LONG-RANGE SOUND CHANNEL PROPAGATION 



213 



4B50 4950 5050 



VELOCITV OF SOUND IN FT PER SEC 



RANSE IN KILOYARDS 



Figure 17. Velocity distribution and schematic ray paths tor a typical deep sound channel. 



flection from the bottom. Leaving the detailed con- 

 sideration of bottom reflections until Sections 9.4.1 

 and 9.4.2, we shall consider here a number of phe- 

 nomena which are due to the presence of a "sound 

 channel" at a certain depth in the ocean. A sound 

 channel can be briefly described as a stratum of the 

 ocean within which the velocity of sound first de- 

 creases with increasing depth, passes through a min- 

 imum, and then increases. The importance of such a 

 region is due to the fact that if a source of sound is 

 placed in it, all rays emitted within a certain range of 

 initial directions will remain confined to the sound 

 channel, executing periodic oscillations in depth. 



All, or nearly all, of the experimental results which 

 have been obtained so far on long-range transmission 

 in deep water can be interpreted in terms of ray 

 paths. Ray paths which lie in a sound channel are of 

 especial importance, and since these rays have rather 

 complicated characteristics, Section 9.3.1 will be de- 

 voted to a theoretical discussion of them. By using 

 the facts established there as a foundation, the ex- 

 perimental results to be given in Section 9.3.2 can be 

 concisely discussed and interpreted. 



9.3.1 Deep Sound Channels 



Sound channels may occasionally occur at shallow 

 depth, when there is either a layer of isothermal water 

 or a layer with a positive temperature gradient under- 

 neath a surface layer in which the temperature 



gradient is negative. The more common configura- 

 tion of an isothermal layer immediately beneath the 

 surface is, moreover, very similar to a sound channel, 

 in that many rays undergo alternate upward refrac- 

 tion and surface reflection, and so remain confined to 

 the isothermal layer out to indefinitely large ranges. 

 Of more importance, however, for long-range trans- 

 mission in deep water, is a deep sound channel which 

 always occurs, except in polar regions, at a depth of 

 the order of 4,000 ft or less. This deep sound channel 

 is due to the fact that at all ordinary latitudes there is 

 an extensive thermocline within which the tempera- 

 ture gradient is negative and below which the tem- 

 perature gradient is so slight that the effect of in- 

 creasing pressure suffices to make the velocity of 

 sound increase with depth. 



Figure 17 shows velocity minima of both shallow 

 and deep types, and several ray paths emanating from 

 a source located at the depth of the lower minimum. 

 It will be convenient to refer to the ray which be- 

 comes horizontal at the depth of minimum velocity 

 as the "axis" of a sound channel. If the thermal 

 gradient is discontinuous, as in Figure 17, the num- 

 ber of different rays connecting any two points on the 

 axis is infinite, since the range per cycle, that is, the 

 horizontal distance traversed by a ray while travers- 

 ing 1 c of its oscillation in depth, approaches zero 

 as the ray approaches the horizontal. This is illus- 

 trated by the three rays labeled I. If the velocity- 

 depth curve is smooth near the minimum, this will 



