HR: 0800h
AN: OS21B-1219    [Abstracts]
TI: The Effect of Bubbles on Optical Backscatter Sensors
AU: * Puleo, J A
EM: jpuleo@coastal.udel.edu
AF: University of Delaware, Department of Civil and Environmental Engineering Center for Applied Coastal Research , Newark, DE 19716 United States
AU: Johnson, R
EM: rjohnson@ocean.washington.edu
AF: University of Washington, School of Oceanography Box 357940 , Seattle, WA 98195 United States
AB: Nearshore field studies typically couple measurements of suspended sediment concentration (SSC) with advective velocity to yield a sediment transport rate. While a variety of methodologies exists, one of the most widely used instruments for measuring SSC is the optical backscatter sensor (OBS). With OBS's, photodiodes quantify backscattered infrared light by scatterers (sediment) as a voltage time series that is later transformed to concentration using a calibration curve. The difficulty with this calibration procedure is that it is typically performed in a benign laboratory environment with sediment being the only influence characteristic of the study site. In contrast, nearshore field measurements occur in an environment that often contains bubbles (among other material) that may influence the signal. Some debate exists as to the effect of bubbles on optical devices, and simple optical theories suggest that bubbles may potentially amplify the backscatter signal causing inaccuracies in predicted sediment transport rates and sediment transport model calibrations. To determine if bubbles do influence OBS measurements, a laboratory study using a standard OBS and a wide variety of bubbles sizes and distributions (spanning the range of sizes found in previous surf zone studies) is conducted. Measurements are taken in fresh, salt and synthetic salt water with and without bubbles and with and without sediment to assess whether any false signal amplification due to bubbles exists. Our conservative estimate indicates that the backscattered signal is amplified by up to 20 % in the presence of bubbles depending on water type, bubble size, bubble volume concentration, sediment concentration and sediment type.
DE: 4546 Nearshore processes
DE: 4594 Instruments and techniques
DE: 3020 Littoral processes
DE: 3094 Instruments and techniques
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting