Chapter 16 — FUNDAMENTALS OF OCEANOGRAPHY 



of sound transmission through the sea medium 

 and the bottom. 



First, the upper portion of the bottom 

 material may be similar in velocity characteris- 

 tics to the adjacent bottom waters. However, 

 with increasing pressure due to depth, the 

 sediments are compacted and their density 

 increased. This causes the sound velocities to 

 exceed the velocity in water adjacent to the 

 bottom. The net effect is to extend the depth 

 of the sound conducting layer. 



Second, in many places the bottom is com- 

 posed of layers of different acoustic properties. 

 Such layers can channel the transmitted sound 

 signals. In this manner the signal may be 

 transmitted through the bottom over fairly long 

 ranges, even through shallow waters. 



Third, the physical and thus the acoustic 

 properties of the bottom materials are changing 

 along the transmission path, and these changes 

 may have an effect on sound transmission. 



As an example of how similar sound velocity 

 structures affect shallow and deepwater trans- 

 mission, consider the following: 



In deep water, where strong negative gradi- 

 ents exist, downward refraction results in 

 shadow zones. In this situation a target would 

 be detected only at close range. In shallow 

 water, however, the refracted sound strikes 

 the bottom and is repeatedly reflected, so that 

 the shadow zone becomes isonified (completely 

 blanketed with sound rays by these overlapping 

 reflected paths). Consequently, longer detection 

 ranges are possible (fig. 16-7). 



Now let us consider a shallow water medium 

 where the following conditions exist: A per- 

 fectly smooth ocean surface and bottom, the 

 sound velocity of the bottom material exceeds 

 that of the adjacent water, the water has iso- 

 velocity strucutre, the sound source and 

 receiver are near the surface, and the sound 

 source produces only a single frequency. 



This means that all ray paths are .straight 

 lines, and their direction changes when they 

 strike the bottom or surface. Further? they 

 leave the reflecting surface at an angle equal 

 to the incident angle. Then, the evaluation of 

 sound intensity becomes a matter of considering 

 the interference effects between multiple reflected 

 rays and direct rays. 



Deepwater Transmission 



In deep water, sound may travel from source 

 to receiver by surface ducts, sound channel, 

 convergence zone, and bottom bounce. Figure 

 16-8 shows examples of deepwater transmission 

 paths. 



SURFACE DUCTS. — A surface duct (fig. 

 16-8 (A)) exists in the ocean if the following 

 conditions are met: 



1. The temperature increases with depth. 



2. An isothermal layer is near the surface. 



In condition 1, sound velocity increases as 

 temperature increases; in condition 2 there is 

 no temperature or salinity gradient, and pressure 

 causes sound velocity to increase with depth. 

 The greater the depth of the duct, the greater 

 is the difference between surface velocity and 

 the velocity at depth, and the greater are the 

 number of rays entrapped. The efficiency of 

 the surface duct in transporting sound also 

 is dependent upon the smoothness of the sea 

 surface. 



Variations in temperature and the resultant 

 variations in velocity are of utmost importance 

 where the surface duct is utilized, since a 

 change in temperature of 0.4° F per 100 feet 

 makes the difference between an excellent duct 

 and no duct. Horizontal velocity variations may 

 seem negligible, but they can bring about 

 complete deterioration of the duct if the 

 variations occur between the sound source and 

 the target. 



SOUND CHANNELS. — In the deep ocean, 

 temperature generally decreases with depth to 

 a little above 0° C at approximately 700 fathoms 

 in the Atlantic Ocean and to about 5° C near 

 500 fathoms in the Pacific Ocean. The velocity 

 of sound is dependent mainly on temperature 

 and decreases to a minimum at the depth cited; 

 but deeper than 500 to 700 fathoms, velocity 

 increases as a result of pressure. 



The zone between these points of similar 

 velocity constitutes a sound channel as illus- 

 trated in figure 16-8 (B). The depth at which 

 sound velocity is at a minimum is called the 

 axis of the sound channel. If an omnidirectional 

 sound source and receiver are placed at the 

 axis of the sound channel, a cone of rays is 

 produced at angles above and below the axis. 

 Those rays which start at angles above the 



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