Remote nautical bottom estimation for the safety of navigation offshore Amazon River mouth

Abstract

In muddy areas, the fluid sediment layer can reach a level of “nautical bottom” that can contact with a ships keel causing either damage or unacceptable effects on controllability and maneuverability. Consequently, the minimum depth and allowed draught need to be determined for the safety of navigation. An acoustic remote sensing technique is proposed to facilitate navigation safety applications through the determination of sediment layer properties. It uses range and frequency-dependent features of the vertical waveguide characteristic impedance, as defined by the ratio of pressure and vertical particle velocity (or pressure gradient) at a given frequency. Such ratio can circumvent inversion uncertainty due to insufficient knowledge of complex time-varying ship noise spectrum because of its source spectral level independent. Real ship noise data recorded on a compact array offshore at the mouth of the Amazon River in Brazil, 2012, are processed by a global optimization based inversion scheme. The inverted results demonstrate that technique can estimate the effective water depth through resolving sediment layers properties, especially the density. The promising results demonstrate the feasibility of this technique to facilitate safety navigation applications at port areas.