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.