54 



0.25 



0.2 



0.15 



0.1 



0.05 







-0.05 



-0.1 



-0.15 



Water Surface 











/^^^.^^^v^^^ 











^/^^^ ^^^''^ 













^^ 













X^ 











>\ 



V 







Original Record 

 LFI Estimate 







Ns. 













1 





1 1 1 









1.2 1.4 1.6 1.8 



2 

 time 



2.2 2.4 2.6 



2.8 



Figure 3.18: Measured and predicted water surface near the crest of a sharp wave in 

 the flume 



clear in the pressure record; applying the LFI method to the pressure record results 

 in predictions that capture the dominant low frequencj^ mode, and miss the higher 

 frequency mode. If the higher frequency information is not in the local segment of 

 the record, it is not possible to reconstruct it. 



In the previous section, the LFI method was applied to pressure records generated 

 by Fourier steady wave theory. These records had only a single free mode, and thus 

 the LFI method was able to identify that mode. In addition, in shallow water, there 

 is little decay with depth of the wave action, and the LFI method could be expected 

 to be effective, as it was on the Fourier record. In deep water, the method can be 

 effective if the pressure gauges are located near enough to the surface. For the deep 

 water steady wave presented previously (figure 3.6) the pressure record was taken 

 from a depth of one wave height below the water surface, one tenth of the total 

 depth. At this depth, there is adequate information to identify the action at a wide 

 range of frequencies. 



Given the limitations of the data, a segment of the record was chosen in which 

 there did not appear to be a substantial high frequency component to the water 



