TRANSMISSION IN ISOTHERM AI- WATER 



101 



4640 4890 4940 



SOUND VELOCITY IN FT PER SEt 



2000 



DATE. 

 TIME 



2-2B-I944 



1705 



BT CLASS MIKE 



WATER DEPTH 2200FM 



SEA 2_ 



SWELL 3_ 



WIND FORCE 4 



6000 



4000 

 RANGE IN YARDS 



Figure 13A. Sample transmission anomaly in isothermal water. 



only about half the runs were made with shallow 

 hydrophones, between 16 and 30 ft, and the other 

 half with deeper hydrophones, usually below the 

 thermocline, this result apparently applies for sound 

 transmitted below the thermocline as well as for 

 sound in the isothermal layer. This linearity of the 

 transmission anomaly for deep hydrophones is dis- 

 cussed again in Section 5.3.2. 



It is perhaps surprising that the transmission 

 anomalies should be straight lines out to long ranges 

 when the isothermal layer is at most a few hundred 

 feet thick. In a completely isothermal layer the up- 

 ward bending of the sound, caused by the increase of 

 pressure with depth, should give rise to a shadow 

 zone near the surface at 3,000 yd for a thermocline 

 starting at 150 ft below the projector and at 6,000 yd 

 for one starting at a depth of 600 ft below. While 

 sound reflected from the surface would penetrate this 

 shadow zone computed for the direct sound, some 

 drop in transmission might nevertheless be expected 

 at the shadow boundary. 



It is possible that shght negative gradients of 

 about 0.1 F in 30 ft were present in most of these 



measurements since slight gradients are common off 

 San Diego and are very difficult to measure exactly 

 with the bathythermograph. Such a slight gradient 

 would offset the effect of pressure on sound velocity 

 and give nearly straight-line propagation out to 

 considerable range. 



The observed results could also be quaUtatively 

 explained on the assumption that the temperature 

 in the isothermal layer is not completely constant, 

 but varies irregularly from point to point. It was 

 shown in Section 5.1.3 that the microstructure ob- 

 served in regions of sharp temperature gradient can 

 broaden the sound beam in the vertical direction by a 

 hundred feet in several thousand yards. If micro- 

 structure of similar effectiveness were present in the 

 isothermal layer, this alternate up-and-down bending 

 from microstructure would wash out the pressure 

 effect entirely and would enable some direct sound 

 to travel to an indefinite range in the isothermal layer. 

 Since a fraction of this soimd would be bent down into 

 the thermochne at all ranges, the linearity of the 

 transmission anomaly curve at depths below the 

 thermocline might also be explained on this basis. 



