3-D Underwater Acoustic Imaging by an Efficient Frequency Domain Beamforming

To generate underwater volumetric acoustic images a planar array is the minimal requirement, as a linear aperture does not allow one to discriminate signals coming from a 3-D space. Therefore, the number of signals acquired by the receiving array and then processed by an adequate algorithm is often huge, giving rise to a computational load prohibitive for a real-time image generation. An efficient frequency beamforming implementation based on the use of the Chirp Zeta Transform (CZT) has been proposed in literature in case of linear equally spaced array. The contribution proposed in this paper is aimed at extending such an implementation to a planar array. In particular, as the direction of a beam inside a 3-D space involves the use of two steering angles, the choice of such angles is fundamental to extend the CZT beamforming to a 2-D array: a conventional formalism does not allow one to express the main computation as a discrete convolution, and thus to exploit the advantages of the fast Fourier transform. The conducted analysis, in terms of computational complexity and system performances, compares the developed solution with traditional beamforming algorithms implemented in time domains, showing several advantages of the proposed method.