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.

► The acoustic properties of suspended marine sands vary with mineralogical composition.
► We present backscatter measurements for common light and heavy mineral sands.
► We formulate and test generic scattering expressions for sands of diverse composition.
► The generic expressions enable accurate acoustic inversion for mixed sediments.