124 



DEEP-WATER TRANSMISSION 



RAY DIAGRAM 



I- 100 



200 



BT INFORMATION 



4840 4890 4940 



SOUND VELOCITY IN FT PER SECOND 



DATE 3-22-194 4 

 TIME FSOO 



BT CLASS 



WATER DEPTH 650FM 



NAN 



SEA 



SWELL 

 WIND 



FORCE I 



4000 

 RANGE IN YARDS 



6000 



Figure 42. Sample transmission anomaly plot; NAN pattern with deep isothermal layer below. 



Insufficient data are available for NAN patterns with 

 D2 between 20 and 30 ft to yield much information on 

 the change of transmission anomaly with hydrophone 

 depth for that case. The scatter of the individual 

 points averaged to yield Figure 43 is moderately 

 large. Since only about ten runs were averaged to 

 give each curve, these average curves have a probable 

 error of between 2 and 3 db. 



Attenuation Coefficient at Shadow Boundary 

 The mechanism by which sound penetrates the 

 shadow zone is of theoretical and practical interest. 

 Information about this mechanism may be obtained 

 by comparing the rate of increase of transmission 

 anomaly near the shadow boundary with that com- 

 puted from the diffraction of sound. The slope of the 

 transmission anomaly beyond the break is very high, 

 usually between 20 and 70 db per kyd when the sharp 

 temperature gradient extends all the way to the sur- 

 face (D2 less than 10 ft). We shall call this slope a' and 



may regard it as a sort of "local attenuation coef 

 ficient," that is, 



. dA 



a' = 



dR 



(15) 



The local attenuation coefficient defined by equation 

 (15) is not to be confused with the actual attenuation 

 coefficient characterizing the transmission from the 

 sound source to the range R, defined as 1,000A/R. 



The observed slope beyond the break is to be com- 

 pared with the local attenuation coefficient at the 

 shadow boundary which would result from diffrac- 

 tion. From Section 3.7, we have the following formula 

 for a' in the case of a linear velocity gradient. 



In formula (16) c is the sound velocity in yards per 

 second; dc/dy is the velocity gradient in feet per 

 second per foot; and a' is in units of decibels per yard. 



