248 



16 resistance wave gauges, starting at Station 6, increasing with each station 

 number offshore, and ending at Station 21. The pore pressure gauges start at 

 Channel 17 and continue through Channel 26. The remaining channels are 

 devoted to electromagnetic current meter measurements. Block 11 contains 

 the first 256 data points for Channel 2. This format continues with buffer 

 number one containing blocks 10 through 73, which are the first 256 data 

 points for Channels 1 through 64. Buffer 2 contains the next 64 blocks, with 

 data points 257 through 512 for Channels 1 through 64. This pattern repeats 

 to the end of the data file. 



Sample raw data. Time series samples from a pore pressure gauge and 

 the co-located resistance wave gauge at Station 9 are shown in Figures 12-7 

 and 12-8. The time series are for the first 80 blocks or 21 min of Run 

 A0618A. Wave conditions for the run were a TMA spectrum with a peak 

 period of 3.0 sec and a zero-moment wave height of 0.6 m. The unsealed 

 time series (Figures 12-7 and 12-8) identify the randomness of the 

 measurements and the visual zero offset corresponding to wave setdown. 

 Channel 24 is the pore pressure gauge, and Channel 4 is the wave gauge. 



Sample interpreted data. Samples of the spectra from a co-located pore 

 pressure and wave gauges are shown in Figures 12-9 and 12-10, respectively. 

 Figures 12-9a and 12-10a represent the first 512 sec of a 2,400-sec record, 

 and Figures 12-9b and 12-10b represent the last 512 sec. Figures 12-9 and 12- 

 10 are expressed in units of fF/sec, with feet of water surface elevation for the 

 wave profile and feet of head for the pressure data. All spectra were smoothed 

 with a moving box-car filter, using 13 frequency intervals in the averaging 

 process. Total energy beneath the spectral density curve E is noted in the 

 upper right-hand corner of each figure. 



Comparison of Figures 12-9 and 12-10 shows a considerable reduction in 

 pressure head transmitted into the seabed. Energy is reduced by more than a 

 factor of four, reducing the amplitudes by more than a factor of two. This 

 behavior reflects pressure attenuation both in the water column and within the 

 sediment. 



Comparing the beginning and ending of each run (Figure 12-9, a to b, 

 Figure 12-10, a to b), a small reduction in total energy is noted. The 

 reduction is due to the effect of terminating the wave generator motion while 

 continuing to sample waves. A small but noticeable increase in low-frequency 

 energy occurs from the beginning to the end of the run. The seiching mode 

 amplifies in time, and this is reflected in both records. Note that the seiching 

 mode represents a greater fraction of the pressure energy density because 

 higher frequency pressure components are attenuated in the water column and 

 sand bed, leaving the low-frequency component as a more significant fraction 

 of the distribution. 



Summary of data characteristics. Tabular values of the mean and stan- 

 dard deviation of the two time series are presented in Table 12-6. Statistics 

 are computed in 256-sec blocks. A reduction in both statistics at the end of 



Chapter 1 2 Pore Pressure and Sediment Density Measurements 



