110 



DEEP-WATER TRANSMISSION 



weakened, however. This section discusses sound re- 

 ceived by a hydrophone in or below the thermochne ; 

 emphasis is placed primarily on thermochnes a hun- 

 dred feet thick or more. Almost all the data available 

 for these conditions are at a frequency of 24 kc. 

 Although some observations have been made at higher 

 frequencies, especially 60 kc, practically no relevant 

 information is available at sonic frequencies. 



5.3.1 



Echo-Ranging Trials 



The importance of the thermocline in weakening 

 sound which passes through it was first shown in 

 practical echo-ranging runs. The earliest and most 

 extensive data of this type were collected by the 

 British. ^^ In each run, a surface vessel echo-ranged on 

 a submarine at continually increasing or decreasing 

 range. The maximum range at which echoes could be 

 obtained was noted, together with the temperatures 

 and salinities at fixed depth intervals. Among various 

 effects produced by refraction, the most striking was 

 the reduction in maximum echo range resulting when 

 a submarine submerged below the thermocline. This 

 reduction in range is called layer effect. 



The quantitative importance of layer effect is evi- 

 denced by the fact that in 40 out of 68 trials reported 

 in reference 28 the maximum range decreased as the 

 submarine dove from periscope depth down to about 

 100 ft. Of the 28 trials in which layer effect did not 

 appear, all but 5 were made in water with very weak 

 temperature gradient, and 3 of these 5 exceptions 

 occurred in shallow water. In 12 of the 68 trials there 

 was a difference of more than 9 F between the tem- 

 perature at projector depth and the temperature at 

 the top of the deep submarine. In these 12 trials the 

 maximum range on the deep submarine varied be- 

 tween 20 and 90 per cent of the range found at peri- 

 scope depth, the average being 65 per cent. 



Similar results were obtained in echo-ranging trials 

 made by the USS Semmes (AG24, ex-DD189) on four 

 fleet-type American submarines.-^ Below the isother- 

 mal layer, which was 150 ft thick, was a sharp thermo- 

 chne, as shown in Figure 22. When the submarine 

 submerged to 250-ft keel depth or deeper, the 

 maximum echo range was consistently about half the 

 maximum echo range observed when the submarine 

 was at periscope depth. 



5.3.2 



Sample Transmission Runs 



such as the change of reverberation level with range, 

 the general explanation is that the sound intensity 

 below the layer is less than above. This result is borne 

 out by detailed transmission measurements. A sample 

 plot of measured transmission anomalies for deep 



100 



150 



20 



Although the detailed interpretation of these echo- 

 ranging results involves many complicated factors, 



450 



50 55 60 65 70 



TEMPERATURE IN DEGREES F 



Figure 22. Temperature-depth record for Semmes 

 tests (deep layer). 



and shallow hydrophones is shown in Figure 23, 

 representing a run made by UCDWR. The computed 

 ray diagram is also shown. 



The signals measured to give Figure 23 were 

 coherent" at all ranges, reproducing moderately well 

 the outgoing pulse. Some of the weaker signals re- 

 ceived below the layer were characterized by "rever- 

 beration tails," representing incoherent sound arriv- 



'» A received signal is called coherent if its envelope repro- 

 duces faithfully the outgoing pulse. An incoherent received 

 signal will in general have a ragged envelope and a length in 

 excess of the length of the original outgoing pulse. Since no 

 received signal portrays the envelope of the outgoing signal 

 completely without distortion, coherence is a question of 

 degree. 



