TRANSMISSION THROUGH A THERMOCI.INE 



111 



RAY DIAGRAM 



-J 1 



HYDROPHONE DEPTH A 



4890 4940 4990 



SOUND VELOCITY IN FT PER SEC 



DATE 3-2-1944 



TIME 1103 



WATER DEPTH 2230 FM 



WIND FORCE 5 + 



2000 



4000 

 RANGE IN YARDS 



6000 



Figure 23. Sample transmission anomalies above and below thermocline. 



ing after the direct signal. This scattered sound 

 usually appears whenever the amplification is made 

 sufficiently great. Since this incoherent sound is 

 relatively more prominent when sharp downward re- 

 fraction is present, it is discussed in Section 5.4.1. 



It is evident from Figure 23 that the plot of trans- 

 mission anomaly against range below the layer was 

 approximately a straight line. This result is quite 

 general, as noted in reference 13, where it was found 

 that for all runs made with isothermal water in the 

 top 30 ft of the ocean the transmission anomaly plots 

 were approximately straight lines. This analysis in- 

 cluded both deep and shallow hydrophones, and the 

 conclusions are therefore valid for hydrophones either 

 above or below the layer. 



On the basis of the simple ray diagrams shown in 

 Section 5.1.2, this result is rather surprising since the 

 transmission anomaly should rise to a very high 

 value at the shadow boundary, as shown in Figure 23 

 by the dashed line, taken from a UCDWR internal 

 report.™ This dashed line represents the anomaly 

 computed by ray tracing methods, as explained in 



Section 3.4.2. An absorption of 4 db per kyd has also 

 been included. The consideration of sound reflected 

 from a flat ocean surface will not change the com- 

 puted anomaly appreciably. In particular, the shadow 

 boimdary will not be much affected; for the isother- 

 mal layer shown in Figure 23, rays leaving the pro- 

 jector at an upward angle less than 2.08 degrees will, 

 after reflection, become horizontal before reaching 

 the bottom of the isothermal layer, while the steeper 

 rays will penetrate the thermocline at ranges of not 

 more than about 3,000 yd. 



Thus, to explain the straight-line anomaly curves 

 found below the layer, some mechanism must be in- 

 volved which is not included in the simple ray theory. 

 Either an irregular sea surface or thermal micro- 

 structure explains the results qualitatively since 

 either mechanism will take sound from the isothermal 

 layer at all ranges and deflect some of it sufficiently 

 sharply so that it will pass out of the surface layer 

 into the thermocline below. At present, it is not possi- 

 ble to state whether either mechanism can explain 

 the facts quantitatively. 



