FACTORS AFFECTING DEEl'-WATEK TRANSMISSION 



87 



1000 



3000 



4000 5000 



DEPTH IN METERS 



6000 



7000 



eooo 



FiGiRE 1. Distribution of depths in the sea. 



a submarine or typical surface vessel cannot ordi- 

 narily be detected more than 3,000 yd from the pro- 

 jector in water of any depth, except under unusually 

 favorable conditions. All supersonic projectors are 

 highly directional with not much energy radiated at 

 angles more than G degrees from the axis. If the bot- 

 tom is more than 150 fathoms below the projector, 

 and the water is isothermal, ^'ery little of the energy 

 in an echo-ranging pulse will reach the bottom at 

 ranges le,ss than 3,000 yd or return to the surface at 

 ranges less than 6,000 yd. Thus, the echo from targets 

 near maximum range will contain very little bottom- 

 reflected sound; however, the background for such 

 echoes inay contain some bottom reverberation. If 

 sharp temperature gradients are present in the upper 

 layer of the ocean, the soimd beam will be bent down 

 more sharply, and a considerable amount of bottom- 

 reflected sound could reach a target 3,000 yd away in 

 water 150 fathoms deep. The bottom reverberation 

 in such conditions may be quite intense even at 1,500 

 yd. To insure that bottom-reflected sound cannot re- 

 turn an echo in practical echo ranging, a depth of 

 more than 200 fathoms is required, while twice this 

 depth is required to eliminate bottom reverberation. 

 For various types of tilting beam equipment, sound 

 scattered from the bottom can be important even in 



somewhat deeper water. For most echo-ranging situa- 

 tions, however, 100 or 150 fathoms is a more repre- 

 sentative dividing line between deep and shallow 

 water. 



It makes very little difference whether the point of 

 division is taken as 150 fathoms, or 100 fathoms, as 

 has been done in the manuals of echo-ranging predic- 

 tion issued by the Navy,'- or 200 fathoms, as has 

 been suggested. Water depths between 100 and 1,500 

 fathoms are quite uncommon. Figure 1 shows the 

 distribution of depths in the sea.^ It is evident from 

 Figure 1 that almost all of the ocean bottom is either 

 less than 100 fathoms, about 200 meters, below the 

 surface, or more than 1,500 fathoms, about 3,000 

 meters, below the .surface. 



Water which is deep for echo ranging may be 

 shallow for sonic listening, since average listening 

 ranges are so much longer than average echo ranges, 

 and since sonic listening gear is nondirectional. Lis- 

 tening ranges are often greater than 10,000 yd. Ex- 

 cept in the deepest parts of the ocean, sound arriving 

 from such long ranges will contain bottom-reflected 

 sound. Sonic gear is usually nondirectional in a verti- 

 cal plane, at least at low frequencies, and bottom-re- 

 flected sound in 2,000 fathoms may contribute ap- 

 preciably to the received signal. Thus, for sonic lis- 



