200 



resolution data were collected. Once this was known, the appropriate 

 calibration for that range was applied. Because the interface could possibly 

 change ranges for even a single data point, this determination of the range was 

 performed for each data point in the time series. 



Data file format 



During the course of the two weeks that data were collected, 83 files of 

 both slow instrument data and ACP data were collected. Because several data 

 collection periods often occupied a single official wave run, the naming 

 convention for the data files differs slightly from the set standard. All data 

 are stored in the binary file format used by the Matlab software package. One 

 file in this format contains one or more variables. One data file contains each 

 of the variables listed in Table 10-3. 



Each variable starts with a fixed length 20-byte header that contains 

 information describing certain attributes of the variable. For purposes of this 

 report, the information in Table 10-4 is sufficient to describe the header and 

 the additional information found in the data file. 



Sample raw data 



Figures 10-3 through 10-6 show the uncalibrated signals from the pressure 

 sensor, pore pressure sensor, u channel of the EMCM, and OBS located 

 56 cm from the bed. The data are from about 2 min into run 12. Digital 

 values from the data logger are plotted against the sample number. Since data 

 were sampled at a rate of 4 Hz, each sample number represents a 0.25-sec 

 time increment. Spikes in the high-gain pore pressure plot, Figure 10-4, 

 result from the interface changing sampling ranges as described previously. 

 Figure 10-7 shows the ripple pattern that has been digitized from the 

 videotape after run 56. 



Uncalibrated data from the ACP are shown in Figure 10-8. The vertical 

 axis in this plot represents the digital value returned from sampling the return 

 voltage from the ACP. The sediment concentration at some location on this 

 axis is a function of the intensity of the return as well as the distance from the 

 sensor head and the concentration above the point. The bottom left axis 

 represents the distance from the sensor head, and the bottom right axis 

 represents the elapsed time from profile to profile. 



Sample interpreted data 



Figures 10-9 through 10-12 display calibrated versions of the same data 

 shown in the previous section, from run number 12, for the pressure sensor, 

 pore pressure sensor, u channel of the EMCM, and OBS located 56 cm from 

 the bed. Shown in Figure 10-13 is the ripple pattern shown in Figure 10-7 



Chapter 10 Intermittent Near-Bed Sediment Suspension 



