Identification of shear wave parameters of viscoelastic solids by laboratory measurements of Stoneley-Scholte waves

International audienceThis paper deals with the problem of viscoelastic solid characterization by acoustical means and in particular with the recovery of the shear wave parameters. It has been previously shown, in the Underwater Acoustics field, that the shear wave parameters of the sea floor could be recovered by using inverse techniques applied to propagation characteristics of interface waves such as Stoneley-Seholte waves, which can propagate in water/sedi ment configurations. The goal of the study presented in the paper is then to test models, commonly used for seabed identification, on media whose properties arc well-controlled by laboratory tank experiments, contrary to in situ bottoms. It is shown that the viscoelastic medium parameters can also be identified from the characteristics of interface waves, generated experimentally in laboratory on very attenuating synthetic materials. The paper presents results about the estimated shear wave parameters obtained from both numerical and experimental data by applying Brent's method on the characteristics of the interface waves. The observation and the discussion of differences between theoretical and experimental results are the goa] of the paper. The study presented here validates the forward model previously developed and it can be considered as a first step towards the direction of acoustic classification of sea bottoms

DE-COHERENCE EFFECTS IN UNDERWATER ACOUSTCS: SCALED EXPERIMENTS

International audienceWe reproduce, using scaled experiments in a water tank, the effects of scattering phenomena responsible for the degradations of sonar system performances in oceanic environment (typically, the small sound speed fluctuations associated with linear internal waves). We reproduce a wide panel of scattering effects, spanning from " simple " phase aberrations up to radical changes in the sound field structure (appearance of caustics). An experimental protocol was developed. It consists in transmitting a high-frequency wave train (ultrasonic pressure field around 2MHz) through wax lenses with randomly rough faces, that induce distortions comparable to those that would be observed at sea at around 1kHz in the case of a lower frequency acoustic signal travelling through a linear internal wave field. Using a 3-D printer, we were able to manufacture lenses with a randomly rough face characterized by its amplitude and vertical and horizontal correlation lengths. The dependence of the various parameters involved in the experiment (related to the object, distance of propagation, frequency, …) were studied using simulation programs allowing to measure the average number of eigen rays and the phase difference between the extreme micro paths. Those two quantities are useful to compare our results to what was obtained in the literature, in particular to Flatté's dimensionless analysis. The propagation through the lenses was then studied in a water tank using virtual arrays (automatic displacements of a hydrophone). We represent the results using the acoustic envelop in order to observe wave front distortions or appearance of caustics. Measurements of the coherence function and, hence, of the radius of coherence, are carried out. Finally, we observe degradation of the performances of a localization algorithm

PROPAGATION OF ACOUSTIC WAVES THROUGH A SPATIALLY FLUCTUATING MEDIUM: THEORETICAL STUDY OF THE PHYSICAL PHENOMENA

International audienceThe authors focus on the effects of phenomena, such as linear internal waves, that are responsible for fluctuations of the depth-dependent sound speed profile and, hence, induce distortions of the resulting acoustic pressure field and degradation of the associated sonar performances. The main goal of this study is to develop a scaled experiment configuration able to provide some results representative of this kind of distortions. To do so, a theoretical study of the phenomenon has first been carried out: we obtained an expression for the standard parabolic equation applied to the Fourier transform of the moments of order 2 and 4 in 3D medium. Various simulation programs were developed and used for the following purposes: validating or discarding some relationships given by Flatté through his classical dimensionless analysis (ΛΦ plane); tracing rays through an acoustic lens featuring a plane face and a randomly rough face and propagating an acoustic wave through the same object in order to anticipate for the shape of the distorted pressure field, including diffraction effects. We were able both theoretically and experimentally to induce acoustic scattering that mimics, at reduced scale and frequencies around 2MHz, the correlation properties and the corresponding array performance that would be observed at sea, after propagation through a linear internal wave field, or reflection on a rough sea surface

EXPERIMENTAL STUDY OF THE INFLUENCE OF SPATIAL INHOMOGENEITIES IN UNDERWATER ACOUSTIC PROPAGATION

International audienceThe authors investigate here the problem of acoustic wave transmission through a spatially fluctuating medium. Although experimental and analytical study are available in the literature, the objective is here to reproduce in tanks some phenomena, such as linear internal waves, that are responsible for horizontal fluctuations of the depth dependant sound speed profile and de-coherence effects of the propagated acoustic signals. The idea is to use acoustic lenses, or wax plates presenting a specific profile, to obtain ultrasonic pressure fields comparable to what can be observed in the case of lower frequency acoustic wave travelling through linear internal waves. Analytical studies allowing to compare dimensionless quantities relative to the measured field with Flatté's classical typology are developed as a support for the experiment. We believe that being able to reproduce these phenomena in controlled environment will be of great help not only to understand and anticipate the perturbations observed on the acoustic wave fronts, but also to work on some corrective signal processing techniques. We focus here on the observation of the wave fronts of the perturbed signals and on the influence of the perturbations on a focalization algorithm

RAFAL: RANDOM FACED ACOUSTIC LENS USED TO MODEL INTERNAL WAVES EFFECTS ON UNDERWATER ACOUSTIC PROPAGATION

International audienceWe present here an experimental protocol to reproduce the effects of linear internal waves (LIW) on acoustic wave propagation in a very controlled and reproducible manner. In fact, the experiment consists in propagating an ultrasonic wave through an acoustic lens presenting a plane input face and a randomly rough output face. The so-called RAFAL (Random Faced Acoustic Lens) was designed so that the roughness of the output face induce resulting acoustic pressure field featuring typical characteristics of propagation though LIW.To ensure representativeness of our model, we conducted analytical calculations leading to dimensionless parameters equivalent to the ones developed by Flatté (strength parameter Φ and diffraction parameter Λ). In our case, the strength parameter was calculated after evaluation of the phase of the average acoustic field propagated through the RAFALS, whereas our diffraction parameter was evaluated using the phase sensitivity kernel. On the other hand, we calculated the ratio of correlation length of the acoustic field to wavelength. Measurements were conducted on several RAFALS, corresponding to various realistic configurations. The regimes of saturation (full and partial) and unsaturation were explored. The results are presented in terms of order 2 (coherence function) and order 4 (intensity) statistics and demonstrate the accuracy of our experimental scheme with respect to real scale simulations and simplified theory. Other representations, such as phasors, also show a very meaningful behavior

INFLUENCE OF DE-COHERENCE EFFECTS ON SONAR ARRAY GAIN: SCLAED EXPERIMENT, SIMULATIONS AND SIMPLIFIED THEORY COMPARISON

International audienceOur study focuses on the subject of acoustic wave propagation through spatially fluctuating ocean. The fluctuations are here linear internal waves (LIW) and we developed an experimental protocol in water tank in order to reproduce the effects of LIW on ultrasound propagation. The present paper gathers the results obtained in terms of coherence function (second-order moment) for various configurations. Typical regimes of the ΛΦ plane developed by Flatté were explored, resulting into coherence function becoming narrower as the saturation increases. We also relate the coherence function to an array gain degradation parameter, δAG, which accounts for how the system performance will be mitigated in a given configuration. δAG was calculated for various sizes of vertical linear array (VLA) and showed an important dependence on the VLA's length. Typically, in any case (scaled experiment, computer simulations and simplified theory), we note that the longer the VLA, the greater the corresponding δAG. Moreover, as the saturation induced by medium fluctuations increases, δAG increases as well. This highlights the need for corrective signal processing techniques when large VLAs are used in a fluctuating environment. Signal processing techniques from various domains (e.g. adaptive optics, radio) are also studied

Panorama des opérateurs de focalisation et applications

Au cours de ces dernières années, plusieurs méthodes de traitement de signaux à large bande ont été proposées. Ces méthodes sont une extension des algorithmes de signaux bande étroite. Elles sont basées essentiellement sur la technique de focalisation. La recherche d'une amélioration du rapport signal sur bruit a conduit au développement de nombreux opérateurs de focalisation. Certains travaux ont montré que les opérateurs unitaires sont optimaux. Ces opérateurs sont construits à partir des vecteurs propres des matrices interspectrales des signaux enregistrés. L'objectif principal de ces opérateurs est de conserver la propriété du bruit blanc après focalisation. Dans cette étude, nous nous proposons de nouveaux opérateurs de focalisation en présence d'un bruit gaussien de structure de corrélation spatiale inconnue. Pour cela nous utilisons les vecteurs propres de la matrice des cumulants des signaux reçus. La focalisation utilisant les opérateurs proposés améliore considérablement le rapport signal sur bruit et permet une meilleure localisation de sources. De plus elle conduit à une réduction du temps de calcul comparée aux méthodes classiques. Nous avons comparé les divers algorithmes en utilisant des données réelles d'acoustique sous-marine. Les résultats obtenus montrent l'efficacité, en terme de localisation, des algorithmes proposés

Inversion géoacoustique passive en milieux petits fonds à partir de signaux représentatifs des émissions de cétacés

International audienceCet article présente un procédé d'inversion géoacoustique passif à partir de signaux représentatifs de vocalises de mammifères marins et d'un unique hydrophone en milieux petits fonds. Notre méthode utilise au mieux la propagation en multitrajets des vocalises et leur signature temps-fréquence pour extraire un observable permettant d'estimer par inversion les propriétés du fond marin. Il s'applique aux vocalises présentant au moins trois trajets résolus (direct, fond, surface) ce qui implique une portée d'environ 300 m autour de l'hydrophone pour des hauteurs d'eau de 100 m. A partir d'outils temps fréquence, chaque émission est localisée et permet d'estimer le couple (rasance, coefficient de réflexion) lui correspondant. Le cumul d'un ensemble d'émissions permet de mesurer une courbe du coefficient de réflexion en fonction des angles de rasance. Cette donnée alimente un algorithme d'inversion géoacoustique mis en oeuvre à travers un algorithme génétique. Dans cet article, nous détaillons la théorie de la méthode et nous étudions ses performances. Nous démontrons la validité de la méthode sur des signaux génériques réels émis par une source contrôlée dans le Golfe du Lion (rampes de fréquences de 1 kHz à 2 kHz de durée 10 ms). Ces données permettent de valider la méthode sur des signaux aux caractéristiques très proches de celles de vocalises de mysticètes (baleines à bosse par exemple). Le cas des sifflements de delphinidés (ex : dauphins communs), plus hauts en fréquence, est également abordé

Time-scale analysis of acoustic scattering by elastic spherical shells for impulse sources. Analyse temps-echelle de la diffusion acoustique par des coques spheriques elastiques en regime impusionnel.

Ce journal (Journal d'Acoustique) n'exsie plus depuis 1996 (Edition de Physique).Mis en ligne avec l'aimable autorisation de l'éditeur.International audienceTime-frequency methods, such smoothed Wigner-Ville transforms, have already been used in studies of the possibility of characterizing scatterers of simple geometric shape. We present here results obtained with the help of a time-and-scale method, the so-called wavelet transform. This transform is used here to analyze results from a scattering experiment on spherical elastic shells, in water. The results obtained are as yet preliminary and semi-quantitative. They indicate however convincingly that certain physical quantities such as type of waves, group velocity, and dispersion laws can be estimated with the help of wavelet methods

Extraction of Modulation laws of Elastic Shells by the use of the Wavelet Transform.

9 pagesInternational audienceThis paper isconcerned with the characterization of elastic targets immersed in a fluid and submitted to an acoustic impulse. Time-frequency methods have already been used in the case of scatterers of simple geometric shape. We have chosen the wavelet transform for its particular properties, such as linearity and local analysis at Df/f=Cst. We have developped an algorithm based on the behavior of the phase of the transform, which enables us to extract modulation laws (related to the dispersion law of the phase velocity), even for close echoes. In the case of spherical elastic shells,we have applied this method to both experimental and simulated signals. We point out the good agreement between theoretical ans experimental results