Northrop 



23003 'N, 153050 'W in about 5 km of water; TERITU was at 22027.8 'N, 

 15^002. 5 'W; and YAQUINA was at 22028.1 'N, 153Ol0.1'W. Only the 

 FLIP data will he discussed in this report. 



METHCD 



Three crystal hydrophones (AX-58-C) were suspended at a depth 

 of ahout 250 ft (75 m) from floats connected by cable to FLIP. The 

 hydrophones were isolated from cable- suspension noise by a series 

 of neutrally-buoyant floatsS and were streamed from FLIP on indi- 

 vid-ual cables out to distances of from 500 to 1000 ft (150 to 300 m) 

 (Fig. 2). This method of hydrophone suspension from FLIP has been 

 foundS to reduce the ambient noise level at the hydrophones to that 

 measured for sea state two^^5 in the frequency range of interest 

 (U-i^-00 cps). The hydrophone had a sensitivity of -79 dB re 1 volt 

 per microbar. The signals were passed through a frequency dividing 

 network" and split into two bands, 3-20 cps and 20-200 cps. Higher 

 frequencies were passed on to a third stage anrplifier and rectified 

 prior to recording. Signals in the low- and intermediate-frequency 

 bands were passed through a ij-0 dB (variable) amplifier. Each 

 amplifier had high- and low-gain outputs with 20 dB separation 

 between channels. The signals were recorded on an oscillographic 

 camera, a pen recorder, and magnetic tape. 



Analysis of Data 



Arrival times and signal levels were picked on the 

 records and total path distances computed from the elapsed time 

 between shot and receiver, assuming a speed of sound in water of 

 l.ij-7 km/sec. The events recorded after the direct hydroacoustic 

 wave were correlated, where possible, with known submarine promi- 

 nences by the use of a calibrated scale in conjunction with a 

 36 in. diameter globe. For each reflection travel time, one end 

 of the scale was placed at the shot position, one at the receiver 

 position, and an arc of possible reflectors traced. Two assijmp- 

 tions were made in analysis: (l) that the hydroacoustic waves 

 traveled along great circle routes between source, reflector, and 

 receiver with no horizontal refraction, and (2) that the main 

 packet of energy traveled via continuously refracted SOFAR (Sovmd 

 Fixing and Ranging) paths in the deep sound channel. 



RESULTS 



The Precursor 



The earliest arrival on the FLIP records was a low- 

 frequency (~5 cps) precursor that emerged from the background 

 23 sec ahead of the direct hydroacoustic wave (Figs. 3 and h) . The 

 precursor probably corresponds to the ground wave of normal-mgde 

 propagation and the main peak to the "rider" wave of Pekeris.° 



30 



