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with standard 5V4-in. (13.3-cm) U-brackets. The seats were attached to an 

 aluminum cross brace with Ve-in. (1.59-cm) bolts that allowed for the precise 

 alignment of the instruments with respect to each other. The pressure sensor 

 was situated between the ACP and BASS seats and was affixed to the cross 

 brace using two plastic cable ties, with the transducer located just above the 

 midpoint of the ACP cylinder. 



The aluminum cross brace was in turn attached to a section of 1-in. 

 (2.54-cm) galvanized steel pipe which ran up to the offshore moveable cart. 

 The pipe was held in place with two 514-in. (13.3-cm) U-brackets at a 

 particular vertical level depending on the cart location along the length of the 

 channel. Guy wires, to prevent cross -channel instrument movement, were 

 attached to the aluminum cross brace and ran diagonally toward the side walls 

 to the bottom offshore corners of the offshore moveable cart. To prevent on- 

 offshore movement caused by the waves passing through the instrument 

 mounting assembly, another section of 1-in. (2.54-cm) galvanized steel pipe 

 was attached to the vertical steel pipe near the cross brace and connected to 

 the lower offshore edge of the cart. 



All of the extra apparatus such as PC's, power supply, oscilloscope, mag- 

 netic tape drive, and other tools were located close to but outside the channel 

 wall, between Stations 18 and 19. 



instrument characteristics 



ACP. Because the ACP is designed as a depth sounder, it is specified to 

 have a range of 6 to 10 m; however, use of the instrument in the presence of 

 suspended sediment causes a high attenuation of its acoustic signal with dis- 

 tance from the transducer face, and thus limits the range of the instrument. 

 Close to the transducer, however, the so-called near-field response to reflec- 

 tors is quite sensitive. The reflected response of scatterers in each range bin 

 to the transducer face can be modeled to convert acoustic signals to sediment 

 concentrations, and the conversion process is discussed in the data analysis 

 section of this chapter. 



An external gain control of the receiver pre-amplifier allows for fine-tuning 

 of instrument response to optimize for different sampling environments. The 

 gain is adjusted to ensure that reflection from the sediment-water interface 

 registers a full-scale output while maintaining statistically significant output 

 from suspended particles within the water column sampling volume. The 

 resulting output is a 0- to 5-V analog signal time trace, which is eventually 

 digitized into 8-bit, 256-increment (hexadecimal 00 to FF) reflected intensity 

 values and fed directly onto the data bus of the system microprocessor. 



BASS. The BASS velocimeter package contains its own microprocessor to 

 control the sampling process for all 16 transducer pairs of the four vertically 

 aligned BASS sensor array frames. Each measurement along a transducer pair 

 axis takes 100 msec to cross the sampling volume. Because one measurement 



Chapter 9 The Ohio State University Measurements 



