FORWARD SCATTERED LOW-FREQUENCY SOUND 

 FROM THE SEA SURFACE 



W. I. Roderick 



Naval Underwater Systems Center 



New London Laboratory 



New London, Connecticut 



Low- frequency propagation over long ranges can have propa- 

 gation paths that interact with the time-varying sea 

 surface. Theoretical predictions and experimental obser- 

 vations of specularly reflected CW acoustic signals indicate 

 that the long gravity waves on the sea surface modulate the 

 amplitude and phase of the incident signal. The Doppler 

 spectrum of the modulated signal consists of a discrete 

 frequency component centered at the carrier and a continuous 

 spectrum that is positioned symmetrically about the carrier. 

 The continuous spectrum consists of energy that has been 

 scattered close to the specular direction and that, when 

 summed with the specularly reflected signal produces 

 amplitude and phase modulation. A review is given of 

 important contributions to our understanding of the forward 

 scattered Doppler spectrum and its functional relationship 

 to geometrical, acoustical, and sea surface parameters. 



This paper is an informal review of one particular aspect of 

 forward scattered sound from the sea surface and that is the Doppler 

 spectrum that would be received in the specular direction. The 

 Doppler spectrum is the spectrum resulting from amplitude and phase 

 modulation of an acoustic signal reflected and scattered from a time 

 varying surface. 



About 1965, Wysor Marsh looked at two separate aspects of 

 scattering, one of which was the Doppler spectrum (Marsh and Kuo, 

 1965) . It is interesting to look at that report written 9 years 

 ago and at a time when there had been no prior direct measurement of 

 the Doppler spectrum in the specular direction. Wysor observed in 



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