Yann Stéphan

Neural Direct Approaches for Geoacoustic Inversion

This paper presents different neural network approaches for geoacoustic inversion. The basic idea of neural inversion is to approximate the inverse function from a set of behaviors, i.e. relations between acoustic fields and geoacoustic parameters. In this work, such methods have been applied in two different forms: a global approach which aims to estimate all parameters from all data, and a hierarchical approach in which the most sensitive parameters are estimated before the least sensitive. The methods are tested using synthetic data. Statistical results, as well as benchmark results show that such approaches are efficient and have similar performances.

Passive geoacoustic inversion with a single hydrophone using broadband ship noise

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.

Localization in Underwater Dispersive Channels Using the Time-Frequency-Phase Continuity of Signals

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.

On the consideration of motion effects in the computation of impulse response for underwater acoustics inversion

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.

Passive acoustic tomography: new concepts and applications using marine mammals: a review

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.

A VARIATIONAL APPROACH FOR GEOACOUSTIC INVERSION USING ADJOINT MODELING OF A PE APPROXIMATION MODEL WITH NON LOCAL IMPEDANCE BOUNDARY CONDITIONS

The adjoint model method of control theory is known to give accurate and efficient data assimilation processes in oceanography and meteorology. However, it has rarely been applied in underwater acoustics for inversion purposes. Based on the back-propagation of the mismatch between observations and their predictions, the adjoint model can produce the corrections to the associated direct forward model input parameters. In this paper, the adjoint of a parabolic equation propagation model with non local impedance boundary conditions at the water sediment interface is used in order to determine an acoustically equivalent representation of the seabed. The bottom is represented by these boundary conditions that play the role of the control parameter.

Motion effect modeling in multipath configuration using warping based lag-Doppler filtering

The estimation of the impulse response (IR) 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 [1]. Indeed, some interference appears in the wideband ambiguity plane because of the multipath propagation. A warping-based lag-Doppler filtering method is proposed allowing us to accurately estimate the IR of the channel.

Tidal Effects on Source Inversion

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.

Acoustic monitoring of the Ushant Front: a feasibility study

Oceanic modeling and prediction are highly dependent on the availability of satellite remote sensing and hydrographic in situ measurements to provide reliable and accurate results. In coastal environments the data assimilation is a difficult problem due to the lack of data, the strong coupling between state variables and forcing and the frequent model failures encountered in the modeling. In particular, the scarcity of satellite measurements due to cloud cover makes the operational data assimilation an hard task. Acoustic tomography data can provide valuable environmental informations to complete the standard data set, on temporal and spatial scales suitable to a regional circulation model. This work presents a feasibility test of acoustic data assimilation in a basic feature model of the Ushant thermal front, west off Brittany. The proposed scheme considers the regular measurements of full-field acoustic data on a vertical array of receivers that are assimilated in the feature front model to continuously track time-evolving front parameters. The assimilation scheme is based on Kalman filtering. Nonlinear extensions of Kalman filters are required to deal with the nonlinearity between the front parameters and the acoustic measurements. Simulation results based on realistic environmental scenarios show that the developed scheme is able to track temporally the main parameters of the front.

High-Resolution Geoacoustic Characterization of the Seafloor Using a Subbottom Profiler in the Gulf of Lion

Subbottom profilers are commonly used to explore the first sediment layers below the seafloor. Recent narrowbeam profilers achieve improved performances in terms of signal-to-noise ratio (SNR) and resolution. Thus, the potential of these systems for near real-time geoacoustic characterization of sediments is high and is worth being specifically explored. This paper presents several methods to estimate geoacoustic parameters such as the absorption, the reflectivity, and the impedance contrast. These procedures are tested against real data collected with the SBP 120 subbottom profiler during the CALIbration MEthodology for Recognition of the Ocean bottom (CALIMERO) experiment. It is shown that the absorption and impedance contrast estimates are fully consistent with in situ measurements, which tends to confirm the possibility of near real-time characterization of sediment layers.