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

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

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

Dimensional analysis adapted to scaled experiments

The authors focus here on the study of a scaled experiment. The intrinsic objective is to reproduce the effects of medium fluctuations on underwater acoustic propagation. To do so, an adaptation of the derivation of the dimensionless parameters generally used to define the regimes of fluctuations is proposed. The aim of the present paper is to present of the calculations leading to the evaluation of these parameters. The procedure is based on the analytical calculation of the sound field propagated through an acoustic lens presenting a plane input face and a randomly rough output face. Statistics on the sound field (first and second-order moments) and sensitivity kernels lead to the evaluation of the so-called strength and diffraction parameters, as well as the ratio of acoustic correlation length to the wavelength. Continuity between our scaled experimental protocol and realistic oceanic configurations is therefore ensured

Un banc d'essai ultrasonore pour reproduire la dégradation de performance sonar en milieu marin fluctuant

This thesis focuses on wave propagation in random media. Especially, the ocean medium is subject to many sources of fluctuations. The most critical ones were found to be internal waves, occurring frequently and inducing fluctuations of the spatial distribution of the sound speed field. Because of the fairly long period of this phenomenon as compared to the propagation time of acoustic waves for sonar applications (typically for frequencies of 1 to 15 kHz and propagation ranges of 1 to 10 km), the process can be considered frozen in time for each stochastic realization of the medium. The intrinsic objective of this project is to develop and benchmark corrective signal processing techniques allowing to mitigate the degradation of performance induced by the medium fluctuations. The development of testbenches allowing to reproduce the effect of atmospheric turbulence on optic waves propagation under laboratory conditions lead to considerable advancements in the field of adaptive optics. We therefore see a vivid interest in being able to reproduce the effects of internal waves on sound propagation in controlled environments. An experimental protocol in a water tank is proposed: an ultrasonic wave is transmitted through a randomly rough acoustic lens, producing distortions of the received wavefront. The induced signal fluctuations are controlled by tuning the statistical parameters of the roughness of the lens. Especially, they are linked to dimensional parameters allowing to classify the configurations into regimes of fluctuations and to predict the statistical moment of the acoustic pressure up to the fourth order. A remarkable relevance of our experimental scheme is found when compared to theoretical and simulation results.The degradation of classical signal processing techniques when applied to our acquired data highlights the need for corrective detection techniques. A review of the existing techniques in other domains is proposed.Ces travaux concernent la propagation d’ondes en milieu aléatoire. En particulier, le milieu océanique est sujet à de nombreuses sources de fluctuations. Les plus importantes sont les ondes internes, très fréquentes et entrainant des fluctuations de la distribution spatiale du champ de célérité du son. En raison de la longue période de ces phénomènes comparée au temps de propagation des ondes acoustiques pour des applications sonar (typiquement pour des fréquences dans la bande 1-15 kHz et pour des distances de 1 à 10 km), le processus peut être considéré figé dans le temps pour chaque réalisation stochastique du milieu. L'objectif intrinsèque de ce projet est de développer et valider des techniques de traitement correctif promettant de compenser la dégradation de performances induites par les fluctuations du milieu.Le développement de bancs d'essais permettant de reproduire les effets de la turbulence atmosphérique sur la propagation d'ondes optiques a permis des avancées considérables dans le domaine de l'optique adaptative. Nous voyons donc un fort intérêt dans la possibilité de reproduire les effets des ondes internes sur la propagation du son en environnement contrôlé. Un protocole expérimental dans une cuve d'eau est proposé: une onde ultrasonore est transmise à travers une lentille acoustique aléatoirement rugueuse, ce qui produit des distorsions du front d’onde reçu. Les fluctuations des signaux reçus sont contrôlées en modifiant les paramètres statistiques de rugosité de la lentille. Ces paramètres sont reliés à l’analyse dimensionnelle permettant de classifier les configurations étudiées selon des régimes de fluctuations et de prédire les moments statistiques du champ acoustique jusqu'à l'ordre quatre. Une excellente correspondance est observée entre notre protocole expérimental et des résultats théoriques et numériques.La dégradation des performances des techniques de détection classiques appliquées à nos données expérimentales souligne le besoin de techniques correctives. Un état de l’art des techniques existantes dans divers domaines est proposé

Direct regressions for underwater acoustic source localization in fluctuating oceans

International audienceIn this paper, we show the potential of machine learning regarding the task of underwater source localization through a fluctuating ocean. Underwater source localization is classically addressed under the angle of inversion techniques. However, because an inversion scheme is necessarily based on the knowledge of the environmental parameters, it may be not well adapted to a random and fluctuating underwater channel. Conversely, machine learning only requires using a training database, the environmental characteristics underlying the regression models. This makes machine learning adapted to fluctuating channels. In this paper, we propose to use non linear regressions for source localization in fluctuating oceans. The kernel regression as well as the local linear regression are compared to typical inversion techniques, namely Matched Field Beamforming and the algorithm MUSIC. Our experiments use both real tank-based and simulated data, introduced in the works of Real et al. Based on Monte Carlo iterations, we show that the machine learning approaches may outperform the inversion techniques

Localisation d'une source acoustique en petits fonds à l'aide d'une antenne plane verticale

Dans cet article, nous analysons les signaux d'une campagne en mer en petits fonds près de la côte orientale de la Corse. Cette expérimentation, menée en novembre 2016 par la DGA sur le R/V JANUS II, fait partie du projet ALMA (Acoustic Laboratory for Marine Applications). Elle consistait à émettre successivement des formes d'ondes complexes composées d'impulsions de modulation de fréquence linéaire et de signaux à fréquences continues, à partir d'un pinger acoustique puis à enregistrer les signaux sur une antenne acoustique passive à environ 9 km. Cette antenne est formée de quatre réseaux de lignes verticales portant chacun 32 hydrophones. Lors des travaux antérieurs sur l’analyse des capacités de détection liée aux fluctuations environnementales, nous avons pu observer que les variations de la célérité dégradent les performances de la formation de voies. Nous présentons ici une nouvelle méthode de localisation performante. La structure du champ sonore reçu est élaborée en appliquant une formation de voies dans le domaine temporel suivie d’un traitement cohérent par inter corrélation entre la réplique source - ici une LFM large bande - avec le signal reçu. Ces traitements permettent d'obtenir une veille panoramique dans le plan site retard et d’extraire les détections. La hauteur des lignes de l’antenne verticale et la large bande passante permettent de séparer efficacement les nombreux trajets sonores dans des environnements petits fonds. Nous estimons ensuite la position de la source en comparant les mesures associées aux détections avec les sorties du modèle de rayons ‘Bellhop’. Les techniques de localisation basées sur la distance de Haussdorff, qui ont montré des résultats prometteurs dans des approches similaires où l'inadéquation environnementale était probable, sont alors explorées. Les performances de cette méthode sont évaluées sur des données acquises dans configurations environnementales diverses en termes de niveau de bruit et de stationnarité