AEROGRAPHER'S MATE 3 & 2 



very effectively., In contrast, a soft mud bottom 

 is an especially poor reflector. 



A smooth rock bottom is perhaps the best 

 reflector because the amount of energy absorbed 

 is small. In most areas, rock bottoms are 

 irregular and, consequently, reflect sound in 

 many directions,, Much of the reflected energy 

 is scattered back to the sound source, creating 

 possible interference and consequent masking of 

 the target signal. This type of interference is 

 referred to as "reverberation." 



Reverberation 



If the surface and bottom of the ocean were 

 absolutely smooth and no suspended matter 

 (including fish) were in the water, there would 

 be no reverberation. However, irregularities in 

 the ocean surface, bottom, and the water itself 

 are capable of scattering the sound pulse and 

 echoes. That portion of the scattered sound 

 which returns to the sound source is 

 reverberation. 



SEA SURFACE TEMPERATURES 



Sea surface temperature (SST) observations 

 are plotted on charts and analyzed to depict 

 the relative warm and cold areas of the sea 

 surface. SST charts provide fundamental infor- 

 mation for the conversion of environmental data 

 into operational data of concern to the Anti- 

 submarine Warfare (ASW) and other interested 

 users. This information may be in the form 

 of sonar range predictions for ASW tactics or 

 recommendations on more favorable convoy 

 routes. Fog may be forecast to develop or 

 dissipate on the basis of the SST pattern (air- 

 sea temperature difference). 



In search and rescue (SAR) operations, the 

 SST chart is useful in indicating survival or 

 operating time in water of differing tempera- 

 tures. (See table 16-1.) SST charts may be 

 used in conjunction with layer depth charts 

 when determining the depth at which submarines 

 may or may not be detected readily and areas 

 in which sonar ranging may be restricted or 

 enhanced. 



Attenuation 



Sound energy propagated through a volume 

 of sea water undergoes some loss of energy 

 because of a factor referred to as "attenuation"; 

 that is, absorption and scattering. In the passage 

 of sound through water, some of the energy 

 is converted into heat; this is called absorption. 

 Scattering loss results from reflectors in the 

 water that may vary in size from minute air 

 bubbles to a whale. 



MAJOR OCEANOGRAPHIC 

 PARAMETERS 



Many parameters (measurements) must be 

 considered when attempting to present a 

 complete picture of the ocean environment. It 

 would be beyond the scope of this training 

 manual to attempt to discuss all of them. How- 

 ever, some of these parameters are of more 

 interest than others, depending upon their 

 intended use. From a military viewpoint, among 

 the more important parameters to be considered 

 are the sea surface temperature (SST), the 

 mixed layer depth (MLD), and the subsurface 

 temperature gradients. The decoding, plotting, 

 and analysis of this information were discussed 

 in chapter 10 of this manual. 



MIXED LAYER DEPTH 



The concept of the "three-layered ocean" 

 was mentioned earlier in this chapter. The 

 mixed layer, the main thermocline, and the 

 deep layer were mentioned at that time. Of 

 these three layers, the mixed layer is the most 

 variable in its properties (primarily depth), 

 and therefore demands considerable attention. 



Variation in the depth of the mixed layer 

 may occur as a result of several different 

 factors; for example, day-to-day (diurnal) 

 heating and cooling. Under extreme conditions 

 these may have magnitudes of as much as 

 3°C. Usually, the day-to-day variation averages 

 about 0.5° C. This is due to heating during the 

 day, and this additional thermocline of a diurnal 

 nature will probably occur within 30 feet of 

 the surface with a maximum gradient occurring 

 during the late afternoon. These conditions may 

 sometimes result in what is referred to as 

 the "afternoon effect." This is the solar heating 

 of the surface water, which causes a shallow 

 negative temperature gradient. 



The net result of this condition is down- 

 ward refraction of sound rays and reduction 

 in near surface ranging. 



378 



