600 



400 



200 - 



'- / 

 / 



• South Haven 

 o Columbia Light 

 « South Pass 



200 



400 



( s ) 



600 



Figure 43. Comparison of modulation period estimated from spacing 

 between MRS constituents and from wave steepness. 



The three Columbia Light data points in Figure 43, which fall closest to 

 the 45° line, are from the 1300- and 1508.5-hour records. The corresponding 

 field data time series show evidence of the approximate 400-second modulation 

 indicated by Tj^pg. The three Columbia Light data points with Tw^g approxi- 

 mately equal to 150 seconds are from the 1500-hour record. The corresponding 

 field data time series shows clear evidence of this modulation time scale and 

 no evidence of 400-second modulation. However, the 1508.5-hour record has 

 some evidence of 150-second modulation. Values of Tj^mg from South Pass are 

 consistently between about 100 and 200 seconds, with one exception. This mod- 

 ulation time scale is consistent with prominent groups in the field data time 

 series. Similarly, Twgc values from South Haven are reasonably consistent 

 with prominent groups in the field data time series. 



From an examination of cases in which wave grouping is most clear, it is 

 evident that the MRS analysis represents prominent modulational time scales by 

 identifying two or more high-amplitude constituents separated by a frequency 

 equal to the reciprocal of modulation period. Thus, the extent of wave group- 

 ing and the modulation period are related to the energy content and frequency 

 spacing of dominant frequency constituents in a record. The evidence supports 

 hypothesis (e), with the provision that spectral resolution is sufficient to 

 identify constituents involved in the modulation process. Figures 16 to 29 

 show clearly that a 0.011-hertz bandwidth spectrum masks information on 

 modulation time scales. 



64 



