TRANSMISSION WITH NEGATIVE GRADIENTS NEAR SURFACE 



121 



RAr DIAGRAM 



HYDROPHONE OEPTH~A~ 

 HYDROPHONE DEPTHO 



HYDROPHONE DEPTH D 



BT INFORMATION 



50 60 70 



TEMPERATURE IN DEGREES F 



RANGE IN YARDS 



Figure 37. Sample transmission anomaly plot; 



puted range to the shadow boundary is discussed. 

 This is followed by a more detailed statistical investi- 

 gation of average transmission anomaly curves for 

 different types of temperature patterns and for dif- 

 ferent hydrophone depths. Finally, separate discus- 

 sions are given of the observed slope of the transmis- 

 sion anomaly near the shadow boundary and of the 

 sound observed in computed shadow zones. 



Computed Range to the Shadow Boundary 



When the temperature gradient extends right up 

 to the surface, and a shadow zone is predicted from 

 the simple theory, the range to the break in the ob- 

 served transmission anomaly curve might be ex- 

 pected to equal the computed range to the shadow 

 boundary. More frequently, however, the range to 

 the break is less than this value, as shown in Figures 

 34, 36, and 37. A detailed comparison between the 

 ray diagram and the transmission anomaly curves is 

 complicated by the variability of temperature condi- 

 tions, already discussed in Section 5.1.3. However, a 



brief analysis of some of the data has been carried 

 out, which averages the range to the shadow bound- 

 ary computed from bathythermograms taken at the 

 beginning and end of the transmission run. 



The resulting plot of these data is shown in Figure 

 38, taken from reference 34. This figure includes data 

 taken during two days of measurements. The dashed 

 line has been fitted visually to the observed points. 

 The correlation between the observed and the theo- 

 retical values is moderately good, considering the 

 variability of the temperature structure. However, 

 the observed ranges to the shadow boundary seem 

 to be systematically less than the predicted values, an 

 effect which is difficult to attribute to changes in 

 ocean temperature or errors in reading the bathy- 

 thermograph slide. The data presented in Section 

 9.2.3 on the change in shape of the pulse in the shadow 

 zone seem to indicate that the range to the break in 

 the transmission anomaly curve is correctly identified 

 as the boundary of the shadow zone. Thus, Figure 38 

 may present a real discrepancy, although the amount 



