I. Introduction
With the increasing demand for subsea resource exploration, the importance of underwater investigation has expanded to numerous marine applications, including physical, biological, archaeological, and industrial aspects [1]. Providing high-resolution 3-D images beyond the optical visibility range makes 3-D underwater imaging one of the most attractive types of technology. In recent years, 3-D sonar systems have been widely used in underwater investigations involving seafloor detection, robotics applications, environment studies, and navigation [2]–[6]. However, the large 2-D aperture required to achieve a high resolution causes two critical issues in the development of 3-D sonar systems: the high hardware cost and computational burden associated with the large number of elements.