Observations and Measurements of Oeean Waves 



39 



in many important respects, they are all based upon the following principles. 

 Surface waves induce pressure fluctuations in the entire column of water 

 between the surface and the sea bottom. For any given depth of water and 

 wave height the amplitude of these fluctuations depends on the wave period 

 in such a manner that waves of very short period are virtually eliminated. 

 The pressure fluctuations at the bottom are measured by an underwater unit. 

 A slow leak placed in this underwater instrument permits the selection of 

 wave periods, as that the instrument will eliminate very long waves, as, for 

 example tidal waves. The pressure fluctuations are converted into electrical 

 modulations which are transmitted by means of a cable to a shore recorder 

 (see Fig. 24). The natural wave is modified in three stages: (a) waves of short 



Fig. 24. Wave recorder (schematic). 



period are eliminated by hydrodynamic filtering; (b) waves of very long 

 period are removed by the slow leak in the underwater unit device; (c) the 

 remaining waves are recorded and then subjected to a harmonic analysis, 

 for further study. 



The effect of the hydrodynamic filtering can be characterized by the ratio 

 Ap/Ap , in which Ap and Ap respectively are the amplitudes of the pressure 

 fluctuation at the bottom, immediately beneath the surface where h is the 

 water depth. From the equation (II. 9) it is easy to deduct that 



Ap_ 



1 



cosh*/z 



when y. = 27r/A. According to the equation (II. 7), however 



g , 



a" = 



ItiV 

 T 



tanh^/z 



and the hydrodynamic filtering effect can be computed as a function of depth 



