G. N. Kuznetsov

Noisiness estimation of moving objects by identification of an acoustic model of the sea bottom

We consider methods for calculating the waveguide transfer function, which affects the accuracy in estimating free space of the noisiness of a source in shallow water. The use of a developed sea-bottom model and absolutely calibrated emitters predicted the sound pressure, recalculated to the emission point, with accuracy no worse than 2.5–3 dB. An increase in the prediction accuracy recalculated to the free-space of the values was established with the use of sound pressure measured in zones of interference maxima.

Estimation of the depth of a stationary sound source in shallow water

The noise immunity of the method for estimating the depth of a stationary sound source in an oceanic waveguide, based on the information about the amplitude ratio of selected wave field modes, is considered. It is shown that the error in recovering the source depth is always limited and tends to a stationary value with an increase in the noise level. The results of a computational experiment with a single detector and a horizontal linear antenna for the low-frequency region are reported. The behavior of the depth estimate in dependence of the mode numbers and detector depth is analyzed for the limiting case of intense noise. The stability of the method with respect to variations in the speed-of-sound profile is demonstrated.

Experimental study of the directional characteristics of a vector-scalar array

We analyze the spatial spectra of a vector-scalar array when the signals are processed using methods with different resolution. The method of presenting the signals used in the research allowed us to apply the previously developed method of detecting and estimating the parameters of signal sources obtained in the calculation of the theoretical characteristics of detecting the signal sources and realization of the algorithms of signal processing. We compared the resolution of vector-scalar and scalar arrays with the same aperture. The experiments were carried out in stationary conditions and in the towing regime. During the towing regime, the accuracy of determination of the source location was controlled using the GPS receivers. It was shown experimentally that the signal to noise ratio at the output of the receiving array is three times greater if the vector-scalar array is used rather than the scalar one. The level of the lateral background appeared to be three times smaller during operation with the vector-scalar array than with the scalar one even in the towing regime. The results of measurements are confirmed by theoretical calculations.

The Field of an Equivalent Multipole Composite Radiator in a Waveguide

The possibility of replacing a spatially extended volume low-frequency sound source, which forms a three-dimensional directional field in the far-field zone, by an equivalent multipole source is considered. Developing the ideas of L.M. Brekhovskikh, who obtained the solution for a monopole, exact and simplified models of the source field are constructed in the form of superpositions of multipoles of various orders. The models are considered in the integral, mode, and ray approximations applied to the unbounded space and homogeneous and plane-layered waveguides. The results of computations are presented, and the conclusion is made that the contribution of the vertical dipole and quadrupole components to the interference structure of the field amplitude in a waveguide decreases with an increase in the distance from the source. The phase structure is found to be stable regardless of the type of the complex multipole source.

The possibility of using the equivalent plane wave model to increase the efficiency of taking bearings of low-frequency signals in shallow water

The possibility of approximating the sound field in the region of interference maxima using the equivalent plane wave model with the actual amplitude and the average “effective” phase velocity calculated or measured by the phase gradient at the array aperture is discussed. The method is substantiated by studying the mode, interference, and phase structures of the low-frequency sound field along with the spatial responses of an extended linear array. For bottom-moored or towed geophysical arrays whose sizes are large compared to the wavelength, both the necessity and the possibility of reducing the error in taking the bearing of a sound source in a waveguide are justified. The use of the proposed model is recommended for approximate matching of the array to the transfer function of the waveguide to reduce the bearing error.

Wave method for estimating the sound source depth in an oceanic waveguide

An experimental estimation of the sound source depth, based on measuring the amplitude ratio for neighboring wave field modes, has been performed on the Pacific shelf under the following conditions: path length about 10 km at a sea depth of about 53m; pneumatic source signal in the band of 200±10 Hz. The predictive parameters of the bottom have been reconstructed from the envelope of the signal received by a single detector. The noise immunity of the algorithm and its sensitivity to variations in the bottom parameters have been analyzed. The experimental results are found to be in good agreement with the theoretical predictions.

Estimating the acoustic parameters of a model of a shallow-water seafloor using a priori geological and geophysical information and the Wigner transform

The results of estimating the acoustic parameters of a laminated bottom in a shallow-water region, which are averaged along the propagation path, are presented. The results of experiments on the propagation of low-frequency acoustic pulses in a water layer served as the initial information for obtaining estimates. The Wigner transform and a priori geological and geophysical information were used to estimate the parameters of normal waves and acoustic characteristics of the seafloor.

Direction finding and suppression of vector-scalar sound signals in shallow water taking into account their correlation and mode structure

The correlation of low-frequency sound signals from towed tonal low-frequency sources at the output of the scalar and vector channels is studied in shallow water. The correlation of the scalar field and signal received by a horizontally oriented vector receiver on average is 0.92–0.99; correlation with the signal received by a vertical vector receiver decreases to 0.66–85. When scalar fields or horizontal projections of the vibration velocity vector with application of the aperture synthesis algorithm are used, 3–5 normal waves are isolated; when the vertical component is used, 7–9 modes. It is demonstrated that the high signal correlation ensures direction-finding accuracy and suppression of strongly noise-emitting moving sources by 20–30 dB or more if the cardioid is directed at the source according to the zone of the minimum.

Noise source localization in shallow water

The possibilities of the method for detecting a noise source, estimating its radial velocity and its distance from the receiver, based on the double Fourier transform of the interference pattern formed during motion, have been considered for a single-receiver configuration. It is shown that, as compared to a signal with a uniform spectrum, the limiting values of the input signal-to-noise ratio, at which the noise source can be localized, increase by about half. The results of a computational experiment are presented. A comparative analysis of the noise immunity of the algorithm applied to signals with uniform and noise spectra is performed.

Spectrogram and localization of a sound source in shallow water

The paper describes an approach to detecting a sound source and estimating the radial velocity and distance from the receiver, based on repeat Fourier transformation of the interference pattern formed during motion. The obtained spectrogram contains localized domains of the spectral density of single modes. We estimate the localization domain and spectral density distribution and discuss the resolution of moving sound sources. We present the results of a field experiment and consider the interference immunity of the approach for localizing a source using a single receiver.