in a common 

 cabinet which is located alongside the instrument panel in the helicopter. 



Fig. 123. — Helicopter installation: (right) amplifier recorder installed in control 

 compartment, and (left) close-up view of equipment, (a) instrument panel, (b) amplifier, 

 (c) ink recorder. (Courtesy of Lundberg Explorations, Limited.) 



(Figure 122.) The detecting unit is electrically connected to the electronic 

 section by means of a shielded multi-conductor cable. 



Navigation and Mapping at Low Elevations, — The magnetic data 

 would be quite useless without correlation of the geographic and topo- 

 graphic features. Aerial photographs are most useful for this purpose, as 

 well as in planning a survey. However, a good topographic map may be 

 sufficient. 



On the map or photograph, course lines at suitable intervals are laid 

 out, to be followed by the helicopter pilot. The altitude is decided upon 

 after studying the topographic relief and taking into consideration the 

 geological conditions as well as the geometrical dimensions of the magnetic 

 features. 



