Northrop 



Similar precvirsors have been observed from large underwater explo- 

 sions in the Pacific (C. T. Johnson, personal communication) and 

 earthquake T phases. 9 



The Direct Wave 



The main hydroacoustic wave was recorded on FLIP at 

 061^25^173 GMT indicating a velocity of l.kj km/sec, which is the 

 speed of sound in the SOFAR channel. Although the shot depth was 

 slightly below the depth of minimum velocity, energy was introduced 

 into the SOFAR channel because of the sound velocity structure in 

 the shot area (Fig . 5 ) • The sound energy traveled outward from 

 the soijrce via a combination of Surface/Bottom reflected (SR/BR) 

 paths and continuously refracted (REIR) paths. Over the long ranges 

 utilized in this experiment, the SR/BR rays have reflection losses 

 of 1 or 2 dB at each reflection and eventually become unimportant 

 (the rays converge about every 35 miles, indicating at least 50 

 surface reflections and an equal number of bottom reflections over 

 the path to Hawaii). Thus, only RRR sound rays need be considered 

 in this analysis of the underwater acoustic signals received on 

 FLIP. Sound energy was received at the off -axis FLIP hydrophones 

 at a level of about 66 dB above background noise (Fig. 6). An 

 oscillographic record of the direct arrival (Fig. k) shows disper- 

 sion in the low- and intermediate-frequency bands. Peaks in the 

 high-frequency band correspond to individual sound ray arrivals 

 before the main (overload) arrival. After the main arrival, the 

 first and second bubble pulse arrivals are evident in the high- 

 frequency (rectified) trace. 



Topographic Reflections 



In this section, the prominent reflections are identified 

 from arrival times and signal levels of Figs. 3 and 6. The 

 location of these reflectors is shown in Fig. 1. 



The first reflection appears on the record before the complete 

 decay of the reverberation from the direct wave and is difficult 

 to identify. However, the observed travel time is approximately 

 equal to that computed for a reflection from the Mendocino Escarp- 

 ment. This reflection is followed 2 min later by a long-duration 

 return from the continental slope of the North American continent 

 (Reflector A of Fig. l). 



The next series of reflections is from the Hawaiian Arch 

 (Reflector B of Fig. l). The reverberation lasts for 25 min 

 (Fig. 6) and includes reflections from Hawaii, Maui, Molokai, Oahu, 

 Kauai, Nihoa, Necker, and Midway islands, as well as the many 

 reefs in the vicinity of French Frigate Shoals and Gardner 

 Pinacles. Individual, short duration reflectors from the neigh- 

 borhood of Mellish Seamoimt (290N, 171°W) are also present in the 

 data. 



31 



