characteristics. The filtered spectrum is then inverse Fourier transformed to 

 generate a filtered time series. 



65. A clear danger in this method is that the possibility arises to 

 remove enough of the spectrum that insufficient energy- containing spectral 

 lines remain to recover enough of the wave height distribution to have 

 reasonably good statistics. This is especially true where the parent time 

 series is short so that the raw spectral frequency bands are wide. Figure 3 

 indicates that, for spectra that are not too broad to begin with, at least 

 40 lines must remain to obtain h'^'''°' to within about 2 percent and at least 

 150 lines must remain to obtain hU/ioo) ^^ within about 5 percent. 



67. There may be advantages to spectral filtering as well. If there is 

 truly noise in the time series such that the spectrum becomes broad in its 

 presence, then filtering can remove the noise and make the spectrum narrower. 

 If such a filter reduces spectral width from an equivalent Af/f^ of order 

 1.0 to something less than about 0.5, a clear improvement in all statistics is 

 suggested by Figure 3. 



40 



