SUPERSONIC TRANSMISSION 



139 



water siiirc this lonmila was derived in Section 5.2.2 

 without tiiking intoaccount the contribution of sound 

 reflected from bottom and surface. For the higher 

 .supersonic fre(iuencies, this fear is freciuently un- 

 justified. As a matter of fact, in the presence of a 

 well-refiectinp; bottom, equation (2) provides a better 

 fit to the ol).ser\ations, considering all types of re- 

 fraction conditions, than in deep water. This appar- 

 ent paradox can be explained easily. For 24-kc sound, 

 for instance, it is known that the attenuation in the 

 direct beam amounts to 4 or 5 db per kyd. Beyond a 

 range of 2,000 yd, the second term in the expression 

 (2) increases less rapidly than the first term. As a 

 result, modifications in the second term of equation 

 (2) due to changes in the geometry of spreading are 

 insignificant compared with the first, or absorption, 

 term — at least at ranges where the effect of the 

 bottom might be expected to become noticeable. On 

 the other hand, deviations from equation (2) in deep 

 water are common in the presence of downward re- 

 fraction in the shadow zone. These deviations are 

 mitigated by the appearance of bottom-reflected 

 sound in the shallow-water sound field at long range. 

 It is, therefore, convenient to plot and to analyze 

 transmission anomaly in supersonic shallow-water 

 transmission, since at short range the sharp inverse 

 square drop is taken out of the transmission loss, 

 while at long range the variations of transmission loss 

 and transmission anomaly are not very different (see 

 Figure 1 of Chapter 4). 



At sonic frequencies the situation is different, since 

 the attenuation as determined in deep-water trans- 

 mission experiments is very small, certainly less than 

 1 db per kyd. As a result, the first term of expres- 

 sion (2) does not overpower the second term even at 

 ranges of the order of 10,000 yd. Moreover, sonic 

 sources and receivers tend to be nondirectional, and 

 bottom-reflected sonic sound tends to become im- 

 portant at shorter ranges than does bottom-reflected 

 supersonic sound. It may, therefore, be expected that 

 the contribution of bottom-reflected sound will sig- 

 nificantly affect sonic transmission at all ranges of 

 operational importance for all refraction conditions 

 except sharp upward refraction. At sonic frequencies, 

 a modification of the inverse square law to take bot- 

 tom-reflected sound into account thus is more neces- 

 sary than at frequencies above 10 kc. If both the sur- 

 face and the bottom were perfectly reflecting, sound 

 energy would spread only in two dimensions, and as 

 a result, the sound field decay at long range should be 

 approximated by an inverse first power law. Actual 



interfaces permit .sound to "leak" across, and the 

 power law of sound field decay must be obtained by 

 fitting a curve to the observations. Even under these 

 circumstances, however, the consideration of trans- 

 mission anomalies based on the inverse .square law 

 .should reveal the e.s.sential features of sound tran.s- 

 mission and may be preferred on the grounds of uni- 

 formity of approach. In addition, a plot of transmis- 

 sion anomaly has the practical advantage that a 

 more open decibel scale is po.ssible than for trans- 

 mission loss. 



6.2 



SUPERSONIC TRANSMISSION 



To study the acoustic properties of various sea 

 bottoms, both UCDWR and WHOI have carried out 

 transmission and reverberation runs in shallow water. 

 The purpo.se of these experiments has been both to 

 measure specific parameters characterizing the sea 

 bottom and to obtain information on the overall 

 properties of the sound field encountered in shallow 

 water. Reverberation experiments are discussed in 

 detail in Chapters 11 to 17 of this volume; but it is 

 necessary to refer to them in this chapter, because 

 they have incidentally furnished tentative values for 

 the reflection coeffi.cients of sea bottoms for slant 

 incidence.^ 



6.2.1 Acoustic Properties of Sea 

 Bottoms 



Types of Sea Bottoms 



Analysis of observed echo and listening ranges, 

 which began in 1941, indicated that ocean bottoms 

 could be roughly subdivided into a few geological 

 types with fairly consistent reflection characteristics 

 for each type. The classification of bottoms for sound 

 ranging purposes has been standardized and includes 

 the following: SAND, SAND-AND-MUD, MUD, 

 ROCK, STONY, and CORAL. These bottom types 

 are described as follows.^ 



SAND Firm, relatively smooth bottom. 



SAND-AND-MUD Relatively firm, smooth bottom. 



MUD Soft, smooth bottom. 



ROCK Rough, broken bottom. Includes 



bedrock, outcrops, and areas covered 

 by boulders. 



STONY Hard bottom, commonly rough. Pre- 



dominantly cobbles, gravel, and 

 shells. Varying amounts of sand and 

 mud commonly present. 



CORAL Hard bottom, with sandy patches, 



irregular to smooth. Includes var- 

 ious marine forms which secrete 

 masses of lime covering the bottom. 



