Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 951 



consequence, the hydrophones are suspended from radio-equipped buoys 

 which communicate the impulses to a shore recording station or recording 

 vessel. 



Explosions of mines and torpedoes or shell hits may be located by sub- 

 marine sound-ranging equipment up to distances of 100 km. 



"Q. Direction-finding and noise-detection. Marine noise-detection and 

 direction-finding are applied (1) in navigation, to determine a ship's posi- 

 tion by taking bearings of one or two sound sources of known position 

 (submarine transmitters on buoys, lighthouses, shore stations); (2) in the 

 detection of enemy craft from surface ships and submarines, and in de- 

 tecting the approach of friendly ships to avert collisions with emerging 

 submarines ; (3) in the surveillance of straits and harbors during periods of 

 poor visibility. With the exception of the last application, the detecting 

 devices are mounted on board ship and therefore an immediate difficulty 

 arises from the high noise level caused by the ship's engines, propellers, 

 rush of water, and activities on board ship. This interference may be 

 partially reduced by sound insulation and electric or mechanical filtering; 

 however, the only really effective means of separating the noises to be 

 detected from these accidental noises is by directional hearing. 



Among the noises produced by other ships, most important is probably 

 that caused by the propeller as the result of the collapse of air bubbles. 

 This noise is so characteristic that a practiced listener may determine the 

 number of propellers and blades, the type of engine, and therefore the type 

 of ship to which he is listening. Other noises are produced by the ship's 

 engines, by pumps, and by generators, and are transmitted through the 

 hull to the water unless special precautions are taken. The frequency 

 range of a ship's noises is considerable; for practical purposes, the band 

 from 500 to 2000 is most suitable. 



In the early days of marine direction-finding, two receivers of the Broca 

 type were used with a rotatable base of comparatively small length. Later 

 this was replaced by two or more receivers on both sides of the ship, con- 

 nected to a mechanical time-delay compensator of the trombone type 

 previously described. This system has recently been superseded by elec- 

 trical coil microphones with electrical compensators, as described on 

 page 939. 



7. Echo-sounding. Echo-sounding is undoubtedly the most widely used 

 marine-acoustic method. Its advantages in speed and accuracy over the 

 mechanical wire-sounding method are obvious. The method is more than 

 a mere means of measuring depth. Because of the speed and completeness 

 with which the topography of the ocean floor may be mapped, it is an aid 

 in navigation since in many cases the ship's position may be determined 

 accurately from a bottom contour map. Echo-sounding has been applied 



