TK ANSMISSION WITH NEGATIVE GRADIENTS NEAR SURFACE 



129 



.03 



.04 



.05 



16 



O 

 o 



25 



50 33 



DEPTH Dj TO THERMOCLINE IN FEET 



Figure 47. Attenuation coefficient above the thermocline. 



Although the scattered sound observed at ranges 

 between 1,000 and 2,500 yd is readily explained, the 

 coherent signals received in the shadow zone at 4,000 

 yd are less simply explained. These coherent signals 

 could be produced by a suitable variation of the 

 scattering coefficient m with depth similar to those 

 found in deep scattering layers. A preliminary 

 UCDWR analysis of reverberation measurements 

 made with a vertical projector at the same time as 

 transmission measurements indicates that this hy- 

 pothesis is correct; the scattering coefficients found 

 by these two methods agree to within a few decibels. 



5.4.2 Weak Temperature Gradients 

 in Top 30 ft 



When the temperature gradients in the top 30 ft 

 are intermediate — CHARLIE patterns — it has been 

 clearly demonstrated in Table 1, that at least half of 

 the transmission anomaly curves are approximately 

 straight lines while the others have more complicated 

 shapes. Thus the type of transmission likely to be en- 

 countered is highly unpredictable. This observational 

 result may be in part caused by the rapid variability 

 of temperature conditions for this type of pattern; 

 small changes of temperature, of the sort very com- 

 mon near the surface, can change the theoretical ray 

 diagram completely in a matter of minutes. Various 

 methods have been developed for analyzing the trans- 



mission conditions to be expected with these patterns. 

 Since the average transmission loss observed with 

 shallow MIKE patterns is very similar to that ob- 

 served for CHARLIE patterns, these two tempera- 

 ture types are combined in the present discussion. 

 Most of this section refers to average results obtained 

 at 24 kc. Some special temperature distributions are 

 discussed at the end of this section. 



Attenuation Coefficients 



In reference 13, an attenuation coefficient was 

 found from the slope of each straight-line transmis- 

 sion anomaly graph. Attempts to correlate these 

 coefficients with various temperature differences 

 either in the surface layers or in the thermocline were 

 not very successful. However, a significant correla- 

 tion was found with the depth Dt to the thermocline. 

 The plots showing this correlation are reproduced in 

 Figures 47 and 48. A least-squares solution gave the 

 following equations of best fit : 



Above the thermocline a = 3.5 -|- 



Below the thermocline a = 4.5 + 



170 

 Dt 

 260 

 Dr 



(25) 



(26) 



Although these mean curves are unquestionably sig- 

 nificant, only half of the individual points lie within 

 2 db of the values predicted from equations (25) and 

 (26). 



