A computer time standard system similar to that in the OBS data collection 

 system was installed in the computer used to collect all of the capacitance 

 wave gauge signals. At the beginning of each run the clock was to be 

 sampled to provide the exact time at which the data collection began. Using 

 this initial time and assuming that the interval between samples was 1/16 sec, 

 the time each data point was collected could be determined. Unfortunately, 

 there was a problem with this procedure, and the time which appears in the 

 header of the file is not the time of the first sample. An accurate method to 

 determine the exact time at which data collection for the wave gauge began 

 has not yet been identified. 



Pre-processing and filtering. Hardware filtering of the OBS signal was 

 performed to remove high-frequency electrical noise on the data lines. The 

 A/D units, which represent the line voltage at the converter, were not 

 processed in any way before being sampled and saved to disk. 



Data Analysis 



Reduction and interpretation methods 



Original data files were written to the hard disk of a PC and then transfer- 

 red to cartridge tapes. These files contained 35 columns of data. The first 

 column was the time for each sample in seconds from midnight, and the sec- 

 ond was a ground reference channel. The next 28 columns were data from 

 stationary OBS arrays, and the final five channels were the data from the 

 mobile OBS array. 



The first step in the data reduction process was to convert the files from 

 binary to ASCII format. Data for the mobile OBS array were then converted 

 from A/D units to scientific units using the calibration constants presented in 

 Table 7-1. The conversion was performed by: (a) dividing the raw data R (in 

 A/D units) by 204.8 (volts per A/D unit), (b) multiplying by the gain G 

 (g/C/V), and (c) adding the offset O iglt). A mean background concentration 

 C B was determined from the data collected prior to the start of the run under 

 still-water conditions using the above procedure. This mean background 

 concentration was subtracted during the conversion procedure so that all 

 sensors would have the same zero reference, so converted data are given by: 



Cte/«) = G — — + O - C B (7-1) 



w 204.8 * 



Converted data files were created containing six columns of data, com- 

 prised of the time and the data from each of the five sensors in the mobile 

 OBS array. These files had the same name as the raw data file with the ex- 

 tension changed to .CON, and the files were subsequently saved on a single 

 tape. 



Chapter 7 Sediment Suspension Measurements from a Platform 



129 



