NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 Inverse parameters estimation of the functionally graded materials using surface waves measured with a laser interferometer Laiyu LU 1 , Mathieu CHEKROUN 1 , Odile ABRAHAM 1 , Géraldine VILLAIN 1 and Valerie MAUPIN 2 1 Laboratoire Central des Ponts et Chaussées, Route de Bouaye BP4129 44341 Bouguenais Cedex, Nantes, France 2 Department of geosciences, University of Oslo, P.O.Box 1047, Blindern, 0316 Oslo, Norway Abstract In this paper, an inverse scheme of the surface waves is developed to invert for the mechanical parameters of functionally graded visco-elastic materials, in which the properties, that is the velocity and the quality factor of body waves, are supposed to be as continuous functions of depth. This scheme is validated by investigating a numerical model with continuous variation that can be expressed by a known smooth function. Then, in laboratory the surface waves are measured on mortar samples with a laser interferometer. The phase velocity and attenuation are then extracted from the seismograms. Based on the phase velocity and the attenuation, the profiles of body wave velocity, as well as the quality factor, are obtained as a continuous variation with depth by back-calculation. Résumé Dans cet article, un problème inverse est développé pour l'inversion des courbes de dispersion des ondes de surface dans des milieux continument variables avec la profondeur aux propriétés visco élastiques représentées par une vitesse de propagation et un facteur de qualité pour les ondes de volume. L'algorithme est validé sur des données numériques avec des variations des propriétés mécaniques qui suivent une fonction analytique. Il est ensuite appliqué sur des données obtenues en laboratoire avec un interféromètre laser sur des dalles de mortier. Les profils des vitesses de propagation des ondes de volumes et du facteur de qualité avec la profondeur sont obtenus avec les courbes de dispersion des vitesses de phase et de l'atténuation mesurées. Keywords Inversion, FGM, Surface wave, mortar Introduction Concrete is characterized by its extremely heterogeneous nature since, as a common construction material, it is a mixture composed of cement or asphalt as well as other materials such as aggregates and water. Non destructive evaluation of concrete is a major issue for monitoring the durability of civil engineering structures, especially for the cover concrete, which is directly subjected to aggressive attacks from the outside. Ultrasonic waves are often used to characterize such material properties. Some experiments show that dispersion and damping of waves are influenced by both variation of grain size and water/cement ratio[1,2]. However, it is difficult to give a quantitative estimation for these properties based on the measured ultrasonic signals, especially for the attenuation. The main reason is the multiple-scattering of the waves in such a heterogeneous media due to the random distribution of the pores, air bubbles and aggregates, when the wavelength has the same order of