HAMILTON: TIME VARIATIONS OF SOUND SPEED OVER LONG PATHS IN THE OCEAN 



the signal- amplitude cutoff is in excess of 30dB and occurs in about 

 0.02 seconds. The trace shown is a Sanborn direct-writing hot-pen re- 

 corder in the log-audio mode (i.e., the log of the rectified audio sig- 

 nal) . The ray diagram shown is from the original Ewing and Worzel 1947 

 SOFAR paper. For this Sargasso Sea sound-speed profile with the Sargasso's 

 500m thick, near surface layer of 18°C water, the first arrivals of the 

 SOFAR signal travel along paths that are near bottom grazing and 

 through the 18°C near-surface water. 



In Figure 3 is shown the bottom hydrophone array off Antigua over 

 which SOFAR charges were dropped. The water is 3,000 fathoms deep. 

 Three hydrophone signals are needed to locate and time an underwater ex- 

 plosion. With six hydrophones in this array, there is redundant data 

 for greater system reliability and for greater time and position accuracy. 

 One SOFAR charge could be located relative to another in the central 

 area of this array with a precision of 30 feet. This shot-position pre- 

 cision on a transmission path of 1,000 miles to a fixed hydrophone 

 means the error in the relative sound-speed measurement due to source- 

 charge positioning errors is of the order of 0.04 feet per second. 



In our SCAVE tests, and we ran about 25 or 28 of them, we 

 chartered a small boat in Antigua as a SOFAR charge drop boat. This 

 boat was the type of yacht you could charter for about $2,000 a 

 week. Normally a one-week charter was required to set up aboard, 

 sail to the hydrophone area and drop SOFAR charges for two hours, 

 return and offload. On one SCAVE, we dropped SOFAR charges every 

 hour for 24 hours, and on another occasion every hour for eight 

 days. 



Figure 4 is a typical record for an overhead SOFAR signal on 

 these bottom hydrophones, illustrating the S/N ratio and system fre- 

 quency response that made the 30-ft shot-position precision possible. 



11 



