DETERMINING SUSPENDED SEDIMENT PARTICLE-SIZE INFORMATION FROM ACOUSTICAL AND OPTICAL BACKSCATTER MEASUREMENTS

During the winter of 1990-1998 an Acoustic BackScatter System (ABSS), five Optical Backscatterance Sensors (OBSs) and a Laser In Situ Settli ng Tube (LISST) were deployed in 90 m of water off the California coas t for 3 months as part of the Sediment Transport Events on Shelves and Slopes (STRESS) experiment. By looking at sediment transport events w ith both optical (OBS) and acoustic (ABSS) sensors, one obtains inform ation about the size of the particles transported as well as their con centration. Specifically, we employ two different methods of estimatin g ''average particle size''. First, we use vertical scattering intensi ty profile slopes (acoustical and optical) to infer average particle s ize using a Rouse profile model of the boundary layer and a Stokes law fall velocity assumption. Secondly, we use a combination of optics an d acoustics to form a multifrequency (two frequency) inverse for the a verage particle size. These results are compared to independent observ ations from the LISST instrument, which measures the particle size spe ctrum in situ using laser diffraction techniques. Rouse profile based inversions for particle size are found to be in good agreement with th e LISST results except during periods of transport event initiation, w hen the Rouse profile is not expected to be valid. The two frequency i nverse, which is boundary layer model independent, worked reasonably d uring all periods, with average particle sizes correlating well with t he LISST estimates. In order to further corroborate the particle size inverses from the acoustical and optical instruments, we also examined size spectra obtained from in situ sediment grab samples and water co lumn samples (suspended sediments), as well as laboratory tank experim ents using STRESS sediments. Again, good agreement is noted. The labor atory tank experiment also allowed us to study the acoustical and opti cal scattering law characteristics of the STRESS sediments. It is seen that, for optics, using the cross sectional area of an equivalent sph ere is a very good first approximation whereas for acoustics, which is most sensitive in the region ka approximately 1, the particle volume itself is best sensed. In concluding, we briefly interpret the history of some STRESS transport events in light of the size distribution and other information available. For one of the events ''anomalous'' susp ended particle size distributions are noted, i.e. larger particles are seen suspended before finer ones. Speculative hypotheses for why this signature is observed are presented.