Acoustic scattering from a suspension of flocculated sediments

A series of controlled laboratory experiments have been conducted to investigate the backscatter of high frequency sound (3-5 MHz) from suspensions of fine sediment in its unflocculated (primary) state and at various levels of flocculation. The size and fall-velocity distribution of the flocs was determined using an optical system and a settling tube, thus allowing floc density to be determined. The measurements have conclusively demonstrated that the acoustic properties of the flocculated particles are not solely controlled by the primary particles; some aspect of the floc structure is influencing the scattering characteristics. The overall trend is for the form function (Ks) to increase as the degree of flocculation increase. This trend was also observed in the total scattering cross section (inline image) but this result is dependent on the assumption that viscous absorption for flocculated particles is negligible. The measured scattering properties are compared to the predicted values from two theoretical models, the elastic (ES) and fluid sphere (FS) models. While the results show that, in their current form, neither model is capable of adequately representing the scattering characteristics of a suspension of flocculated particles, the two models did provide upper (ES) and lower (FS) bounds to the measurements. In terms of the operational use of acoustics to measure the concentration of flocculated sediments, empirical relationships could be fitted to the observations but, until a better theoretical understanding of how sound interacts with flocculated particles is achieved, the fitting of such empirical relations may be somewhat premature

An overview on the use of backscattered sound for measuring suspended particle size and concentration profiles in non-cohesive inorganic sediment transport studies

For over two decades, coastal marine scientists studying boundary layer sediment transport processes have been using, and developing, the application of sound for high temporal–spatial resolution measurements of suspended particle size and concentration profiles. To extract the suspended sediment parameters from the acoustic data requires an understanding of the interaction of sound with a suspension of sediments and an inversion methodology. This understanding is distributed around journals in a number of scientific fields and there is no single article that succinctly draws together the different components. In the present work the aim is to provide an overview on the acoustic approach to measuring suspended sediment parameters and assess its application in the study of non-cohesive inorganic suspended sediment transport processes

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

In this study, an acoustic backscatter system was used with single broadband transducers utilising narrowband excitation at multiple frequencies of 2.00, 2.25 and 2.50 MHz, to determine the scattering properties of three sizes of glass particles (40, 78 and 212 µm) in liquid suspensions. A calibration procedure was developed to initially measure the transducer constants, and form function and scattering cross-section values were calculated experimentally. Determined values aligned well with theoretical predictions, where viscous absorption was found to be important for the smallest glass particle size. A logarithmic translation of the signal attenuation gave a linear response, with respect to concentration, up to a maximum measured concentration of 125 gl−1 for the two smallest glass species. However, attenuation data for the largest species were only linear up to ~ 40 gl−1, attributed to significant multiple particle scattering causing an increase in the noise floor. Additionally, a procedure was developed to fit measured attenuation data to a nearfield correction factor correlation, improving measurements in restricted geometries and highly attenuating suspensions. Concentration profiles were produced using both single and dual frequency inversion methods and were found to be accurate up to ~ 25–40 gl−1, after which multiple scattering effects caused errors in the measured backscatter, and instability in the inverted profiles. Additional scatter observed in the dual frequency inversions was modelled in terms of the ratio between the attenuation coefficients at each frequency and compared to the experimental error. A ratio < 0.6 between the attenuation coefficients is suggested to sufficiently minimise errors in the dual frequency inversion