The acoustic properties of in-situ measured suspended sediments and their implications on concurrent ADCP response – Case studies of the Portuguese inner shelf

Hydrodynamic and Sediment Transport Patterns in the Minho and Douro Estuaries (NW Portugal) Based on ADCP Monitoring Data: Part 1-Tidal Sediment Exchanges

“…Gartner (2004) stated that, for 1200 and 600 kHzfrequency systems, the combination of the scattering and viscous loss terms of sediment sound attenuation [56] account for little attenuation when compared with fluid absorption, unless the particle size is very small or concentrations are very high, neither of which were reported in ECOIS observations [23]. Additionally, and corroborating this statement, the results presented by Santos et al (2020) showed that sound attenuation due to generally coarse suspended particles in the Portuguese coastal zone was two orders of magnitude lower than the sound attenuation due to the surrounding fluid, for concentration ranges in the same order as the ones measured in both estuaries [54]. Given these results, the attenuation of sound by sediment can be considered negligible, removing PSD dependence on the 2TL term in the equation.…”

“…Detailed information about each one of these terms can be consulted in a number of references ( [23,25,54,55], among many others).…”

“…The latter, in the case of the ECOIS datasets, and although some limited information is available, is assumed to be unknown. Therefore, in order to convert the ADCP acoustic backscatter output to estimated concentrations using a single-frequency system, some assumptions and simplifications regarding particle size have to be made [54].…”

“…The exponent in Equation ( 2) contains a term for the measured acoustic backscatter, corrected for geometric spreading and sound absorption by water (FCB-fluid corrected backscatter). The slope (B) and intercept (A) coefficients of the equivalent linear relation in Equation (3) (calibration parameters) can be estimated by fitting the logarithmic values of known concentration values to the corrected acoustic signal (FCB) [23,54]. To this end, SSC values determined at the fixed station in each estuary were depth averaged for each sampling period (hour) and interpolated in order to obtain values representative of the time of the passing of the ADCP in the fixed station transect, following the method described by Wall et al (2006) [52].…”

“…Determined regression coefficients (R 2 ) were in the order of 20-60% with higher significance (p-values < 10%) in the Douro estuary in 2005 and the Minho in 2006 when the valid calibration points were greater than 10 (Table 1). For similar attempts of converting ADCP backscatter into SSC, Wall et al (2006) reported R 2 = 86% (using a point-integrated water sampler), Gartner (2004) reported R 2 values ranging from 23-85% by crossing ADCP backscatter with a calibrated OBS output, and Santos et al (2020) reported R 2 ranging from 30-60% for ADCP FCB/LISST concentration output [23,52,54]. Although R 2 values reported in this study were in the same order of magnitude as these published studies, it is acknowledged that the regression significance (p-value) fell short of a desirable value.…”