TRANSMISSION THKOUGH A THERMOCLINE 



109 



BT INFORMATION 



SOUND FIELD DATA 



4890 4940 4990 



SOUND VELOCITY IN FT PER SEC 



en 20 



4000 6000 8000 10,000 12,000 14,000 



RANGE IN YARDS 



Figure 21. Sample transmission anomaly out to short range. 



though practically all the sound which strikes the 

 surface will be reflected back into the water, its 

 direction will usually be affected by the water waves 

 on the surface. A glance at sunlight reflected from 

 the ocean surface shows how a sound beam may be 

 reflected in a variety of directions at a rough surface. 

 It is possible, for example, that the surface reflects 

 sound predominantly downward, with little surface- 

 reflected sound reaching a shallow hydrophone at 

 ranges of several hundred yards or more. While some 

 observational and theoretical studies of this problem 

 have been attempted, the transmission anomaly at 

 several hundred yards is still uncertain. One would 

 expect theoretically that the anomaly might lie any- 

 where from to —3 db. This corresponds to an un- 

 certainty of 6 db in computed echo levels from targets 

 of known target strength. This important gap in 

 transmission information will presumably be filled in 

 when more transmission measurements have been 

 made with the help of one of the several calibration 

 methods discussed in Section 4.3. In most of the runs 

 made up to 1944, however, neither the instrumenta- 

 tion nor the caUbration procedure was completely 



reliable. For example, the observed values of the 

 anomaly at 500 yd vary from —6 db to -|-15 db 

 when the water is isothermal to a depth of at least 

 40 ft. Thus, one may readily believe that the absolute 

 values of the transmission anomaly for the majority 

 of the runs available may be systematically in error 

 by as much as 3 db. 



5.3 TRANSMISSION FROM AN 



ISOTHERMAL LAYER THROUGH A 

 THERMOCLINE 



As pointed out in Section 5.1, the ocean is rarely 

 isothermal to great depths. In the more typical case, 

 an isothermal layer overlies a sharp negative gradient, 

 or thermocline, whose top may be at a depth any- 

 where from less than a hundred to many hundreds 

 of feet. When the isothermal layer is a hundred 

 feet thick or more, sound transmission above the 

 thermocline is apparently independent of the depth 

 or sharpness of the thermocline, at least out to ranges 

 of several thousand yards. Sound transmitted to 

 points in or below the thermocline maybe appreciably 



