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

Hausdorff Distance Applied On Real Data Experiment For Underwater Localization

International audience— This paper addresses the problem of localizing and tracking a surface or underwater vessel with the technique called as Hausdorff Distance. Two proposed approaches, based on TDOAs comparison, were used for 2-D localization, in range and depth, with one sensor only, and have been successfully applied to localize a motionless unknown target in a tank's experiment. Results in terms of the localization accuracy have been obtained with real signal and the performance of the proposed localization techniques have been demonstrated and confirmed by simulation with respect of signal-to-noise ratio and compared with the correlation techniques used nowadays for single hydrophones

Understanding deep-water striation patterns and predicting the waveguide invariant as a distribution depending on range and depth

Author Posting. © Acoustical Society of America, 2018. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 143 (2018): 3444, doi:10.1121/1.5040982.The Waveguide Invariant (WI) theory has been introduced to quantify the orientation of the intensity interference patterns in a range-frequency domain. When the sound speed is constant over the water column, the WI is a scalar with the canonical value of 1. But, when considering shallow waters with a stratified sound speed profile, the WI ceases to be constant and is more appropriately described by a distribution, which is mainly sensitive to source/receiver depths. Such configurations have been widely investigated, with practical applications including passive source localization. However, in deep waters, the interference pattern is much more complex and variable. In fact the observed WI varies with source/receiver depth but it also varies very quickly with source-array range. In this paper, the authors investigate two phenomena responsible for this variability, namely the dominance of the acoustic field by groups of modes and the frequency dependence of the eigenmodes. Using a ray-mode approach, these two features are integrated in a WI distribution derivation. Their importance in deep-water is validated by testing the calculated WI distribution against a reference distribution directly measured on synthetic data. The proposed WI derivation provides a thorough way to predict and understand the striation patterns in deep-water context.This work was funded by Delegation General de l’Armement and by Thales Underwater Systems. We warmly acknowledge D. Fattaccioli (DGA) for his scientific support. J.B.’s contribution was supported by ENSTA Bretagne (France) and by the Investment in Science Fund at WHOI (USA)

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

Nrf2-interacting nutrients and COVID-19 : time for research to develop adaptation strategies

There are large between- and within-country variations in COVID-19 death rates. Some very low death rate settings such as Eastern Asia, Central Europe, the Balkans and Africa have a common feature of eating large quantities of fermented foods whose intake is associated with the activation of the Nrf2 (Nuclear factor (erythroid-derived 2)-like 2) anti-oxidant transcription factor. There are many Nrf2-interacting nutrients (berberine, curcumin, epigallocatechin gallate, genistein, quercetin, resveratrol, sulforaphane) that all act similarly to reduce insulin resistance, endothelial damage, lung injury and cytokine storm. They also act on the same mechanisms (mTOR: Mammalian target of rapamycin, PPAR gamma:Peroxisome proliferator-activated receptor, NF kappa B: Nuclear factor kappa B, ERK: Extracellular signal-regulated kinases and eIF2 alpha:Elongation initiation factor 2 alpha). They may as a result be important in mitigating the severity of COVID-19, acting through the endoplasmic reticulum stress or ACE-Angiotensin-II-AT(1)R axis (AT(1)R) pathway. Many Nrf2-interacting nutrients are also interacting with TRPA1 and/or TRPV1. Interestingly, geographical areas with very low COVID-19 mortality are those with the lowest prevalence of obesity (Sub-Saharan Africa and Asia). It is tempting to propose that Nrf2-interacting foods and nutrients can re-balance insulin resistance and have a significant effect on COVID-19 severity. It is therefore possible that the intake of these foods may restore an optimal natural balance for the Nrf2 pathway and may be of interest in the mitigation of COVID-19 severity

High fidelity simulation tools for performance assessment of underwater acoustic communications modems

International audienceNew attractive techniques, such as multi-carrier modulations, efficient channel coding, iterative reception algorithms, and use of multiple antennas at reception or transmission, are now emerging in underwater acoustic communications (UAC). According to the published results, these techniques could provide, within the next years, a significant breakthrough with respects to the critical issue of robust and high bitrate (a few kbits/sec) tactical communications with submarines and/or AUV). Thales Underwater Systems (TUS), as a UAC equipments manufacturer and integrator, is therefore very interested in performance assessment of these techniques, either by implementing the corresponding signaling schemes and algorithms, or by testing them as black boxes from external suppliers. As long as possible this assessment has to be done without redoing expensive at sea experiments, but by using simulated data based upon previous experiments and/or high fidelity acoustic propagation modeling. A critical point is that UAC is likely one the most demanding applications for underwater acoustic propagation modeling. Indeed, apart from an accurate knowledge of the Signal-to-Noise ratio, the prediction and simulation of the UAC channel require a comprehensive modeling of the its time and spatial fluctuations (UAC channel modeled as a doubly dispersive Ricean channel). Constraints on the equipments (e.g. rms and peak transmitted power) have also to be taken into account. Specific tools which address the above aspects will be presented, from the computation of Shannon channel capacity under the above assumptions to high fidelity simulations tools relying on an explicit modeling of scattering of sound by the moving rough sea surface (e.g. NARCISSUS [1] developed by TUS). Emphasis will finally be laid upon a new innovative stochastic replay technique [2], which consists in simulating a random channel whose response has exactly the same statistical properties as a previously at sea recorded channel response, thus allowing perfect comparison of different modulation schemes or trimmings under the same environmental conditions