In modern radar, sonar and wireless communication systems digital beamforming technology based on sensor arrays is widely used to suppress unwanted interfering signals and detect interested target signals．Beam pattern synthesis for arbitrary arrays and robust beamforming for imperfect arrays are two crucial technical issues associated with digital beamforming．In a complex volumetric underwater acoustic transducer array, due tostructural shadowing and scattering. the practical beam pattern significantly deviates from its designed pattern, which would greatly degrade the detection and direction. finding performance of the sonar system. An efficient method is proposed to design beam patterns for arbitrary arrays and is applied to robust beamforming for this sonar system. Sea experimental results show that the proposed robust beamforming method achieves a much lower sidelobe level and a much higher direction. finding accuracy than the conventional beamforming technique.