sonar equation. Three components of the acoustic signal pathway must be 

 modeled: the incident beam from the transducer face to the scattering field, 

 the backscattered signal from the scatterers to the transducer, and the 

 attenuation of the signal during each transit. The first two components can be 

 approached from known solutions obtained by acoustic wave theory and are 

 geometry-dependent. The attenuation component is the result of both the 

 medium (parameterized by the temperature, pressure, viscosity, and fre- 

 quency) and the effect of signal dispersal due to the density of scatterers. 

 Effects of noise from the reflected signal and the processing hardware of the 

 instrument must also be taken into account. The model has been verified with 

 test data supplied by the instrument manufacturer as well as laboratory and 

 field data. 



BASS. Any calibration of the acoustic sensors of the BASS system is done 

 by adjusting the integrator current sources on the microprocessor board within 

 the BASS electronics package and was performed by the manufacturer prior to 

 shipping. The BASS velocity interpretation is self-calibrating due to the 

 redundant acoustic interpretive method it employs. Final interpretation of 

 BASS data depends on shifting recorded values by capacitive zero offsets. 

 Zero offsets are determined by taking several minutes of data in still water, or 

 because still water is elusive in field conditions, taking a "bagged" zero offset 

 by wrapping the BASS frame and transducers in an isolating sheathing, such 

 as plastic, which can still the water within the sensor sampling volume. These 

 should be obtained prior to or after any BASS deployment. Zero offsets 

 obtained before and after SUPERTANK are used in the data conversion and 

 analysis section of this report. 



Pressure sensor. Internal calibration of the pressure sensor is performed 

 by the manufacturer at the factory, and there is no need to obtain any offset 

 values. Measurements of pressure values from calm periods before and after 

 each wave run were stored and examined. This provides an exact reference of 

 the instrument position below the still-water level, and thus the water depth, 

 since the position of the instrument array was varied along the length of the 

 channel. 



Sampling and time reference 



ACP. The controlling circuitry of the ACP is specified to sample profiles 

 at rates up to 200 Hz, but at SUPERTANK, 32 complete profiles (110 range 

 bins each) were obtained each second. The 16-msec sampling of each vertical 

 increment of the profile dictates the 1-cm height of the range bins, and the 32 

 values for each bin are averaged providing one complete profile per second. 

 The purpose of averaging is to minimize configurational noise, due to the 

 changing orientation of individual scatterers within the insonified volume. 

 Based on commonly encountered flow rates it has been judged that the 1-Hz 

 stored data are adequate to reveal appreciable changes in concentration. The 

 value of 32 Hz as the actual sampling rate was chosen as a compromise 



Chapter 9 The Ohio State University Measurements 



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