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

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

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

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

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

Time-Frequency Representations matched to guided waves

Recent progress on time-frequency representation (TFR) are based on adapted methods. We develop an original methodology based on physics of propagation in Underwater Acoustics. We present first the methodology for guided waves. We build then matched TFR to the Perfect Waveguide and to the Pekeris Waveguide which both describe wave propagation on Ultra Low Frequency (1-100 Hz) on UA shallow water environment. We test those methods on real pressure signals. We evaluate finally the built TFR and we study their limits.Les progrès récents en représentation temps-fréquence (RTF) reposent sur la mise au point de méthodes adaptées aux signaux traités. Nous développons ici une méthodologie originale dans laquelle la RTF est supervisée par les connaissances a priori issues de la physique de la propagation des ondes en Acoustique Sous-Marine (ASM). Nous présentons d'abord la méthodologie générale des RTF adaptées applicables aux signaux propagés dans un guide d'ondes. Nous construisons ensuite des RTF adaptées au guide parfait (guide sans perte) et au guide de Pekeris [1] qui décrivent tous deux la propagation des ondes d'Ultra Basse Fréquence (1-100 Hz) en ASM dans un environnement petit fond. Nous testons ces méthodes sur des signaux réels. Nous évaluons enfin les RTF construites et exposons leurs limites

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

Acoustic signals generated by filamentation of ultrashort TW laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.Comment: 9 pages, 12 figure

Utilisation d'une source laser pulsée à haute energie comme source acoustique large bande en milieu liquide Source acoustique générée par un laser pulsé intense

International audienceIn this work the non-linear opto-acoustic problem which consists in generating an acoustic signal in water from an intense ultra short laser pulse has been studied. The acoustic source obtained could be related to the phenomenon of filamentation which produces a contraction of the initial beam accompanied by the formation of plasma. Relatively recent work has shown that lasers of this type could be used to produce remote acoustic sources with interesting applications to underwater acoustics. The spectrum of the sound source obtained was investigated and its directivity pattern in both planes (plane of the filament and plane 96 TS. Volume 33 – n° 1/2016 perpendicular to the filament) was measured. The sound level of the source as a function of energy, duration, and wavelength of the laser pulse was also measured.Ce travail est relatif à l'étude expérimentale d'un problème d'opto-acoustique non linéaire, consistant à générer un signal acoustique dans l'eau à partir d'un laser pulsé térawatt (TW). La source acoustique obtenue a pu être reliée au phénomène de filamentation qui produit une contraction du faisceau initial, accompagnée de la formation d'un plasma. Des travaux relativement récents ont montré que les lasers de ce type pouvaient être utilisés pour produire des sources acoustiques déportées, avec des applications intéressantes pour l'acoustique sous-marine. Le spectre de la source acoustique obtenue a été étudié à l'aide de plusieurs hydrophones couvrant une très large bande de fréquence, et le diagramme de directivité a été mesuré dans deux plans (plan du filament et plan perpendiculaire au filament). Le niveau acoustique de la source en fonction de l'énergie, de la durée, et de la longueur d'onde de l'impulsion laser, a également été étudi

Etudes theoriques sur les interferometres pour la detection des ondes de gravitation

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc