Measurements of the backscattering characteristics of suspensions having a broad particle size distribution

Acoustic backscatter systems (ABS) can be used to non-intrusively measure profiles of both the concentration and particle size of suspended sediments in the marine environment. Inversion of ABS measurements into sediment size and concentration requires knowledge of two scattering parameters, namely the total normalised scattering cross-section, &chi;, and the form function, f. &chi; quantifies the acoustical scattering by a given particle over all angles, relative to its cross sectional area, and represents attenuation due to particle scattering losses. f describes the backscattering characteristics of a particle relative to its geometrical size. In recent years, a number of studies have presented measurements of f and &chi; for populations of sediments sieved over narrow size ranges, thereby essentially providing values for nominally a single particle size in suspension. In the present study, we extend these works by looking at the impact that a broad particle size distribution has on the form of f and &chi;. Here we model and measure the average form function for a broad size distribution (&sigma; = ±0.35a0, where &sigma; is the standard deviation about the mean particle radius, a0) of suspended glass spheres, whose scattering characteristics are well documented. The model is in close agreement with the provisional measurements, and suggests that for populations of suspended glass spheres with broad size distributions, the form function increases by about 40% in the Rayleigh regime (&lambda; >> 2&pi;a0, where &lambda; is the wavelength of the sound in water), whilst decreasing by a factor of around 25% in the geometric regime (&lambda; << 2&pi;a0), relative to that obtained for populations with a nominally single size in suspension. The output from this work has direct implications for the calculation of particle size and concentration profiles, obtained from acoustic backscatter data collected on suspensions of marine sediments at sea

Measurements of the scattering characteristics of sediment suspensions with different mineralogical compositions

Acoustic studies of suspended sediments often assume the dominant mineral in suspension is quartz, the density and intrinsic scattering properties of which are implemented when inverting acoustic backscatter data collected at sea. However, compositional analysis studies of suspended and sea-bed particulate material show a wide range of mineral species contribute to the inorganic fraction of sediments in the marine environment. Whilst no theoretical framework exists to predict the acoustic properties of irregularly shaped sediment grains, the density, compressional, and shear wave velocities of common marine mineral species can vary by up to a factor of two. In this study, we present and compare measurements of the intrinsic scattering parameters, namely the normalized total scattering cross section, χ, and the backscatter form function, f, obtained from homogenous suspensions of irregularly shaped sand sized grains of both magnetite and quartz. Our preliminary measurements suggest that in the geometric scattering regime, χ is enhanced for magnetite sands by ~ 100 % relative to quartz. Similarly, measurements of the form function for magnetite sands are enhanced by ~ 33 % relative to quartz in the geometric regime, though no measurable difference was observed in the Rayleigh regime. The implications of these results for acoustic backscatter data collected at sea are discussed

Acoustic measurements of boundary layer flux profiles over a sandy rippled bed under regular waves

The study of boundary layer sediment transport processes requires contemporaneous measurements of the bedforms, the flow and the sediment movement. Obtaining these three parameters, at the required temporal-spatial resolutions, has been traditionally difficult, especially within a few centimetres of the bed. To circumvent some of the deployment of an acoustic backscatter system, ABS, an acoustic ripple profiler, ARP, and an acoustic Doppler velocity profiler, ADVP, to measure sediment entrainment processes above a rippled bed under regular waves. High resolution acoustic observations of the suspend sediment concentration, flow and bedforms have been collected. Here we report on some of the initial results obtained from this study

Field deployment and evaluation of a prototype autonomous two dimensional acoustic backscatter instrument: The Bedform And Suspended Sediment Imager (BASSI)

The processes of sediment entrainment, transport and deposition over bedforms are highly dynamic and temporally and spatially variable. However, most measurements of these processes tend to be collected in a vertical line at one spatial location above the bed; one dimensional in the vertical, 1D-V. Such measurements capture the temporal signature, and, to a lesser degree, the spatial variability when bedforms migrate, and they have contributed greatly to our understanding of sediment transport processes. It is generally acknowledged, however, that such 1D-V systems provide a limited description of the spatially three dimensional processes occurring at the turbulent and intra-wave time scales. It would undoubtedly facilitate the interpretation of fundamental sediment processes above bedforms if other spatial dimensions could be simultaneously interrogated. To this end a multi-frequency acoustic array has been developed to measure suspended sediments and bedforms over a horizontal transect of the bed in the vertical, providing two dimensional observations in the vertical and horizontal, 2D-HV. This new acoustic instrument, the BASSI; Bedform And Suspended Sediment Imager, has been deployed in the River Dee tidal estuary in the UK. The design of the BASSI and results from the deployment are presented and its performance assessed against more conventional instrumentation. Measurements and images of 2D-HV suspended sediments and bedforms are provided to illustrate the capability and future use of the BASSI for the investigation of sediment transport processes

Measuring suspended sediment and its wave and turbulence forcing in the Dee estuary

Coastal areas support many human activities and represent a very important habitat for many marine and bird species. The Dee estuary, located in the eastern Irish Sea, is 20 km long, 8 km wide at the mouth and is characterized by a 10 m tidal range. Suspended and seabed sediments in the Dee contain a diverse assemblage of non-cohesive and cohesive sediments, and therefore the threshold of motion at the bed could be a complex process, dependent on several factors. In this paper, we present data collected during 2 deployments in the Dee, including both acoustic and optical instruments to study the link between the hydrodynamics, turbulence, and suspended sediments. Suspended sediment concentration is clearly controlled by tides following the flood/ ebb and spring/neap cycle. Moderate wave events were observed to increase sediment concentration, though mainly near the bed. High concentrations of silt and very fine sand were found that could support the flocculation processes during the flood and ebb cycle. Sediment concentrations at heights above 1 m from the bed do not present direct relation with bottom stress

Interpreting acoustic backscatter from suspended sediments of different and mixed mineralogical composition

Measurements are presented from a multi-frequency acoustic backscatter study of aqueous suspensions of sand sized particles having different and mixed mineralogical compositions. Seven different mineral sands are examined: quartz, crushed shell fragments, aragonite, muscovite mica, olivine, zircon, and magnetite. For each of these sands, measurements of the normalised total scattering cross section and backscatter form function were obtained. These measurements show some significant differences in the acoustic scattering properties between the different mineral sands. If the composition is erroneously assumed to be quartz when inverting the acoustic backscatter data collected from these different mineral sands, relatively poor estimates of mean size and suspended sediment concentration are obtained. To overcome this problem, the scattering measurements are used to construct a generic scattering description by normalising the scattering properties by grain density and averaging over the different minerals examined. This generic scattering description is evaluated by testing its application to the inversion of the acoustic backscatter data obtained from the seven different mineral sands, as well as two different mixtures each containing multiple mineral sands spanning size distributions typical of those encountered in the marine environment. These inversion results show close agreement to independent estimates of suspended sediment concentration and mean size

Scattering from suspended sediments having different and mixed mineralogical compositions: Comparison of laboratory measurements and theoretical predictions

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

Measurements and inversion of acoustic scattering from suspensions having broad size distributions

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