Location: Watershed Physical Processes ResearchTitle: Acoustic Measurement of Suspended Fine Particle Concentrations by Attenuation) Author
Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 11/20/2009
Publication Date: 12/1/2009
Citation: Carpenter, W.O., Chambers, J.P., Wren, D.G., Kuhnle, R.A., Diers, J.A. 2009. Acoustic Measurement of Suspended Fine Particle Concentrations by Attenuation. USDA ARS national Sedimentation Laboratory Research Report No. 67, 62 p. (Funded research project completion report) Interpretive Summary: Knowledge of sediment concentration is important in the study of streams and rivers. Small particles, such as those in the clay size range, can make up a substantial fraction of the transported material. Acoustic measurements make continuous monitoring of sediment transport possible, without the need for personnel on site at streams during storms, at night, on weekends, etc. The current paper describes measurements of the amount of acoustic signal lost due to various amounts of clay in water. This information will be used to design and calibrate an acoustic system to be used for measurements in the field. As a result of the work, the range for the transmit and receive transducers will be 18 cm, allowing good sensitivity to low concentrations and ability to measure high concentrations of suspended fine particles. This work is a step towards a comprehensive acoustic system for remote, autonomous measurement of both sand and finer particles transported by streams and rivers.
Technical Abstract: Knowledge of sediment concentration is important in the study of streams and rivers. The work presented explores the appropriate frequency and transducer spacing for acoustic measurement of suspended particles in the range of 0.1 – 64 microns. High frequency (20 MHz) acoustic signal attenuation was used to measure the concentration of fine sediment particles in a laboratory environment. A small tank with a pitch-catch transducer configuration was used to measure the attenuation caused by kaolinite, bentonite, and silt concentrations over a range of distances (180 – 357 mm). As much as a 35 dB change in signal level was observed over a range of kaolinite and bentonite concentrations (1 – 14 g/L) and silt concentrations (1.5 – 5.5 g/L) over the 180 mm to 357 mm range. Additional measurements to assess the limits of detection of the setup were performed at 180 mm for kaolinite concentrations (0.01 – 0.7 g/L), bentonite concentrations (0.01 – 0.7 g/L), and silt concentrations (0.006 – 0.7 g/L). The data suggest that a fixed distance of 180 mm between the transducers will be capable of measuring a wide range of concentrations. The ultimate goal of this project is the fabrication and deployment of an autonomous instrument for field studies of fine sediments transported by streams and rivers.