The application of holography to the analysis of size and settling velocity of suspended cohesive sediments

George W. Graham and W. Alex M. Nimmo Smith

Limnol. Oceanogr. Methods 8:1-15 (2010) | DOI: 10.4319/lom.2010.8.1

ABSTRACT: Suspended cohesive sediments commonly occur in flocculated form. The flocculation process significantly increases particle size, settling velocity, and settling flux in comparison to unflocculated particles. To better understand suspended particles, a new instrument for in situ imaging using digital inline holography is presented and evaluated. With a resolution of 7.4 µm per pixel and a field of view of 7.4 × 7.4 mm, the instrument generates sharply focused images of particles from approximately 20 µm to 7 mm in diameter at up to 25 frames per second. A significant advantage of holography over current imaging systems is that in-focus images are obtained over a substantial depth of field. Contained in small-diameter cylindrical housings, the instrument presents minimal flow disruption and is easily deployable. Digitally recorded holograms are reconstructed and analyzed for number and size of particles in a fully automated manner. To assess the systems' potential, particle size distributions for two grades of quartz sand are compared with those from a laser diffraction particle sizer and are found to exhibit good similarity. To simultaneously estimate particle size and settling velocity, a modification to the system is demonstrated in which settling particles are automatically tracked and sized. The resulting relationships between settling velocity and effective density as functions of particle size are characteristic of suspended cohesive sediments and empirical power-law descriptors of these relationships show excellent agreement with previously published studies.