TK VINSMISSION WITH NEGATIVE GRADIENTS NEAR SURFACE 



135 



RAY DIAGRAM 



BT INFORMATION 





J 200 



300 



HYDROPHONE DEPTH A 



HYDROPHONE OEPTHO 



— SENDING SHIP U 



— RECEIVING SHIPlI 

 /) 



1/ 



SOUND FIELD DATA 



4840 4890 4940 



SOUND VELOCITY IN FT PER SECOND- 



4000 

 RAN.GE IN YARDS 



Figure 55. Peaked transmission anomaly with sound channel unlikely. 



channel although barely perceptible changes in the 

 temperature-depth record would make it so. Never- 

 theless, the anomalously high intensity at long range 

 in the shallow hydrophone is very striking. If an 

 absorption coefficient of 4 db per kyd is assumed, the 

 increase of intensity at 6,000 yd shown in Figure 54 

 is about 25 db. Until a more accurate means for de- 

 termining ocean temperatures is available, it cannot 

 be decided whether peaks of this type are actually 

 the result of the focusing action predicted in sound 

 channel theory. 



However, it is suggestive that an examination of 

 all cases in which the anomaly curves show peaks of 

 10 db or more shows that the temperature-depth 

 curves for most of these are very similar to the curves 

 which would, on the simple theory, give rise to sound 

 channels. Out of the many hundreds of runs made off 

 San Diego, only about 25 show these peaks. In almost 

 all of these the thermocline is below 100 ft with nearly 

 isothermal water above, and the receiving hydro- 

 phone is at shallow depth. Moreover, in most cases 

 slight negative gradients are present close to the sur- 



face. A few significant exceptions are present, as for 

 example the run shown in Figure 55 where the deep 

 hydrophone shows a peak of 20 db at 4,500 yd. As an 

 example of the transitory nature of most such peaks. 

 Figure 55 may be compared with Figure 42, which 

 plots the run immediately preceding and shows no 

 trace of any peaks. Also on one day (March 15, 1944) 

 the shallow hydrophone showed a marked peak 

 throughout the day while the temperature records 

 only occasionally showed the deep layer of constant 

 temperature required for a sound channel. Thus, 

 while the evidence suggests that sound channels may 

 in fact occur, there may quite possibly Ije other 

 factors, still unexplored, that play a part in producing 

 anomalously high intensities at certain ranges. 



5.4.3 



Transmission at 60 kc 



The effects produced by temperature gradients on 

 the transmission of underwater sound have been 

 thoroughly explored only at 24 kc. A few measure- 

 ments are available, however, at 60 kc. 



