excess should be greater than or equal to 10 meters per second for success- 

 ful convergence zone operation. This insures enough rays getting to the 

 surface for a zone strong enough to be potentially useful. (A velocity 

 excess identically equal to zero would allow the bottom grazing ray to 

 pass; if slightly greater, the first convergence zone ray would pass.) 

 Whether this condition exists or not is simply determined from the velocity 

 profile. Figures A. 4 and A. 5 in Appendix A are ray plots illustrating 

 convergence zone conditions. 



The SOFAR path is the propagation path which employs the deep 

 sound channel whose axis is normally located at depths of 1000 to 1500 

 meters. Propagation conditions are excellent and signals can travel dis- 

 tances of the order of a thousand kilometers with only cylindrical 

 spreading loss. Although SOFAR propagation is a multipath phenomenon 

 with different paths arriving at different times on a given signal, it has 



an abrupt ending corresponding to — , the time to travel in a great circle 



o 



along the channel axis where V is the velocity at the channel axis. 



° o 



X . 



The time of travel along longer paths is less than y since the 



o 

 sound travels at a speed greater than V . Because of this abrupt ending, 



accurate timing is possible, and if the time of arrival of a signal is 



measured at more than one location (say 3), the geographical position of 



the source can be determined in a manner similar to LORAN. 



The deep sound channel is characterized by a velocity profile 



which shows first a decrease in velocity with increasing depth below 



the surface or surface layer and then a more gradual increase in velocity 



with increasing depth. The depth of the minimum velocity occurs near 



1300 meters in the mid-latitudes of the North Atlantic. Data from 



3rthur ZD.1Uttlc.5lnr. 



