3-D acoustic imaging with a thin lens

Too often, underwater video, photographic, and laser images are obscured by turbid water. Acoustic systems that are able to pass energy through turbid water typically generate images with insufficient resolution and contrast. This paper discusses an experiment that provided satisfactory images of objects at short range using acoustic energy at 3 MHz. The imaged objects had different shapes (cylinders and plates) with small features such as bolt patterns and ribs. The experiment took place in an acoustic tank with objects 1.8 meters in front of a biconcave, thin lens 20 cm in diameter. The lens generated a conical, focused beam 0.16° between the -3 dB points with side lobes more than 50 dB below the main lobe. The system mechanically scanned and interrogated the targets at 0.5 cm intervals from left to right and from top to bottom. Targets were imaged in 3-D perspective with clearly distinguished bolt patterns and ribs. The paper compares images acquired in different levels of water turbidity and with different beam patterns and different image processing techniques