The interpretation and evaluation of a 3-MHz acoustic backscatter device for measuring benthic boundary layer sediment dynamics

Abstract

For ultrasonic backscatter devices to be of aid in studies of turbulent sediment transport dynamics, the procedures for converting signal to concentration estimates should be as accurate as possible, and the space and time resolution should be as high as possible. Signal conversion for the 3-MHz system considered here takes into account nearfield beam pattern effects on propagation and attenuation. Enhancing resolution involves explicit consideration of various sources of noise and interference. The results of field deployments demonstrate that, with signal conditioning and ensemble averaging, signal-to-noise ratios can be achieved that allow turbulent variation in concentration to be measured. Two possible sources of error in the conversion procedure are: (1) attenuation due to scatters is not measured in situ, but must be estimated from the data, and (2) with a single-frequency beam, the effects on scattering response of concentration variation and particle-size variation cannot be uncoupled.

Plume Measurement System (PLUMES) Calibration Experiment

“…First, the range dependency has a characteristic peak close to last axial maximum of the transmitted pressure field. This is in contrast to the results reported by Libicki et al (1989), where the same model produced a monotonically decreasing function. The cause for this discrepancy is not known, but may be related to shading techniques employed in their 3-Mhz system.…”

Acoustic imaging of cohesive sediment resuspension and re-entrainment in the Changjiang Estuary, East China Sea

“…5. Possible errors in the procedure for converting acoustic signal to concentration estimates include: (1) attenuation due to scatter (Libicki et al 1989); (2) uncoupled scattering response of concentration and particle-size variation; (3) organic materials in suspension (Vincent et al 1991); (4) attenuation by high-concentration suspension; and (5) the point-sampling method. As a result, the uncertainty in the present ASSM concentration estimations is believed to be approximately 16% for the absolute concentration.…”

Laboratory measurements of current flow using cross correlation on acoustic backscattering from suspended sediments

“…These advances have been supported by laboratory measurements on the scattering by suspensions of spheres 11 and marine sands, [12][13][14] and the deployment of acoustic backscatter systems in the marine environment. [15][16][17][18] The result of these works shows that for incoherent scattering with suspension attenuation accounted for, but not multiple scattering, the ensemble average square backscattered pressure P 2 for a piston transceiver can be written as…”