Applicability of Acoustic Concentration Measurements in Suspensions of Artificial and Natural Sediments Using an Acoustic Doppler Velocimeter

Abstract

For the investigation of turbulence and particles, interaction measurement systems are required, which are able to measure velocity and concentration fluctuations simultaneously. Acoustic Doppler Velocimeters (ADV) are widely used for velocity and turbulence measurements in natural and artificial flows. Based on the acoustic sonar theory, a model is presented that correlates the ADV’s Signal-to-Noise Ratio (SNR) and the suspended solids concentration of several natural (Ems Estuary, Lake Eixendorf, Lake Altmühl) and artificial sediments (Chinafill, quartz powder, bentonite, metakaolin) for the range 0.001 g/L–50 g/L. Within the presented method, the sound absorption in water and on particles is considered in a continuous approach for sampling frequencies up to 100 Hz. The widely-used log-linear relation between the SNR and the concentration, which is only valid for low concentrations, was extended for the high concentration regime. Measurement results show a similar behavior of the SNR with respect to varying suspended solid concentrations for different sediments. However, the analysis of the fit parameters shows systematic differences depending on the type of sediment. It is concluded that the proposed model is applicable as well for laboratory use as for measurements in rivers and estuaries. Finally, we discuss the reliability of the results and the methodology with regard to measurements in rivers, lakes, and estuaries.

Experimental validation of acoustic inversions for high concentration profiling of spherical particles, using broadband technology in the Rayleigh regime

“…Indeed, a decrease in the signal-to-noise ratio with increasing concentration (and hence attenuation) has been observed previously by other authors [50], [51] and is attributed to the fact that high levels of attenuation will be measured by the transducer as an incoherent signal (i.e. additional noise).…”

Characterizing the Composition of Sand and Mud Suspensions in Coastal and Estuarine Environments Using Combined Optical and Acoustic Measurements

“…We can mathematically describe acoustic backscatter using the sonar equation, which balances the difference between energy emitted and received by the sensor with energy lost on the return trip of an acoustic pulse [Hoitink and Hoekstra, 2005]. The sonar equation is presented here in form similar to [Hoitink and Hoekstra, 2005;Salehi and Strom, 2011;Chmiel et al, 2018]: integrated over the travel distance. BI is the volume backscatter strength [dB] and is a function of SSC and particle characteristics:…”

“…We can mathematically describe acoustic backscatter using the sonar equation, which balances the difference between energy emitted and received by the sensor with energy lost on the return trip of an acoustic pulse (Hoitink & Hoekstra, 2005). The sonar equation is presented here in form similar to (Chmiel et al., 2018; Hoitink & Hoekstra, 2005; Salehi & Strom, 2011) …”

“…The attenuation terms ( α s and α w ) are higher at larger concentrations and greater distances (Thorne et al., 1993) but can be neglected below 1,000 mg/L (Chmiel et al., 2018) and O (10 cm) from the sensor (Pomázi & Baranya, 2020). In this study, we thus neglect attenuation, given the small distance between source and measuring volume (15 cm) and low concentrations expected at our study site in Ameland (<1,000 mg/L).…”

“…Equation can be further simplified as where c ′ is a constant depending on instrument characteristics and b ′ is a variable depending on suspended particle properties (e.g., size, shape, density, and elasticity). The log linear relation between SNR and SSC is only valid for concentrations less than 1,000 mg/L (Chmiel et al., 2018; Salehi & Strom, 2011); beyond this threshold, particle absorption losses reduce the recorded backscattering signal.…”