Thomas Planès

Locating a small change in a multiple scattering environment

This article presents an imaging technique to locate a weak perturbation in a multiple scattering environment. We derive a formula to predict the spatiotemporal decorrelation of diffuse coda waves induced by an extra scatterer. Locating this new defect is formulated as an inverse problem which is solved by a maximum likelihood approach. Using elastic waves in the 50–400 kHz frequency band, we recover the position of a millimetric hole drilled in a concrete sample with a precision of a few centimeter. Note that the size of the defect is comparable to the size of the myriads of heterogeneities constituting the sample.

Locating a weak change using diffuse waves: Theoretical approach and inversion procedure

We describe a time-resolved monitoring technique for heterogeneous media. Our approach is based on the spatial variations of the cross-coherence of diffuse waves acquired at fixed positions but at different dates. The technique applies to all kind of waves, provided that waveforms can be acquired with a sampling frequency much larger than the wave frequency. To locate and characterize a weak change that occurred between successive acquisitions, we use a maximum likelihood approach combined with a diffusive propagation model. We characterize this technique, locating a weak change using diffuse waves, called LOCADIFF, with the aid of numerical simulations. In several illustrative examples, we show that the change can be located with a precision of a few wavelengths and that its effective scattering cross-section can be retrieved. We investigate how the accuracy and precision of the method depends on the number of source-receiver pairs, on the time window used to compute the cross-correlation and on the errors in the propagation model. Applications can be found in nondestructive testing, seismology, radar, and sonar location.

Diffuse ultrasound monitoring of stress and damage development on a 15-ton concrete beam.

This paper describes the use of an ultrasonic imaging technique (Locadiff) for the Non-Destructive Testing & Evaluation of a concrete structure. By combining coda wave interferometry and a sensitivity kernel for diffuse waves, Locadiff can monitor the elastic and structural properties of a heterogeneous material with a high sensitivity, and can map changes of these properties over time when a perturbation occurs in the bulk of the material. The applicability of the technique to life-size concrete structures is demonstrated through the monitoring of a 15-ton reinforced concrete beam subject to a four-point bending test causing cracking. The experimental results show that Locadiff achieved to (1) detect and locate the cracking zones in the core of the concrete beam at an early stage by mapping the changes in the concretes micro-structure; (2) monitor the internal stress level in both temporal and spatial domains by mapping the variation in velocity caused by the acousto-elastic effect. The mechanical behavior of the concrete structure is also studied using conventional techniques such as acoustic emission, vibrating wire extensometers, and digital image correlation. The performances of the Locadiff technique in the detection of early stage cracking are assessed and discussed.

Imaging multiple local changes in heterogeneous media with diffuse waves

This study focuses on imaging local changes in heterogeneous media. The method employed is demonstrated and validated using numerical experiments of acoustic wave propagation in a multiple scattering medium. Changes are simulated by adding new scatterers of different sizes at various positions in the medium, and the induced decorrelation of the diffuse (coda) waveforms is measured for different pairs of sensors. The spatial and temporal dependences of the decorrelation are modeled through a diffuse sensitivity kernel, based on the intensity transport in the medium. The inverse problem is then solved with a linear least square algorithm, which leads to a map of scattering cross section density of the changes.

Locating and characterizing a crack in concrete with diffuse ultrasound: A four-point bending test

This paper describes an original imaging technique, named Locadiff, that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre-existing cracks, and/or to the development of diffuse damage at the tip of the crack. After giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change, the article introduces the inversion procedure that produces the three dimensional maps of density of changes. These maps are interpreted in terms of mechanical changes, fracture opening, and damage development. In addition, each fracture is characterized by its effective scattering cross section.

LOCADIFF: Locating a weak change with diffuse ultrasound

This article presents an imaging technique to locate a weak perturbation in a multiple scattering environment. We focus on concrete probed by ultrasound. We derive a formula to predict the spatiotemporal decorrelation of diffuse coda waves induced by an extra scatterer. We solve the inverse problem with a maximum likelihood approach. Using elastic waves in the 50-400 kHz frequency band, we recover the position of millimetric holes drilled in a concrete sample with a precision of a few centimeters.

Diffuse ultrasound monitoring of stress and damage development on large scale concrete structures

This paper describes the use of an ultrasonic imaging technique (Locadiff [1]) for the NDT&E of concrete structures. By combining coda wave interferometry and a sensitivity kernel for diffuse waves, Locadiff can monitor the elastic and structural properties of a heterogeneous material with a high sensitivity, and can map changes of these properties over time when a perturbation occurs in the bulk of the material. The applicability of the technique to life-size concrete structures is demonstrated through monitoring 2 reinforced concrete strutures [2-3]. Locadiff achieved to detect and locate the cracking zones in the core of concrete and monitor the internal stress level in both temporal and spatial domains by mapping the variation in velocity caused by the acoustoelastic effect. The mechanical behavior of the concrete structure is also studied using conventional techniques such as acoustic emission, vibrating wire extensometers, and digital image correlation. [1] E. Larose et al., “Locating a small change...

Locating a small change in a multiple scattering environment (LOCADIF): Application to monitoring concrete.

(1) CNRS and Universite J. Fourier, LGIT, GRENOBLE Cedex 9, France (eric.larose@ujf-grenoble.fr), (2) Laboratoire de Physique et Modelisation des Milieux Condenses, CNRS & Universite J. Fourier, BP 166, 38042 Grenoble, France., (3) Centre Europeen de Recherche et d’Enseignement des Geosciences de l’Environnement, CNRS & Universite Aix Marseille Europole Mediterraneen de l’Arbois, BP 80, 13545 Aix en Provence, France