Spatial Focusing of Low-Mode Acoustic Signals in a shallow Sea with Underwater Flow

Abstract

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...

Experimental study of mode selection in shallow-water sea

“…IN ACOUSTIC TOMOGRAPHY OF SHALLOW WATER SEA The efficiency of an acoustic system for tomogra phy of a shallow water sea substantially depends on how well the transmission and reception of probing signals are matched to the variability of the underwater (UW) waveguide [1][2][3][4][5][6][7][8]. Solving such a problem involves the optimal selection of the positions and sizes of antenna arrays and adaptive tuning of their aperture distributions to the waveguide characteris tics,which vary in time; reverberation disturbances and noise; and variations in the shape of the transmit ting and receiving arrays under the influence of under water currents [9][10][11][12][13][14][15][16].…”

“…In practice, detailed data on the waveguide are often absent. In such a case, one can use waveguide models that are horizontally uniform along the path [7]. Figure 3 shows the results of optimally compressing the mode signals compen sating for intermode dispersion.…”

“…Figure 3 shows the results of optimally compressing the mode signals compen sating for intermode dispersion. The numerical model corresponded to the measurement conditions in the shallow water sea [7,8]. Because the acoustic path was rather short, 17 km, the mode pulses with a band of about 10 Hz could hardly be resolved in their arrival times due to the disbalance of spectra of the mode pulses with replicas under the influence of the inter mode dispersion.…”

“…Because the acoustic path was rather short, 17 km, the mode pulses with a band of about 10 Hz could hardly be resolved in their arrival times due to the disbalance of spectra of the mode pulses with replicas under the influence of the inter mode dispersion. To resolve the mode pulses, we used the dispersion dependences of a uniform waveguide with a perfectly rigid bottom [7]. The waveguide parameters were specified with allowance for correct interpretation of the specifics of pulse propagation, in particular, the correct sequence of their arrivals and information on the angular spectra of modes (Fig.…”

Optimal selection of mode signals by vertical arrays in plane-layered waveguides in the presence of undesired signals

“…This method is based on the use of low-mode signals excited by vertical arrays [4][5][6][7][8][9][10][11]. Numerical simulations and the corresponding experiments indicate that the quality of selection of the low-mode acoustic signals in the presence of noise in LMPAT schemes is determined by the quality of the signal-processing algorithms which should take into account the mode structure of the received and interference (undesired) acoustic signals, as well as their statistical spatial and temporal characteristics [1][2][3].…”