Abstracts

Monitoring fine suspended sediment in streams using high frequency acoustic signal attenuation measurements

Author(s): Kajdan, T.; Carpenter Jr., W.; Chambers, J.; Goodwiller, B.; Surbeck, C.

This research focuses on a new device to measure fine suspended sediment particles in rivers and streams using acoustic signal attenuation measurements. The purpose of this device is to create a high frequency signal attenuation system that will be able to measure concentrations continuously without having to physically visit the site in order to obtain the samples. This system will be non-intrusive as well as relatively cost-effective in comparison with the methods that have previously been adopted. Two 20 MHz transducers will be aligned in a pitch-catch configuration at a distance based upon near field effects and noise level caused by the interaction between the transmitting and receiving transducers. The 20 MHz acoustic device will be able to measure particles ranging from 0.2 to 65 µm in size. A vast attenuation database has been created at the National Center for Physical Acoustics (NCPA), located in Oxford, Mississippi, to account for this range of particle sizes. The database includes acoustic signal attenuation measurements of bentonite, kaolinite, illite, and silt, which were used to broaden the sediment range. The database consists of peak-to-peak voltage data that was captured by the receiving transducer during the experiment and displayed and analyzed using LabView 2010. This voltage data corresponds to known particle concentrations. A digital signal processing (DSP) board will be used to process the data obtained from the 20 MHz transducers in the field. The attenuation signal that is measured in the field will be compared with the database so that a sediment concentration can be determined. The 20 MHz acoustic device will be calibrated at the NCPA using the attenuation database. Once the calibration is complete, the 20 MHz acoustic device will be deployed at Harris Bayou, located in Coahoma County, Mississippi. This particular bayou has relatively steady base flow, which is ideal for the acoustic device. The steady base flow of this bayou ensures that the signal produced by the transmitting transducer will not reflect off of the surface of the water causing inaccurate attenuation data. The digital signal processing board will be mounted in the gauging station housing that is currently being used by the United States Geological Survey (USGS). The results that are obtained from the field will be compared with concentrations that the USGS finds from its grab samples.

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