Ship strike acoustics

Abstract

The confluence of Lloyds mirror effect together with acoustical shadowing and spherical spreading pose significant detection challenges for whales and manatees. Direct measurements of approaching vessels using horizontal and vertical hydrophone arrays demonstrate how the noise from vessels can become indistinguishable from ambient noise. Geometric scattering is also estimated for various hull dimensions as the acoustical shadows cast ahead of ships provide no warning to animals near the surface within the shadow zone. Animals at sufficient depths, or outside the shadow boundaries that hear approaching vessels, may seek refuge near the surface or directly in the path of vessels where it is relatively quiet. Speed reductions proposed to reduce collisions do not address the underlying acoustical challenges marine mammals face. Field measurements support predictions that noise intensity is proportional with vessel speed to the 5th power. In multiple ship environments the acoustical masking challenges are greatest and noise from slow vessels operating in marine mammal corridors can mask the sounds of distant faster moving vessels. A low intensity bow-mounted projection system has been designed to selectively fill-in acoustical shadows with modulated ship noise to mitigate masking and near surface effects, and neutralize the dangerous ambiguity posed by acoustical shadows.