Studies Of Reverberation Rejection Based On Time Reversal Processing

In shallow water, reverberation is the prominent interference to active sonars, which constrains the range and system performance severely. Reverberation is produced by the sonar itself, and it is also related to the character of transmission channel. So the suppression of reverberation is always a difficult problem. Time reversal is a process that utilizes the reciprocity of wave transmission in static medium and can focus acoustic energy at origin of the probe signal, without the knowledge of medium and transducer array. So it provides a new method to echo enhancement and reverberation suppression: Time reversal processing can be utilized to focuses acoustic energy on target, thereby resulting in the enhancement of target echo and the reduction of reverberation form the boundaries. This dissertation studies the application of time reversal processing in reverberation suppression. The following are the main contents of the dissertation:1. The ocean reverberation theory and the ray-normal mode reverberation model are discussed. The formula for average intensity of ocean bottom reverberation is deduced on the basis of normal mode theory for monostatic case.2. The principle of the time reversal processing and its spatial and time focusing are studied. Then the realization of time reversal processing in shallow water (Time Reversal Mirror) and its focal properity are analysed via computer simulation.3. The iterative time reversal processing and the decomposition time reversal operator method are studied. The convergence of the iterative time reversal processing and the principle of the decomposition time reversal operator are analyzed theoretically by transfer matrix. Then their selective focusing capabilities are analyzed via computer simulation.4. The application of time reversal processing in echo-reverberation ratio enhancement is studied. Firstly, the capability and the stability of time reversal processing for echo-reverberation ratio enhancement are studied by computer simulation in shallow water. Then the time reversal nulling method is given by extending the source and receiver array to normal excitation weight vector. Lastly, a new time reversal processing approach with constraints of reverberation null for echo enhancement is proposed, combining the time reversal focusing and nulling.