A Study Of Characteristics Of Ocean Ambient Noise Based On Fiber Optic Vector Hydrophone

Ambient noise is part of the inherent acoustic field in ocean, which contains distinguishing features of the marine environment, and also will interfere the detected signals. Therefore, we can inverse ocean parameters using ocean ambient noise, moreover, for ocean ambient noise is not correlated in a certain range, the signal detection probability of sonar systems can also be improved by suppressing ambient noise with hydrophone arrays. The vector hydrophone is a ble to suppress the isotropic noise effectively because of its intrinsic dipole directivity and decompose the acoustic field into horizontal and vertical parts by measuring the particle velocity of different directions. Compared with conventional vector hydrophone, fiber optic vector hydrophone has higher sensitivity, less transmission loss, more convenient for network connecting and long-time monitoring etc. In this paper, on the basis of vector fiber optic hydrophone, the directivity, correlation and spectrum properties of ocean ambient noise field were studied by theoretical analysis, simulation and experiment:1. The spatial correlation matrices of acoustic pressure and particle velocities of single point and vertical uniform linear array were derived; the basic concepts and physical interpretation of spectrum properties characterized by noise spectrum level, coherence function, coherence & diffuse component, phase spectrum were introduced; the spatial gain of vector hydrophone in isotropic noise field was derived when motion-induced self-noise was taken into account.2. A vector ocean ambient noise model was built based on the parabolic equation, and the horizontal & vertical correlation, vertical directivity and noise spectrum level of point and surface noise source were simulated, which verified the correctness of the model.3. By the method of vector beamforming, the directivity of ocean ambient noise field was researched, and the experimental data were processed. The results show that in shallow water off-shore, horizontal ambient noise is anisotropic without rain, mainly coming from the open sea, while the directivity is almost isotropic with rain; meanwhile, the relationship of the field directivity and the spatial gain was analyzed.4. The ambient noise in shallow water is measured by single fiber optic vector hydrophone with four components in the sea test, and the experimental results ofrainless and raining days are compared. In rainless days, for sound pressure and X channel below 200 Hz, the coherent coefficient is quite large and the phase difference is stable in the presence of the noise energy flow along horizontal direction, which mainly comes from ship noise; for sound pressure and Y channel above 800 Hz, the coherent coefficient is quite large in the presence of noise energy flow vertical to the coastline, which may be the pneumatic noise from open sea. In raining days, coherent coefficient of each channel is very small because of the "freezed" noise energy.