Dual-frequency ADCPs measuring turbidity

Abstract

A pair of self-contained acoustic Doppler current profilers (SC-ADCPs) operating with different frequencies were moored on a muddy sea bottom at about 20 m depth in the Bay of Vilaine off the French Atlantic coast. With their acoustic beams oriented upwards, the SC-ADCPs ensonified most of the water column. The results of several months of in situ recorded echo intensity data spanning 2 years (2003 to 2004) from the dual-frequency ADCPs are presented in this paper. The aim was to estimate suspended particle mass concentration and mean size. A concentration index CI is proposed for the estimation of particle concentration. Based on theory the CI-unlike the volume backscatter strength-does not depend on particle size. Compared with in situ optical data, the CI shows reasonable precision but not increased with respect to that of the highest-frequency backscatter strength. Concerning the mean particle size, despite a lack of quantitative validation with optical particle-size measurements, the method yielded a qualitative discrimination of mineral (small) and organic (large) particles. This supports the potential of dual-frequency ADCPs to quantitatively determine particle size. A cross-calibration of the transducers of each ADCP shows that a specific component of the precision of the backscatter strength measured by ADCP depends on the acoustic frequency, the cell thickness and the ensemble integration time. Based on these results, the use of two ADCPs operating with distinctly different frequencies (two octaves apart) or a single dual-frequency ADCP is recommended.

Dynamics of an Equatorial River Plume: Theory and Numerical Experiments Applied to the Congo Plume Case

“…(3), the Coriolis frequency acts in favor of trapping by the surface for a near-equatorial plume. This feature is observed in the BIOZAIRE cruises (see salinity section of (Jourdin et al 2006), where the Congo plume is surface trapped with no evidence of coastal current (it would have been necessarily noticeable if the plume was bottom influenced). The shape of the surface bulge is also observed to spread like a lens in the satellite observations of CDM absorption length of GlobColour data (Maritorena et al 2010) and sea surface salinity of SMOS data (Boutin et al 2012) (see Fig.…”

“…In fact, in its first approximation, the temperature would not influence density much, as the thermocline is not very marked and lies under a sharp halocline due to the freshwater discharge (Jourdin et al 2006;Vangriesheim et al 2009). The propagation speed of the gravity internal waves in a two-layer model is formally c i 5…”

Quantification and Analysis of Suspended Sediments Concentration Using Mobile and Static Acoustic Doppler Current Profiler Instruments

“…In Figure 4, it can be seen that the water condition near the bottom was categorized as turbid with visibility ranging from 5 to 10 m. Based on several factors that have been described, the transmission losses correction factors (absorption and distance) became important because they were related to sound signal propagation. The factors affecting sound propagation recorded by ADCP were distance, medium, and coefficient of attenuation [18,19].…”

On the variability of near-bed floc size due to complex interactions between turbulence, SSC, settling velocity, effective density and the fractal dimension of flocs

“…Previous studies have shown that turbidity measurements obtained by optical (e.g., XR620 and the OBS-3A turbidity and temperature monitoring systems) and acoustic techniques can be combined with in situ water samples to determine SSC variability in the bottom boundary layer and throughout the water column (Yang et al 2007;Yuan et al 2008;Jourdin et al 2014;Shi et al 2014). In the present study, the XR620 was immersed in tanks filled with water samples collected in the field.…”