Besides the spectral information, statistical information pertaining to the 

 spectrum is presented. Segments refers to the number of individual segments 

 processed. The run length (sec) is the total time length of the run in seconds. 

 Iskip refers to the number of data points skipped between selected data points, 

 e.g., 1 means every data point is analyzed, 2 means every other data point is ana- 

 lyzed, etc. This option allows for a choice in effective sample rate. The effec- 

 tive sample rate is output next. The segment size is the number of data points in 

 each segment based on the effective sample rate. The number of degrees of freedom 

 is useful in calculating the confidence level of the results. The statistical 

 bandwidth is also presented. Finally, the mean and standard deviation of the time 

 history are presented for each channel. Also, the standard deviation of the 

 spectral data for each channel accompany those of the time history. 



The second display, as seen in Table 2, presents the spreading, based on any 

 of the three earlier mentioned techniques. The energy is distributed by direction 

 and frequency. The data are variances relative to direction, but are densities 

 relative to frequency. This gives the units of length-squared/hertz or length- 

 squared-seconds. An optional method of viewing a portion of the results is 

 available in graphic form, as seen in Figure 6. This is operational for frequency 

 versus energy densities, mean direction and rms spreading. The graphic output of 

 energy spreading of Table 2 is not yet operational. 



DISCUSSION 

 LINEAR WAVE THEORY AND NONLINEAR EFFECTS 



One of the assumptions made during the process of data analysis of the 

 Wave-Track buoy is the linear wave theory. The buoy responds to the wave par- 

 ticle velocities beneath the water surface and traces the particle displacement. 

 From the linear wave theory, the rate of change of tilt angle can be related to the 

 surface elevation as indicated in Equation (32). 



The verification of linear theory of wave particle velocities has been 

 reported in papers based on the results of field investigations. '' ' According 

 to a recent paper by Battjes and Heteren, the particle velocity, based on linear 

 theory predictions from measured surface waves and the direct field measurements 

 of particle velocity agree within +5 percent in both amplitude and phase. The 

 agreements are validated for various sea states such as wind waves from young seas 

 to old swells. However, no agreement in range has been found for other sea states 



16 



