Digital Near Field Beamforming for Efficient 3-D Underwater Acoustic Image Generation

To generate volumetric underwater acoustic images a planar array of sensors is required to collect the signals coming from a 3-D scene. The method most frequently used to perform the spatial elaboration of the acquired signals is the beamforming algorithm. In general, owing to the huge number of sensors and the massive number of beam signals, computational load becomes prohibitive for real-time image generation. An efficient frequency beamforming implementation based on the use of the Chirp Zeta Transform (CZT) has been proposed in the literature for the case of a scene placed in the far-field and involving linear arrays. In this paper, we propose an algorithm to perform CZT beamforming on the wideband signals collected by a planar array and generated by a scene placed in the near-field. This extension becomes simpler and feasible thanks to (i) a non-conventional method of defining the azimuth and elevation angles; (ii) the Fresnel approximation in the delay computation; (iii) an adequate organization of the different operations. In terms of computational complexity, the analysis compares the solution developed with traditional beamforming techniques, showing several advantages of the proposed method.