Temperature field in inhomogeneous strongly anisotropic medium with sources

Using an “on-average exact” asymptotic method in a first approximation we constructed an analytical solution to the problem on the temperature field of heat sources in an anisotropic layer and ambient medium with dominant vertical thermal conduction. Results on temperature calculations in application to oil-gas formations are presented. It is shown that in the Laplace-Carson space, the zero and first expansion coefficients of the exact solution in terms of the asymptotic parameter coincide with those constructed on the base of the proposed method.     

Imaging through an inhomogeneous layer with an acoustic antenna array

A method is proposed for obtaining images through a layer of an inhomogeneous medium by using an antenna array scanning in angle or space. The method is based on the wave front inversion, which allows one to form an undisturbed sound field on the object of location in an inhomogeneous medium. This property makes it possible to suppress the effect of the thin inhomogeneous layer on the signals observed at the array output. The technique consists in a mutual processing of two received signals, one of which is obtained by locating the objects through the inhomogeneous medium and the other is obtained by locating the same objects, in the same medium, by the front-inverted wave. The mutual processing procedure consists in using the first received signal to form a filter for the second signal. The method is tested by a numerical simulation.     

Phase transition in strongly inhomogeneous ferromagnets

A field theoretical model is proposed to describe the critical behaviour of a strongly inhomogeneous spin system with a position dependent concentration of magnetic atoms C(R) and magnetisation M(R). Assuming a finite number of $\text{n}$ Fouriermodes $\text{C}{}_{\mathrm{<mtext>Q</mtext>}}{}_{\mathrm{<mtext>v</mtext>}}\text{v}\text{= 1}$,..., $\text{n}$, to express C(R), the quenched randomness requires to interpret {$\text{Q}{}_{\mathrm{<mtext>v</mtext>}}\text{|}\text{Q}{}_{\mathrm{<mtext>v</mtext>}}\text{|}{}^2$} on a set of invariant or marginal lengths. As consequence, M(R) can be described by n Fourier-modes M_Qv, where $\text{n ?}\text{n}$. For short range spin-spin interaction, we find for strong inhomogeneity, i.e. large n, the critical exponent between those of the related homogeneous system and those of the spherical model.     

Hole-burning NMR in strongly inhomogeneous fields

Different pulse sequences for frequency-selective NMR in the highly inhomogeneous fields of single-sided NMR are explored. A modified Hahn-echo is used to burn a hole in the spectrum of the detected echo. The hole diminishes following molecular dynamics on the scale of the echo time. Preliminary experiments were performed on pure water and natural rubber with the NMR-MOUSE. The results demonstrate the feasibility of hole burning to study slow molecular dynamics by mobile NMR in strongly inhomogeneous magnetic fields.     

Beam propagation in a randomly inhomogeneous medium

An integrodifferential equation describing the angular distribution of beams is analyzed for a medium with random inhomogeneities. Beams are trapped because inhomogeneities give rise to wave localization at random locations and random times. The expressions obtained for the mean square deviation from the initial direction of beam propagation generalize the “3/2 law.”     

Wave propagation in a weakly inhomogeneous medium

We develop an asymptotic method to describe the n-dimensional wave propagation through nonlinear inhomogeneous media characterized by dissipative and (or) dispersive systems of partial differential equations. This method is used to study the bidimensional propagation of surface water waves of varying depth.     

Permittivity of a randomly inhomogeneous medium

A theory of permittivity of suspension-type systems is developed that allows one to calculate such optical parameters of inhomogeneous systems as the length of scattering and the transport length. It is shown that, in the Born approximation, which takes into account two-particle correlations in the arrangement of scattering particles, the theoretical and experimental data are in agreement only to within tens of percent. The contribution of three-particle correlations to the permittivity of a system of solid spheres is determined. It is shown that, in describing the optical properties of suspensions with a large difference between the refractive indices of the medium and the particles, it does not suffice to replace the Rayleigh-Gans form factor by the Mie form factor, even under a restriction to two-particle correlations.     

Selective saturation in strongly inhomogeneous magnetic fields

The possibility to produce selective saturation by nuclear magnetic resonance (NMR) sequences of low-power radio-frequency pulses in strongly inhomogeneous magnetic fields is explored. The saturation of parts of the sensitive volume is produced by a particular pulse sequence with reduced amplitude distribution and the spectrum of the recorded signal is compared with the simulated spectrum. The spectra of the recorded free induction decays and echo signals are in good agreement with the simulated spectra of the pulse sequence, which demonstrates the effect of the selective saturation. The results obtained are an important step towards the development of new mobile and lowpower NMR equipments operating with inhomogeneous magnetic fields.     

Wave scattering in a multiscale random inhomogeneous medium

In this paper, using the Fock method of the fifth parameter and weighted Fourier-transform with respect to the coordinates of the source and observer, an integral representation is obtained for the wave field in a randomly inhomogeneous medium without invoking the assumption about small-angle propagation. Random trajectory variations to a first approximation are taken into account in calculating the partial wave phase (the expression under the integral sign). The expressions for the field in a medium with different-scale irregularities and for the scintillation index, obtained using this integral representation, are compared with known results. The good agreement with results from the theory of single scattering in a medium with background irregularities, and with investigations of the scintillation index made in terms of Rytov's method and path integrals, indicates that it is possible to use the approach developed in this study to describe the effects of simultaneous influence of different-scale irregularities.     

Radiation transfer in a three-dimensionally inhomogeneous stochastic medium

Within the framework of a small-angle iteration method the propagation of radiation in a three-dimensionally inhomogenous stochastic scattering medium is considered. The results of the analysis show a significant dependence of the average light field on the statistical structure of scattering parameters of the medium. Deceased. Institute of Applied Optics, National Academy of Sciences of Belarus, 11, Belynitskii-Birulya Str., Mogilev, 212793. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 234–240, March–April, 1999.