The application of acoustic backscattering, for measuring near-bed high-resolution profiles of suspended sediment concentration and particle size, has advanced substantially in recent years. To interpret these suspension observations and link them with the hydrodynamics, similarly detailed flow observations would be advantageous. In the present work backscattering from suspensions of marine sediments has been investigated to examine the potential of using the backscattered signal to also measure high-resolution near-bed vertical profiles of horizontal current flow. The approach adopted has been to attempt to utilize the temporal coherence in the suspension field, and to employ cross correlation of the backscattered signal between pairs of horizontally separated transducers to measure current profiles. To investigate this technique a series of laboratory measurements have been conducted for a variety of experimental arrangements, and the outcome from these measurements is reported in the present work. An examination of the suspension field has been conducted, and cross correlations between pairs of transducers investigated. Using cross-correlation time lags, current profiles have been calculated, and the results assessed using laser Doppler flow measurements. Comparison of the acoustic cross-correlation flow measurements with the Doppler reference measurements show a high degree of agreement, and strongly support the use of the cross-correlation approach to accurately measure vertical profiles of the mean horizontal current.
Shallow, narrow and busy channels do not easily allow conventional current measurements in real time. The goal is to investigate the possibility of using data of a horizontally mounted ADCP for measuring flow velocity and direction in a river or channel. Measurements were in the Port of Rotterdam. Two horizontal acoustic beams point fiam one shore to the &er. During WO tidal cycle s h u l m m w were carried with a vessel mounted vertical ADCP on three transects. performed during one month with a horizontally mounted ADCP in a 400m wide waterway in the Port of Rotterdam. Two horizontal acoustic beams point from one shore to the other. We compare the horizontal ADCP data with the data of a vessel mounted ADCP. It has been shown that the horizontal ADCP averages flow over a certain part of the water column.We propose a method to compute river flow averaged over the total cross section from horizontal ADCP data. The horizontal ADCP results compare well with vessel mounted ADCP data for two tidal cycles.
A method is described to simultaneously extract the bottom backscattering strength and bottom reflection loss as a function of grazing angle from measurements of monostatic reverberation. The least-squares difference between a parametrized model and data is minimized with respect to the parameters using generic algorithms or simulated annealing. The reverberation data are from broadband sources and omnidirectional receivers deployed at a flat-bottomed deep-water area in the Mediterranean. The reverberation model is simple: the propagation is described by straight-line ray paths (although the effective angles are corrected for the sound-speed profile); and parametrized functions are used for the bottom scattering strength and reflection loss. The parameters can also include the water depth, sound speed and gradient, source level, and pulse length. Where they can be compared, the parameters resulting from the inversion show good agreement with the measured ones, leading confidence to the procedure. As well, when the resulting parameters and actual sound-speed profile are input to the generic sonar model, the calculated and measured reverberations are in good agreement. a)Currently at Rijkswaterstaat, P.O. Box 3006, 2280MH Rijswijk, The Netherlands.
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