Locating structural changes in a multiple scattering domain with an irregular shape.

Locadiff is a method for imaging local structural changes in a random, heterogeneous medium. It relies on the combination of a forward model to calculate the sensitivity kernel of the source-receiver pairs, with an inversion method to determine the position of the changes. So far, the sensitivity kernel has been evaluated based on an analytical solution of the diffusion equation, which lacks the flexibility to handle problems where the domain has boundaries with an irregular shape. Moreover, the accuracy of the previous inversion method, based on linear algebra tools, was very sensitive to the values of the inversion parameters. This paper introduces a more generic approach to solve both these issues. The first problem is tackled by the implementation of a numerical method as an alternative for solving the diffusion equation. The second problem is tackled by the introduction of enhanced optimization algorithms to improve the stability of the inversion. This improved version of Locadiff is validated via both numerical examples and experimental data from an actual civil engineering problem.

Characterizing horizontally-polarized shear and infragravity vibrational modes in the Arctic sea ice cover using correlation methods.

The deployment of three drifting seismic stations on the Arctic sea ice during the winter of 2014-2015 with station inter-spacing between 30 and 80 km enables the characterization of the coherent seismic wavefield at these scales through the use of array methods. Two distinct vibrational modes are observed, corresponding to the fast and non-dispersive horizontally-polarized shear (SH) mode and the slow and dispersive flexural, infragravity mode (ice swell). The excitation of these two modes is not synchronous. The activation of the infragravity mode is linked to the arrival of energetic, dispersive wavetrains that can be readily seen on individual spectrograms, and that, as previous studies have shown, are likely to have their origins in distant storms. In contrast, the SH mode is excited at other time intervals and cannot be isolated on the recording of single stations due to the broadband and emergent nature of these wavetrains; given the horizontal polarization of these waves, the authors hypothesize that SH waves are caused by episodes of rapid SH deformation along major leads located outside the station network. The existence of horizontally-polarized waves propagating over long distances opens the possibility of monitoring ice deformation at the scale of the Arctic basin with unprecedented time resolution.

Monitoring ice thickness and elastic properties from the measurement of leaky guided waves: A laboratory experiment.

The decline of Arctic sea ice extent is one of the most spectacular signatures of global warming, and studies converge to show that this decline has been accelerating over the last four decades, with a rate that is not reproduced by climate models. To improve these models, relying on comprehensive and accurate field data is essential. While sea ice extent and concentration are accurately monitored from microwave imagery, an accurate measure of its thickness is still lacking. Moreover, measuring observables related to the mechanical behavior of the ice (such as Young's modulus, Poisson's ratio, etc.) could provide better insights in the understanding of sea ice decline, by completing current knowledge so far acquired mostly from radar and sonar data. This paper aims at demonstrating on the laboratory scale that these can all be estimated simultaneously by measuring seismic waves guided in the ice layer. The experiment consisted of leaving a water tank in a cold room in order to grow an ice layer at its surface. While its thickness was increasing, ultrasonic guided waves were generated with a piezoelectric source, and measurements were subsequently inverted to infer the thickness and mechanical properties of the ice with very good accuracy.

Measuring the wavenumber of guided modes in waveguides with linearly varying thickness.

Measuring guided waves in cortical bone arouses a growing interest to assess skeletal status. In most studies, a model of waveguide is proposed to assist in the interpretation of the dispersion curves. In all the reported investigations, the bone is mimicked as a waveguide with a constant thickness, which only approximates the irregular geometry of cortical bone. In this study, guided mode propagation in cortical bone-mimicking wedged plates is investigated with the aim to document the influence on measured dispersion curves of a waveguide of varying thickness and to propose a method to overcome the measurement limitations induced by such thickness variations. The singular value decomposition-based signal processing method, previously introduced for the detection of guided modes in plates of constant thickness, is adapted to the case of waveguides of slowly linearly variable thickness. The modification consists in the compensation at each frequency of the wavenumber variations induced by the local variation in thickness. The modified method, tested on bone-mimicking wedged plates, allows an enhanced and more accurate detection of the wavenumbers. Moreover, the propagation in the directions of increasing and decreasing thickness along the waveguide is investigated.

Accurate measurement of guided modes in a plate using a bidirectional approach.

Measuring guided wave propagation in long bones is of interest to the medical community. When an inclination exists between the probe and the tested specimen surface, a bias is introduced on the guided mode wavenumbers. The aim of this study was to generalize the bidirectional axial transmission technique initially developed for the first arriving signal. Validation tests were performed on academic materials such a bone-mimicking plate covered with either a silicon or fat-mimicking layer. For any inclination, the wavenumbers measured with the probe parallel to the waveguide surface can be obtained by averaging the wavenumbers measured in two opposite directions.

Relationship between impulse oscillometry and spirometric indices in cystic fibrosis children.

BACKGROUND: The aim of our retrospective study was to determine the relationship between impulse oscillometry (IOS) data and spirometric tests in cystic fibrosis (CF) children. methods: Thirty CF children aged 4-19 years have performed lung function tests (LFT). A subset of 15 patients repeated LFT on five separate occasions. IOS parameters were respiratory resistance (Rrs), reactance (Xrs) and impedance at 5 Hz (R5, X5, Zr) and the resonant frequency (Fres). Spirometry indices (SI) included forced expiratory volume in 1 sec (FEV(1)), forced expiratory flow during the middle half of FVC (FEF(25-75)) and forced vital capacity (FVC). RESULTS: An inverse relationship was observed between raw values of R5, Zr, Fres and SI respectively, and X5 correlated positively with SI. Although significant, these correlations were poor. Receiver operating characteristic curves (ROC) were constructed to identify cutoff points for IOS parameters to discriminate between children according to predefined FEV(1) thresholds (percent predicted), generally used to categorize the level of lung function impairment. No acceptable cutoff points can be found for IOS parameters. Trends analyses in the subgroup of 15 patients showed a significant decline of FEV(1) between the first and the fifth evaluation. None of the IOS indices demonstrated a consistent tendency, apart from a slight decrease of Fres. CONCLUSION: IOS measurements presented an insufficient sensitivity to detect and follow bronchial obstruction in CF patients.

Influence of material viscoelasticity on the scattering of guided waves by defects.

Guided waves are potential candidates for the nondestructive evaluation of viscoelastic structures due to their relatively long range of propagation. The major drawback is the difficulty in interpreting the scattered waves especially at high frequency-thickness values since many modes then exist. Moreover, in damping material waveguides, each mode of the scattered field has its own attenuation. Viscoelastic material characterization has been widely investigated by many authors in the past, but very few are treating multimodal scattering by discontinuities in viscoelastic guide. The scattering of a pure fundamental mode incident on a trough in a viscoelastic plate is investigated in this paper, over a relatively large frequency range, with up to five scattered modes. A hybrid two-dimensional finite element and modal projection method is used, based on modal orthogonality, to obtain the relative energy fluxes of each mode. Experiments are also made to validate the numerical predictions.

Reflection and transmission coefficients for guided waves reflected by defects in viscoelastic material plates.

The paper presents a Fourier transform-based signal processing procedure for quantifying the reflection and transmission coefficients and mode conversion of guided waves diffracted by defects in plates made of viscoelastic materials. The case of the S(0) Lamb wave mode incident on a notch in a Perspex plate is considered. The procedure is applied to numerical data produced by a finite element code that simulates the propagation of attenuated guided modes and their diffraction by the notch, including mode conversion. Its validity and precision are checked by the way of the energy balance computation and by comparison with results obtained using an orthogonality relation-based processing method.

A Bayesian approach for high resolution imaging of small changes in multiple scattering media.

This paper introduces a Bayesian approach to achieve high-resolution imaging of sub-wavelength changes in the presence of multiple scattering. The approach is based on the minimization of a cost function defined by the decorrelations induced in the measured waveforms by the apparition of a local changes. Minimization is achieved via a Monte Carlo Markov Chain (MCMC) algorithm combined to an analytical model that computes the sensitivity kernel of the medium. In the inversion procedure, the parameters to infer represent the physics of the problem, such as the diffusivity in the medium and/or the geometrical features of the reflector (position and scattering cross-section). The method is successfully compared to the linear inversion approach initially proposed for the so-called Locadiff imaging method through several examples, both numerical and experimental.

3-D reconstruction of sub-wavelength scatterers from the measurement of scattered fields in elastic waveguides.

In nondestructive testing, being able to remotely locate and size defects with good accuracy is an important requirement in many industrial sectors, such as the petrochemical, nuclear, and aerospace industries. The potential of ultrasonic guided waves is well known for this type of problem, but interpreting the measured data and extracting useful information about the defects remains challenging. This paper introduces a Bayesian approach to measuring the geometry of a defect while providing at the same time an estimate of the uncertainty in the solution. To this end, a Markov-chain Monte Carlo algorithm is used to fit simulated scattered fields to the measured ones. Simulations are made with efficient models where the geometries of the defects are provided as input parameters, so that statistical information on the defect properties such as depth, shape, and dimensions can be obtained. The method is first investigated on simulations to evaluate its sensitivity to noise and to the amount of measured data, and it is then demonstrated on experimental data. The defect geometries vary from simple elliptical flat-bottomed holes to complex corrosion profiles.

Ultrasonic imaging algorithms with limited transmission cycles for rapid nondestructive evaluation.

Imaging algorithms recently developed in ultrasonic nondestructive testing (NDT) have shown good potential for defect characterization. Many of them are based on the concept of collecting the full matrix of data, obtained by firing each element of an ultrasonic phased array independently, while collecting the data with all elements. Because of the finite sound velocity in the test structure, 2 consecutive firings must be separated by a minimum time interval. Depending on the number of elements in a given array, this may become problematic if data must be collected within a short time, as it is often the case, for example, in an industrial context. An obvious way to decrease the duration of data capture is to use a sparse transmit aperture, in which only a restricted number of elements are used to transmit ultrasonic waves. This paper compares 2 approaches aimed at producing an image on the basis of restricted data: the common source method and the effective aperture technique. The effective aperture technique is based on the far-field approximation, and no similar approach exists for the near-field. This paper investigates the performance of this technique in near-field conditions, where most NDT applications are made. First, these methods are described and their point spread functions are compared with that of the Total Focusing Method (TFM), which consists of focusing the array at every point in the image. Then, a map of efficiency is given for the different algorithms in the near-field. The map can be used to select the most appropriate algorithm. Finally, this map is validated by testing the different algorithms on experimental data.