P. I. Korotin

Experimental studies of sound diffraction by moving inhomogeneities under shallow-water conditions

The experimental data on the sound propagation and diffraction by moving test inhomogeneities under lake conditions are presented. It is shown that the diffracted signals under multimode propagation are adequately described by simplified theoretical models proposed earlier. The detection of the diffracted signals against the background of a fluctuating direct signal is demonstrated for the reception by a horizontal or vertical array. It is also shown that the direct and diffracted signals observed in the lake are similar in their characteristics to the signals in a shallow sea, which allows one to use the lake experiment for testing various underwater acoustic techniques intended for shallow-sea conditions.

Numerical time reversal of waves

A numerical time reversal of waves is proposed instead of the conventional time reversal of wave procedure used in underwater acoustics. In the numerical method, the test sound source and the receiving arrays are used, as in the conventional method, but the transmission of the received signals after their time reversal into the same medium, as well as the measurement of the field obtained in this way at the point of the test source, is replaced by computations. To use the proposed technique for obtaining the same results as those provided by the conventional time reversal of waves, the teset source should be placed at different depths. A simplified numerical algorithm with the test source operating at a single depth is proposed and justified. This version of the time reversal of waves is successfully applied to the experiment in the Barents Sea. In contrast to the conventional method, the proposed technique allows one to study the stability of the sea medium with currents.

The use of receiving arrays for measuring underwater noise levels of moving sources

A method of measuring underwater noise levels of moving ships with the use of a vertical receiving array is considered. An algorithm is proposed for synthesizing the weight vector that provides a given measurement accuracy with maximal noise suppression for the cases of an interference with a known structure and an a priori unknown interference (the adaptive synthesis). Results of both numerical simulation of the method and its experimental testing under actual sea conditions are presented.

Focusing of a field in a hydroacoustic waveguide into a given depth interval

We propose and test in a field experiment a method that uses an emitting vertical array to focus a field to a given depth interval in a vertical section of an underwater sound channel at the observation distance. The amplitude-phase distribution of signals at the array elements is found by solving the variational problem of maximizing the ratio of mean sound field intensities inside and outside the chosen area of the section. The introduction of an additional restriction on the range of allowable grazing angles of excited waves has made it possible to obtain a solution in the form of a wave beam propagating without bottom reflections. Such solutions can be used for field focusing without information on the bottom parameters.

Focusing of a sound beam in an underwater waveguide using a vertical antenna

We propose a method for calculating the amplitude-phase distributions of signals on the elements of an emitting antenna for generating narrow wave beams and field focusing at given points of the waveguide. The efficiency of the method is confirmed by the data of a field experiment conducted in Lake Ladoga.

A method for determining the contribution of an individual source to the vibrational and acoustic field of complex mechanical structures

A calculation-experimental method is presented for determining the contribution of individual mechanisms to the vibrational and acoustic field of complex structures. The method is based on measuring vibrations in a set of points on the vibration energy propagation path from an operating mechanism, as well as on the subsequent calculation of the dynamic forces and external field using a numerical model of the structure. Experimental implementation of the method has shown that the contribution of an individual mechanism can be evaluated with high accuracy, including in the presence of vibrations at its base induced by external sources, the magnitude of which may significantly exceed the eigenvibrations of the mechanism.

Coherent aperture synthesis by an incoherent signal

The procedure of aperture synthesis by an incoherent source during displacement of the receiving system in space is considered. The advantages of the method are the possibility of aperture synthesis with the help of antenna arrays consisting of two receivers and its applicability to both the source of a quasimonochromatic signal and that possessing a continuous spectrum. The method is applicable with the same limitations as the known methods using the procedures of signal phasing. The results of mathematical simulation of the synthesis upon location of two sources with close frequencies at different distances and in the presence of noise, as well as the results of the in situ experiment with a reverberation interference, are given. The algorithm for decreasing the time of aperture synthesis without resolution deterioration is proposed and tested in situ.

Experimental implementation of a method for searching for sources of vibroactivity and maintenance defects in complex structures

A method suggested earlier for searching for sources of vibroactivity and maintenance defects in complex mechanical structures has been experimentally implemented. The method is based on joint use of a numerical finite-element design model and coherent vibration measurement at several control points. The search for defects is implemented by analysis of vibration field measurements at a control point related to scattering on the arising defect. Results are given from a study of the method’s efficiency depending on the number of control points for measuring vibrations and the type of the source’s spectrum.

Spatial extension of the acoustic time reversal region

According to the data of a full-scale shallow-water experiment (in the Barents Sea, at sea depths of about 120 m), a considerable gain in the signal-to-noise ratio is obtained for an acoustic signal received from a source at a distance of 12 km when matching with the medium is performed by the signal from the same source at a distance of 10.5 km. To interpret this experimental fact, a numerical simulation is performed to determine the size of the region of signal focusing due to the time reversal of waves in an ideal waveguide with a soft bottom. It is shown that, for narrowband signals, within a distance of ±5 km along the path from the point of emission of the reversed signal, a regular interference pattern whose maxima are comparable with the principal maximum is observed throughout the whole waveguide depth. For a spectrum width from 100 to 300 Hz, only the principle maximum with an extension of about 100 m is observed at a single depth.

Experiment on estimating the coordinates of an emitter on the Black Sea shelf

The paper presents the results of an experiment, conducted in Sukhumi Bay, on estimating the depth of a wideband emitter and its distance to a vertical reception array moored close to shore. The coordinates of the emitter were reconstructed from acoustic measurement data. The solution to the formulated problem was complicated by the absence of reliable information on the bottom relief and bottom parameters. It is demonstrated that under such conditions, matched field processing can be applied to estimate the coordinates of the emitter. For this, when solving the inverse problem, the components of signals without bottom reflection were used as the input parameters.