56 



surface record, and thus the effect of the high frequency decay should cause few 

 problems, and the LFI method can be applied. 



While this particular segment was chosen to minimize the effects of the high 

 frequency decay, it was not possible to eliminate it entirely. Figure 3.18 shows the 

 measured water surface and the LFI predictions of the water surface in three windows 

 in the vicinity of a sharp crest in the record. The solution parameters are: J = 3, 

 To = 0.3r,, and I = N = 4. Both the pressure and the velocities were measured 

 at an elevation of 0.8m below the MWL, with the mean water depth 1.55m. The 

 LFI method located the water surface fairly accurately, although it clearly missed 

 the very sharp peak of the crest. Varying the solution parameters did not improve 

 the accuracy at the crest. In fact, a smaller window width resulted in a discontinuity 

 in the water surface between windows. The inability of the LFI method to capture 

 the sharp crest is likely due to the missing high frequency information. If the high 

 frequency part of the signal is missing from the examined pressure record, there is no 

 way to recapture that information. In order for the method to be more effective in 

 this situation, the pressure would have to be measured at a depth closer to the water 

 surface. 



Figure 3.19 shows the results of the LFI method applied to the entire segment of 

 the record from which the crest was taken. The water surface is captured fairly well, 

 as are both the vertical and horizontal velocities. 



3.6 Discussion 



The given results demonstrate the potential of the LFI method in the interpre- 

 tation of submerged pressure traces in a variety of conditions. In the case of steady 

 waves, the LFI method accurately computed the detail of the wave, using only data 

 from a small window in time. In particular, the method was able to capture the 

 pronounced sharp crest of a steep, shallow water wave. 



The method did not perform as well on laboratory records, failing to capture 

 some of high frequency detail in the water surface. This is due to the limitations of 

 subsurface pressure records, where much of the high frequency information is missing 



