ESTIMATING RANGE DEPENDENT SEDIMENT THICKNESS AND REFLECTION COEFFICIENTS FROM BATHYMETRIC DATA FOR IMPROVED SONAR PERFORMANCE MODELLING

Abstract

Knowledge about the underwater environment is critical to anti-submarine warfare operations in littoral waters. The sonar settings and depth selections can significantly affect the predicted sonar performance. Bathymetry and sound speed profiles can be found or measured on site, whereas the geoacoustic properties of the seabed usually rely on sporadic historical data. Sediment thickness is important for low- to-mid-frequency active sonars, which cannot be found in the traditional sediment databases where only the top layer is sampled. The thickness of sediment deposit in an area will, among other things, depend on the shearing angle of the sediment and the slope of the bottom profile. Utilising available environmental information, such as granular size and bathymetry, an estimate of geoacoustic properties along the bottom profile can be made. When using a sonar performance model, a reflection coefficient for the bottom interface is required. The reflection coefficient is frequency-dependent and depends on the substructure of the bottom. The available information is utilised in a simple and fast calculation which gives the environmental parameters along a bottom profile to be used in a sonar performance model. The goal is to improve the accuracy of the predicted sonar ranges without making extensive measurements of the sea floor. We find that there seems to be a strong correlation between the slope of the sea bottom and the sediment thickness in the area of the North Sea west of Marstein. Furthermore, the background level towards the Norwegian coast is modelled, using a range dependent sediment profile, and compared to sonar data from the same region. Finally, the modelled background level with a range dependent sediment profile is compared to previous models with range independent sediment profiles.