Experiment Procedures 



Sequence of events 



Data were collected from the instrumentation using two independent 

 computer systems because of the high speed of collection and the large 

 quantity of data generated by the ACP. In a typical data run, the collection 

 software on the AT computer was started 15 sec before data collection was to 

 begin. The AT then signaled the second system, Onset Computer Tattletale 

 Model 6 data logger, that collection was to begin. After this original 

 synchronization, the two systems ran independent of one another. The data 

 logger then turned power on to each of the slow instruments, which consisted 

 of the EMCM, both OBS, the pressure sensor, and the pore pressure sensor. 

 This allowed almost 15 sec for the instruments to warm up. During the 

 warm-up period, headers to the data files were created which specified the 

 starting and ending times of collection and the number of points to be 

 sampled. After the warm-up period, data collection from the ACP began on 

 the AT, and, simultaneously, data collection from the slow instrumentation 

 began on the data logger. On both systems, data were collected and stored in 

 memory until the end of the data run, at which point data were saved to disk. 



Calibration and zero reference 



The calibration facility of the ACP and OBS sensors consists of a cylinder 

 and a circulation system, as seen in Figure 10-2. The cylinder is made of 

 Plexiglas and has four jet holes on the wall at its top and a funnel at its 

 bottom. The circulating system consists of a centrifugal pump, circulation 

 tube, and four jets. The pump circulates a sand-water mixture to create as 

 uniform a sand concentration in the cylinder as possible. The jets help make 

 the concentration more uniform. The funnel prevents accumulation of sand at 

 the bottom. 



The total volume of water is measured. Then dry sand is weighed and 

 added to water in an accumulative manner to create from low to high 

 concentration while the pump circulates the mixture continuously. 



The relationship between the backscattered acoustic intensity and the sus- 

 pended sediment concentration is expressed by the following nonlinear equa- 

 tion 



A C(z) = V(zY exp 



f k t * *, C(0 dC 



(10-2) 



192 



Chapter 10 Intermittent Near-Bed Sediment Suspension 



