Under certain conditions, propagation losses may greatly exceed the 

 more normal ones cited above. The presence of large quantities of scatterers 

 or absorbers such as bubbles, fish with swim bladders, squid, certain 

 euphausiids and other small crustaceans, and even plankton in large numbers 

 may greatly increase absorption loss particularly at certain frequencies, 

 related to specific scatterers. Accurate prediction of such losses must be 

 based on detailed oceanographic observations of the specific area under 

 consideration. A second factor which may lead to abnormally high propagation 

 losses is vertical refraction, or bending of the sound beam. This occurs when 

 temperature and/or salinity varies with depth. For example, higher 

 temperatures near the surface, as often happens because of solar heating of 

 the ocean surface, cause downward bending of sound rays so that the loss 

 effect of an absorptive bottom is accentuated. 



Not all departures from standard progagation appear as excess loss. 

 Under some conditions propagation is enhanced such as to give rise to higher 

 sound levels than predicted by the simple expressions (1) and (2) above. One 

 such mechanism involves penetration of sound (particularly low frequency) 

 energy into the ocean bottom which may act as a better sound path than the 

 water above. Sound so conducted may leak back into the ocean far down the 

 path and appear again as underwater sound. A second mechanism is the so 

 called "meciaphone effect," whereby sound transmission over a downward sloping 

 bottom, as from the continental shelf out into deep water, is enhanced because 

 consecutive bottom reflections tend to direct the sound into the horizontal 

 path. 



G-ig 



