| Reverberation is a combination of scattering from surface,volume,bottom,and sub-bottom.The changing of reverberation spatial characteristics can be related to variations of bottom sediment and topography through reverberation imaging and/or comparison between model predictions and reverberation imaging data,which provides background information and theoretical basis for reverberation prediction and target detection.This thesis deeply investigates the modeling of bistatic bottom reverberation and scattering imaging.The modeling and methodologies proposed in the thesis are verified through theoretical derivation,simulation analysis,imaging system design,and experimental data processing.Aiming at the problems of low resolution,low precision and tow-ship noise in the conventional low-to-mid frequency bistatic bottom reverberation imaging algorithm design,the thesis proposes a broadband deconvolution beamforming method to achieve high-resolution imaging,and theoretically explaines the influence of coherence sources on this method.To suppress the tow-ship interference in the near field,the Fresnel approximation is applied to a linear high-resolution spatial filter based on the deconvolved conventional beamformer.This thesis also proposes an arrival paths classification method according to the depth of scattering footprints and the distance between scattering footprints and the source/receiver array to improve the accuracy when mapping reverberation beam-time response to Cartesian grid.Bottom reverberation contains both effects of propagation and scattering and cannot directly reflect the bottom scattering capability.This thesis proposes a relative scattering strength imaging method with a data smoothing window.Through optimal design of the window,the relative scattering strength can be effectively extracted to achieve wide-area imaging.This thesis also compares the model predictions and measured reverberation data in different sea areas to verify the detection capability of bottom sediment variation for mid-to-low frequency imaging sonar systems and explain the beam-time response mechanism of bistatic bottom reverberation.Finally bottom scattering coefficient imaging is realized according to differences between data and model predictions.In order to support the model-based scattering coefficient imaging method,this thesis establishes a bottom reverberation model considering disperation of normal mode in time domain.By analyzing the mechanism of mode dispersion,the conditions for the average group velocity hypothesis are given to simplify the reverberation model.Meanwhile,the Doppler effect is considered in the frequency-domain reverberation model.The time-domain sequences and beamtime response predicted by the reverberation model agrees with the measured data,which proves the accuracy of the model established in this thesis.In the end,this thesis establishes a reverberation model that systematically takes into account bottom topography,sediment,dispersion and Doppler effects.The model achieves long-range bottom imaging and scattering characteristic analysis for low-to-mid frequency system while simplifying the model and imaging algorithms as much as possible.It provides significant theoretical and model support for acoustic experimental design and data analysis,and helps improve the accuracy and explainability of bistatic bottom imaging.Thus the thesis has both theoretical and practical significance for the development of low-to-mid frequency underwater detection technology. |
