Focusing of acoustic fields in a randomly inhomogeneous waveguide by using the time reversal method

We perform theoretical analysis of the method of field focusing in a randomly inhomogeneous waveguide using reradiation of the received signal with time reversal. The simplest case where point sources and receivers are used for emission and reception is considered. As an example, the waveguide is chosen which simulates an underwater sound channel with refractive-index fluctuations caused by random internal waves. In underwater acoustics, the considered method of field focusing is usually applied at relatively short distances that are shorter than or about 10 km. This work deals with much longer paths, along which sound waves propagate under conditions of well-developed ray and wave chaos. Main attention is given to studying the width of the focal spot and the field amplitude at its center. It is shown that the amplitude distribution in the vertical section of the focal spot and the peak amplitude value at its center can be estimated analytically using the stochastic ray theory.     

Focusing of a wave beam in an underwater sound channel

The use of a vertical radiating antenna array for generation of a wave beam propagating in an underwater sound channel along the reference ray trajectory is discussed. The method for selecting the starting field in the antenna aperture for maximum compression of a beam in the specified vicinity of the reference ray is proposed. The estimates showing up to what distances a beam can propagate while remaining narrow as compared to the range of depths between the rotation horizons have been obtained. The problem concerning the distances from the antenna array at which a beam can still be effectively focused in the vicinity of the selected reference ray point is investigated.     

The possibility of using a vertical array for estimating the delays of sound pulses at multimegameter ranges

The majority of schemes used for acoustic monitoring of temperature fields in the ocean are based on measuring the variations of the so-called ray arrival times, i.e., the travel times of sound pulses along different ray trajectories connecting a source and a receiver. The solution of the inverse problem is considerably hindered by the fact that, in the case of signal detection by a point receiver, it is possible to resolve only the sound pulses propagating along steep rays. To a large extent, this is caused by the phenomenon of ray chaos, which is fully developed at distances of about one thousand of kilometers. The present study shows that the use of a vertical receiving array provides an opportunity to loosen this restriction. An appropriate space-time processing procedure is proposed. The procedure is based on the characteristics of ray arrival distribution in the time-depth plane that remain stable even under the conditions of ray chaos.     

Interferential algorithm for ocean bottom diagnostics

The efficiency of determining sound velocity, sound density, and acoustic attenuation factor in the bottom is evaluated by a numerical experiment, using the method of interferential ocean bottom tomography proposed by the authors. Simulation is performed in the presence of random noise when the source power is unknown.Deceased.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 3–4, pp. 202–205, March–April, 1995.     

Reconstruction of the average ocean temperature from the measured travel times of sound pulses

An approximate formula expressing the variations of ray travel times in terms of range-averaged characteristics of temperature inhomogeneities is derived. This formula essentially reduces the underdetermination of the inverse problem, which appears in the reconstruction of climatic variations of the average ocean temperature from acoustic measurement data. The efficiency of the suggested approach is confirmed by numerical simulations.     

Formation of the planar structure of ammonium dihydrogen orthophosphate in porous glass

Multiple cycles of porous glass impregnation by aqueous solutions of ammonium dihydrogen orthophosphate with subsequent dehydration lead to gradual accumulation of the salt in the carrier. In the course of progressive filling of the surface of glass in channels with the salt a region with considerably elevated electrical conductivity was found. The correlation between the amount of salt and the specific surface area of glass points to the formation of a continuous planar structure of ammonium dihydrogen orthophosphate in glass pores.     

Temperature dependence of protonic conductivity of porous glasses saturated with solutions of cesium dihydroorthophosphate in orthophosphoric acid

Wide-pore-glass plates saturated with solutions of cesium dihydroorthophosphate in orthophosphoric acid demonstrate a monotonic rise in the protonic conductivity in the temperature range 373–523 K, with σ ≥ 10^–2 Ω^–1 cm^–1 reached.     

The ray approach for analyzing the modal structure of the sound field in a range-dependent waveguide

Simple ray-approximation formulas are obtained for the mode amplitudes in a waveguide with range-dependent parameters. The idea of the proposed approach is based on the mode expansion of the complex field amplitude determined using the geometrical-optics approximation. A specific example of calculating mode amplitudes is analyzed for a deep-water sound channel with a sound speed profile nonadiabatically varying with distance. The results of the calculation are compared with the numerical solution obtained for the same problem by the parabolic equation method.