Research On Ocean Ambient Noise Modeling

Ocean ambient noise is the interference to underwater sound channel, and it has great effects on the design and use of all kinds of sonar systems; And furthermore, ambient noise field is useful structure. Some environmental parameters can be inversed by its spatial information. The ocean noise field is a complicated field changing with space and time. In shallow water, sound emanating from a noise source undergoes multiple reflections between the sea surface and the sea floor. The ocean ambient noise field is a stochastic process of many such noise sources and the respective interactions of their wave fields with the environmental boundaries. Spectrum level and vertical directionality of the noise are two of major factors affecting the performance of underwater sound systems.Ambient noise model consists of a sound source model and a sound propagation model. The static continuous dipole source model is adopted in this paper, the assumption that uncorrelated noise sources are randomly distributed on the infinitely large plane just below the ocean surface is used. In the presence of the pressure- release surface, monopole sources will couple into the water as dipoles. Three kinds of noise sources are concluded in this paper: the noise sources of wind and rain are distributed on an infinitely large plane, and the noise sources of ships are distributed on an annulus. The source levels can be divided by empirical formulas. A hybrid model of normal mode method (for far field) and ray method (for near field) is used in the propagation model, and the KRAKEN program is used in the computation of normal modes and bottom reflection coefficients.This paper is based on the basic wave equation, then a noise model applicable for range-independent shallow water is divided, and the effect of sound speed profile, frequency, bottom and the distribution of sources on the noise spectrum level and vertical directionality of the surface sources (such as wind, rain and ships) is discussed. The work in this paper is as following: Firstly, green function are obtained from the wave equation; Then according to the static source model, the normal mode and ray expressions of spectral level and vertical directionality expressions are derived; Last proper coefficients are chosen and the simulation work for the model is done, and the noise spectrum level of measured noise data are computed. The results of our model are basically in agreement with the results of T.C. Yang's and the results in RANDI model.