Project

ERATO research project

Goal: covert REA using geoacoustic inversion and data fusion

Updates
0 new
0
Recommendations
0 new
0
Followers
0 new
5
Reads
0 new
57

Project log

Yann Stephan
added 6 research items
Ocean acoustic tomography is a technique for observing the dynamic behaviour of ocean processes by measuring the changes in travel time of acoustic signal transmitted over a number of ocean paths. Its ability for large scale integrated measurements makes tomography a candidate for a permanent and continuous observational network for monitoring oceanic circulation at basin scale. However the use of tomography data in studies involving numerical modelling raises several theoretical and practical difficulties. The variational approach is proposed to assimilate the tomography data collected during the THETIS experiments in the primitive equation model of the western Mediterranean Sea within the EUROMODEL project
The purpose of this study is to theoretically evaluate the performances of a vertical array in a fluctuating medium by the mean of arrival times-angles diagrams. First, pertinence of using the eikonal approximation to monitor the sound speed structure in a fluctuating ocean and especially to probe meso-structure in the northeastern Atlantic Ocean is discussed. Then, a study of the evolution of tomographic parameters: travel time, temporal spreading, temporal resolution, angular density...based on rays theory is conducted through a synthesis of recent theoretical studies and a parametric approach. Its aim is to quantify and precisely define the undesirable effects induced by the fluctuating medium. These results are compared with those of a statistical study, and then efficiency of an array processing dealing with a joint assessment of arrival angles and times is investigated
The purpose of this study is to theoretically evaluate the performance of a vertical receiving array in a fluctuating medium by means of arrival time-angle diagrams. First, the suitability of using the eikonal approximation to monitor the sound speed structure in a fluctuating ocean and especially to probe meso-structure in the North-East Atlantic Ocean are discussed. Then, a study of the evolution of tomographic parameters: travel time, temporal spreading, temporal resolution and angular density, based on ray theory is conducted through a synthesis of recent theoretical and experimental studies. Its aim is to quantify and precisely define the undesirable effects induced by the fluctuating medium. These results will be compared with those of a statistical study, and then the effectiveness of an array processing scheme dealing with a joint assessment of arrival angles and times will be investigated. Finally, the feasibility of realizing predicted improvements are considered, particularly for the case of the North-East Atlantic Ocean.
Yann Stephan
added 6 research items
Internal tides commonly occur along ocean coasts. They are internal waves driven by the usual tidal force and generated by scattering at a sharp bottom feature such as the shelf break. The internal tides are dramatic features with crests typically 10-30 km apart and wave heights of 20 m. On the ocean surface they cause only a gentle ripple about 10 cm high but they affect the shine of the surface. As a result, astronauts often see them as the tides propagate away from the shelf break. In June 1996, a shallow-water tomography experiment (INTIMATE 96) was conducted off the coast of Portugal to observe these internal tides and learn about their acoustic effects. A source was towed around a vertical hydrophone array to produce acoustic sections along several slices. The experiment also provided an ideal opportunity for testing model-based source tracking. Acoustic sections taken parallel to the Portuguese coast allowed us to understand the propagation physics in a range-independent area. With this we have been able to develop a matched-field algorithm suitable for use in the far more complicated downslope direction which, being perpendicular to the crests of the internal tides, also experiences strong ocean-temperature variations. We will discuss both the approach and the source tracking.
In the summer of 1996, an experiment was conducted off the coast of Portugal to study the effects of internal tides on sound propagation. This experiment—called INTIMATE '96 (Internal Tide Investigation by Means of Acoustic Tomography Experiment)—has provided a great deal of insight about the variability of pulse transmission over space and time. In contrast to a common view of shallow-water propagation as complicated and unpredictable, we find a steady pattern of echoes. The echo-pattern stretches and shrinks in a systematic way with the tides and allows us to infer the components of the first few oceanographic modes. We also used the echo-pattern to track the source over a period of several days. During this period the isotherms in the ocean wavered by 20 m as a result of the tides, providing a challenge for model-based tracking. We will discuss these acoustic results with emphasis on the source tracking.
Yann Stephan
added 2 research items
The first-order small slope approximation is applied to model the scattering strength from a rough surface in underwater acoustics to account for seafloor for high frequencies from 10 kHz to hundreds of kilohertz. Emphasis is placed on simulating the response from two-dimensional anisotropic rough surfaces. Several rough surfaces are described based on structure functions such as the particular sandy ripples shape. The scattering strength is predicted by the small slope approximation and is first compared to a well known bistatic method, interpolating the Kirchhoff approximation and the small perturbations model, assuming that the rough interface is isotropic. Results obtained from the two different mod-els are similar and show a higher level in the specular direction than in the other directions. For an isotropic surface, changing the propagation plane gives similar results. Then, SSA, which lets us adapt the structure function of the roughness straight away, is tested trough several anisotropic surfaces. In a longitudinal direction of ripples, the scatter-ing strength is mostly in the specular direction, whereas in the transversal direction of ripples, the scattering strength prediction shows high values for different angular directions. Thus the scattering strength is spread in a very different way strictly related to the particular features of the ripples. Combine our results, indicates the importance of taking into account the anisotropy of a surface in a scattering prediction process, taking into account the positions of the emitter and of the receiver which are naturally significant when predicting scattering strength.
Seismic velocities together with sediment/rock compaction models are of common use in the oil industry to predict reservoir pressures and to detect and identify any potential hazard associated with over-pressured formations. Excess pore pressure is a key parameter for geohazard assessment, nevertheless this approach is very seldom applied to shallow offshore engineering as it requires expensive ground-truth boreholes. The simultaneous availability in the Bourcart-Herault canyon intertluve (Gulf of Lion - NW Mediterranean) of 1) geotechnical and sedimentological data (300 m boreholes) from the PROMESS project; 2) in situ pore pressure measurements; and 3) High Resolution (FIR) seismic geophysical data showing the presence of active structures in relation to fluid circulation, allows the importance of a multi-disciplinary approach to shallow geohazard assessment to be illustrated through a case study. The velocity field obtained from HR seismic data is interpreted in light of the neighbouring boreholes and resulting velocity anomalies are translated in terms of fluid overpressure and free gas content. Additional in situ surface geotechnical measurements (piezometers) are then taken into account and the role of these parameters is discussed.
Yann Stephan
added 4 research items
Experimental observations of acoustic propagation through a Soliton Wave Packet (SWP) show an abnormally large attenuation over some fre- quencies, that was found to be signicantly time dependent and anisotropic. Nevertheless, by remarking the problem of signal attenuation, the approach used in most of the studies can be considered as "static" since no additional eects were taken into account as a SWP evolves in range and time. Hy- drographic and acoustic data from the INTIMATE'96 experiment clearly exhibit traces of the presence of soliton packets, but in contrast with known observations of attenuation its frequency response also reveals a sudden increase of signal amplitude, which may be due to a focussing eect. This
In the summer of 1996, an experiment was conducted off the coast of Portugal to study the effects of internal tides on sound propagation. This experiment —called INTIMATE '96 (Internal Tide Investigation by Means of Acoustic Tomography Experiment)—has provided a great deal of insight about the variability of pulse transmission over space and time. In contrast to a common view of shallow-water propagation as complicated and unpredictable, we find a steady pattern of echoes. The echo-pattern stretches and shrinks in a systematic way with the tides and allows us to infer the components of the first few oceanographic modes. We also used the echo-pattern to track the source over a period of several days. During this period the isotherms in the ocean wavered by 20 m as a result of the tides, providing a cha l- lenge for model-based tracking. We will discuss these acoustic results with emph a- sis on the source tracking.
Yann Stephan
added a research item
The method presented in this paper assumes that the received signal is a linear combination of delayed and attenuated uncorrelated replicas of the source emitted waveform. The set of delays and attenuations, together with the channel environmental conditions, provide sufficient information for determining the source location. If the transmission channel is assumed known, the source location can be estimated by matching the data with the acoustic field predicted by the model conditioned on the estimated delay set. This paper presents alternative techniques that do not directly attempt to estimate time delays from the data but, instead, estimate the subspace spanned by the delayed source signal paths. Source Localization is then done using a family of measures of the distance between that subspace and the subspace spanned by the replicas provided by the model. Results obtained on the INTIMATE'96 data set, in a shallow-water acoustic channel off the coast of Portugal, show that a sound source emitting a 300-800-Hz LFM sweep could effectively be localized in range or depth over an entire day
Yann Stephan
added 5 research items
Invert acoustic data using sparse arrays - at the limit with a single hydrophone - is a challenging task. The final goal is to obtain a rapid environmental assessment with systems both easier to deploy and less expensive than full vertical arrays. In this paper, it is shown that using a known broadband source signal and an array with few hydrophones, ocean acoustic tomography can be performed, even in a complex internal waves induced highly variable ocean. The inversion approach presented herein is based on an arrival matching processor and a genetic algorithm search procedure. Due to the poor accuracy on the a priori knowledge of the source range, source depth and water depth, the inversion procedure was split in two stages: in the first stage the geometric parameters where estimated and in the second stage sound speed estimates where obtained. This procedure was applied to field data, acquired during the INTIMATE'98 sea trial, in a shallow water area off the coast of France in the Gulf of Biscay. That area is expected to have a relatively high internal wave activity, specially during the summer. A 4 sec long, 700 Hz bandwidth linear frequency modulated signal was transmitted from a ship suspended sound source and received on a 4 element vertical array at a range of approximately 10.5 km, over a relatively range-independent area. The results from the inversion of the acoustic data are in line with those obtained by concurrent non-acoustic data like GPS source range, measured source depth, XBT casts and temperature sensors.
Inversion methods based on spatial coherence of multi-tone CW data have been applied extensively in underwater acoustics to estimate geoacoustic profiles. This paper presents results for a different approach based on using the time coherent information in a broadband signal measured at a single hydrophone. The inversion is formulated as an optimization problem to determine the geoacoustic model that provides the best fit between measured signal waveforms and replica waveforms that are calculated using ray theory. An efficient hybrid search method that combines simulated annealing with a local downhill simplex algorithm is used to search the multi-dimensional model parameter space that includes both the geoacoustic and geometrical parameters of the experiment. The method is applied to data from the INTIMATE'98 shallow water experiment to estimate the sound speed and density at the sea floor. The inversion obtained stable estimates of the most sensitive geoacoustic model parameters that were in good agreement with ground truth data that were obtained for the site.
Yann Stephan
added a research item
This paper discusses inversions for geoacoustic parameters of very shallow water channels (depth from 10 to 30 meters) using monochromatic sounds radiated by moving ships. Our approach requires a very simple system and allows to take into account several ships. By using the movement of the ships we are able to extract radial wave numbers of the channel at several frequencies and thus to fill dispersion curves. We propose to invert those dispersion curves in order to estimate the geoacoustic parameters. We describe and validate our approach on synthetic realistic data and on real very shallow water at sea trial data conducted in the south of Barcelona. Keywords: Geoacoustic inversion, Passive tomography, ship, wave number, shallow water. RÉSUMÉ: Dans le contexte de la tomographie acoustique passive, nous proposons une méthode d'estimation des propriétés d'un canal océanique très petits fonds (profondeur de 10 à 30 mètres) à partir des raies spectrales rayonnées par des navires en transit. La solution est développée afin de conduire à un système de mesure de simplicité maximale et permet la prise en compte de plusieurs navires dans le procédé d'inversion. A partir de l'observation acoustique de radiales centrées sur un hydrophone de mesure, les raies rayonnées par les navires et le mouvement de ceux-ci sont utilisés pour extraire les nombres d'onde radiaux du canal aux fréquences rayonnées. La concaténation des nombres radiaux d'onde pour une ou plusieurs fréquences issues d'un ou plusieurs navires constitue les courbes de dispersion du milieu qui sont inversées pour fournir les propriétés du canal. Une description des algorithmes et de leurs implémentations est effectuée et leurs validations sont envisagées sur des données synthétiques réalistes ainsi que sur des données réelles issues d'une campagne expérimentale très petits fonds réalisée au sud de Barcelone à partir des navires d'opportunité. Les résultats produits sont encourageants et permettent de dresser des perspectives d'amélioration pour la méthode proposée. Mots clés : Inversion Géo-acoustique, tomographie passive, navires d'opportunité, nombres d'onde radiaux, canal très petit fond.
Yann Stephan
added 3 research items
Processing marine-mammal signals for passive oceanic acoustic tomography or species classification and monitoring are problems that have recently attracted attention in scientific literature. For these purposes, it is necessary to use a method which could be able to extract the useful information about the processed data, knowing that the underwater environment is highly nonstationary. In this context, time-frequency (TF) or time-scale methods constitute a potential approach. Practically, it has been observed that the majority of TF structures of the marine-mammal signals are highly nonlinear. This fact affects dramatically the performances achieved by the Cohen's class methods, these methods being efficient in the presence of linear TF structures. Fortunately, thanks to the warping operator principle, it is possible to generate other class of time-frequency representations (TFRs). The new TFRs may analyze nonlinear chirp signals better than Cohen's class does. In spite of its mathematical elegance, this principle is limited in real applications by two major elements. First, as we will see, its implementation leads to a considerable growth of the signal length. Consequently, from operational point of view, this principle is limited to short synthetic signals. Second, the design of a single warping operator can be inappropriate if the analyzed signal is multicomponent. Furthermore, the choice of "adapted" warping operator becomes a problem when the signal components have different TF behaviors. In this paper, we propose a processing method of marine-mammal signals, well adapted to a real passive underwater context. The method tries to overcome the two aforementioned limitations. Also, the first step consists in data size reducing by the detection of the TF regions of interests (ROIs). Furthermore, in each ROI, a technique which combines some typical warping operators is used. The result is an analytical characterization of the instantaneous frequency laws (IFLs) of- signal components. The simulations on real underwater data show the performances of this method in comparison with classical ones
Ocean Acoustic Tomography (OAT) uses powerful active emissions of repetitive signals causing problems when acoustic discretion is required as in military operations. In this paper, we propose to develop a new concept of OAT, called Discreet Acoustic Tomography (DAT), which is based on a stealthy acoustic signals emission. An appropriate global procedure to synthesize a signal waveform in accordance with the compromise between interception probability and accuracy in channel parameters estimation, is proposed. Finally, this procedure was applied to a realistic scenario of which objective consists in hiding a synthetic signal in ship noise with the constraint to have an accurate estimation of the channel parameters. Results obtained illustrate the interest and the potential of the proposed method.
This paper presents the new concept of passive acoustic tomography which allows ocean data collection with a passive acoustic remote sensing process. The originality lies in using acoustic sources of opportunity such as surface noise, radiated ship noise and marine mammal calls. Such use of passive tomography is a promising way to reduce acoustic emissions in oceans. A review is first presented, including the description of new concepts of covert active, assisted passive and autonomous tomography, followed by applications on real world data. Under the assumptions of multipath propagation and measurements performed by a sparse network of hydrophones, a time–frequency processor is proposed to simultaneously estimate the source location and the impulse response of the propagation channel for marine mammal calls used as opportunistic sources (multipath structure, time delay and attenuation are estimated). Promising results are obtained on real data coming from the Laurentian channel where wideband beluga calls (1 to 3 kHz) are measured by a sparse network of 6 bottom hydrophones.
Yann Stephan
added 2 research items
We propose a new method to localize low-frequency calls in 2D in shallow waters from a sparse array of hydrophones using modal propagation modelling. An analysis of modal propagation modelling of transients signals in shallow water environment shows that the dispersive behaviour of the waveguide can be exploited to design a robust localization scheme without requiring any knowledge of the acoustics properties of the environment (bottom and water column) nor any simulation of propagation, The localization scheme also does not require synchronization of the array and is therefore independent of any clock drift. Promising results are obtained for Northern right whale gunshot calls from 'Bay of Fundy data set of the 2003 Workshop on Detection and Localization of Marine Mammals Using Passive Acoustics.'.
In the context of the passive geoacoustic inversion, this manuscript proposes a new Inversion Method suitable for very shallow water environments (10 to 30 m) from the broadband noise produced by ships of opportunity. The solution is dedicated to a small number of hydrophones and is easy to implement. The interference patterns generated by the ship movement and the propagation properties are exploited to extract the relative dispersion curves on a chosen bandwidth then the geoaoustic inversion is performed, Here, we describe our algorithms and validate them on realistic synthetic data as well as on real data from a very shallow water trial performed off the Southern coast of Barcelona, Spain, Valuable results are obtained and allow to draw some way of improvement.
Yann Stephan
added 2 research items
The estimation of the impulse response of a propagation channel is necessary for a large number of acoustic applications: underwater communication, detection and localization, etc. Basically, it informs us about the distortions of a transmitted signal in one channel. This operation is usually subject to additional distortions due to the motion of the transmitter channel-receiver configuration. This paper points on the effects of the motion while estimating the IR by matching filtering between the transmitted and the received signals and introduces a new motion compensation method. Knowing the transmitted signal, the apparent speed of each propagation path can be estimated using wideband ambiguity function [2]. Indeed, some interference appears in the wideband ambiguity plane because of the multipath propagation. A warpingbased lag- Doppler filtering method is proposed allowing us to accurately estimate the IR of the channel.
Underwater channel is an example of a natural environment potentially characterized by signals generated by various sources : underwater mammals, human activity noise, etc. In order to take advantage of these sources, the concept of passive acoustic tomography has been introduced. According to this concept, the environment parameters could be extracted from the analysis of the received signals. While the signal's parameters are intimately related to physical parameters of the environment, their accurate extraction is crucial. That is, this task is complex while we work in completely passive context and when we deal with a large diversity of underwater signals. Generally, signals issued from underwater environment have complex time-frequency structures : non-linear time-frequency and multi-components. Two typical non-linearities are generated by the dispersive systems and the relative motion between transmitter and receiver. Despite the origin of these phenomena, the signal approach proposed in this paper will provide a unique framework for parameters extraction. This approach is based on the time-frequency-phase coherence of any natural non-linear time-frequency component. Taking advantage of this coherence, the non-linear structures can be efficiently extracted and used for physical parameters estimation.
Yann Stephan
added a research item
The estimation of the impulse response (IR) of a propagation channel may be of great interest for a large number of underwater applications: underwater communications, sonar detection and localization, marine mammal monitoring, etc. It quantifies the distortions of the transmitted signal in the underwater channel and enables geoacoustic inversion. The propagating signal is usually subject to additional and undesirable distortions due to the motion of the transmitter channel-receiver configuration. This paper shows the effects of the motion while estimating the IR by matched filtering between the transmitted and the received signals. A methodology to compare IR estimation with and without motion is presented. Based on this comparison, a method for motion effect compensation is proposed in order to reduce motion-induced distortions. The proposed methodology is applied to real data sets collected in 2007 by the Service Hydrographique et Océanographique de la Marine in a shallow water environment, proving its interest for motion effect analysis. Motion compensated estimation of IRs is computed from sources transmitting broadband linear frequency modulations moving at up to 12 knots in the shallow water environment of the Malta plateau, South of Sicilia.
Yann Stephan
added 2 research items
A single-hydrophone geoacoustic inversion procedure using a calibrated source is proposed and tested with experimental shallow water data. In a first step the number of sediment layers, their thicknesses and velocities are estimated using a conventional seismic approach. In a second step, the amplitude variations of each reflected arrival are used to estimate the reflection coefficients versus the incidence angle. The results are compared to numerical model solutions. With this second step, velocity estimations are refined in order to predict the observed critical angles. Attenuation and density are estimated from the absolute level of the reflection coefficient. Finally, experimental and synthetic data generated after inversion are compared.
We propose the use of discrete time-scale transformations of acoustic signals to characterize the wideband effect of underwater environment propagation. The representation depends on the Mellin transform, and it can be used to efficiently process the effect of the underwater environment. For sparse environments, we also consider a new approach with reduced computational complexity. The approach is based on a warping lag- Doppler filtering technique in the wideband ambiguity function plane to separate ray paths and estimate their parameters. We validate the signal representation and filtering approach using real experimental data from the BASE07 experiment.
Yann Stephan
added 2 research items
In September 2009, during two days, a geo-acoustic experiment, named ERATO-09, was conducted by SHOM in the Bay of Biscay. During the experiment, the presence of common dolphins (Delphinus delphis) has been noticed. In this paper, we retrospectively evaluate the feasibility of passive acoustic monitoring to derive interactions between active acoustic scenarios and dolphin behaviour. Our study is based on measurements obtained by two autonomous hydrophones moored in the experimental area. We first evaluate the additional noise induced by the emissions in these two measurement points. The increment relative to ambient noise is 10 dB 1μPa2Hz ref-1 on average with peak values of 30 dB 1μPa2Hz ref-1. To assess the possible impact of acoustic emissions on dolphins behaviour (escape, attraction, vocal adjustment), we have developed a detection algorithm. This algorithm can detect whistles of common dolphins within a range of 1 km and with a detection success of 98%. Three active scenarios have been used to analyse the acoustic behaviour of dolphins in terms of whistle rates in relation to the power and type of anthropogenic sounds. The results do not show clear relationships between whistle activity and anthropogenic emissions. This is partly due to the scarcity of events that could be analysed and the short duration of the campaign. Nevertheless, this study shows the feasibility and interest of a passive acoustic monitoring system in conjunction with active experiments and identifies powerful analytical tools. This type of acoustic study should regularly be implemented in the future in order to consistently increase the data set for a more conclusive statistical analysis.
We investigate a characterization of underwater acoustic signals using extracted time-scale features of the propagation channel model for medium-to-high frequency range. The underwater environment over these frequencies causes multipath and Doppler scale changes on the transmitted signal. This is the result of the time-varying nature of the channel and also due to the relative motion between the transmitter-channel-receiver configuration. As a sparse model is essential for processing applications and for practical use in simulations, we employ the matching pursuit decomposition algorithm to estimate the channel time delay and Doppler scale change model attributes for each propagating path. The proposed signal characterization was validated for sparse channel profiles using real-time data from the BASE07 experiment.
Yann Stephan
added 4 research items
Ocean modeling and forecasting require significant hydro-graphic and satellite measurements to provide accurate and valid results through assimilation methods. In coastal environment, this task is made more complex because of the strong coupling between state variables and forcing, the lack of data and the frequent model break. In addition, in the region of Ushant, cloud cover often makes it impossible to use satellite measurements. Acoustic tomography can provide complementary measures in time and space scales relevant to regional modeling. In this context, this paper develops an approach to acoustic data assimilation based on Kalman filters using a feature model to parameterize the thermal front of Ushant. The simulation results show that this approach could acoustically track the front position, when present.
In September 2009, during two days, a geo-acoustic experiment, named ERATO-09, was conducted by SHOM in the Bay of Biscay. During the experiment, the presence of common dolphins (Delphinus delphis) has been noticed. In this paper, we retrospectively evaluate the feasibility of passive acoustic monitoring to derive interactions between active acoustic scenarios and dolphin behaviour. Our study is based on measurements obtained by two autonomous hydrophones moored in the experimental area. We first evaluate the additional noise induced by the emissions in these two measurement points. The increment relative to ambient noise is 10 dB 1μPa2Hz ref-1 on average with peak values of 30 dB 1μPa2Hz ref-1. To assess the possible impact of acoustic emissions on dolphins behaviour (escape, attraction, vocal adjustment), we have developed a detection algorithm. This algorithm can detect whistles of common dolphins within a range of 1 km and with a detection success of 98%. Three active scenarios have been used to analyse the acoustic behaviour of dolphins in terms of whistle rates in relation to the power and type of anthropogenic sounds. The results do not show clear relationships between whistle activity and anthropogenic emissions. This is partly due to the scarcity of events that could be analysed and the short duration of the campaign. Nevertheless, this study shows the feasibility and interest of a passive acoustic monitoring system in conjunction with active experiments and identifies powerful analytical tools. This type of acoustic study should regularly be implemented in the future in order to consistently increase the data set for a more conclusive statistical analysis.
Time-frequency representations constitute the main tool for analysis of nonstationary signals arising in real-life systems. One of the most challenging applications of time-frequency representations deal with the analysis of the underwater acoustic signals. Recently, the interest for dispersive channels increased mainly due to the presence of the wide band nonlinear effect at very low frequencies. That is, if we intend to establish an underwater communication link at low frequencies, the dispersion phenomenon has to be taken into account. In such conditions, the application of the conventional time-frequency tools could be a difficult task, mainly because of the nonlinearity and the closeness of the time-frequency components of the impulse response. Moreover, the channel being unknown, any assumption about the instantaneous frequency laws characterizing the channel could not be approximate. In this paper, we introduce a new time-frequency analysis tool that aims to extract the time-frequency components of the channel impulse response. The main feature of this technique is the joint use of time-amplitude, time-frequency, and time-phase information. Tests provided for realistic scenarios and real data illustrate the potential and the benefits of the proposed approach.
Yann Stephan
added 4 research items
The continuous monitoring of coastal processes presents a great interest both from the environmental and economic viewpoints. Passive acoustic tomography can be a good candidate to provide synoptic measurements over wide areas while a range-dependent inversion scheme allows to achieve a reasonable spatial resolution along each vertical slice section. This work develops a feature-oriented parameterization scheme for acoustic tomography purposes, enabling the tracking of the main structure of a thermal front. A Kalman algorithm filters sequentially acoustic data recorded on a vertical array, in a frequency regime corresponding to a useful part of ship noise spectrum.
-------------------------------------------------------------------------------- One of the most challenging applications of time–frequency representations deals with the analysis of the signal issued from natural environment. Recently, the interest for passive underwater context increased, basically due to the rich information carried out by the natural signals. Taken into account the non-linear multi-component time–frequency behaviour of such signals, their analysis is a challenging problem. In this context, the analysis of underwater mammal's whistles is aimed to extract, accurately and adaptively, their main time–frequency components. In this paper, we define a time–frequency-phase tracker which is composed of three steps. The first one consists of modelling the short-time segments of the vocalization by a set of third order polynomial phase modulations. The second step consists in the fusion of local polynomial phase modulations according to a local phase continuity criterion. Finally, in the third step, the detected time–frequency track is used to design the time–frequency filter, in charge of extracting the samples corresponding to the detected track. This procedure is then iterated until all component of interest are extracted. Tests provided for realistic scenarios and real data taken in Bay of Biscay at September 2009 containing whistles of common dolphin Delphinus delphis illustrate the potential and the benefits of the proposed approach.
Yann Stephan
added a research item
An inversion scheme is proposed, relying upon the inversion of the noise of a moving ship measured on a single distant hydrophone. The spectrogram of the measurements exhibits striations which depend on waveguide parameters. The periodic behavior of striations versus range are used to estimate the differences of radial wavenumber between couples of propagative modes at a given frequency. These wavenumber differences are stacked for several frequencies to form the relative dispersion curves. Such relative dispersion curves can be synthesized using a propagation model feeded with a bottom geoacoustic model. Inversion is performed by looking for the bottom properties that optimize the fit between measured and predicted relative dispersion curves. The inversion scheme is tested on simulated data. The conclusions are twofold: (1) a minimum 6 dB signal to noise ratio is required to obtained an unbiased estimate of compressional sound speed in the bottom with a 3 m s(-1) standard deviation; however, even with low signal to noise ratio, the estimation error remains bounded and (2) in the case of a multi-layer bottom, the scheme produces a single depth-average compressional sound speed. The inversion scheme is applied on experimental data. The results are fully consistent with a core sample measured around the receiving hydrophone.
Yann Stephan
added 2 research items
The scattering strength of isotropic and anisotropic rough surfaces was experimentally and theoretically investigated for high frequencies about 500 kHz. Emphasis was placed on studying the response from three two-dimensional rough sur-faces which roughness was either isotropic (characterized by a Gaussian distribution) or anisotropic (characterized by a modified-sine surface). Theoretical predictions rely on the first-order small slope approximation either including a Gaussian structure function or a quasi-periodic structure function. The combination of true data and theoretical results indicates the importance of taking into account the anisotropy of a surface in a scattering prediction process. It is shown that the scattering strength varies a lot depending on the propagation plane. In the longitudinal direction of ripples, scat-tering strength is mostly in the specular direction, whereas in the transversal direction of the ripples, the scattering strength is spread in a very different way related to the particular features of the ripples, with several maxima and min-ima independent of the specular direction. Contrary to the isotropic surface, the scattering strength from an anisotropic rough surface is modified from one propagation plane to another, which explains why the entire rough surface should be taken into account without any simplification as it is often seen when dealing with scattering models. Compared to such a surface, positions of the emitter and of the receiver are naturally significant when measuring scattering strength.
To improve the performance prediction of low frequencies sonar (Anti Submarine Warfare)we can use geoacoustic information coming from different kind of sensors. However, geoacoustic and scattering properties estimation by inversion of received acoustic signals remains very difficult and strongly dependent on the system of measurement. Indeed the interaction between an acoustic wave and the sediment is heavily dependent on frequency, measurement angle and micro roughness of seafloor. Therefore, fusion of geoacoustic models inverted from different sonar systems with wide diversity of insonification angles and frequencies (single beam echosounder (SBES), multibeam echosounder (MBES), sidescan sonar (SSS) and subbottom profiler(SBP)) allow an extended description of the acoustic properties of the seafloor and the first sediment layers. In this paper, we propose a characterization method based on the Dempster Shafer theory of evidence to fuse geoacoustic models in order to classify the seafloor and estimate geoacoustic parameters.
Yann Stephan
added 3 research items
Le concept de tomographie acoustique de bassin, né il y a une trentaine dʼannées, a suscité un engouement très fort à la fin du vingtième siècle. Cet engouement est sensiblement retombé depuis et le concept nʼa pas débouché sur de réelles applications opérationnelles. Lʼintérêt très fort suscité depuis une dizaine dʼannées pour les zones côtières, que ce soit pour des applications civiles ou militaires, a fait émerger de nouveaux concepts de tomographie comme lʼévaluation rapide de lʼenvironnement et la tomographie côtière, active et discrète. Quels sont les évolutions prévisibles à court et moyen termes de ces différents concepts ? Cet article propose des pistes de réflexion pour les développements futurs des concepts et systèmes de tomographie du milieu marin. ABSTRACT The concept of basin scale ocean acoustic tomography, born thirty years ago, generated a strong enthusiasm in late twentieth century. This enthusiasm fall as the concept did not led to operational applications. Since a decade the interest for coastal areas, either for civilian or military applications rose again with the new concepts of rapid environmental assessment and coastal tomography, active and discrete. What are the expected evolutions in the short and medium terms of these concepts? This paper explores some future developments for concept and systems of maritime tomography.
Monitoring the environment with non intrusive acoustic techniques is the subject of many research initiatives worldwide. In the framework of the scientific interest group "Europôle Mer (GIS-EM)", ENSIETA, SHOM and Océanopolis co-organized the SERENADE workshop in Brest. This workshop has enabled to provide a state of the art in the field of acoustic monitoring of marine environment, to share the points of view from various scientific communities and to identify prospects for collaborations in the next future.
The concept of basin scale ocean acoustic tomography, born thirty years ago, generated a strong enthusiasm in late twentieth century. This enthusiasm fall as the concept did not led to operational applications. Since a decade the interest for coastal areas, either for civilian or military applications rose again with the new concepts of rapid environmental assessment and coastal tomography, active and discrete. What are the expected evolutions in the short and medium terms of these concepts? This paper explores some future developments for concept and systems of maritime tomography.
Yann Stephan
added a project goal
covert REA using geoacoustic inversion and data fusion