Expansion of Field of View for Near-Field Fast Beamforming in 3-D Acoustic Imaging Based on the Optimized Subregion Approach

Real-time underwater 3-D acoustic imaging employs various fast-beamforming methods that significantly reduce the computational cost. However, in the near-field region, these methods rely on a popular model based on the Fresnel approximation, which has a narrow field of view (FOV) boundary of approximately 26^\circ. The FOV of the near-field region is very limited compared with that of the far-field region. Therefore, in this study, an optimized subregion approach is proposed to eliminate the FOV limitation for fast beamforming to improve the FOV of the near-field region. First, the FOV is divided into subregions, and within each subregion, a linear approximation is adopted to simplify the time-delay expression, with the approximation error limited to a reasonable threshold. Furthermore, the least-squares method and coordinate rotation techniques are employed, and the FOV for each subregion is reshaped to an ideal shape. Subsequently, a nested nonuniform fast Fourier transform is proposed to implement fast beamforming, and subregions can be computed in parallel. The results demonstrate that the proposed approach overcomes the limited FOV that exists for near-field fast beamforming in 3-D acoustic imaging and has a computational complexity comparable with those of existing algorithms. In addition, this approach supports an irregular planar array and maintains a satisfactory performance.