A Ivakin. Particle size and shape distributions

Abstract — Coarse particles in the sediment samples collected during the sediment acoustics experiment (SAX04) in shallow water off Fort Walton Beach, Florida, were analyzed in the 0 to –5 phi sieve size range, at quarter-phi intervals. Conventional size frequency histograms were determined and compared with results obtained by others at the SAX04 site. In addition, the coarse content was analyzed separately for the two populations, quartz (sand) and carbonate (shell) particles, using independent measurements of total weight and number of particles in each size interval. With this approach, four size distributions were measured, instead of a single one as in traditional sieving techniques. This analysis resulted in a modified technique, which can be used to improve geoacoustic characterization of the sediment. A particle shape factor was introduced for a simple quantification of the difference between the sieve size and true size (equivalent spherical particle diameter). It is shown that shells and shell fragments have a shape factor significantly different from that for sand particles. The size distributions for the two populations are also shown to be different. Empirical relationships are established between the shape factor, sieve size, and true size, for both carbonate and quartz particle populations. Using these relationships, the true size distributions, required in acoustic applications, were determined by correction of traditional sieve size distributions. These corrections are based on replacement of the “spherical shape” assumption used in traditional techniques by the “typical shape ” assumption, using the shape factors measured for the two populations. Finally, to test the corrected distributions, they were compared with those obtained using a new approach allowing estimation of the true size distributions without particle sieving and/or separating into size intervals (bins). The true size distributions obtained in this work will provide input parameters and ground truth for model/data comparisons using SAX04 acoustic backscattering measurements. Index Terms—Coarse sand, gravel, shell fragments, sieve size, true size (equivalent diameter), particle size and shape distributions