Impact of sound attenuation by suspended sediment on ADCP backscatter calibrations

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Abstract. Although designed for velocity measurements, acoustic Doppler current profilers (ADCPs) are widely being 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. We aim 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.

Introduction
Quantifying mass concentration of suspended particulates in natural environments is typically accomplished using surrogate measurements, since direct analysis of samples is too labor intensive to capture large-scale dynamics in time and in space [Wren et al., 2000; Gray and Gartner , 2009]. Acoustic profilers can yield non-intrusive, collocated, and simultaneous measurements of mass concentration of suspended particulate matter [Young et al., 1982; Thorne and Hanes, 2002]. Acoustic Doppler current profilers (ADCPs) were originally designed for flow measurement. While manufacturers store ADCP backscatter for quality checking of the velocity measurements, many researchers have adopted the ADCP backscatter as a surrogate measure of suspended mass concentration [e.g. Dinehart and Burau, 2005a, b]. Over the past decade, a variety of studies on geophysical surface flows have relied on the use of ADCPs to quantify variation of mass concentration of suspended matter, in the context of sediment transport research [e.g. Souza et al., 2004;

Kostaschuk et al., 2005; Wargo and Styles, 2007; Wall et al., 2008; Bartholoma et al., 2009; Defendi et al., 2010] and environmental monitoring [e.g. Hoitink, 2004]. ADCP backscatter calibrations have been found to be highly site-specific and eason
dependent [e.g. Gartner , 2004; Hoitink and Hoekstra, 2005], which can be attributed to the sensitivity of the acoustic response to particle size, density, shape and composition of scatterers in the target volume. Reichel and Nachtnebel [1994] were among the first to investigate the relation between ADCP backscatter and suspended sediment concentrationin a fluvial environment; they concluded that a mono frequency instrument such as the ADCP cannot separate effects due to particle concentration from those due to size distribution. In effect, when density, shape and composition of the suspended particles can be assumed constant, the acoustic backscatter mainly depends on the number of scatterers and their size [e.g. Medwin and Clay, 1998; Vincent, 2007; Marttila et al., 2010]

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