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Fisheries survey ECOAO 13, including the collection of surface sediment samples and acoustic propagation measurements, was carried out off the Senegalese coast in February 2013. The bottom acoustic properties were determined by geoacoustic inversion using the noise of R/V Antea as a noise sound source of opportunity. The vessel moved on a straight course with constant engine speed (1627 RPM) while the generated noise field was recorded by a vector sensor array deployed from a reinforced hull inflatable boat launched from the vessel. The multi-wavelength array, designed and built at EHL, is composed of four pressure sensors with different spacing combinations and two pressure gauges. The inversion approach estimates the bottom geoacoustic properties from observation of range-dependent features of vertical impedance at multiple frequencies and optimization based on a genetic algorithm. As compared to our previous two-element array, deployed offshore Amazon, the newly designed array provides more comprehensive observables and therefore more reliable geoacoustic inversion results. For this first trial, the vertical impedance data at the spectral lines of 140 Hz, 280 Hz and 321 Hz were extracted from pairs of adjacent array elements and jointly processed. The bottom compression speed (1640 m/s) and density (1.96 g/cm3) deduced from the posterior probability density functions suggest a sandy seabed. The acoustic inversion results are in general agreement with the grain size analysis of two sediment grab samples, which indicate that more than 80% of the material is composed of well-distributed sand particles of different sizes, the remaining part being shell fragments and silt/clay mixtures.

A submersible digital holographic microscope (DHM) Holoflow@Sea was deployed during AWA fisheries surveys conducted in March 2013 and 2014 in the large upwelling ecosystem of the Canaries Current. The microscope collected about half a million of holograms of microorganisms and other particles suspended in the water column. Selected new results of reconstruction of phyto- and zooplankton species using improved algorithms with respect to previously published results are presented. Amplitude images are more detailed and, for some species, phase images provide useful additional information about morphology and composition. Extensive analysis confirms that the submersible DHM is suitable for in-situ imaging of organisms in the size range 2 μm–200 μm and species identification. When compared to standard procedures the microscope allows to rapidly monitor planktonic communities over a wide area. It contributes to a better understanding of the organization and functioning of the ecosystem and in particular the link between biogeochemistry cycles and small pelagic fish stock dynamics.

We present the first results obtained by a newly developed submersible digital holography microscope (DHM), Holoflow@Sea, to enable continuous in-situ monitoring of ocean or fresh water bodies in a less intrusive manner. The microscope features an off-axis configuration with reduced-coherence illumination. The optics is designed to image plankton and particles in the size range 2 μm200 μm within a water volume of 1 mm x 1 mm x 2 mm. The prototype was successfully deployed for the first time over the continental shelf of Senegal during a fisheries survey carried out in March 2013. The objective was to combine several laboratory techniques used for plankton and particle identification (highperformance liquid chromatography, flow cytometry and optical microscopy) on discrete collected samples with DHM images taken in situ at locations with different environmental conditions. Hologram data were acquired inside an upwelling cell, i.e., new water, and along the coast, i.e., old water, as well as off the upwelling cell at the continental shelf border. Preliminary results of holographic reconstruction are encouraging, with the distinctive morphology of some phytoplankton species allowing easy identification to genera level. Challenges are recognised with the identification of small spheroid organisms. Analyses are underway to allow comparison with traditional methods of plankton identification and evaluate the benefit of additional in-situ observations obtained by holography microscopy. The preliminary results already demonstrate the potential of DHM for in-situ studies of plankton and particles.

International audience; The Ushant thermal front is a seasonal phenomenon which occurs from May to October in a shallow water environment (100m) of the Iroise Sea (off the coast of the northwestern France). It corresponds to the boundary separating a well mixed inner shelf water from an open sea stratified water. To determine the dynamic of the front -or more basically the presence of a stratified or homogeneous water column- the possibility to use a shipping lane as a continuous acoustic source is studied. The originality of this work is to use a single receiver. Simulation results of sounds radiated by a shipping lane in a shallow water environment are presented, both for stratified and homogeneous water column. The corresponding pressure fields show a mean level difference in the frequency band 50-300 Hz. This feature will be used in the future as an observable to differentiate both environments, and thus passively detect the Ushant thermal front. One of the issue to get the mean level offset is to record the shipping lane noise without isolated ship interferences. As a consequence, an optimum mooring position to track the thermal front is suggested from the analysis of real acoustic data, acquired around the Ushant traffic lane during 2010, coupled with AIS (Automatic Identification System) data, and a MARS-3D oceanographic model output. Keywords: Passive acoustic inversion

National audience; 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.

Shallow water naval operations require detailed knowledge of the environmental properties. In addition to parameters such as water depth, knowledge about the sediment properties is of high importance for a wide range of operations. In this context, the MREA BP'07 experiment was carried out in the Mediterranean Sea in 2007. Measurements employed a large set of sensors, thereby providing all information required to fully describe the environment. This paper focuses on multibeam echosounder (MBES) measurements, which were taken not only to provide information about the water depths, but also to provide the backscatter strength as a function of angle. These backscatter data are employed to infer sediment parameters. To this end, a comparison of measured and modeled backscatter strengths is conducted. Use is made of a model that accounts for both scattering at the water-sediment interface and scattering and scattering at the inhomogeneities in the sediment, i.e. volume scattering. In practice, the measured backscatter strength values are affected by the imperfect MBES calibration. This impedes a direct model-data comparison, unless the effects of miscalibration are eliminated. Therefore, a calibration curve is derived by optimizing spectral strength and volume parameter, according to known mean grain sizes at bottom grab positions. After having corrected the measured backscatter strength for these systematic effects, inversions are carried out to estimate the sediment parameters (grain size, spectral strength, and volume parameter) at various locations in the research area.

In the context of the recent Maritime Rapid Environmental Assessment/Blue Planet 2007 sea trial (MREA/BP07), this paper presents a range-resolving tomography method based on ensemble Kalman filtering of full-field acoustic measurements, dedicated to the monitoring of environmental parameters in coastal waters. The inverse problem is formulated in a state-space form wherein the time-varying sound-speed field (SSF) is assumed to follow a random walk with known statistics and the acoustic measurements are a nonlinear function of the SSF and the bottom properties. The state-space form enables a straightforward implementation of a nonlinear Kalman filter, leading to a data assimilation problem. Surface measurements augment the measurement vector to constrain the range-dependent structure of the SSF. Realistic scenarios of vertical slice shallow-water tomography experiments are simulated with an oceanic model, based on the MREA/BP07 experiment. Prior geoacoustic inversion on the same location gives the bottom acoustic properties that are input to the propagation model. Simulation results show that the proposed scheme enables the continuous tracking of the range-dependent SSF parameters and their associated uncertainties assimilating new measurements each hour. It is shown that ensemble methods are required to properly manage the nonlinearity of the model. The problem of the sensitivity to the vertical array (VA) configuration is also addressed.

For shallow-water naval operations, obtaining rapidly an accurate picture of the environmental circumstances often is of high importance. Hereto a multi-sensor approach is required. In this context, the MREA/BP'07 experiment has been carried out south of Elba (Mediterranean Sea), where several techniques of environmental characterization covering the fields of underwater acoustics, physical oceanography and geophysics have been combined [Le Gac&Hermand, 2007]. The required information typically concerns water-column properties, sea surface roughness, and sediment geo-acoustic properties. Estimating these geo-acoustic parameters from inversion of acoustic data received on drifting sparse arrays has proved to be a promising approach. Part of MREA/BP'07 was therefore dedicated to this type of measurement. For validating the resulting geo-acoustic estimates sediment samples were collected. Additionally, measurements were carried out using a multibeam-echosounder. This system provides depth information, but also allows for seafloor classification. The classification approach taken is model-based employing the backscatter data. It discriminates between sediments in the most optimal way by applying the Bayes decision rule for multiple hypotheses, implicitly accounting for backscatter-strength ping-to-ping variability. Here, results of seafloor classification using the multibeam data and a preliminary comparison with the sediment sample analysis and the geo-acoustic parameter estimates as obtained from the drifting arrays are presented.

This paper discusses the principle of geoacoustic inversion. That will demonstrate the need for validation with experimental data. The RNLNC has been involved in two recent sea trials that will be described in brief. Results for the geoacoustic parameter characterization for typical examples will be shown and discussed. Finally, the paper presents conclusions, future recommendations and references for further reading.

During the spring of 2006, hydrographic survey vessel HNLMS "Snellius" ran an extensive multibeam and sidescan survey of the Saba bank, a large submerged atoll located in the north-eastern Caribbean. The survey provided an excellent opportunity for a number of small-scale geoacoustic inversion experiments in a shallow water environment which attracts many divers for its unique tropical ecosystem and rich marine wildlife. The feasibility of a rapid deployment of ocean-acoustic sensors and equipment was demonstrated for the purpose of an environmental assessment of the area southwest of the small volcanic island of Saba. The environmental impact was kept to a minimum by exploiting the hydrographic ship as a sound source of opportunity that was moving along the survey lines and passing by a sparse vertical line array that was deployed from a rubber boat at anchor. Several low-frequency narrowband tones were identified for the inversion process that provided an accurate account of the experimental geometry in terms of moving source and receiving array positions, and detailed geoacoustic properties of the sea floor and sub-seafloor.