Importance of Underwater Acoustic Imaging Technologies for Oceanographic applications –A brief review

Underwater Acoustic imaging (UAI) is an interdisciplinary area covering Physics, Mechanical, electrical, engineering, signal processing and computer Science. Synthetic Aperture SONAR’s (SAS) is a matured technology at present, though real time underwater imaging techniques provide challenges to the designer. Low cost acoustic cameras for underwater imaging which are compact have been reported in literature with recent advances in hardware technology and the development of the wide band underwater transducer technologies over a period of time. Low frequency wide band transducer technology and SAS processing techniques do have an advantage in achieving a reasonable resolution since SAS processing makes the resolution independent of the frequency. The array configuration, number of sensors do play a key role in the compact SONAR system designs. Low cost compact SONAR designs do have applications in the Autonomous Underwater Vehicles (AUVs) used for search, recovery applications in dams and shallow water areas and aquaculture in addition to defence applications. Delay And Sum beam forming (DAS), low complex real 3-D imaging systems with cross arrays have also been realized compared to the complex hardware requirements using 2D planar array designs. Multifrequency (MF) algorithm for the transmitting process and a parallel sub array (PS) algorithm for the receiving beam forming have solved real time problem of the cross array which was a bottleneck earlier. Multibeam synthetic aperture SONARs (MBSAS) and 3-D imaging of objects have also been reported. Synthetic Aperture SONAR is a methodology suitable for sea bed survey operations due to its large area coverage rate and centimetre resolution, with detailed imagery in the regions where the normal optical camera cannot provide clear pictures. Present trend in the research is to achieve miniaturized SONAR systems for AUVs which have restrictions on the power and endurance. The design in shallow water areas is really a challenge where multipath reflections are an issue in addition to the issue of Signal to Noise Ratio (SNR). A brief review of the 2D/3D imaging technologies, algorithm developments and their relevance to under water imaging for shallow water are presented in the paper.