Abstract
A method is developed in this paper to calculate the spatial gain of a vertical line array when the plane-wave assumption is not applicable and when the oceanic ambient noise is correlated. The proposed optimal array gain (OAG), which can evaluate the array’s performance and effectively guide its deployment, can be given by an equation in which the noise gain (NG) is subtracted from the signal gain (SG); hence, a high SG and a negative NG can enhance the performance of the array. OAGs and SGs with different array locations are simulated and analyzed based on the sound propagation properties of the direct-arrival zone (DAZ) and the reliable acoustic path (RAP) using ray theory. SG and NG are related to the correlation coefficients of the signals and noise, respectively, and the vertical correlation is determined by the structures of the multipath arrivals. The SG in the DAZ is always high because there is little difference between the multipath waves, while the SG in the RAP changes with the source-receiver range because of the variety of structure in the multiple arrivals. The SG under different conditions is simulated in this work. The “dual peak” structure can often be observed in the vertical directionality pattern of the noise because of the presence of bottom reflection and deep sound channel. When the directions of the signal and noise are close, the conventional beamformer will enhance the correlation of not only the signals but also the noise; thus, the directivity of the signals and noise are analyzed. Under the condition of having a typical sound speed profile, the OAG in some areas of the DAZ and RAP can achieve high values and even exceed the ideal gain of horizontal line array 10 logN dB, while, in some other areas, it will be lowered because of the influence of the NG. The proposed method of gain analysis can provide analysis methods for vertical arrays in the deep ocean under many conditions with references. The theory and simulation are tested by experimental data.
Highlights
The direct-arrival zone (DAZ) and reliable acoustic path (RAP) are important acoustic ducts in deep water
A concern that arises when considering a deep ocean, which is range independent, is that the spatial gain of the vertical array is influenced by the signal distortion that results from the multipath effect and the vertical correlation of the noise [5,6]
A vertical line array that consists of 16 elements with an equal spacing of 5 m was used, and the array center was fixed at a depth of approximately 432 m
Summary
The direct-arrival zone (DAZ) and reliable acoustic path (RAP) are important acoustic ducts in deep water. A concern that arises when considering a deep ocean, which is range independent, is that the spatial gain of the vertical array is influenced by the signal distortion that results from the multipath effect and the vertical correlation of the noise [5,6]. In [5,7], the approach to calculating the spatial gain is proposed when the arrival signals at the vertical array are not plane waves but the ambient noise is assumed to be Gaussian white. The simulation results were consistent with the experimental results measured in the South China Sea. This paper is organized as follows: in Section 2, the propagation properties of the acoustic field in the DAZ and RAP are studied based on ray theory.
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