The Prodigal Bubble - Celebrating the Return of the Bubble to a Recirculating Suspended Sediment Tower

Abstract:

The measurement and processing of high frequency acoustic backscatter profiles at multiple frequencies is an established technique for measuring suspended sediment load and equivalent mean particle size through the water column. The technique relies on the fact that the intensity of sound scattered by suspended particles is related not only to the particle size, but also the incident acoustic frequency. It is exploited by compact instruments that are deployed on seabed frames, vessels, and in laboratory flumes. However, one of the most significant influences on measurement accuracy comes from the presence of bubbles in the water column. Often found in suspended sediment study areas such as shallow wave-affected waters or highly turbulent flows, they can be of comparable acoustic cross-section to the suspended sediment particles and may dominate backscatter.

Acoustic backscatter instrument calibrations and experimental measurements are typically carried out in a recirculating suspended sediment tower where, through careful design, a near homogeneous suspension can be established. Great attention is paid to the choice of fittings and recirculating pumps to ensure that no air is introduced into the flowing liquid, and lengthy periods of degassing are necessary to ensure no bubbles remain before measurements are taken.

To tackle the problem of sediment backscatter signal contamination, a new research project is investigating how to detect and quantify bubbles present in sediment suspensions, with the aim of decomposing the backscatter signal into its sediment and bubble components. The first step in evaluating new acoustic techniques is to welcome back the bubble to the sediment tower from its long exile so it can become the focus of observations.

The paper describes the recirculating suspended sediment tower and the newly introduced bubble generation and observation apparatus, which is being used to generate and observe controlled bubble populations. Bubble detection and measurement techniques are described, and initial results using a commercial acoustic backscatter profiler are presented.