Yu. A. Polovinka

Experimental studies of pulsed signal propagation from the shelf to deep sea

Results of an experimental study of low-frequency broadband pulsed signal propagation in a waveguide that includes the shelf zone, the continental slope, and the deep sea region are presented. Using phase-manipulated signals with central frequencies of 366 and 600 Hz, pulsed characteristics are measured at six points along the propagation track, the maximal distance from the source being 368 km. It is experimentally demonstrated that, in the presence of a negative sound velocity gradient in the near bottom layer on the shelf with a small bottom slope, the choice of the source position at the shelf bottom near the shoreline provides the formation of a continuous illumination zone in the deep sea near the USC axis and a stable pulsed characteristic with two main sound energy arrivals. The propagation velocity of the pulse that is last to arrive is identical (within the measurement error) to the velocity of sound on the USC axis at the point of reception. Possibilities for practical application of the results obtained from the experiment are discussed.

Time reversal technique for gas leakage detection

The acoustic remote sensing of subsea gas leakage traditionally uses sonars as active acoustic sensors and hydrophones picking up the sound generated by a leak as passive sensors. When gas leaks occur underwater, bubbles are produced and emit sound at frequencies intimately related to their sizes. The experimental implementation of an acoustic time-reversal mirror (TRM) is now well established in underwater acoustics. In the basic TRM experiment, a probe source emits a pulse that is received on an array of sensors, time reversed, and re-emitted. After time reversal, the resulting field focuses back at the probe position. In this study, a method for enhancing operation of the passive receiving system has been proposed by using it in the regime of TRM. Two factors, the local character of the acoustic emission signal caused by the leakage and a resonant nature of the bubble radiation at their birth, make particularly effective scattering with the conjugate wave (CW). Analytical calculations are performed for the scattering of CW wave on a single bubble when CW is formed by bubble birthing wail received on an array, time reversed, and re-emitted. The quality of leakage detection depends on the spatio-temporal distribution of ambient noise.

Oscillations of a gas inclusion near an interface

The paper fills the gap in studying acoustic manifestations of a gas inclusion near an interface and makes it possible to analytically describe the behavior of a bubble at a small (comparable with its size) distance from the boundary. It is shown that the use of a specific (bispherical) coordinate system allows separation of variables and leads to a modified Rayleigh equation. Explicit dependences of natural frequency and damping on distance to the boundary and on physical parameters of contacting media are obtained.

Scattering from a pair of closely spaced bubbles

Acoustic scattering by a single bubble is the most basic problem for bubble detection. For a two-phase flow, the subject of forced oscillations of a pair of bubbles is important because it controls how bubbles interact with each other. Attempts to incorporate interactions between bubbles have traditionally used a multiple scattering methodology. The proposed approach uses a bi-spherical coordinate system and is limited to a description of a sufficiently long-wave acoustical field, so that the bubbles are homobaric, and the medium in the vicinity of the bubbles can be considered incompressible. The choice of a specific coordinate system allows the authors to take into account the internal symmetry inherent in this problem and provides a partial summation on only the most important contributions to the multiple scattering series. A closed form solution was derived for the scattered acoustic field that determines its parametric dependence on bubbles sizes and the separation distance.

Acoustic Manifestations of a Gas Inclusion Near an Interface

The study continues previous investigations based on the use of specific (bispherical) coordinates in describing the behavior of bubbles in the presence of bounding surfaces. Explicit dependences of the first oscillation modes and the scattered field on bubble size, distance to the boundary, and physical parameters of contacting media are obtained. It is shown that, as the distance to the boundary decreases, dipole oscillations acquire resonance nature and become comparable in amplitude with radial oscillations. This effect is of applied value for modern ultrasonic cleaning techniques, because it leads to a considerable increase in bubble-generated microstreaming.

A hydroacoustic complex and procedure for measuring hydrophysical parameters of the environment in shallow water areas

A procedure, hardware implementation, and software for remote monitoring of the hydrophysical parameters of the environment from data of the pulse hydroacoustic sounding on stationary horizontal routes in shallow water areas are described. Vertical distributions of profiles of the sound speed, temperature, and surface level were measured under natural conditions on the shelf of the Sea of Japan and in the Korea Strait in 2011. The results obtained in real time measurement with a high sampling rate can be used for prompt monitoring of the environmental parameters within the context of problems of ecological monitoring and protection against natural and man-caused threats in the shelf area, in bays, straits, closed seas, and artificial reservoirs.

Application of pseudorandom signals for underwater distance measurement on a shelf

In this paper, the results of experiments conducted on the Sea of Japan shelf are applied and discussed. The aim of these experiments was to study regularities in the formation of impulse characteristics for wave conductors under variable hydrological conditions. It has been shown that, in order to determine the distance and coordinates of underwater devices precisely when working on a shelf, one should identify the first and last impulses in impulse characteristics for the wave conductor, measure their arrival times, and multiply them to calculate the values of effective sonic velocity along the propagation routes of these impulses.