K = e " ±LL f 2 

 Z g 



Figure A2 presents the wave height attenuation as a function of frequency alone 

 for depths of 75 and 100 feet. 



Thus, for a low frequency swell of .05 cycles per second (20-second period) only 

 20 percent of the surface wave height will be attenuated at 75 feet, and only 26 per- 

 cent will be attenuated at 100 feet. If the submarine behaves as a particle, the 

 heave amplitude of the neutrally buoyant, hovering submarine should be something 

 like a simple harmonic motion in phase with the simple harmonic wave at the surface 

 (neglecting for the moment, all other components of submarine motion). In any event, 

 a correction must be applied to either the original data or possibly to the spectrum. 



In general, the submarine will execute motion in all six degrees of freedom. The 

 effect of pitching and rolling will tend to increase the path length over which the sound 

 beam has to travel; this tends to make the recorded wave motion appear higher than the 

 actual wave motion, which overcomes some of the difficulty mentioned in the preceding 

 paragraph . 



Both these difficulties can be ameliorated If submarine data are recorded at a depth 

 of 200 or even 300 feet. However, with increased depth, there will be some "skipping" 

 of data since, occasionally, a return echo will be too weak to gate the time interval 

 counter . 



52 



