events due only to wave activity could be isolated from the turbidity present 

 even during periods of calm. In addition to determining the background 

 concentration during these periods, the still-water level was determined using 

 the mean return from the pressure sensor. Knowledge of the water level 

 allowed accurate determination of wave energy spectra from the calibrated 

 pressure signal, as described in the analysis portion of this chapter. 



Calibration of the pressure sensor was accomplished by collecting data 

 from the sensor at different gauge-measured pressures of compressed air. The 

 EMCM was calibrated by the manufacturer prior to the experiment, so cali- 

 bration constants were provided by the manufacturer. 



To collect pore-pressure measurements with the maximum resolution the 

 instrument was capable of providing, an interface was designed which allowed 

 measurements to be made over five software selectable ranges. A low- 

 resolution sample from the sensor was used to determine the appropriate range 

 for high-resolution measurement. Each of the selectable ranges, as well as the 

 low-resolution portion of the interface, were calibrated separately by varying 

 hydrostatic pressure. 



Sampling and time reference 



As stated previously, two separate data acquisition systems were used for 

 data collection. The first system consisted of a high-speed data acquisition 

 card in a 386 AT computer. Data from the ACP were collected on this 

 system at a maximum rate of 250 kHz. As many as 512 data points were 

 collected at 250 kHz, these points making up a single profile. At this rate, 

 each data point, or bin, in the profile represented the concentration over a 

 vertical section 3 mm in height. It should be noted that, although the 

 sampling rate allows a maximum resolution of about 3 mm per bin, the actual 

 resolution is further limited by the pulse width of the ACP. Away from the 

 bed, where high spatial resolution was not necessary, four adjacent bins were 

 averaged reducing the maximum spatial resolution of the concentration to a 

 vertical section 1.2 cm in height. This spatial averaging was performed in 

 most of the data runs. Bins close to the bed were not spatially averaged. In 

 addition to spatial averaging, each set of 25 consecutive profiles was av- 

 eraged, finally yielding four profiles per second averaged in both time and 

 space. 



The first data acquisition system served as a master system, in that starting 

 data collection on this system also simultaneously started acquisition on the 

 second system. The second system was the Tattletale model 6 data logger 

 sampling data from two OBS, one pressure sensor, one pore pressure sensor, 

 and one EMCM, at 4 Hz. 



Chapter 10 Intermittent Near-Bed Sediment Suspension 



195 



