Title: Acoustic measurement of suspensions of clay and silt particles using single frequency attenuation and backscatter Authors
Submitted to: Applied Acoustics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 2014
Publication Date: November 1, 2014
Citation: Carpenter, W.O., Goodwiller, B.T., Chambers, J.P., Wren, D.G., Kuhnle, R.A. 2014. Acoustic measurement of suspensions of clay and silt particles using single frequency attenuation and backscatter. Applied Acoustics. 85:123-129. Interpretive Summary: Sediment measurements are important for many areas of stream and river management, such as detecting changes in watershed erosion and evaluating the effects of mitigation efforts. In many streams, suspended sediment transport is dominated by a few significant storms each year. These flood events are hard to predict and frequently occur at night, making it difficult and expensive to collect physical sediment samples. Traditional sampling techniques, including manually deployed isokinetic samplers and automatic pumping samplers, yield samples that are widely spaced in time and small in number. Ultrasonic measurement systems have the potential to measure the concentration of particles with a high degree of both spatial and temporal resolution, making them ideal for addressing the needs of those who rely on sediment data. In the work described here, high frequency acoustic signals are used to estimate the concentration of sediment suspended in water. The goal of the work is to be able to use low-cost acoustic devices to monitor sediments in streams, reducing the need for personnel to be on site during inclement weather and drastically improving the quality of sediment concentration data by allowing for it to be recorded continuously over time.
Technical Abstract: The use of ultrasonic acoustic technology to measure the concentration of fine suspended sediments has the potential to greatly increase the temporal and spatial resolution of sediment measurements while reducing the need for personnel to be present at gauging stations during storm events. The conversion of high-frequency attenuation and backscatter amplitudes to suspended silt and clay concentration has received relatively little attention in the literature. In order to improve the state of knowledge, a laboratory investigation was undertaken by the National Center for Physical Acoustics in cooperation with the USDA-ARS National Sedimentation Laboratory. In these experiments, two immersion transducers were used to measure attenuation and backscatter from 20 MHz acoustic signals propagated through suspended clay (smectite and kaolinite) and silt particles. The resulting data includes attenuation values for a wide range of concentrations (0.3 – 14 g/L) and particle sizes (0.011 – 20 micron diameter). Attenuation curves for each particle were compared to the theoretical attenuation curves developed by Urick (1948) and Sheng and Hay (1988) for scattering as presented by Landers (2010). In addition, it was found that the backscatter signal could be used to discriminate between suspensions dominated by clay or silt.