Green function Retrieval and Time-reversal in a Disordered World

We apply the theory of multiple wave scattering to two contemporary, related topics: imaging with diffuse correlations and stability of time-reversal of diffuse waves, using equipartition, coherent backscattering and frequency speckles as fundamental concepts.Comment: 1 figur

Signature of wave localisation in the time dependence of a reflected pulse

The average power spectrum of a pulse reflected by a disordered medium embedded in an N-mode waveguide decays in time with a power law t^(-p). We show that the exponent p increases from 3/2 to 2 after N^2 scattering times, due to the onset of localisation. We compare two methods to arrive at this result. The first method involves the analytic continuation to imaginary absorption rate of a static scattering problem. The second method involves the solution of a Fokker-Planck equation for the frequency dependent reflection matrix, by means of a mapping onto a problem in non-Hermitian quantum mechanics.Comment: 4 pages, 1 figure, reorganized versio

Filtered Kirchhoff migration of cross correlations of ambient noise signals

International audienc

Parabolic and white-noise approximations for elastic waves in random media

International audienc

Random backscattering in the parabolic scaling

International audienc

Time-Reversal Aperture Enhancement

Time-reversal refocusing for waves propagating in inhomogeneous media have recently been observed and studied experimentally in various contexts (ultrasound, underwater acoustics, ...), see for instance [9]. Important potential applications have been proposed in various fields, for instance in imaging or communication. However, the full mathematical analysis, meaning both modeling of the physical problem and derivation of the time-reversal effect is a deep and complex problem. Two cases that have been considered in depth recently corresponds to one dimensional media and the parabolic approximation regime where the backscattering is negligible. In this paper we give a complete analysis of time-reversal of waves emanating from a point source and propagating in a three dimensional randomly layered medium. The wave transmitted through the random medium is recorded on a small time-reversal mirror and sent back into the medium, time-reversed. Our analysis enables us to contrast the refocusing properties of a homogeneous medium and a random medium. We show that random medium fluctuations actually enhances the spatial refocusing around the initial source position. We consider a regime where the correlation length of the medium is much smaller than the pulse width, which itself is much smaller than the distance of propagation. We derive asymptotic formulas for the refocused pulse which we interpret in terms of an enhanced effective aperture. This interpretation is in fact comparable to the super-resolution effect obtained in the other extreme regime corresponding to the parabolic approximation. However, as we discuss, the mechanism that generates the super-resolution is very different in these two extreme situations

Enhanced and specular backscattering in random media

International audienc

Characterization of Local Turbulence in Magnetic Confinement Devices

A multifractal analysis based on evaluation and interpretation of Large Deviation spectra is applied to plasma edge turbulence data from different devices (MAST and Tore Supra). It is demonstrated that in spite of some universal features there are unique characteristics for each device as well as for different confinement regimes. In the second part of the exposition the issue of estimating the variable power law behavior of spectral densities is addressed. The analysis of this issue is performed using fractional Brownian motion (fBm) as the underlying stochastic model whose parameters are estimated locally in time by wavelet scale spectra. In such a manner information about the inertial range as well as variability of the fBm parameters is obtained giving more information important for understanding edge turbulence and intermittency