Modeling And Characteristic Of Long-range Bottom Reverberation In Shallow Water

It had been long before the beginning for researches about low frequency distant bottomreverberation in shallow water. With the ever ceasing development of underwater acoustictechnology, more deeply analysis and understanding from the view of its physical mechanism,the long-range bottom reverberation characteristic and the complicated boundary scatteringprocess，is much more demanded for the design of low frequency active sonar or multi-staticsonar system, which would be available for long-range target detection.Therefore, this dissertation is focused on the analysis about physical mechanism andmodeling of the low frequency long-range bottom reverberation (LFLBR) in shallow water.Firstly, the coupled modes reverberation model based on physical scattering mechanism isintroduced, and is extended to range-dependent waveguide. Then, the decaying rule of thereverberation level (RL) and the space coherence of LFLBR are discussed systematically. Inaddition to the mentioned theoretical researches, much LFLBR measurement is carried out intypical shallow water such as the Yellow sea and the South China Sea. The validity of thetheoretical modeling and analysis is thoroughly confirmed by data processing.Ⅰ. Theoretical modeling work:1. The reverberation field received by hydrophones is considered as the interaction ofincident modes with rough boundary or volume in-homogeneities in bottom medium bymeans of coupled mode method, which can explain the complicate scattering process ofbottom medium.2. According to roughness spectrum of the boundary surface and distribution of thesediment in-homogeneities, the formula of bottom reverberation is derived directly. Then, theFourier integration is used for the reverberation calculation of impulsive source with anysignal spectrum. In addition, the analytical expressions of RL are presented for both mono-and bi-static situation in range-independent waveguide.3. While for the range-dependent waveguide with varying depth, the adiabatic modesolution is introduced to derive the mathematical expressions for horizontal factors in slopingenvironment. This approach can also be extended to handle3D problems, which gives resultsobeying principle of reciprocity in all cases.Ⅱ. The discussion of characteristics for engineering application1. Beginning with the reverberation intensity, the RL of the multi-static sonar, for whichlittle work had been done before, is demonstrated systematically. The difference between the mono-static and the bi-static system is compared. Moreover, the decaying rule of RL forbi-static case in range-dependent waveguide is studied. The investigation indicates that theinfluence of sea floor inclination on RL should be considered.2. In thesis, the reverberation process is represented as a linear system without secondarysources. The sound channel effect and properties of sound propagation in shallow water areconsidered during the discussion of the RL for bi-static case. At the same time, the detailedphysical mechanism of the rough interface and the effect of sediment in-homogeneities on RLare displayed respectively.3. The space coherence of LFLBR is summarized by the coupled mode reverberationmodel. The time/space/frequency properties of the vertical, longitudinal, and transversecorrelation coefficients are predicted all-sided. It is indicated that transverse correlation isgreater than the longitudinal correlation, and it is most uncorrelated in vertical space.4. The surface effect on the space coherence of LFLBR is manifested. The mechanism ofsea surface effect on LFLBR is pointed out by describing the transfer of energy betweendifferent modes. It is indicated that the propagation effect of the shallow water waveguide isaffected seriously by the irregular upper boundary. Hence, higher modes are attenuated morequickly, and that will increase its space coherence.Ⅲ. Series of bottom reverberation measurements in shallow water:Three sea experiments for the measurement of bottom reverberation were conducted in2009,2010and2011respectivly. A large amount of high quality measured data was collectedin selected area of the Yellow sea and the South China Sea. The validity of the modelpredicted results is confirmed by data processing.