154 



a summary, and further data are presented in Appendix I (contained in Vol- 

 ume II of this report). 



Experiment Apparatus 



Each instrument used during SUPERTANK is physically described along 

 with an explanation of its principle of measurement. The instrument system 

 was a laboratory version of the ARMS field data collection package designed 

 and developed at OSU. ARMS, its components, and its operating principles 

 have been described previously by Bedford (1989) and van Evra and Bedford 

 (1992). 



Instrument description 



Acoustic Concentration Profiler. The Acoustic Concentration Profiler 

 (ACP) is a device used by the OSU Coastal Engineering Laboratory to obtain 

 high-resolution vertical profiles of suspended sediment in the water column up 

 to 1.5 m above the bottom. Originally designed as a high-resolution depth 

 sounder by the Edo Corporation (1981), the instrument has been modified to 

 enhance performance near the transducer and allow for range-gating of the 

 near-field reflected acoustic signal. These modifications include a diamond- 

 shaped ceramic transducer which produces an extremely narrow beam width 

 and reduction of side lobes which are present in all instruments of this type. 



The ACP transducer and electronics are contained in a 29-cm-long by 

 14-cm-diam cylindrical pressure housing made of stainless steel, shown in 

 Figure 9-1. One endcap has the acoustic transducer mounted into its external 

 face within a protective rubber potting compound which allows the transducer 

 to vibrate without excessive flexure. The other endcap is attached to an inter- 

 nal circuit chassis which holds three circuit boards containing the transmitting 

 and receiving electronics. On the outside of this endcap is a 10-pin underwa- 

 ter electrical bulkhead connector which links the instrument via an underwater 

 cable to control and signal-conditioning hardware. The external hardware 

 supplies power, triggering, and gain control to the ACP, receives the analog 

 signal output from the ACP, and processes the data for presentation to a stor- 

 age device. 



The ACP operates by transmitting a 3-MHz acoustic pulse, which inson- 

 ifies a narrow conical cross section of the water column from the face of the 

 transducer to the bottom sediments. Immediately after the pulse has been 

 transmitted, the transducer is internally switched over to a receiving circuit, 

 which allows the transducer to act as a hydrophone. Detected return signals 

 are amplified and filtered internally, and the resulting analog voltage trace is 

 sent out of the ACP to the conditioning hardware. Output signals are then 

 digitized according to the distance travelled, or range gated, 110 times, thus 

 partitioning them into volumetric bins approximately 1 cm in depth. 



Chapter 9 The Ohio State University Measurements 



