Modelling And Experiment Studies On Very Low Frequency Vector Acoustic Field Based On Subsurface Buoy

Vector hydrophone can simultaneously and colocatedly measure the acoustic pressure and three orthogonal components of particle velocity, which may provide more comprehensive information for further underwater acoustic signal processing and also inspire new thoughts and methods in solving undersea problems. With the development and improvement of the vector hydrophone's manufacture skill and vector signal processing, the related research on the application of vector hydrophone has become one of the most important issues in the field of underwater acoustics. However, the research on physical properties of vector acoustic field propagation is rarely reported, especially the theoretical study and experiment of VLF (very low frequency) vector acoustic field. Due to the influence of ocean current, tide, storm etc, subsurface buoy, as the platform of vector hydrophone, will suffer attitude changes including rotation, pitch, roll etc. Meantime, it also introduces self vibration and this vibration-coupling problem has caught more and more attention. With the demands of low frequency vector hydrophone in many aspects of underwater acoustic field, the calibration of low frequency vector hydrophone in the water tank is gradually calendared. So it is necessary to study the vector acoustic field in water tank in order to provide guidance for the calibration of vector hydrophone.In this dissertation, after reviewing the research on engineering application of vector hydrophone and its suspension system, ocean vector acoustic field propagation and geoacoustic inversion, and measurement technology in the water tank, it focuses on the following aspects for further theoretical and experiment study, including vector properties of shallow water acoustic propagation, geoacoustic inversion, integrated monitoring technology of subsurface buoy and vector acoustic field in the water tank.According to the normal mode theory, it derives the expressions of acoustic pressure, particle velocity and active acoustic intensity in shallow water vector acoustic field. Then the propagation properties of sound pressure, particle velocity and their mutual phase relation are study through numerical simulation and verified by sea trial data. The geoacoustic inversion via matched field processing on the basis of joint processing of acoustic pressure and vertical particle velocity is studied considering the information complementarity of them. The sensitivity of seabed density and sound speed for four cost functions used by matched field inversion (MFI) via both broadband and single frequency signal are compared in different conditions. Finally, the MFI inversion is conducted by use of the information of sound pressure and vertical particle velocity. Underwater integrated monitoring system is designed to study the attitude and synchronous vibration of subsurface buoy. Basic structure of such integrated monitoring svstem is introduced, and the experiment data of three set of subsurface buoy are analyzed using correlation theory and spectral analysis theory. The analysis of synchronous data could provide some suggestion to improve the design of vector hydrophone suspension device, subsuface buoy and mooring system. In addition, damping performance of first order and second order suspension system are analyzed with a simplified model, which is verified vibration test.The low frequency vector acoustic field in the water tank is studied to assist the calibration of low frequency vector hydrophone. In order to solve the low-speed calculation via classic pressure field expression, an efficient procedure for computation of the sound field in water tank is proposed, which converts a triple sum infinite expression to a double sum one and makes the eigenvalues in three orthogonal directions related to each other. In addition, another improved strategy is used to avoid the problem of computation overflow in PC implementation. Numerical simulation and tank trial have verified its effectiveness and correctness. Finally, this fast numerical solution is extended to calculate the particle velocity field and some numerical results are given for further use.