Uso do Backscatter para identificar árras do fundo marinho com o mesmo sedimento

Abstract

According to the Standard S-32, which is the reference dictionary of the International Hydrographic Organization (2022), Hydrography is a branch of applied sciences, mainly focused on safety of navigation. Thereby, it is naturally interested in providing browsers with the most faithful description of the different navigability spaces (oceans, seas, rivers, lakes, coastal areas). Among the information made available to navigators on the sea charts, is the nature of the seabed whose most widespread method of obtaining consists of collecting samples “in situ”, for visual and laboratory analysis. Apart from the logistics required to complete this task, the main limitation of this method is linked to the extrapolation of the nature of the collection point to the areas neighboring it. A subsidiary method consists in analyzing the intensity of the return acoustic pulses (backscatter) recording after insonification of the bottom during bathymetric surveys with multibeam sounders and to develop classification criteria in relation to the nature of this bottom from parameters considered representative. However, it would benefit from being normalized or standardized (abusively dependent on the operator for the elimination of dependencies angles, the choice of the parameters to be used for developing the classification criteria, the definition of these classification criteria, the definition and characterization of soil types in depending on the parameters chosen, resolutions of the areas to be used, etc.), so that under the same conditions, we can always achieve the same results. Nevertheless, it has been found that in relation to the characterization of the nature of an insonified surface, if we adopt the return intensity of the acoustic pulse after insonification, its angle of incidence on the bottom and the depth (bathymetry) as parameters, we manage to find the nature of the said area (hard,medium or soft). These are the parameters which have been chosen in the analysis of the backscatter of the area of the island of ITACURUÇA, whose results on the nature of the bottom obtained numerically were confronted with satisfaction with samples collected on the site. Three different classifications criteria (configurations), developed from the chosen parameters were used in this study in order to have an idea of their influence on the result, example giving, the nature allocated to the zone. The method used to develop these criteria was to choose them in the gradually restrictive form, or gradually moved away (away from one standard deviation, two and then three) from the average values of the parameters adopted for their elaboration. Finally, it appeared after analysis of the results obtained in each of the configurations that the methodology used in defining the criteria can be an excellent way to increase the reliability as to the attribution of the nature of an area. Also, the introduction of backscatter analysis in the procedures in force (technical instructions) in the hydrography and navigation directorate of Brazil and in the different hydrographic services, could, at first, improve the ability to choose the points of interest where samples from the bottom should be collected as this activity is mandatory during hydro oceanographic surveys. In a second time, as soon as this method will be more standardized, it could be used as the main method of identification of seabed areas with the same sediments.