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Acoustic scattering from a suspension of flocculated sediments
Author(s) -
MacDonald Iain T.,
Vincent Christopher E.,
Thorne Peter D.,
Moate Benjamin D.
Publication year - 2013
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/jgrc.20197
Subject(s) - flocculation , settling , suspension (topology) , scattering , materials science , absorption (acoustics) , light scattering , backscatter (email) , rheology , mechanics , mineralogy , optics , composite material , chemistry , physics , environmental science , thermodynamics , mathematics , environmental engineering , engineering , telecommunications , homotopy , pure mathematics , wireless
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 (σ 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.