Measurements Of Fluvial Sediments Using A Single-Frequency Acoustic Attenuation System
Suspended sediments are a global-scale pollutant that threaten the integrity of our waterways and pose an environmental threat to many aquatic ecosystems. Innovative techniques are required to continuously monitor this pollutant. Ultrasonic acoustic measurements have the potential to improve the temporal resolution of fluvial sediment data. This work investigates the use of a prototype field grade single-frequency acoustic attenuation system that was developed at the National Center for Physical Acoustics (NCPA), located at the University of Mississippi. This field-deployable unit consists of two 20 MHz immersion transducers that are aligned in a "pitch-catch" configuration at a fixed distance of 18 cm. A graphical user interface was designed in LabVIEW® to allow users to control the input and output parameters. The received (attenuated) signal is digitized by a digital signal processing (DSP) board that is fixed within the system. The acoustic attenuation (DSP) system was calibrated in a recirculation tank at the NCPA. The calibration results were compared with experimentally verified attenuation data recorded by Carpenter Jr. et al. (in press). A calibration curve valid for silt-sized particles (D50 = 20 μm) was developed. Following the calibration, the acoustic device was deployed at Harris Bayou, located in Coahoma County, Mississippi. The transducers were submerged in the bayou, and continuous in situ measurements of fine suspended sediments were conducted. The data recorded in the field were used in an empirical conversion scheme to determine suspended sediment concentration. After the field data were evaluated, it was hypothesized that changes in water temperature were drastically affecting the acoustic response of the DSP system. A temperature calibration was conducted to verify the hypothesis. The acoustically predicted SSC data were compared with grab samples taken in predetermined intervals throughout the data collection period. Statistical methods were used to validate the accuracy of the acoustic prediction. The DSP system shoa high level of reproducibility in laboratory experiments. However, field results were not conclusive, and the acoustic predictive model did not agree with the grab samples. It was shown that additional parameters, such as water temperature, must be considered for use in field applications.
Acoustic, Attenuation, Fines, Fluvial, Sediment, Ultrasound, Civil Engineering
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