Dual-frequency ADCPs measuring turbidity

Type Article
Date 2014-08
Language English
Author(s) Jourdin Frederic1, Tessier Caroline2, Le Hir Pierre3, Verney RomaricORCID3, Lunven Michel3, Loyer Sophie1, Lusven Andre1, Filipot Jean-Francois4, Lepesqueur Jeremy1
Affiliation(s) 1 : SHOM, F-29228 Brest, France.
2 : DHI, F-44300 Nantes, France.
3 : IFREMER, F-29280 Plouzane, France.
4 : France Energies Marines, F-29200 Brest, France.
Source Geo-marine Letters (0276-0460) (Springer), 2014-08 , Vol. 34 , N. 4 , P. 381-397
DOI 10.1007/s00367-014-0366-2
WOS© Times Cited 14
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.
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