140 



SHALLOW-WATER TRANSMISSION 



In the actual classification of sea bottoms, the 

 criterion established for estimating the relative firm- 

 ness or softness of the bottom was grain size, as de- 

 termined by mechanical analysis. The size limits 

 were set as follows (Division 6, Volume 6). 



MUD 90%byweightsmallerthan0.062mm. 



SAND-AND-MUD Between 10% and 90% smaller than 

 0.062 mm. 



SAND Less than 10% smaller than 



0.062 mm and 90% smaller than 

 2.0 mm. 



STONY Rounded or angular pieces of rock 



more than 2.0 mm and less than 

 10 cm, which appear to represent 

 glacial drift or other transported 

 material. 



ROCK Rocks of a size greater than 10 cm 



or pieces broken from rock ledges or 

 where bottom photographs show 

 projecting rocks or rock ledges. 



CORAL Samples containing calcareous masses 



of coral, algae, or other lime secreting 

 organisms, or bottom photographs 

 showing their e.xistence. 



This classification has been reasonably satisfactory 

 from the acoustical standpoint except in the case of 

 mud, for which it is now likely that texture alone is 

 not an adequate criterion. 



Although the bulk of experimental work on sound 

 transmission in shallow water has consisted of trans- 

 mission runs, some special experiments have been 

 made to determine numerical reflection coeflficients 

 of sea bottoms. 



Reflection Coefficients 



It may be gathered from the discussion in Section 

 6.1.1 that different values of the reflection coefficient 

 are to be expected for sound incident vertically on 

 the bottom and for slant rays. Although the deter- 

 mination of reflection coefficients for vertical inci- 

 dence has some interest at sonic frequencies, the most 

 important situations at all frequencies, from an opera- 

 tional point of view, involve slant rays. To obtain the 

 reflectivity of the sea bottom for slant incidence, 

 three different experimental methods have been con- 

 sidered. 



The most direct method uses transmission runs 

 with very short signals (10 msec), which often permit 

 the separate reception of the direct signal and the 

 bottom-reflected signal. (The surface-reflected signal 

 cannot be resolved for the usual projector depth of 

 16 ft, but would be resolvable if the projector could 

 be lowered to several hundred feet.) No coefficients 



of reflection resulting from these experiments have 

 been reported. It is possible to make a very crude 

 estimate of the reflection coefficient from published 

 standard transmission runs in those cases where the 

 curve showing the transmission anomaly plotted 

 against range indicates at least two well-marked 

 peaks corresponding to single and double bottom re- 

 flections. Reading the level difference between con- 

 secutive peaks and correcting for transmission loss 

 due to absorption between reflection (say 4 or 5 db 

 per kyd) leads to the following estimates: (1) for 

 SAND, the loss through reflection amounts to be- 

 tween and 6 db per reflection, corresponding to in- 

 tensity reflection coefficients between 1 and 0.25; (2) 

 for MUD, the loss is between 10 and 30 db per reflec- 

 tion, corresponding to coefficients of intensity reflec- 

 tion between 10~^ and 10~'. The wide spread in each 

 of these estimates indicates both the uncertainty of 

 the estimate in a given case and the wide varia- 

 bility among ocean bottoms falling within one clas- 

 sification. 



The second method involves the measurement of 

 bottom reverberation. If an echo-ranging transducer 

 is tilted downward about 30 degrees, a peak of the 

 reverberation is received at the range at which the 

 sound beam strikes the bottom. Sometimes, over 

 well-reflecting bottoms, a secondary peak is observed, 

 at a range at which the sound beam, specularly re- 

 flected first by the bottom and then by the surface, 

 strikes the bottom a second time. This secondary 

 reverberation peak has been observed over a coarse 

 sand bottom, and the average amplitudes of princi- 

 pal and secondary peaks have been determined by 

 UCDWR.'' If reasonable assumptions are made con- 

 cerning the transmission loss between the primary 

 and the secondary reverberation peak, and if the re- 

 flection at the sea surface is assumed to be perfect (a 

 calm sea and a low wind), then the reflection coeffi- 

 cient of the sea bottom can be estimated. It was found 

 be somewhere between 0.25 and 1.0, with the most 

 probable value 0.5. These values were obtained for 

 coarse sand, probably the bottom with the highest 

 reflectivity. 



Reflection coefficients have been estimated as 0.031 

 for foraminiferal SAND, between 0.005 and 0.025 for 

 SAND-AND-MUD, and 0.0017 for MUD.^ These 

 determinations are not very reliable, because they 

 involve unrealistic assumptions concerning the trans- 

 mission loss between consecutive reflections. In these 

 computations, it was assumed that the transmission 

 anomaly amounted to 1.6 db per kyd of vertical path 



