RODERICK: FORWARD SCATTERED LOW- FREQUENCY SOUND FROM THE SEA SURFACE 



measurements of the relative energy in the Doppler spectrum with 

 supporting oceanographic data. You could fill a bookcase with the 

 various theoretical models that are available in predicting scattered 

 sound from the sea surface. However, I cannot determine whether the 

 present models are adequate enough to produce the absolute value of 

 the coherent as well as incoherent energy of the Doppler spectrum. 

 To my knowledge, there has not been any deep ocean measurement of 

 the absolute value of the Doppler spectrum. Lastly, there are 

 measured asymmetrical sidebands obtained by various experimentalists — 

 theoretical predictions should be made to compare to these results. 



In the remaining time, I would like to give a brief overview of 

 an experimental program in measuring bistatic reverberation from the 

 sea surface presently being conducted in the Block Island-Fishers 

 Island Range. A parametric source (Figure 14) is being used to 

 generate a narrow beam of acoustic energy incident on the sea surface. 

 The source characteristics of wide bandwidth, narrow beam width, and 

 no sidelobes result from the array of virtual sources created by 

 the nonlinear interaction of the acoustic waves in the water medium. 

 The reverberation from the surface is received on the vertical array 

 of transducers and the information is cabled to shore. As you may be 

 aware, the reverberation Doppler spectrum is a function of the sea- 

 surface directional wave spectrum. To obtain an estimate of this 

 spectrum, an array of five upward-looking transducers is used to 

 measure the wave height as a function of time. The near-field 

 characteristics of the transducers are used to isonify a small spot 

 on the surface. The transducers are positioned to obtain equi- 

 spaced cross- power spectral-density functions. 



A typical beampattern of the parametric source measured at a 

 difference frequency of 7 kHz is shown in Figure 15. Note the absence 



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