Acoustic scattering from flocculating suspensions

Acoustic backscatter from sediment suspensions in the marine environment has been limited in application by the lack of understanding of how sound scatters from flocculating particles. To support theoretical development of sound scattering, combined measurements of high frequency acoustic backscatter and particle population characteristics are presented over a range of flocculating suspensions, from natural in-situ muddy suspensions to laboratory controlled pure clay flocs. Field measurements of cohesive suspended sediments were made in the meso-tidal Tamar Estuary, Devon, UK over several tidal cycles during spring tides. Controlled laboratory experiments were conducted using oscillating grid turbulence to suspend kaolin, oxidized, and natural marine sediments sieved below 63 microns. In both field and laboratory cases distributions of floc size and settling velocity were acquired using video techniques (from which effective density was derived) and acoustic backscatter measured over frequencies of 1-4 MHz. Particle measurements were complimented by pumped suspension samples later analyzed for mass and organic content. Measured scattering properties from the various sediments are compared against each other and with predictions from a hybrid elastic-fluid sphere model. Initial results suggest a good agreement with the model for both in-situ field suspensions and oxidized natural sediments from the laboratory.