Acoustic remote characterization of seabed sediments using the Angular Range Analysis technique: The inlet channel of Tagus River estuary (Portugal)

The inlet channel of the Tagus River estuary is located in the biggest urban area of Portugal and connects the river estuary basin to the Atlantic Ocean. Hence, it is a region with extreme economic importance, attested by the presence of the Lisbon port. For this reason, a large amount of resources is invested in acoustic surveys to update nautical charts and monitor the bottom modifications induced by the high hydrodynamics of this river. The use of acoustic remote characterization allows mapping the seafloor, including discrimination of different seafloor types. In this study, we used the Angular Range Analysis (ARA) technique to analyze the backscatter data from four distinct surveys acquired by different multibeam echo sounders systems to estimate the mean grain size of the bottom sediments. The correlation between the mean grain size provided by the ARA and laboratory results present in most cases a moderate to a strong linear relationship between them and the standard deviation of the residuals of the mean grain size between the ARA and the laboratory results varied between 0.52 and 1.01 ϕ. The higher residuals could be explained by regions with poor sediment sorting; changes in the seabed that occurred during the time interval between the hydrographic and sediment sampling surveys; solid input from tributary rivers; dumping of dredged material; positional inaccuracy of the samples; insufficient signal penetration of the multibeam echo sounders system to represent the vertical variation of the samples; and sediment mixing of vertical sedimentary structures during sampling. Therefore, we show that it is possible to apply acoustic remote characterization to highly hydrodynamic areas such as the Tagus River channel.