Impact of sound attenuation by suspended sediment on ADCP backscatter calibrations

Abstract

[1] Although designed for velocity measurements, acoustic Doppler current profilers (ADCPs) are being widely used to monitor suspended particulate matter in rivers and in marine environments. To quantify mass concentrations of suspended matter, ADCP backscatter is generally calibrated with in situ measurements. ADCP backscatter calibrations are often highly site specific and season dependent, which is typically attributed to the sensitivity of the acoustic response to the number of scatterers and their size. Besides being a joint function of the concentration and the size of the scatterers, the acoustic backscatter can be heavily affected by the attenuation due to suspended matter along the two-way path to the target volume. Our aim is to show that accounting for sound attenuation in ADCP backscatter calibrations may broaden the range of application of ADCPs in natural environments. The trade-off between the applicability and the accuracy of a certain calibration depends on the variation in size distribution and concentration along the sound path. We propose a simple approach to derive the attenuation constant per unit concentration or specific attenuation, based on two water samples collected along the sound path of the ADCP. A single calibration was successfully applied at five locations along the River Mahakam, located up to 200 km apart. ADCP-derived estimates of suspended mass concentration were shown to be unbiased, even far away from the transducer.

Variability of residual fluxes of suspended sediment in a multiple tidal-inlet system: the Dutch Wadden Sea

“…For instance, the residual transport of water in the Marsdiep has been estimated with models and observations before (Ridderinkhof 1988;Buijsman and Ridderinkhof 2007;Nauw et al 2014;Duran-Matute et al 2014), and large discrepancies between model and observations can be explained partly by the temporal and spatial limitations inherent to the ferry-borne ADCP measurements (Sassi et al subm.). Furthermore, given the typical uncertainties in the conversion from acoustical backscatter units to suspended sediment concentration (e.g., Sassi et al 2012), one may expect large deviations in long-term estimates.…”

Sediment discharge division at two tidally influenced river bifurcations

“…This procedure was repeated for different levels at each vertical. Details of the calibration procedure can be found in Sassi et al [2012b]. After calibration, spatiotemporal distributions of echo intensity measured with the shipborne ADCP were converted into SSC.…”

Untitled

“…Because ADCP is a surrogate method, the acoustic backscatter data collected still requires calibration using manual samples. Sassi et al (2012) have shown how this can be done efficiently. The resulting Q and SSC data may then be used to analyse hysteretic behaviour, which can indicate the relative contributions of proximal (e.g.…”