Estimation of suspended sediment concentration by acoustic calibration and scattering properties: An experimental and theoretical analysis

Sedimentation is a primary and growing environmental, engineering, and agricultural issue around the world. As the needed data collection to develop solutions to sedimentation issues has declined, acoustic suspended sediment have been used to potentially obtain sediment data more accurately, with higher spatial and temporal resolution, and with less intensive, costly, toilful and dangerous methods. However, the extraction of suspended sediment parameters from acoustic data requires an understanding of the interaction of the sound with a sediment suspension and an inversion methodology. This inversion requires a description of the intrinsic particles scattering properties, which are usually quantified in terms of the normalized total scattering cross section and the backscatter form function, both expressing the attenuation and backscatter of the acoustic signals, respectively. This work presents a laboratory study to examine the relationship between suspended sediment concentrations (SSC) of glass microspheres, and the backscattered signals recorded by a 6MHz Acoustic Doppler Velocimeter (ADV - Nortek Vector). A mixture of glass microspheres and very fine alumina particles was also used to verify the influence of viscous attenuation in the backscattering signals, which somehow is a controversial aspect in the literature. Backscattering predictions were obtained using an inversion model based on adaptations of Beer's law considering the intrinsic particle scattering properties and the velocimeter sampling volume characteristics. Originally, this law was developed to measure the light transmission by a collection of particles. The main goal of this topic was to verify the applicability of this model to calibrate the acoustical device using only a sample of water or superficial bottom sediments. Measurements were taken using a simple experimental apparatus: the ADV was immersed in a 50 liter acrylic tank in which known concentrations of glass microspheres with medium diameters <;D> of 41, 75 and 218μm were mixed by a stirrer and a pump in distilled water to obtain a homogeneous solution. The bimodal size distribution, with one narrow peak in the size range of clay and the other in the size range of fine sand, was used to exploit the acoustic scattering and dissipation properties of the water-sediment mixture. Following the sedimentologists' common practice, for concentrations less than 1500mg/l, a region of linearity between the logarithm of concentration and time-average relative acoustic backscatter (RB) was found. The experimental data was used to develop calibrated equations of the form log(SSC)=c1*RB+c2; R
²
values for all calibrated equations were greater than 0.98, suggesting that properly calibrated relations can yield accurate time-averaged SSC measurements using Vector measured and attenuation corrected RB. With the results obtained, it is concluded that the proposed Beer inversion model can be used reasonably to estimate suspended sediments in concentrations where the attenuation does not affect the acoustic backscatter, and once a consideration have been taken on the incident acoustic intensity. The application of Hanes's formula to a velocimeter rather than a profiler does not provide good results. Lastly, the relative backscattering was found to be proportional to the logarithm of concentration, indicating that the velocimeter is a useful tool for measuring suspended sediment concentration.