Selective Acoustic Focusing Using Time-Harmonic Reversal Mirrors

International audienceA mathematical study of the focusing properties of acoustic fields obtained by a time-reversal process is presented. The case of time-harmonic waves propagating in a nondissipative medium containing sound-soft obstacles is considered. In this context, the so-called D.O.R.T. method (decomposition of the time-reversal operator in French) was recently proposed to achieve selective focusing by computing the eigenelements of the time-reversal operator. The present paper describes a justification of this technique in the framework of the far field model, i.e., for an ideal time-reversal mirror able to reverse the far field of a scattered wave. Both cases of closed and open mirrors, that is, surrounding completely or partially the scatterers, are dealt with. Selective focusing properties are established by an asymptotic analysis for small and distant obstacles

Selective imaging of extended reflectors in a two-dimensional waveguide

We consider the problem of selective imaging extended reflectors in waveguides using the response matrix of the scattered field obtained with an active array. Selective imaging amounts to being able to focus at the edges of a reflector which typically give raise to weaker echoes than those coming from its main body. To this end, we propose a selective imaging method that uses projections on low rank subspaces of a weighted modal projection of the array response matrix, $\widehat{\mathbb{P}}(\omega)$. We analyze theoretically our imaging method for a simplified model problem where the scatterer is a vertical one-dimensional perfect reflector. In this case, we show that the rank of $\widehat{\mathbb{P}}(\omega)$ equals the size of the reflector devided by the cross-range array resolution which is $\lambda/2$ for an array spanning the whole depth of the waveguide. We also derive analytic expressions for the singular vectors of $\widehat{\mathbb{P}}(\omega)$ which allows us to show how selective imaging can be achieved. Our numerical simulations are in very good agreement with the theory and illustrate the robustness of our imaging functional for reflectors of various shapes

Numerical Simulation of Acoustic Time Reversal Mirrors

International audienceWe study the time reversal phenomenon in a homogeneous and non-dissipative medium containing sound-hard obstacles. We propose two mathematical models of time reversal mirrors in the frequency domain. The first one takes into account the interactions between the mirror and the obstacles. The second one provides an approximation of these interactions. We prove, in both cases, that the time reversal operator $T$ is selfadjoint and compact. The D.O.R.T method (french acronym for Decomposition of the Time Reversal Operator) is explored numerically. In particular, we show that selective focusing, which is known to occur for small and distant enough scatterers, holds when the wavelength and the size of these scatterers are of the same order of magnitude (medium frequency situation). Moreover, we present the behaviour of the eigenvalues of $T$ according to the frequency and we show their oscillations due to the interactions between the mirror and the obstacles and between the obstacles themselves

Far field modeling of electromagnetic time reversal and application to selective focusing on small scatterers

International audienceA time harmonic far field model for closed electromagnetic time reversal mirrors is proposed. Then, a limit model corresponding to small perfectly conducting scatterers is derived. This asymptotic model is used to prove the selective focusing properties of the time reversal operator. In particular, a mathematical justification of the DORT method (Decomposition of the Time Reversal Operator method) is given for axially symmetric scatterers

Selective focusing on small scatterers in acoustic waveguides using time reversal mirrors

International audienceWe investigate the acoustic selective focusing properties of time reversal in a two-dimensional acoustic waveguide. A far-field model of the problem is proposed in the time-harmonic case. In order to tackle the question of selective focusing, we derive an asymptotic model for small scatterers. We show that in the framework of this limit problem, approximate eigenvectors of the time reversal operator can be obtained when the number of propagating modes of the waveguide is large enough. This result provides, in particular, a mathematical justification of the selective focusing properties observed experimentally. Some numerical experiments on selective focusing are presented

Space-time focusing of acoustic waves on unknown scatterers

International audienceConsider a propagative medium, possibly inhomogeneous, containing some scatterers whose positions are unknown. Using an array of transmit-receive transducers, how can one generate a wave that would focus in space and time near one of the scatterers, that is, a wave whose energy would confine near the scatterer during a short time? The answer proposed in the present paper is based on the so-called DORT method (French acronym for: decomposition of the time reversal operator) which has led to numerous applications owing to the related space-focusing properties in the frequency domain, i.e., for time-harmonic waves. This method essentially consists in a singular value decomposition (SVD) of the scattering operator, that is, the operator which maps the input signals sent to the transducers to the measure of the scattered wave. By introducing a particular SVD related to the symmetry of the scattering operator, we show how to synchronize the time-harmonic signals derived from the DORT method to achieve space-time focusing. We consider the case of the scalar wave equation and we make use of an asymptotic model for small sound-soft scatterers, usually called the Foldy-Lax model. In this context, several mathematical and numerical arguments that support our idea are explored

Localisation de défauts et applications pour les milieux inhomogènes en propagation d'ondes acoustiques

Cette thèse traite de problèmes inverses en propagation d'ondes acoustiques pour des milieux inhomogènes avec des mesures en champ lointain. Dans la première partie nous nous intéressons à la localisation de défauts, c'est-à-dire à la recherche du lieu où l'indice effectif est différent d'un indice de référence. Nous obtenons une caractérisation du support des défauts à partir des mesures par une extension de la "Factorization Method" d'A. Kirsch. Nous proposons plusieurs méthodes numériques permettant de déterminer la localisation des défauts dont une permettant de traiter le cas de directions d'émission et de réception distinctes. Ces algorithmes sont validés numériquement. Dans une seconde partie, nous considérons la reconstruction des valeurs d'un indice inconnu. A partir de la méthode de localisation de défauts introduite précédemment, nous proposons deux stratégies pour déterminer des zones d'intérêt sur lesquelles la reconstruction est focalisée. Enfin, nous introduisons un nouveau type de fonction coût pour la reconstruction, qui exploite les propriétés établies dans la première partie.In the first part of our work, we are interested in the localization of defects, i.e. the areas where the actual index is different from some reference index. We obtain a characterization of the support of the defects from the measurements by an extension of A. Kirsch's Factorization Method. We propose several numerical methods, one of them allowing us to consider the case of measurements directions which are different from the incidence directions. These algorithms are numerically validated. In a second part, we consider the reconstruction of the values of some unknown index. Using the previous defects localization, we propose two strategies to determine regions of interest on which the reconstruction is focused. Finally, we introduce a new cost function type, for the reconstruction, which capitalizes on the properties demonstrated in the first part

Imagerie ultrasonore de fantômes biologiques par optimisation topologique

L'Energie Topologique dans le Domaine Temporel (TDTE) est une méthode d'imagerie ultrasonore conçue pour le contrôle non destructif. Son analogie avec le retournement temporel a permis de donner une signification physique à la solution mathématique du problème inverse qu'elle définit. Notre objectif a été d'étendre l'utilisation de la méthode à l'imagerie échographique. Pour cette nouvelle application, une approche quantitative a été développée qui extrait les propriétés physiques des tissus imagés. Pour établir les performances et les limites de TDTE et de l'approche quantitative, la répartition spatiale et quantitative de l'énergie topologique a été étudiée théoriquement, puis numériquement et enfin validée expérimentalement. De la même manière, une étude comparative de l'énergie topologique a été effectuée pour des mesures ultrasonores avec et sans guide d'onde.The Time Domain Topological Energy (TDTE) is an ultrasound imaging method developed for Non Destructive Testing. Its analogy with the time reversal has given a physical meaning to the mathematical solution of the inverse problem. Our aim is to extend the use of the method to echography. For this new application, a quantitative approach has been developed that extracts the physical properties of the imaged tissues. To establish the performances and limits of TDTE and of quantitative approach, the spatial and quantitative distribution of the topological energy has been studied theoretically, then numerically and finally validated experimentally. In the same way, a comparative study of the topological energy has been performed for ultrasound measurements with and without wave guide