Exploitation of nonlinear acoustical effects of air bubbles in water for a bubble/target discriminating sonar

Abstract

Target detection by traditional sonar can be undermined by air bubbles in various environments, including the surf zone, within gas-bearing sediments, or within seagrass beds. Bubbles introduce a variety of strong acoustical effects, including the onset of many nonlinear effects at sound pressure levels far below that required in bubble-free water. This paper describes laboratory tank experiments demonstrating a bubble/target discriminating sonar that exploits some of these nonlinear effects. Bubble plumes were generated in the tank by releasing compressed air through a porous ceramic plate. Rigid targets were also positioned at various locations in the tank. A high amplitude driving pulse excited translation of bubbles in the water column yielding Doppler signatures from conventional imaging sonar, while Doppler signatures were absent from the heavy rigid targets ensonified by the same driving signal. For sufficient bubble density, subharmonic generation was also observed in regions of bubbly water, but not from the rigid targets. The goal of this work is to use one or both of these nonlinear effects to identify and reduce clutter caused by bubbles and improve target detection for sonar operation in areas where bubbles are present. [Work supported by ARL:UT and ONR.]