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Morski svijet i morsko dno u denominaciji su općenito slabije zastupljeni od kopna. Na to nedvosmisleno upućuje i (ne)postojeća onomastička terminologija. Postoji unutar „morske” toponimije jedna skupina toponima kojoj dosad nije posvećena gotovo nikakva pozornost. Riječ je o bentonimima – toponimima morskoga dna. Na morskome se dnu nalaze brojne konfiguracije, vidljive i nevidljive, koje ribari, pomorci, a ponekad i kartografi imenuju i u načelu vrlo precizno lociraju. Ovo je prethodno priopćenje o radu koji se priprema i koji želi obuhvatiti sve do danas potvrđene toponime hrvatskoga podmorja te utvrditi jezične postupke pomoću kojih se oblikuju njihovi toponimski likovi. Organizacija i prezentacija građe strukturirane su na iskustvom potvrđenim opažanjima. Najprije se pokušava utvrditi koji referenti, s obzirom na blizinu morske površine, jesu, a koji nisu predmet istraživanja, zatim razriješiti „konfuziju” oko ustaljenih termina koji su često, barem na razini opće prihvaćenosti termina, u sukobu s onim što imenuju (greben, sika, hrid), te razvrstati toponime u podmorju u nekoliko karakterističnih skupina: na one koji identificiraju referente i konfiguracije u plitkome moru, one na razini mora i one u dubini, gdje se dno ne vidi ili se slabo vidi., Le monde et le fond marins attirent moins d’intérêt dans la dénomination que la terre ferme. Témoin en est la non existante terminologie concernant ce champ sémantique. Il y a à l’intérieur de la toponymie » marine « un groupe de toponymes auquel les onomasticiens n’ont jamais attribué une attention adéquate. Il s’agit notamment des benthonymes – toponymes du fond marin. On trouve sur le fond marin un grand nombre de configurations, visibles et invisibles, que les marins, les pêcheurs et, quelquefois, les cartographes, localisent avec précision et leur accordent des noms précis. Les linguistes, cependant, ont rarement réagi de façon affirmative à ce champ de recherche. Nous avons essayé à plusieurs reprises attribuer, à travers quelques contributions, une attention particulière à ce champ sémantique, notamment, avec nos collaborateurs, sur les pages de la bibliothèque Onomastica Adriatica. Tout particulièrement dans le VIIe volume de cette bibliothèque consacré à la toponymie de l’Archipel de Šibenik. Ayant l’intention de faire une recherche approfondie sur ce sujet, notre dessein est de le présenter à cette occasion sous forme d’une note de recherche. Deux buts sont à atteindre. D’une part, établir un fond lexical aussi exhaustif que possible concernant ce groupe particulier de toponymes et, de l’autre, analyser les procédés linguistiques participant à la création des formes toponymiques indispensables à l’identification des référents sous-marins. La visibilité insuffisante, l’approche souvent difficile et l’ignorance de ces configurations ont eu pour conséquence, d’un côté, l’absence des termes communs pour la même réalité physique, et de l’autre, l’emploi de mêmes termes pour les contenus totalement différents. Il est vrai que ce procèdé dénominatif ne représente pas un spécifique touchant uniquement le fond marin. Cependant, on peut affirmer d’ores et déjà, en se fondant sur les résultats de recherches préalablement exercées dans d’autres champs, que la mer et le fond marins donnent la plus grande diversité de solutions. Le corpus des configurations sous-marines n’est pas homogène. La ligne de partage passe entre le visible (en principe le fond près de la côte) et l’invisible (les configurations de pleine mer). Le premier groupe est beaucoup plus complexe et beaucoup plus riche en formes linguistiques. Le second, quoi qu’invisible ou difficilement visible, est plus uniforme en ce qui concerne la structure du nom. Tout d’abord, tous les toponymes sont composés. Ils sont composés selon un modèle de dénomination basé en dernière ligne sur le visible dans l’entourage du référent. Formule en est : nom commun brak ‘haut-fond’ + od ‘de’ + le nom d’une configuration proche (île, pointe, colline, récif…).

Le projet d'exploiter les ressources minérales marines profondes, qui remonte aux années 1960, est revenu au premier plan dans les années 2000, dans un contexte de course aux matières premières et de croissance rapide des économies émergentes, en particulier celle de la Chine. La mine sous-marine constitue une nouvelle frontière économique et technologique pour les firmes minières et le capital transnational tandis que les pays et territoires du Pacifique insulaire s'efforcent d'anticiper cette rencontre en renforçant leurs compétences souveraines et la coopération régionale. Pour les peuples autochtones et les autorités coutumières est en jeu la représentation cognitive et normative des espaces océaniques comme part intégrant de leur univers et monde de vie alimentant des affirmations spécifiques de souveraineté «?non-westphalienne?». La réalisation du potentiel minier sous-marin reste toutefois lente à se dessiner. Cet article se propose d'explorer cette frontière minière encore largement virtuelle via les politiques et les dispositifs mis en place dans ce domaine dans les territoires français du Pacifique, avec un accent sur le cas de Wallis-et-Futuna. Il s'agira d'examiner et d'analyser les effets induits par la rencontre entre politique française, réponses océaniennes et virtualité de la mine sous-marine, en particulier du point de vue des relations des Outre-mer à l'État et à l'information, entre défiance, rumeurs et controverses. On verra à quel point les questions d'incertitude, y compris morale, institutionnelle et ontologique, jouent un rôle structurant dans la compréhension et les négociations des enjeux de l'exploration et de l'exploitation des ressources minérales marines profondes.

Projet MICADO : http://www.agence-nationale-recherche.fr/projet-anr/?solr=run&tx_lwmsuivibilan_pi2%5BCODE%5D=ANR-11-JSV7-0003; International audience; Chemosynthetic mats involved in cycling sulfur compounds are often found in hydrothermal vents, cold seeps and whale falls. However, there are only few records of wood fall mats, even though the presence of hydrogen sulfide at the wood surface should create a perfect niche for sulfide-oxidizing bacteria. Here we report the growth of microbial mats on wood incubated under conditions that simulate the Mediterranean deep-sea temperature and darkness. We used amplicon and metagenomic sequencing combined with fluorescence in situ hybridization to test whether a microbial succession occurs during mat formation and whether the wood fall mats present chemosynthetic features. We show that the wood surface was first colonized by sulfide-oxidizing bacteria belonging to the Arcobacter genus after only 30 days of immersion. Subsequently, the number of sulfate reducers increased and the dominant Arcobacter phylotype changed. The ecological succession was reflected by a change in the metabolic potential of the community from chemolithoheterotrophs to potential chemolithoautotrophs. Our work provides clear evidence for the chemosynthetic nature of wood fall ecosystems and demonstrates the utility to develop experimental incubation in the laboratory to study deep-sea chemosynthetic mats.

International audience; We investigated surface carbonate sediments at 33 sites on the outer shelf and slope around New Caledonia (163°–167°E, 20°–23°S), at water depths of 75–720 m. Four carbonate sedimentary facies are recognized on the basis of sediment size fractions and predominant constituents: Facies 1, encrusted grains (rhodoliths and macroids), bryozoa, and benthic foraminifera; Facies 2, bryozoa, benthic foraminifera, and mud; Facies 3, plankton and mud; and Facies 4, ahermatypic corals. Facies distributions were constrained primarily by water depth, and secondarily by local seafloor geomorphology that, in some areas, allows transport of sediments to deeper water. Because the dominant facies (Facies 1 and 3), as well as lagoon and basin facies, are distributed worldwide at similar latitudes, facies around New Caledonia can be considered as representative of carbonate sedimentary facies distributed in tropical–subtropical regions.

The aim of the paper is to evaluate and map changes in sea-floor elevation in a marine sandy epeiric environment as a tool in research on present-day sea floor sediment dynamics, especially of bank summits and swale floors, and to assess the mobility of banks and sand waves from sounding and hydrographie maps and from hypsometric profiles provided by sequential bathymétrie campaigns along fixed tracks. It provides a review of data acquisition techniques and data processing methods. It focuses on stability maps and on morpho- metric methods, especially on trend maps of elevation change. Superposed profiles and serial profiles allow to study in more detail the displacement of banks, the progression or retreat of bank flanks, sand-wave mobility and other related topics. It shows that each method has its own specific application field but that all are complementary. The paper presents numerous examples of application of different methods to the Flemish Banks. It also mentions the potential of new technologies developed since the 1990s: multi-beam bathymetry, local relief mapping and the interpretation of satellite images. It discusses the specific support of side-scan sonar to bathymétrie profiling and to echogram interpretation., Résumé L'article a pour but d'évaluer et de cartographier les tendances à l'abaissement et au rehaussement du fond marin dans un milieu épi- continental sableux, plus spécialement du sommet des bancs de sable et du fond des chenaux, et d'étudier la mobilité des formes à partir de campagnes bathymétriques séquentielles le long de lignes fixes. Il donne un aperçu des méthodes de traitement et des technologies de collecte de données, devenues disponibles au cours de la période 1980-1995, et par lesquelles une évaluation numérique est devenue possible. Il traite plus en détail de la carte de stabilité, ainsi que des méthodes morphométriques, dont celle des cartes de tendance du changement altimétrique, et livre une synthèse cartographique couvrant de longs suivis avec de très nombreuses données. Les profils superposés et les profils sériels servent surtout à étudier en détail le déplacement des bancs, l'avancement ou le recul des flancs, la mobilité des dunes hydrauliques et autres aspects spécifiques. L'article montre la complémentarité des différentes méthodes. Il présente de nombreux exemples d'application aux Bancs de Flandre pour la période 1987-1994. Il mentionne les possibilités des nouvelles techniques en voie de développement depuis 1995 (bathymétrie à l'écho-sondeur multi-faisceaux, levé de cartes de différences de hauteur, apport des levés au sonar à balayage latéral, interprétation d'images satellitaires) et montre aussi l'apport des levés au sonar à balayage latéral à la bathymétrie opérationnelle et à l'interprétation des échogrammes., De Moor Guy. Évaluation de la morphodynamique sous-marine en Mer du Nord méridionale au cours de la période 1985-1995 / Evaluation of sea-floor sediment dynamics in the Flemish Banks (southern North Sea) between 1985 and 1995. In: Géomorphologie : relief, processus, environnement, Avril-juin, vol. 8, n°2. pp. 135-150.

International audience; Features on the seabed can be mapped from remote sensing multi/hyperspectral imagery, provided that their effects on the measured reflectance spectrum can be made independent of those produced by the atmosphere and water column. The non-linear effect of water column light attenuation can then be corrected to obtain the absolute reflectance of the seabed. Light attenuation by the water column and bathymetry are both determined from the satellite image. The water column attenuation is then removed in order to apply an automated supervised classification, whatever the depth is. We have compared the results obtained with and without the correction of water column attenuation, for two different statistical measures: Euclidean (ED) and spectral angle mapper (SAM) distances. We have applied this methodology to MERIS images acquired on the lagoon of New Caledonia. The best overall accuracy (79%), as compared to in situ data, is obtained with the corrected image and the SAM distance.

International audience; The shallow submarine hydrothermal field of the Prony Bay (New Caledonia) discharges hydrogen- and methane-rich fluids with low salinity, temperature (

International audience; Building high-resolution models of several physical properties of the subsurface by multi-parameter full waveform inversion (FWI) of multi-component data is one of the challenge of seismic imaging for the next decade. The physical properties, which govern propagation of seismic waves in visco-elastic media, are the P and S wavespeeds, density, attenuation and anisotropic parameters. Updating each of these properties is challenging because several parameters of different nature can have a coupled effect on the seismic response for a particular propagation regime (from transmission to reflection). This is generally referred to as trade-off or cross-talk between parameters. Moreover, different parameter classes can have different order of magnitude or physical units and footprints of different strength in the wavefield, which can make the inversion poorly conditioned if it is not properly scaled. These difficulties raise the issue of a suitable parameterization for multi-parameter FWI, where the term parameterization must be understood as a set of independent parameter classes that fully describe the subsurface properties. Many combinations of parameters can be viewed and this choice is not neutral as the parameterization controls the trade-off between parameters and the local resolution with which they can be reconstructed. Once this parameterization is selected, the subset of parameter classes in the parameterization that can be reliably updated during the inversion, must be identified to avoid over-parameterization of the optimization problem. The purpose of this tutorial is to provide a comprehensive overview of the promise, pitfalls and open questions underlying multi-parameter FWI. We first review the main FWI ingredient that controls the trade-off between parameters, namely the radiation pattern of the so-called virtual sources, and some tools for analyzing these trade-offs. Then, we present some illustrative examples of multi-parameter FWI, which should provide some guidelines to choose a suitable parameterization for FWI in visco-acoustic anisotropic media. We conclude by proposing a data-driven and model-driven workflow for visco-elastic anisotropic FWI of multi-component marine data, which has been inspired by a real data case study from Valhall.

SUMMARY Altimetric measurements of sea-surface heights have long been used to locate uncharted marine features. Newly available data sets (Geosat-GM, ERS-1 and Topex/Poseidon), now allow the topography of submarine volcanoes to be accurately restored and a method based upon inverse modelling is presented in this paper. This method allows a joint assimilation of altimetric data and shipborne profiles of bathymetry and/or gravity anomalies when available. Furthermore, uncertainties can be computed together with the topography, which constitutes a noticeable improvement since these parameters are still missing in the usual bathymetric data bases. Tests are performed using simulated data, in order to characterize the errors which may or must occur with the modelled topography of a previously unknown volcano, in particular the errors due to data uncertainties and the lack of precise knowledge of sensitive parameters that are used in the modelling. These errors can be maintained under the 100 m level (rms) for altimetric data with a measurement noise of 5 cm (rms) or less, which is achieved for Geosat, ERS-1 and Topex/Poseidon altimeters. An example of inversion of Seasat data is presented in the case of a seamount in French Polynesia.

The Coriolis backarc troughs in southern Vanuatu are elongate, rhomboidal depressions with steep walls and flat floors, formed by extension. The Futuna Trough is relatively immature and, like Erromango Basin, shows no recent volcanism. Vate Trough is most developed, and its floor is largely covered by lava fields. Between Vate and Banks Islands, the backarc has three distinct characters: in the south it is sediment-draped; centrally, either lava projects through sediment cover or the area is covered by slide debris; in the north, a steep escarpment may result from collision of the arc with the D'Entrecasteaux Ridge.

278 pages

International audience; The seismic background noise observed on the OBS (ocean-bottom seismometer) recordings is well known to be strong. With the recent appearance of broadband OBS, the background noise in the extended frequency range is even more significant. By taking into account the various degrees of freedom of the seismic sensors, it has been shown that the seismometers are not only translational sensors but also rotational ones with a sensitivity that depends on the installation. The strong similarity of the background noise recorded on the two horizontal traces of the broadband Hippocampe OBS data deployed during the Rosmarin program is interpreted as a rotational motion around the vertical axis. This torsion motion is so important that it dominates the other noise seismic contributions like translations and tilt (rotational motion around a horizontal axis) and represents more than 90% of the background noise signal at the periods between 5 and 50 sec. Use of the difference between the two horizontal traces makes it possible to reduce the background noise made up primarily of this torsion motion and thus to enhance the displacement signal but also strongly modify it. An effort should be made to better control the installation of the sensors on the seafloor. A better solution would be to simultaneously record rotations at the same position as that of the seismometer. Such data will permit the correction of the seismic traces of the OBS data and strongly reduce the background noise.

Bathymodiolin mussels occur at hydrothermal vents and cold seeps, where they thrive thanks to symbiotic associations with chemotrophic bacteria. Closely related generaIdasandAdipicolaare associated with organic falls, ecosystems that have been suggested as potential evolutionary ‘stepping stones’ in the colonization of deeper and more sulphide-rich environments. Such a scenario should result from specializations to given environments from species with larger ecological niches. This study provides molecular-based evidence for the existence of two mussel species found both on sunken wood and bones. Each species specifically harbours one bacterial phylotype corresponding to thioautotrophic bacteria related to other bathymodiolin symbionts. Phylogenetic patterns between hosts and symbionts are partially congruent. However, active endocytosis and occurrences of minor symbiont lineages within species which are not their usual host suggest an environmental or horizontal rather than strictly vertical transmission of symbionts. Although the bacteria are close relatives, their localization is intracellular in one mussel species and extracellular in the other, suggesting that habitat choice is independent of the symbiont localization. The variation of bacterial densities in host tissues is related to the substrate on which specimens were sampled and could explain the abilities of host species to adapt to various substrates.

Geochemistry, Geophysics, Geosystems, v. 5, p. Q02009, 2004. http://dx.doi.org/10.1029/2003GC000561; International audience; For a given site, many factors control the seismic risk. Earthquake magnitude, hypocentral distance, rupture mechanism, site effects and site vulnerability are among the most important. This article deals with one of these factors: the depth of the seismogenic zone, in the northern Ecuadorian subduction system, beneath a highly vulnerable site, the city of Esmeraldas and its industrial complex, the Ecuadorian oil refinery and shipping terminal. To address this problem, we analyzed data from a three weeks passive seismological experiment, conducted in the spring of 1998, using 13 Ocean Bottom Seismometers and 10 portable land-stations. A preliminary interpretation of wide-angle data obtained in the fall of 2000, in the Manta area, 100 km South of the study area, unambiguously indicates the presence of a velocity inversion in the Ecuadorian margin velocity structure. This velocity inversion is characterized by a shadow-zone of ∼1 s on the record-sections, and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer II of the subducted oceanic Nazca plate. One-dimensional velocity models are deduced from these wide-angle data and are used for earthquake location in the Esmeraldas area. This highly improved the hypocentral parameter determinations. The updip limit of the seismogenic zone is found at a depth of ∼12 km, 35 km eastward of the trench, and the depth of the seismogenic zone below the Esmeraldas city is found at ∼20 km. This shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area.

A novel moderately thermophilic, hydrogenotrophic, sulfate-reducing bacterium, strain 6N(T) (=DSM 15269(T)=CIP 107713(T)), was isolated from matrixes of Alvinella and Riftia originating from deep-sea hydrothermal-vent samples collected on the 13 degrees N East-Pacific Rise at a depth of approximately 2600 m. It was a Gram-negative, non-sporulating, curved rod, motile with one polar flagellum, that did not possess desulfoviridin. It grew at temperatures ranging from 30 to 60 degrees C, with an optimum at 45 degrees C, in the presence of 0-5 % NaCl (optimum 2 %). Strain 6N(T) utilized only H(2)/CO(2) and formate as electron donors with acetate as carbon source. Sulfate, sulfite, thiosulfate and elemental sulfur were used as terminal electron acceptors during hydrogen oxidation. The G+C content of DNA was 34.4 mol%. Strain 6N(T) grouped with members of the family Desulfohalobiaceae in the delta-subclass of the PROTEOBACTERIA: Its closest phylogenetic relative was Desulfonatronovibrio hydrogenovorans, with only 90 % similarity between the sequences of the genes encoding 16S rRNA. Because of significant phylogenetic differences from all sulfate-reducing bacteria described so far in the domain Bacteria, this novel thermophile is proposed to be assigned to a new genus and species, Desulfonauticus submarinus gen. nov., sp. nov.

Les fonds marins du plateau insulaire de l'archipel guadeloupéen, élargi à Saint-Martin, et de la Martinique ont fait l'objet d'une reconnaissance détaillée, à l'aide de techniques comme le sonar à balayage latéral, la bathymétrie, les prélèvements de sédiments à la benne et les prises de vue. En complément, les images fournies par le satellite SPOT et des photographies aériennes ont permis de cartographier la frange littorale. Les résultats obtenus ont été regroupés sous forme de cartes au 1...

6 pages, 6 figures, New data collected between the northernmost tip of the East Pacific rise (18°05´N, 105°35´W) and the Middle America trench provide evidence that the seafloor, which lacks significant sedimentary cover, has a typical spreading-derived abyssal hill topography. The tectonic fabric of this seafloor is concave to the west, as it is today at the tip of the East Pacific rise. Farther to the east, at the outer wall of the trench, the seafloor topography exhibits a north-south trending fabric. We suggest that this fabric originated along the East Pacific rise, as it reached the trench and possibly subducted beneath the North America plate prior to the development of the complex connection of the East Pacific rise with the Rivera transform

International audience; This paper addresses the automatic cartography of sea-bottom by means of high resolution sonar images. Many texture analysis methods have been developed since now, based on statistical, geometrical or spectral modelling. Nevertheless, few of them are robust toward image rotations. Indeed, the property of rotation invariance is essential in our framework, particularly for obtaining good classification rates. We present in this article five methods for the automatic classification of rotating images, corresponding to four classes of sea-floor: “sand”, “ridge”, “dune” and “wreck”. The first one is an extension of a circular AutoRegressive method, initially proposed by Kashyap et Khotanzad [19], which allows to estimate a reduced number of rotation invariant parameters. The four other methods are based on an original approach, consisting to apply a mathematical transform to a set of parameters describing texture features. Two of them consist in computing the Log- Polar transform to autoregressive (AR) or correlation (COR) parameters. The two others consist in estimating the Zernike moments of autoregressive (AR) or correlation (COR) parameters. Classification rates obtained on sonar images and on Brodatz album are presented and allow to compare the performances of each approach.; Les applications civiles et militaires nécessitant l'identification des caractéristiques de la couche superficielle du fond de la mer sont nombreuses : géologie marine, prospection minière, pêche et gestion de la biomasse, cartographie, guerre des mines, chasse aux mines ... Cet article s’intéresse à la cartographie automatique des fonds marins en imagerie sonar haute résolution. De nombreuses méthodes d'analyse de textures ont été développées jusqu'à présent, utilisant des approches statistiques, géométriques ou spectrales. Cependant, peu d'entre elles fournissent des attributs caractéristiques robustes vis-à-vis des rotations d'images. Cette propriété est pourtant essentielle dans le cadre de l'étude proposée : elle a pour objectif de faciliter et d'améliorer l'apprentissage du classifieur. Nous présentons dans cet article cinq méthodes de caractérisation, robustes vis-à-vis des rotations d'images. La première méthode est une version étendue de la modélisation AutoRégressive (AR) circulaire initialement proposée par Kashyap et Khotanzad, en vue d'en extraire directement un nombre restreint de paramètres caractéristiques significatifs invariants en rotation. Les quatre autres méthodes résultent d'une approche originale qui consiste à appliquer une méthode de traitement d'images à un ensemble de paramètres décrivant une texture, afin de le rendre robuste vis-à-vis des rotations d'images. Les deux premières de ces méthodes consistent à appliquer la Transformation Log-Polaire respectivement aux paramètres issus d'une modélisation autorégressive 2D non-causale et aux paramètres de corrélation associés (nommés paramètres COR). Quant aux deux dernières méthodes, elles consistent en l'application de la méthode des moments de Zernike respectivement à ces deux ensembles de descripteurs. Des résultats de classification expérimentaux obtenus sur images réelles et sur images tournées artificiellement, ainsi que sur des textures issues de l'album de Brodatz, sont fournis pour souligner les performances de chacune des méthodes.

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Réunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Réunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate. Copyright 1999 by the American Geophysical Union.

Not available, Les sondeurs multifaisceaux de basse fréquence (13 khz) émettent un signal de grande longueur d'onde capable de pénétrer dans les sédiments. Le signal retourné renferme ainsi une information sur la nature et les propriétés physiques du fond marin. L'épaisseur prise en compte par un tel signal dépend de la nature des dépôts et de l'angle d'incidence : de 0 à 2 m en présence de sable, elle peut atteindre 10 à 15 m dans un milieu argileux. Cette étude vise à améliorer notre compréhension des phénomènes liant rétrodiffusion et lithologie. Nos recherches ont été conduites sur deux zones : le fan celtique et le système sédimentaire du var. Une caractérisation physique des dépôts a été réalisée et comparée avec l'imagerie acoustique, par le biais du processus de segmentation des images sonars développé par l'Ifremer. Il permet une partition de l'image en zones acoustiquement homogènes et quantifiées par des courbes angulaires de réflectivité. Les relations entre réflectivité et lithologie sont ambigües. Elles ne peuvent être résumées selon les hypothèses classiques associant faible réflectivité à la présence de sédiments fins. La plupart des séquences sableuses de cette étude présentent un niveau de rétrodiffusion égal voire inferieur à celui des dépôts argileux. Des séquences sédimentaires différentes peuvent produire une réponse acoustique similaire. Néanmoins, nous avons pu distinguer deux cas : une configuration lithologique simple dans laquelle la rétrodiffusion dépend de l'enfouissement d'un réflecteur, et une configuration complexe faite d'alternances de niveaux sableux et argileux. Les variations de réflectivité n'y sont pas clairement expliquées, mais elles paraissent liées à l'hétérogénéité du sable. Des anomalies acoustiques ont été rencontrées. Il s'agit de séquences sableuses présentant une réflectivité anormalement basse. Nous proposons une explication basée sur un état de consolidation anormal des sédiments pouvant modifier leurs propriétés acoustiques.