zones ranging in frequency* from 0.049 to 0.270 cycle per minute, 

 which is equivalent to a period of 20.4 to 3. 7 minutes or a wave- 

 length of 2. 04 to 0. 37 nautical mile. When the peaks are wide 

 they are considered zones; for example, 0. 090 to 0. HO cycle per 

 minute (or 1. 11- to 0. 91-mile wavelengths) and 0. 182 to 0.200 

 cycle per minute (or 0.55- to 0.50-mile wavelengths) are con- 

 sidered zones or wide bandwidths. 



The power spectrum curve shows that the greatest power 

 is in the low frequencies without peaks. The number of degrees 

 of freedom is given by v = — - -|- . When 1440 consecutive depth 

 sample values and 144 lags are used, v - 19.5. The correspond- 

 ing ratio of computed to true value falls between 0. 54 and 1. 60 

 for 90-percent confidence limits. 



The ratio of background to peak height was determined by 

 constructing a base line and vertical height (fig. 27). For 

 example, the 3. 7 -minute -period peak of the 19° isotherm depth 

 has a peak-to-background ratio of 82 to 50, or 1. 64, which is 

 significant, whereas the 9. 1 -minute -period peak in the peak 

 zone (fig. 27) has a ratio of 1.38, which is not significant. 



DIRECTIONAL POWER SPECTRUM 



To summarize the power spectrum, the peaks and peak 

 zones were read from the individual power spectrum graphs, and 

 the peak-to-background ratio was computed and listed (fig. 28) 

 for the data sections with the isotherm in the thermocline and 

 (fig. 29) for data sections with the isotherm below the thermocline. 



*Here the sampling chain is moving through a quasi -stationary 

 field of internal waves and the frequencies discussed are fre- 

 quencies of encounter. The wavelengths are nominal and com- 

 puted from the ship speed of 6 knots, assuming that the internal 

 waves are essentially stationary, i.e. moving much slower than 

 6 knots. Broad peaks (or peak zones) should be expected as often 

 as narrow peaks if the internal waves are traveling in all direc- 

 tions; e.g. if internal waves of only a very narrow band of 

 frequencies arrived from all directions, the straight track of the 

 ship would intercept apparent wavelengths corresponding to a 

 broad band of frequencies. 



56 



