A. L. Matveev

Forward Scattering Observation With Partially Coherent Spatial Processing of Vertical Array Signals in Shallow Water

In this paper, we address the problem of detecting an inhomogeneity in shallow water by observing changes in the acoustic field as the inhomogeneity passes between an acoustic source and vertical line array of receivers. A signal processing scheme is developed to detect the perturbed field in the presence of the much stronger primary source signal, and to estimate such parameters as the time when the inhomogeneity crosses the source-receiver path, its velocity, and its size. The effectiveness of incoherent, coherent, and partially coherent spatial processing of the array signals is evaluated using models and data obtained from experiments in a lake. The effect of different bottom types is also considered, and it is shown that partially coherent processing can have a significant advantage depending on the bottom type. Estimates of the minimum input signal-to-noise ratios (SNRs) for which the diffracted signal can be observed are presented.

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.

Comparative analysis of tomographic methods for the observation of inhomogeneities in a shallow sea

Several procedures are proposed that allow one to determine the parameters of motion of an inhomogeneity crossing the propagation path between a source of an acoustic signal and a receiving array without the use of any detailed description of the medium (the sound velocity profile, the bottom parameters, etc.). The potentialities and characteristic features of the proposed approach are analyzed within the framework of a one-dimensional waveguide propagation model. A comparative analysis of the reliability of inhomogeneity observations is performed on the basis of a representative experimental data set obtained from a lake experiment for the cases of using the procedures of coherent space-time processing of signals in antenna arrays and the procedures with a spatial and temporal incoherent accumulation.

Cascade cross modulation due to the nonlinear interaction of elastic waves in samples with cracks

The phenomenon under study consists in that, in an inhomogeneous material with nonlinearity caused by the presence of soft defects (the so-called “nonclassical” nonlinearity), cascade nonlinear effects are fairly strong and may even become comparable to the first-order effects. Similar cascade effects in media with a common nonlinearity of the crystal lattice are much weaker. This difference can be used as an important diagnostic indicator in nondestructive testing. Experimental data obtained for samples with cracks, which exhibit both ordinary modulation and cross-modulation effects, as well as a cascade cross modulation, are presented. The origin of the enhanced level of cascade effects is explained by modeling with the use of a simple model of nonlinearity of an inhomogeneous material containing soft Hertzian contacts.

Experimental and Theoretical Study of the Vertical Coherence of the Sound Field in a Shallow Sea

The combined effect of multiple scattering by random inhomogeneities of a waveguide and bottomcaused sound absorption is known to be a fundamental factor that governs the formation of the sound field in a shallow sea [1, 2]. A number of publications [2–6] present statistical analyses of the fluctuation phenomena that accompany the sound propagation in shallow sea regions. In these publications, most attention is paid to studying the evolution of the intensities of normal waves under the effect of scattering by the random field of internal waves. It is shown that in natural conditions, in addition to the inhomogeneities of the water column, one should take into account the irregular boundaries of the sound channel, which can also affect the correlation characteristics of the sound field. In this paper, we present experimental data on the vertical coherence of the sound field on a fixed path in the Barents Sea. We also compare the experimental data with theoretical calculations based on a model of sound scattering by the rough sea surface.

Source localization in a shallow-water channel with a rough surface

The problem of source localization in a shallow-water sea is considered, in which the fully developed sea is the prevailing sound-scattering mechanism. Using the criterion of the signal-to-noise-ratio maximum, we construct robust algorithms for estimating the parameters of partially coherent signals. The results of testing these methods for solving the inverse problem using experimental data obtained on a stationary path in the Barents Sea are presented. It is shown that under real conditions, the corresponding algorithms are serviceable and provide satisfactory quality of reconstructing a source located at a distance of ∼15 km from an antenna array.

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.

Coherent aperture synthesis for incoherent source

We propose a refinement of the method for aperture synthesis for an incoherent source, imparting selectivity and noise immunity to the method. Instead of measuring the phase difference between the signals of the array receivers, we measure the array-averaged phase difference between neighboring receivers by searching for the maximum of the magnitude of the moving array’s response to the source signal during its direction-of-arrival-scanning (phasing). This procedure is carried out after incoherent accumulation. The gain in noise immunity is determined by the gain in the array with the addition of the gain in incoherent accumulation. The size of this gain in the experiment was on the order of 20 dB. The method retains applicability to wideband signals and the possibility of observing other incoherent signals. We demonstrate the possibility of high angular resolution with the synthesized antenna of the source, which does not possess the perfect time coherency of the source and other sources coherent with it. Owing to phase averaging over the array aperture, we obtain an increase in the method’s stability to reverberation. We present results of simulation and applicability of the method in full-scale experiments.

Determination of the parameters of motion for an underwater object

Experimental data on the observation of a moving underwater acoustic screen by simplest antenna arrays extended in both vertical and horizontal directions are presented. New signal processing algorithms are proposed.

Space-time acoustic holography of moving inhomogeneities

A method for observing weak diffraction responses on the background of a fluctuating signal from a primary cw source is developed and tested. The possibility to visually observe the dynamics of the secondary field caused by the presence of nonstationary perturbations of the water medium is demonstrated.