Shallow Water Acoustic Vector Field And Its Signal Processing

One of the inevitable tendencies of sonar technology is physics of underwater acoustics, signal processing and ocean environment being combined tightly. The direction of modern sonar's development is low frequency and working in shallow water. With respect to lower and lower frequencies, more attentions are paid to acoustic vector sensor. Under this context, the dissertation studies acoustic vector field and its signal processing. The main contributions are as follows, the first two sections of which concern vertical acoustic intensity, the last three sections of which concern horizontal acoustic intensity:(1) The combined descriptions of the pressure field and particle velocity field in Pekeris waveguide, especially the vertical acoustic intensity flux are proposed in this paper. The results of the study show that both the horizontal and the vertical acoustic intensity flux have active and reactive component because of the interference between the normal modes. When an acoustic vector sensor is placed appropriately, the reactive component of the vertical acoustic intensity flux in low frequency acoustic field can be used to tell the source's specified depth, although it can't transport energy. Then the reactive component of the vertical acoustic intensity flux is of importance for vector signal processing. The pressure and particle velocity cross spectra signal processing algorithm is proposed to distinguish the targets.(2) The further study on vertical acoustic intensity flux in broad sense shallow water waveguide is proposed. The results show that the theory of using reactive component of vertical acoustic intensity flux to tell the source's specified depth is valid for varies shallow water environment. Approximate theory analysis provides instruction to the placement of vector sensor, and points out that the polarity distribution of active component of vertical acoustic intensity flux is independent of the vector sensor's depth.Reciprocal relationship is applied to compute the acoustic vector field especially the reactive component of vertical acoustic intensity flux in shallow water. The acoustic field shows characteristics of near field in the near range, and the polarity of reactive component of vertical acoustic intensity flux varies regularly in the far range; the sound speed profile affects the method proposed here slightly; the attenuation of the bottom doesn't effect the method, on the contrary, it is in favor of the method; the interference patterns are different slightly for different frequencies, but the depth of the sensor can be fitted, in other words, one single vector sensor can work in a wide band. The formula of placing depth is proposed. All this conclusions is under the conditions that there are only two trapped normal modes. The condition can be fulfilled only at low frequencies, and the frequency band used by modern sonar is heading this direction.(3) There are much more output channels when the acoustic vector sensor is formed into arrays, which make the computation intense. A few computational efficient algorithms are proposed to apply to acoustic vector sensor array.A generalized transform matrix is constructed, and then the unitary MUSIC algorithm is adapted to acoustic vector sensors array. The observation data are incorporated with their conjugate, and then the covariance matrix can be decomposed in real-valued space. Hence, the proposed algorithm has features of lower computational complexity and better performance. The propagator method without eigen decomposition is applied to wideband coherent sources. The simulations and lake trial data processing show that the new algorithms are valid.(4) The array intensity estimator was proposed on the basis that acoustic vector sensors measure the pressure and particle velocity information of one spatial point simultaneously. After extracting the available spatial spectrum in the beamspace, one can process the acoustic intensity information to get the DOA of the target. When the spatial sampling law can't be fulfilled, the spatial spectrum will lead to cyclically ambiguous DOA estimates. Using the array intensity estimator can solve the ambiguity problem of a sparse acoustic vector sensor array, thus the aperture of the array can be extended without increase the number of the element and then offers enhanced spatial resolution. The performance of the array intensity estimator was analyzed. The simulation experiments show that the algorithm is valid.(5) Estimating the direction of arriving using the pressure and velocity combined processing are well documented in the literature, few attentions have been put into the time and frequency estimates with the combined processing idea. A new ESPRIT algorithm for frequency estimating is proposed based on the pressure and velocity cross covariance matrix, which can reduce the computational burden to 1/27 of the previous algorithm. The Cramer-Rao lower bound is developed for the data model. The simulation shows the validity of the new algorithm.