92 



DEEP-WATER TRANSMISSION 



The chief effect of temperature microstructure is 

 to introduce irregularities into the path of the indi- 

 vidual sound rays. They will be slightly bent away 

 from the average ray path in random fashion, as in- 

 dicated in Figure 8. Considering a sound beam as a 

 whole, we may expect that microstructure will very 

 slightly broaden the beam pattern, although such 

 broadening effects have never been determined with 

 assurance. Within the sound field, local intensities 

 may show deviations from the average values which 

 would be observed in the absence of microstructure ; 

 these local deviations will be discussed in Chapter 7. 

 Another effect of these irregularities is that sound 

 may penetrate with a small but observable intensity 

 into regions which are shadow zones according to the 

 large-scale ray pattern. 



It is possible to estimate the effect which micro- 

 structure will have on the ray trajectories of indi- 

 vidual sound rays. Theoretical analysis shows that 

 with certain simplifying assumptions the rms lateral 

 displacement Ay of a sound ray because of micro- 

 structure is given by the formula' 



^2/ = WbGRK 



(1) 



In this equation G is the rms value of the fractional 

 velocity gradient caused by microstructure, R is the 

 range, and 6 is a quantity having the dimension of a 

 length, which may be called the patch size of the 

 microstructure. Roughly speaking, b is the average 

 distance over which the vertical velocity gradient 

 caused by microstructure retains the same sign. To 

 derive this formula, an expression was first obtained 

 for the lateral displacement of a ray passing through 

 a given microstructure. This expression was then 

 squared and averaged. The square root of the final 

 result gave expression (1). 



It has already been noted that fluctuations of the 

 vertical temperature gradient, amounting to 0.02 F 

 per ft over patches about 100 yd in length, have been 

 reported. If these values for G and for b are substi- 

 tuted into equation (1), it is found that at a range of 

 1,000 yd the rms lateral spreading of the sound beam 

 amounts to about 20 ft; while at 2,500 yd it amounts 

 to 70 ft and at 4,000 yd at 150 ft. These figures indi- 

 cate that at these ranges random spreading of the 

 transmitted sound beam, because of microstructure, 

 will obscure bending of the sound rays due to large- 

 scale vertical temperature structure if the vertical 

 gradient is of the order of 0. 1 F in 30 ft. Actual obser- 

 vation shows that even negative gradients of four 

 times this magnitude often fail to produce clearly 



recognizable shadow zones, although the sound does 

 weaken gradually with increasing range. It is not 

 known at present whether microstructure will fre- 

 quently have a magnitude appreciably in excess of 

 that assumed for the estimate of lateral beam spread. 

 If not, some other cause must be invoked for an ex- 

 planation of why weak negative gradients do not 

 produce shadow zones. 



Since no complete theory exists at the present time 

 capable of explaining in detail the results obtained in 

 transmission runs, much of the discussion of under- 

 water sound transmission must be empirical in char- 

 acter. It is possible, for example, that some of the 

 empirical relationships found between the smoothed 

 temperature-depth curves and the measured trans- 

 mission anomalies result primarily from an oceano- 

 graphic correlation between the temperature micro- 

 structure and the smoothed distribution of tempera- 

 ture with depth. Such observed empirical relation- 

 ships are valuable, but until their basic physical cause 

 is explained they should be used with caution since 

 they may be valid only for the particular time and 

 place in which the observations were made. 



5.1.4 Classification of Bathy ther- 

 mograms 



For practical use of temperature-depth informa- 

 tion some simple method of classifying bathythermo- 

 graph records is essential. Even if the predictions of 

 ray theory were exactly fulfilled, practical require- 

 ments would probably rule out the time and effort 

 required to construct ray diagrams and to compute 

 theoretical intensities. Thus, a set of rules has been 

 devised to classify temperature-depth records by the 

 properties which are acoustically significant. 



Such classifications have also proved useful in 

 transmission research. Since the simple ray theory 

 was clearly inadequate, some other basis was required 

 for comparing measured anomaly curves with the 

 corresponding bathythermograms. In view of the 

 complexity of possible temperature-depth curves, no 

 classification can be entirely satisfactory. All such 

 classifications must be regarded as preliminary until 

 sufficient acoustic information is available to indicate 

 exactly what features of the temperature-depth pat- 

 tern are significant in any situation. 



Present systems of classification are prunarily de- 

 signed to correspond to different types of transmis- 

 sion loss for a shallow projector, about 15 ft. When 



