Three-dimensional image generation and processing in underwater acoustic vision

Underwater exploration is becoming more and more important for many applications involving physical, biological, geological, archaeological, and industrial issues. Unfortunately, only a small percentage of potential resources has been exploited under the sea. The inherent structureless environment and the difficulties implied by the nature of the propagating medium have placed limitations on the sensing and the understanding of the underwater world. Typically, acoustic imaging systems are widely utilized for both large- and small-scale underwater investigations, as they can more easily achieve short and large visibility ranges, though at the expense of a coarse resolution and a poor visual quality. This paper aims at surveying the up-to-date advances in acoustic acquisition systems and data processing techniques, especially focusing on three-dimensional (3-D) short-range imaging for scene reconstruction and understanding. In fact, the advent of smarter and more efficient imaging systems has allowed the generation of good-quality high-resolution images and the related design of proper techniques for underwater scene understanding. The term acoustic vision is introduced to generally describe all data processing (especially image processing) methods devoted to the interpretation of a scene. Since acoustics is also used for medical applications, a short overview of the related systems for biomedical acoustic image formation is provided. The final goal of the paper is to establish the state of the art of the techniques and algorithms for acoustic image generation and processing, providing technical details and results for the most promising techniques, and pointing out the potential capabilities of this technology for underwater scene understanding.