Array gain of conventional beamformer affected by structure of acoustic field in continental slope area

Abstract

Conventional beamforming (CBF) is an important processing step in underwater array signal processing. Previous researches have shown that the sound field structure as manifested by amplitude nonhomogeneity and wave-front corrugation can reduce the array gain of CBF. The acoustic environment of the continental shelf slope area is very complex. For an underwater acoustic array in this area, the amplitude and phase of the received signals will be distortional seriously. Recently, the acoustic field correlation has been the focus of research on the array gain relations with the underwater acoustic filed. However, the attenuation of acoustic field correlation is not the only factor that induces the array gain to decline, and the analyses of the array gain in the shallow water based on normal-mode model are not applicable to the continental slope area. In this paper, the array gain relations with the structure of acoustic field in continental slop area are investigated based on the theory of underwater acoustic signal propagation. The effects of acoustic field on the signal and noise gains are considered respectively. The analytic expressions of the array gain of CBF in an isotropic noise field are derived from the primal definition of array gain, which indicates that acoustic field correlation and transmission loss in continental slope are the intrinsic factors that affect the array gain of CBF. A horizontal uniform linear array (ULA) with a wide aperture receiving signals from a source in the deep water region is considered in the upslope propagation condition. The RAM program is utilized in the numerical simulations to generate the sound field of this specific environment with given parameters. The array gains, the ATLs and the horizontal longitudinal correlation coefficients of the acoustic field corresponding to three different locations are compared. Conclusions can be drawn as follows. 1) The array gain of CBF is determined by acoustic field correlation and the acoustic average transmission loss (ATL), and its maximum is less than 10lg M as the signal waveform distortion. 2) when the ATLs corresponding to hydrophones at two different receiving locations are similar, the correlation of acoustic filed is higher, and the array gain of CBF is larger. 3) When the ATLs corresponding to hydrophones at two different receiving locations are greatly different, the relation between the array gain of CBF and the acoustic filed correlation is no longer positive. The simulation results verify the array gain of CBF relations with the acoustic filed correlation and the transmission loss in the continental slope area.

Effect of the fluctuant acoustic channel on the gain of a linear array in the ocean waveguide

“…The array noise gain of the array with an equal interelement spacing d in an isotropic noise environment has been derived as [3]…”

“…And subject to the topography and acoustic properties of sea-floor, as well as eddies, tides, internal wave, and surface gravity waves, the ocean waveguides are always randomly inhomogeneous, which leads the acoustic channels to suffer from spatiotemporal variation. Besides, in the continental slope area, [3] the ocean bottom is rangedependent, which will result in the more complex acoustic channels as manifested by inhomogenous amplitude and corrugated wavefront. A distorted wavefront or the multipath effect leads the amplitude and phase of the received signals (not the plane waves) to fluctuate.…”

“…[19] In the range-independent waveguide, the signal field is characterized by a set of discrete normal modes of propagation, and the array gain can be formulated by normal modes. But in the continental slope area, [3] the acoustic wave propagation will span the different seas. In this case, the ocean waveguide is not horizontally layered, but it is dependent on the range, which leads the modes to be coupled seriously.…”

“…In Ref. [3] , we have discussed the gain of a linear array relating to the structure of the acoustic field from the acoustic field correlation and the transmission loss. In the present paper, we will provide a new theoretical method to further analyze the array gain affected by the fluctuant acoustic channel, which can be applied to the range-dependent and rangeindependent waveguides.…”

Mesoscale Eddy Effects on Vertical Correlation of Sound Field and Array Gain Performance

“…The research showed that winds and waves played a significant role in influencing array performance. Xie et al [10] analyzed the impact of sound field characteristics on the array gain using conventional beamforming (CBF) in the shelf-slope area, revealing that the array gain was collectively influenced by transmission loss (TL) and sound field correlation at the array location.…”