PORTER: SOFAR PROPAGATION OF WIDE-BAND SIGNALS TO LONG RANGES 



The data discussed here were taken in the western Mediterranean 

 during July 1968 on the cruise CHAIN 82. One-pound charges were 

 dropped and received once per hour at the sound channel axis depth 

 of 100 m. Precise STD lowerings were made permitting detailed 

 correlation of analytical and experimental results. 



ANALYSIS OF SIGNAL STRUCTURE 



Propagation conditions during July are such that there is an 

 extremely sharp SOFAR channel at a depth of 100 m in the western 

 Mediterranean. The ray trace of Figure 1 shows that ray angles be- 

 tween ±10° are wholly refracted when the source and receiver are on 

 the channel axis. Since nearly all surface reflected energy is also 

 bottom reflected, the signal structure at long range exhibits classic 

 SOFAR arrival structure. Note that as the ray angle increases, the 

 ray cycle length is longer and that the ray turning points are in 

 higher sound-velocity water. Thus, rays with long cycle lengths will 

 arrive sooner. 



Figure 2 shows graphic recordings of the arrival structure of 



four shots at ranges between 250 and 380 km. At T , the axial arrival 



o 



time, most of the energy has arrived; afterwards the signal level 



drops abruptly to the noise level. It should be noted that the axial 



arrival is actually much more intense. Some clipping by the graphic 



recorder has been permitted so that the early arrivals can be easily 



seen. For arrival time t < T , it is possible to resolve the records 



o 



into multiple arrivals. Each of these arrival times is characteristic 

 of a particular eigenray defined as a ray leaving the source at such 

 an angle that it crosses the channel axis at the receiver. For the 

 Mediterranean sound-velocity profile, the larger the exit angle from 

 the source, the earlier an eigenray arrives at the receiver. It is 

 evident from both the short records and the ray trace (Figure 1) , 



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