Values of ef for the South Haven and South Pass records are an order of 

 magnitude larger than the modal frequency spacing between constituents. For 

 these records, 2 ef is a better indicator of the range of frequency covered 

 by the high energy part of the spectrum. However, these records represent 

 actively growing waves. The BF instability operates over a range of frequen- 

 cies to a separation /2 ef on either side of the carrier frequency, f . 

 It is possible that the observed constituents in the field records are partic- 

 ipating in resonant exchange of energy fueled by a BF-type instability of the 

 carrier. Since the waves are actively growing, it is quite possible that 

 the instability is being observed in an early stage, before the most unstable 

 mode has dominated as prominent sidebands spaced ef on either side of the 

 carrier. 



Well-developed BF-type sidebands should certainly be present in the 

 Columbia Light swell records if the mechanism is operating. Although the 

 frequency structure is fairly consistent wLth expectations, the amplitude 

 structure is not. The mechanism does not account for the observed large and 

 rapid shifts in energy between constituents. 



Although the field data do not clearly follow all aspects of BF theory, 

 the preceding discussion and the modulation information shown in Figures 40 to 

 43 provide evidence that BF theory gives useful insight on the characteristics 

 of some field records. It is submitted that the evidence is more than 

 circumstantial and that detailed spectral shape may be partially explained by 

 the BF theory (hypothesis c) . 



VII. SUMMARY 



Wave measurements are examined from three relatively deepwater field sites 

 in Lake Michigan, the Pacific Ocean, and the Gulf of Mexico. Approximately 1 

 hour of data representing high waves, unimodal spectra, and nearly constant 

 significant height and peak, spectral period was selected for each site. The 

 data represent actively growing waves at two sites and swell at the third 

 site. Record lengths for analysis vary from 512 to 1,200 seconds. 



Analysis is done in both the frequency and the time domain. The FFT spec- 

 tral analysis procedure is shown to possess limitations for resolving details 

 of the distribution of energy as a function of frequency and for identifying 

 correct phase values for each frequency component. Phases returned by FFT 

 procedures are shown to be subject to erratic variations. These variations 

 are believed to have led previous investigators to conclude that phases are 

 random. 



Shortcomings of the FFT spectral analysis procedures are circumvented by 

 using a MRS technique to identify major frequency constituents. The MRS 

 technique has been used in published meteorological studies and in at least 

 one unpublished laboratory wave study; however, the current study is believed 

 to be the first in which the MRS technique is applied to field wave records. 



Time domain analyses of the field records are focused on extracting wave 

 grouping information directly from the time series. A wave group is concep- 

 tualized as a small area of sea surface containing relatively high energy. 

 Groups are identified as sections of the time series in which the local 

 variance is high relative to the variance of the whole record. Fluctuations 



70 



