FACTORS AFFECTING DEEP-WATER TRANSMISSION 



93 



the tcinpcratiirc iiic reuses vvilh deplh sufficiently for 

 the temperature at some depth below the projector 

 to be greater than the projector temperature, rays 

 leaving the projector at slight downward inclina- 

 tions will in theory be bent back up to the surface 

 again; the transmission anomaly for a shallow hydro- 

 l)hane should therefore be low, although experimental 

 data on this point are lacking. When such positive 

 gradients are present, the temperature pattern is 

 called positive, sometimes denoted by PETER. 



Such patterns may be more completely character- 

 ized by the depth of the layer of maximum tempera- 

 ture and the difference between maximum tempera- 

 ture and the temperature at projector depth. The 

 sharpness of the underljang thermocline may also be 

 acoustically significant. 



Other temperature-depth records are classified by 

 the temperature dilTerence in the top 30 ft. If this 

 difference is 0.3 F or less, the water is said to be 

 isothermal, and the temperature pattern is called 

 mixed, sometimes denoted by the word MIKE. When 

 this difference is greater than 1/100 of the surface 

 temperature the computed range to the shadow 

 boundary is less than 1,000 yd, for projector at 15 ft, 

 hydrophone at 30 ft. For this temperature condition, 

 the predicted shadow zone is commonly observed, 

 and transmission to a shallow hydrophone becomes 

 poor for ranges greater than 1,000 yd. Such a tem- 

 perature distribution is called a sharp negative pat- 

 tern, sometimes denoted by NAN. Temperature dif- 

 ferences intermediate between MIKE and NAN 

 tend to be somewhat variable and are classified as 

 weak negative or changing patterns, denoted by 

 CHARLIE. 



One exception is included in this relatively simple 

 scheme. When the temperature difference from to 

 30 ft is large enough to give a NAN pattern, but the 

 temperature difference from 15 to 50 ft is 0.2 F or 

 less, the pattern is classified as CHARLIE. With such 

 an extremely shallow and negative gradient and with 

 the projector in isothermal or nearly isothermal 

 water, good but variable sound conditions may be 

 expected. This type of pattern is the most favorable 

 for the formation of a sound channel. 



With MIKE and CHARLIE patterns the depth 

 and sharpness of the thermocline would be expected 

 to affect the transmission of sound to a deep hydro- 

 phone. Appropriate methods for characterizing these 

 quantities are discussed in Section 5.3, where the 

 acoustic measurements made with a hydrophone in 

 or below the thermocline are summarized. 



Another more detailed system of classification, 

 which supplements the classification of negative 

 gradients into MIKE, CHARLIE, and NAN pat- 

 terns, has been devi.sed at UCDWR. This system 

 utilizes the depths at which the temperature is 0.1, 

 0.3, 1.0, 5.0, and 10 F below the surface temperature. 

 These depths are called, respectively, Di, D^, Di, Di, 

 and A. 



For statistical analysis, these depths are given code 

 numbers between and 9 by the following numerical 

 scale. 



Code cligit Depth D in feet 



S £> < 5 



1 5 g D < 10 



2 10 S 7) < 20 



3 20 g D < 40 



4 40 g Z) < 80 



5 80 g D < 160 



6 160 g D < 320 



7 320 ^ D 



9 D greater than greatest 



depth reached by bathy- 

 thermograph 



Any bathythermogram may then be coded by giv- 

 ing the code digits corresponding to Di, A, D3, Dt, 

 D5. The surface temperature T is also coded by giving 

 T/10 to the nearest whole number and by placing this 

 digit after the other five and separating it by a deci- 

 mal point. The code numbers for Di through D^ and 

 also r/10 are denoted by rfi, di, ds, di, df,, and de, re- 

 spectively. The series of numbers is then written as 

 d1d2d3didi.de, as for example 23 457.6. The accurate 

 determination of di is very difficult because of the 

 wide trace made by the bathythermograph near the 

 surface; since the variability of this small tempera- 

 ture difference will usually be high, there is some 

 question whether this quantity is usually significant. 



Examples of bathythermograms classified by the 

 two methods are given in Figure 9. These two systems 

 of classification supplement each other and should 

 probably be used together. The code system is 

 probably most useful for surface gradients, where 

 considerable detail is provided. For example, it is 

 shown in subsequent sections that the transmission 

 of sound to a shallow hydrophone depends markedly 

 on ^2 for different NAN patterns. On the other hand, 

 for a fixed ^2, transmission to a shallow hydrophone 

 differs markedly between NAN and MIKE patterns. 

 For deep gradients the code system is somewhat less 

 useful, owing to the very expanded depth scale. For 

 example it is frequently not clear from the present 

 code whether a deep hydrophone is above or below 

 the thermocline. It seems likely that when more com- 



