Northrop 



Recorded signal levels for reflected soixnd are all below that 

 of the direct wave because of increased spreading loss over the 

 greater travel paths involved and because of scattering loss on 

 bottom reflection. The signal level for Hawaiian reflectors is 

 greater than the California Slope reflectors^ probably because of 

 the steeper slopes of the former and the lack of sedimentary cover. 

 More distant reflectors produce levels not inconsistent with spread- 

 ing and attenuation losses observed by Sheehy and Halley. ' A curve 

 of the level vs range computed from Equations (l) and (2) is shown 

 on Fig. 6 to indicate the relative loss for individual reflectors. 



CONCLUSIONS 



1. A low-frequency seismic precursor was recorded 20 sec 

 before the SOFAR arrival. 



2. The main hydroacoustlc wave was recorded at a level near 

 66 dB above ambient noise. 



3. Topographic reflections were recorded for 2-l/2 hours 

 after the main arrival. 



h. Reflections from the continental shelf off California were 

 35 dB below the calculated level for a non-reflected path, whereas 

 the other reflectors were only 20-25 dB lower. 



5. Reflections from many seamounts of the Emperor Seamount 

 Chain were received, as well as from Volshouki Ridge. 



6. Reflections in the 3-20 cps band were 20-25 dB higher 

 than in the 20-i|00 cps band. 



ACKWOWIMDGMENTS 



This work was supported by the Office of Naval Researcn. 

 R. W. Raitt provided some of the records and, with P. Rudnick, 

 gave valuable suggestions in data analysis. Information on the 

 Ocean Bottom Seismograph results was provided by H. Bradner; the 

 TERITU records by A. Furumoto; and the YAQUINA records by R. P. 

 Meyer of the University of Wisconsin, and A. C. Jones of the 

 Marine Physical Laboratory. E. W. Werner gave valuable assistance 

 in data reduction. 



34 



