Fast Broadband Beamforming Using Nonuniform Fast Fourier Transform for Underwater Real-Time 3-D Acoustical Imaging

A broadband 2-D array is necessary for the application of underwater real-time 3-D high-resolution acoustical imaging. However, the number of sensors and beams is so large that the computational load is overly high for generating a 3-D image in real time, when using the conventional broadband beamforming methods. Therefore, a fast broadband beamforming method is needed. In this paper, a fast broadband frequency-domain beamforming method based on nonuniform fast Fourier transform is proposed for underwater real-time 3-D acoustical imaging. The computational load of the proposed method is one or two orders of magnitude lower than that by the conventional frequency-domain direct method. Compared with the conventional time-domain delay-and-sum method, the computational load required by the proposed method will be lowered by approximately three orders of magnitude. Moreover, unlike the chirp zeta transform (CZT) beamforming method which is only suitable for equispaced 2-D arrays or sparse arrays thinned from equispaced 2-D arrays, the proposed method can be applied to arbitrary 2-D arrays. Even for the arrays available for the CZT method, the computational load needed by using the proposed method is still lower than that needed by using the CZT method in most cases.