Chapter 16 — FUNDAMENTALS OF OCEANOGRAPHY 



increases with depth because of the weight of 

 the water above. Water will sink only when its 

 density is greater than the density of the sur- 

 rounding water. In the Tropics as the water 

 is heated it decreases in density. It then expands 

 and spreads northward and southward along the 

 surface, gradually cooling as it approaches 

 higher latitudes. As the warm surface water 

 moves outward away from the Tropics, it is 

 replaced by underlying cold water that flows in 

 underneath from the polar regions. Thus, it is 

 apparent that the deep water of all oceans is 

 ultimately derived from high latitudes, and has 

 much the same characteristics of salinity and 

 temperature as the surface waters of those 

 regions. Water types and masses are discussed 

 later in this chapter. 



SOUND TRANSMISSION 

 QUALITIES 



The most common form of energy presently 

 used for the detection of subsurface craft is 

 underwater sound. Being concerned with under- 

 water sound transmission, we must be aware 

 of the methods by which sound is generated, 

 transmitted, lost, and received so we can better 

 prepare forecasts of sound propagation conditions. 



SOURCE 



209.321 

 Figure 16-4. — Outgoing ping showing shape of 

 beam pattern and divergence of sound rays. 



TEMPERATURE 



209.322 

 Figure 16-5. — Drawing of outgoing ping 

 showing effect of refraction. 



Refraction 



Reflection 



Variations in the temperature and salinity 

 of sea water can profoundly affect sound trans- 

 mission because they produce variation in the 

 speed of sound as it travels from one point to 

 another. This, in turn, causes refraction of 

 sound waves. 



Sound waves travel in straight lines only 

 in a medium in which the speed is everywhere 

 constant. In sea water the speed of sound 

 generally varies with depth. Suppose, for 

 example, the speed increases with depth. In 

 that case every ray of the sound beam will 

 be curved toward the surface. This bending of 

 the sound rays is REFRACTION. (See figs. 

 16-4 and 16-5.) 



The degree of refraction is proportional to 

 the velocity gradient so that a rapid increase 

 in velocity with depth will refract the sound 

 beam sharply toward the surface, while slight 

 positive velocity gradients will cause the beam 

 to bend over a longer arc. 



Sound energy striking some solid surface 

 may be reflected as a mirror reflects light 

 with little loss of intensity, may be scattered 

 in many directions, or may be lost by absorption 

 into the medium. 



The surface of the sea is rarely smooth; 

 therefore, sound energy striking it is seldom 

 reflected specularly (mirror reflection). Instead, 

 only minute elements of the sea surface reflect 

 sound as a mirror; however, because the 

 orientation of each of these "mirrors" is 

 changing continuously, the sound energy is 

 reflected in many directions. 



Because of the acoustic property differences 

 between air and water, almost all sound energy 

 is reflected at the air-sea interface. 



The ocean bottom also may reflect sound; 

 however, the amount of sound energy reflected 

 from the bottom depends upon the type of bottom 

 material. A smooth sand bottom reflects sound 



377 



