64 APPLICATIONS OF GEOPHYSICS IN ENGINEERING [Chap. 6 



able aid in navigation. Sound ranging is used in both the army and the 

 navy for locating enemy guns and for determining the range of their own 

 artillery. Direction finding and noise analysis applies in the detection of 

 airplanes, submarines, and enemy sappers, and the location of pipe leaks 

 in sanitary and pipe-line engineering. Echo sounding methods are cur- 

 rently applied in the merchant marine and the navy for measuring the 

 depth to sea bottom. Occasionally they have been applied in the location 

 of icebergs and in finding the depth to fish shoals. An electrical reflection 

 method based on frequency modulation of ultrashort waves has made 

 possible the airplane terrain-clearance indicator. 



F. Gas Detection 



Gas detection methods are applied in chemical warfare, in pipe-line 

 engineering, and in mine safety work. Detectors have been constructed 

 for both combustible and toxic gases and are used to find leaks in buried 

 gas pipe lines, gases in manholes, mine openings, and the like. 



G. Thermal Detection 



Thermal-detection methods involve the location of objects by their heat 

 radiation. During the war it was found that planes could be detected 

 by the heat issuing from their exhaust pipes; icebergs have been located 

 at appreciable distances by such detection methods. 



