UDC 534.231We analyze the possibilities of increasing noise immunity during observation of low-mode acoustic signals diffracted by a localized inhomogeneity against the noise background in shallow-water oceanic waveguides using vertical transmitting and receiving antenna arrays in the case where their deflections in the vertical direction, arising due to the influence of underwater flows, are assumed known. It is shown that compensation for the array inclination increases significantly the observation-region sizes.Remote diagnostics of shallow-sea inhomogeneities within the framework of long water areas can be performed with the use of low-frequency low-mode acoustic tomography [1][2][3]. Sensitivity and accuracy of reconstruction of the parameters of the observed inhomogeneities by such a technique increase with increasing efficiency of selective excitation and reception of mode signals. An efficient tool for excitation of low-mode signals in the waveguide are vertical arrays of radiators [4][5][6]. For excitation of low-mode acoustic fields in the waveguide, an amplitude and phase spatial distribution corresponding to the radiated mode is created along the transmitting vertical antenna. However, along with the desired mode, other modes, whose excitation degree is determined by both the structure of the hydroacoustic waveguide and the sizes of the array and its position in the channel, are radiated because of the finite aperture of the antenna [7-9]. Owing to the underwater-flow effect, the antenna can be deflected from the vertical direction and change in shape. This makes the radiation patterns of such antennas in the horizontal plane different from the circular one which is typical of strictly vertical arrays. The array deformation also leads to the broadening of the excited-mode spectrum and its dependence on the azimuthal angle [9, 10]. Removal or attenuation of these effects can be provided by keeping the bottom-anchored arrays in the vertical position using a buoy with sufficient positive buoyancy.It should be noted that the influence of such effects on the structure of the excited acoustic fields depends, to a considerable degree, not only on the design of the arrays, but also the conditions of propagation of low-frequency sound in a shallow sea, in particular on the differences in the decay rates of the modes with different numbers [10]. The structure of the received low-mode acoustic signals diffracted by localized inhomogeneities depends on both the structure of the "illumination" signals, which is excited by the transmitting array, and the horizontal distribution of hydrological conditions and acoustic characteristics of the bottom and bottom sediments in the observation region. In particular, the decay rates of the modes depend on the depth distribution of the sound speed, which determines the degree of their interaction with the absorbing bottom. Thus, during observation at long distances, the differences in the decay rates of the modes with different numbers can significantly affect the sensitivit...

UDC 534.231We study the features of formation of low-mode-number acoustic signals in shallow sea by a vertical array matched to the hydroacoustic waveguide structure in the presence of drowned flows that cause the array bending and deflection from the vertical. For other given parameters of the waveguide and the flows, the efficiency of excitation of the low-mode-number signals is shown to be determined by the array position and length. It is demonstrated that the flow-induced array bending and deflection from the vertical introduces inhomogeneity in the structure of the excited low-mode-number fields. In particular, spatial modulation of the radiated signals can appear in the horizontal plane. Possibilities of reduction of the field inhomogeneity are discussed.

No abstract

No abstract

No abstract