No abstract

[1] 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 distributions of the flocs were 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 (K s ) to increase as the degree of flocculation increases. This trend was also observed in the total scattering cross section ( t ) 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.

Measurements are presented from a multi-frequency acoustic backscatter study of aqueous suspensions of irregularly shaped quartz sediments having broad particle size distributions. Using the backscattered sound from a homogenous suspension, measurements of the ensemble backscatter form function and ensemble normalized total scattering cross section were obtained. Three different size distribution types are examined; namely Gaussian, log-normal, and bi-modal distributions, each covering a range of particle sizes similar to those observed in sandy marine environments near the seabed. The measurements of ensemble scattering are compared with theoretical predictions, derived by integrating the intrinsic scattering properties of the sediments over the probability density functions of the size distributions used in the present study. The results show that the ensemble scattering parameters are significant functions of both the width and type of size distribution in suspension. The impact of errors in size distribution width on inversion predictions of both mean size and suspended concentration is also examined. The validity of the theoretical predictions is discussed, along with the implication of the inversion results for using acoustic backscatter data to measure suspended concentration and particle size in sandy marine environments.

Although sound has been applied to the study of sediment transport processes for a number of years, it is acknowledged that there are still problems in using the backscattered signal to measure suspended sediment parameters. In particular, when the attenuation due to the suspension becomes significant, the uncertainty associated with the variability in the scattering characteristics of the sediments in suspension can lead to inversion errors which accumulate as the sound propagates through the suspension. To study this attenuation propagation problem, numerical simulations and laboratory experiments have been used to assess the impact unpredictability in the scattering properties of the suspension has on the acoustically derived suspended sediments parameters. The results clearly show the commonly applied iterative implicit inversion can lead to calculated sediment parameters, which become increasingly erroneous with range, as the sound propagates through the suspension. To address this problem an alternative approach to the iterative implicit formulation is investigated using a recently described dual frequency inversion. This approach is not subject to the accumulation of errors and has an explicit solution. Here the dual frequency inversion is assessed and calculated suspended sediment parameters are compared with those obtained from the iterative implicit inversion.

Laboratory measurements of the acoustic scattering properties of aqueous suspensions of non-cohesive sands having different and mixed mineralogical compositions are presented. Four different types of sand are examined: quartz, crushed shell, magnetite, and muscovite mica. The experimental data obtained for each type of sand are compared with theoretical scattering predictions for spheres having the same physical properties. The results show that for each type of sand, scattering is enhanced in the geometric regime relative to the sphere predictions, and for mica, scattering in the Rayleigh regime is reduced. To provide a theoretical framework for the observations, the applicability of two different modified sphere scattering models previously reported in the literature is evaluated. Measurements of the ensemble scattering properties obtained from mixtures of the different sands are also presented and compared with theoretical predictions. The results show that to accurately predict the scattering properties of compositionally diverse mixed sediment suspensions, it is necessary to know the relative proportions of each mineral present at each size within the size distribution; however, the scattering properties can be approximated for the suspensions studied by considering only the dominant mineral by mass.