TRANSDUCER HORIZONTAL 



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-80 



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Mi 

 N 



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cr 

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 tc 



m 

 q: 

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iLl 



> 

 o 



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-80 



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a. 

 iij 

 m 

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1000 2000 3000 



RANGE IN YARDS 



4000 



Figure 26. Range dependence of reverberation as a 

 function of refraction conditions. 



axis is parallel to the surface, the reverberation ob- 

 served at wind speeds greater than 8 mph is pre- 

 dominantly surface reverberation. At ranges greater 

 than 1,500 yd, the reverberation does not depend 



significantly on wind speed, location, season, or ther- 

 mal structure of the ocean. It seems justifiable, there- 

 fore, to regard the reverberation at ranges greater 

 than 1,500 yd as the characteristic volume reverbera- 

 tion of the ocean. 



Possible Explanations 



The lack of dependence of reverberation on wind 

 speed at ranges greater than 1,500 yd is well estab- 

 lished, but is nevertheless surprising. The masking of 

 the surface reverberation by volume reverberation 

 at this range is in large part due to the rapid decrease 

 of surface reverberation with range. The reason for 

 this decrease is obscure. The following factors have 

 been suggested, in Section VI of reference 1, as possi- 

 ble causes of this rapid decrease of surface reverbera- 

 tion with increasing range: attenuation, variation of 

 the scattering coefficient with angle of incidence on 

 the surface, shadowing effects of waves, and inter- 

 ference effects in a thin surface layer (Lloyd mirror 

 effect). These possible causes will now be considered 

 briefly. 



In Figure 23, the drop between 100 and 1,000 yd in 

 median reverberation level at wind speeds greater 

 than 20 mph is 16 db more than would have been 

 predicted from the —30 log r dependence of equa- 

 tion (43), in Chapter 12. If this change is due to 

 transmission loss, the term A in equation (43) must 

 have a median value of 8 db per kyd. While this value 

 of the attenuation is not impossible, it is significantly 

 greater than the mean attenuation coefficient ob- 

 served in transmission studies off San Diego (see 

 Section 5.2.2), especially since with high wind forces 

 the surface layer tends to become isothermal. If the 

 steep slope in the curve of Figure 24 is due to atten- 

 uation, the attenuation coefficient would have to be 

 as high as 15 db per kyd. The possibility that some 

 of the increased loss is due to attenuation cannot be 

 ruled out; but on the whole the evidence from trans- 

 mission studies does not justify regarding attenua- 

 tion as the primary cause of the rapid decrease of 

 surface reverberation with range. 



If the surface scattering layer is very thin, it can 

 be argued that the scattering coefficient m' should 

 decrease at least as rapidly as sin 6, where d is the 

 grazing angle of the ray incident on the surface. For 

 the total volume of surface scatterers irradiated by 

 the ping at any instant is proportional to Cor, accord- 

 ing to Section 12.3. All the energy reaching this 

 volume must pass through the surface whose cross 

 section in the plane of Figure 27 is AB. Thus, if I is 



