Omni-directional Echo Characteristic And Signal Processing Of Underwater Hemispherical Cylindrical Shell

The characteristics of acoustic scattering vary with factors such as the structure,size,material of target,and external environment,which is an effective basis for underwater target detection and recognition.Small underwater targets generally have a non-centrosymmetric geometric shape,so the acoustic scattering field of the target has high spatial heterogeneity under the condition of active sonar detection.The characteristics of acoustic scattering are very sensitive to the change of incident angle of sound waves,and there is aliasing interference between them.In addition,the target is often in a proud or buried state,and the echo at the receiver will be accompanied by strong reverberation interference,which will affect the detection performance of the target.According to the acoustic scattering mechanism of the underwater target,this paper studies the acoustic scattering characteristics of the target model in the order of theoretical research,numerical simulation calculation,free-field experimental verification,and external-field detection experiment.The main research contents of the paper are as follows:(1)Aiming at that the acoustic scattering echo by underwater target is sensitive to the incident angle of the sound waves,the omni-directional acoustic scattering echo characteristic of the hemispherical cylindrical shell is studied.The echo tomography method is used to reconstruct the complete geometric structure of the target.The reconstruction results show that there is some interference beyond the geometric range of the target at the bottom corner with a small surface curvature radius and the local connection between the hemispherical part and the cylinder.The influence of the target length,material and shell thickness ratio on the elastic resonance characteristics of the target is studied by controlling variables.It is found that the elastic formant interval is related to the incident angle of sound waves,which is less affected by the target length and shell thickness ratio,and is obviously affected by the target material characteristics.The relationship between formant interval and incident angle of the sound waves is modeled,and the results show that the fitting curve can effectively characterize the material characteristics of the target.(2)Aiming at the mutual interference between geometric and elastic acoustic scattering features of small underwater targets,the stainless-steel hemispherical cylinder shell is taken as the research object.According to the formation mechanism and signal characteristics of the target echo by the acoustic scattering,the geometric acoustic scattering echoes and elastic acoustic scattering echoes are modeled respectively,and their energy accumulation characteristics in the fractional domain are theoretically deduced and simulated.Then,a filtering and separation scheme of echo signals based on fractional Fourier transform is designed.The processing results show that the proposed scheme can effectively separate the omni-directional geometric and elastic acoustic scattering echoes with low computational complexity.(3)Aiming at the spatial non-uniform characteristics of the target acoustic scattering field.a cluster-driven method of echo angle inhomogeneity is proposed.The feature vector is constructed by extracting the omni-directional target echo features,and the internal connection of the feature vector in the low-dimensional feature space is improved by using the nonlinear dimension reduction algorithm.The implied angle relationship between echo features is extracted by cluster analysis under unsupervised conditions.The reconstruction results of target local geometric structures corresponding to different categories show that the proposed method can realize the angle intervals division of target local structure based on the target inherent acoustic scattering characteristic.(4)Aiming at that the echo of the proud or buried target is disturbed by the reverberation,a reverberation suppression method based on non-negative matrix factorization is proposed.In consideration of the difference in the time-frequency distributions of the target echo and reverberation,non-negative matrix factorization is used to study the low-rank matrix factorization characteristics of time-frequency distribution features between target echo and reverberation.Given that the time-frequency matrix of the target echo cannot be expressed in the low-rank form when transmitting linear frequency modulation signal,this matrix is processed with low-rank preprocessing through matrix rotation.The components of the target echo,which can be expressed in the form of a low-rank matrix,are preserved.By contrast,the components of the reverberation interference,which cannot be represented in such a form,are removed in the underwater target echoes.The processing results of the lake and sea experiments show that the algorithm can effectively suppress the uniform incoherent reverberation.It also has a strong weakening effect on strong coherent reverberation interference,and can obviously improve the signal-to-reverberation ratio of underwater target echoes.There is a deeper understanding of the omni-directional acoustic scattering characteristics of the hemispherical cylindrical shell through the work of this paper,especially the signal processing flow and algorithm required by the actual target detection are comprehensively analyzed and summarized.The content and conclusion of this paper can be transferred to the study of other shapes of targets,which can provide technical reference for the detection and recognition of underwater targets.