Chapter 5 



DEEP-WATER TRANSMISSION 



TRANSMISSION IN DEEP WATER, where bottom- 

 reflected sound is unimportant, is somewhat 

 simpler to study than transmission in shallow water. 

 Even when the effects of the bottom have been elimi- 

 nated, however, sound transmission in the ocean re- 

 mains an exceedingly complex phenomenon. The 

 theoretical results based on the elementary ray 

 theory and on an idealized ocean stratified in imi- 

 form horizontal layers are seldom realized exactly in 

 the sea. Sometimes this simple picture leads to er- 

 roneous results, even qualitatively. Moreover, the 

 only constant element in underwater sound trans- 

 mission is change. No one ping re.^embles the preced- 

 ing. In this chapter, the rapid fluctuation of trans- 

 mitted sound from one second to the next is ignored 

 and reference is made throughout to averages based 

 on many consecutive pings. However, even these 

 averages sometimes vary considerably. 



Although the theory developed in the previous sec- 

 tions is admittedly imperfect and maj' be incorrect in 

 principle, this theory is nevertheless retained as the 

 framework on which to hang the discus.sions of the 

 observational material. The theory is beheved valu- 

 able, partly in indicating which results may be ex- 

 pected to have general validity beyond the particular 

 conditions under which the results were obtained. 

 Even more important, a discussion of the interrela- 

 tion between facts and theories is essential for an in- 

 telligent formulation of research programs. In the 

 long run, progress in any scientific problem can be 

 achieved most efficiently by formulating hypotheses 

 and then testing them in critical experiments. To lay 

 the groundwork for such future research is, in large 

 part, the objective of the present chapter. 



5.1 FACTORS AFFECTING 



DEEP-WATER TRANSMISSION 



In principle, the propagation of sound can be com- 

 pletely determined if the nature of the medium 

 through which the sound passes is known. In the 

 present section a description is given of the known 

 properties of the sea which are believed to influence 

 underwater sound transmission. 



5.1.1 Meaning of "Deep Water" 



For the purposes of this chapter, water is deep 

 when the bottom has a negligible effect on under- 

 water sound propagation. From a theoretical stand- 

 point this has the very simple meaning that the bot- 

 tom is ignored; the ocean is thought of as extending 

 to infinite depths. From the observational stand- 

 point, this means that only those observations will 

 be con.sidered here on which the bottom is believed 

 to have no effect. 



In general, the bottom can have several effects on 

 underwater sound. Sound energy reaching the bottom 

 may be partly reflected back at various angles into 

 the body of the sea and partly transmitted into or 

 absorbed by the bottom. The relative amounts re- 

 flected and absorbed depend on the depth and the 

 nature of the bottom, prevailing refraction condi- 

 tions, and sea state. This dependence and, generally, 

 the effect of the bottom on sound transmission will 

 be discussed in detail in Chapter G. Furthermore, the 

 presence of the bottom affects the background. Some 

 of the sound reflected backward by the bottom 

 reaches the receiver and gives rise to a ringing sound 

 known as reverberation. 



For most of the observations discussed in this 

 chapter, short pulses of sound are used. With this 

 technique, sound that has traveled to the bottom and 

 has been reflected toward the hydrophone can readily 

 be distinguished from sound that has traveled directly 

 from projector to hydrophone. Thus, for most obser- 

 vations the bottom-reflected sound can readily be 

 distinguished. If the bottom is rough, an appreciable 

 amount of sound may reach the hydrophone after 

 having been scattered from various portions of the 

 sea bottom so that the signal is followed by a long 

 single-frequency train of forward reverberation. 

 Usually, the direct signal is so far above this back- 

 ground of scattered sound that forward reverbera- 

 tion is negligible in the evaluation of transmission 

 observations. 



It is of importance to know when the ocean is ef- 

 fectivelj^ deep in practical applications of underwater 

 sound. With present echo-ranging gear, an echo from 



86 



