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International audience; The French Navy Mine Warfare Office conducted a series of 8 calibrated underwater explosions in December 2018 in the Bay of Hyères, SE France. The charges were ranging from 80 to 680 kg TNT-equivalent weights and the generated mechanical waves have been recorded at 17 seismic stations (composed of velocimeters, accelerometers and/or Microelectomechanical system accelerometers) temporarily deployed within 10 km of the sources. The large sampling rate recordings (> 250 Hz) allow to clearly identify both seismic, hydroacoustic and acoustic waves on the shoreline. The hydroacoustic phases, mostly observed at the rocky sites, rapidly attenuate when propagating on land and seem to be conditioned by the bathymetry and the seabed sedimentary cover. The hydroacoustic onset times can be used very efficiently to infer explosion locations and origin times. The spectrum analyses and site response along the shore and inside a building (church) provide insights on the mitigation of the possible nuisance caused by the disposal of unexploded historical ordnance.

International audience; Unexploded historical ordnance (UXO) from World War II, that is discovered almost every week close to the French coast, must be destroyed quickly after discovery to ensure the safety of divers and ships. The favored destruction method is countermining, i.e., the use of a high-order detonation conducted by exploding an additional donor charge placed adjacent to the UXO. In the framework of a UXO countermining campaign conducted in the Rade d'Hyères (Mediterranean Sea, France) in December 2018, hydro-acoustic and seismic recording systems have been deployed to record the explosion-induced waves in water and the seismic signals on the shore, respectively (POSA project). In this expanded abstract, we present the main observations and focus on the impact of the shallow water environment (whose water depth is less than 50 m), and more specifically on the impact of the unconsolidated sedimentary layer, on the recorded signals induced by the detonation of charges with weights ranging from 80 to 680 kg TNT-equivalent. We also discuss the acoustic-to-seismic wave conversions.

International audience; Insight on wave regimes along the world's coastlines is important for virtually all coastal and nearshore marine activities, installations, planning and protection. Waves are pervasive and the dominant source of energy driving extreme sea levels, the transport of pollutants and sediments, erosion, and a major contributor to risks of flooding. We quantify the global spatio-temporal wave conditions along the world's coasts and evaluate the needs for coastal Earth Observation strategies, with the aim, notably, that the derived scales of change can contribute to optimisation of these strategies. A global dominant timescale of 30 days is found in coastal wave variability that is, on average, spatially correlated just over the synoptic 5 degrees' regional scale (≈ 550 km at the equator). This regional-scale dimension suggests that the timing and design of traditional field surveys and observations relevant to a vast array of coastal activities, and which may be expensive in terms of human resources, may be complemented by information gained from satellite Earth Observation that throws light on spatio-temporal scales of wave-energy change along the world's coastlines.

International audience; Explosive devices from World War II are discovered almost every week on the French coasts and they must be destroyed by the French Navy Mine Warfare Office. The consequences of the counter-mining on the marine environment are complex to evaluate. Depending on the environment geology, the explosive charges and their localization, the seismo-acoustic waves generated by the explosion may cause damage to infrastructures located on the coast, and under specific conditions small submarine landslides. The ANR ASTRID POSA project proposed to address the risk assessment issue on the Mediterranean coast. The main originality of this project lied in the approach used that combines geological measurements, acoustic data recorded within the sea coastal zone and seismic data recorded on the coast, together with numerical simulations of seismo-acoustic wave propagation. We propose here to present the results of such a methodology applied to the counter-mining campaign that occurred in the Rade of Hyères (France) in December 2018. We focus more specifically on the numerical simulations of seismo-acoustic wave propagation from the location of the device explosion to several seismometers deployed on the coast of Hyères. From topographical and sedimentary measurements performed in the Rade of Hyères, physical and geometrical characteristics of the marine seabed have been carefully selected as input data for numerical simulations. In addition, a particular attention has been paid to the measured characteristics of the device explosion (source). The numerical simulations have then been conducted using a spectral-element method. The impact of the explosive device charge and its location (on the seabed or in the water column), together with the impact of the marine environment properties, on the simulated signals have been studied. Finally, the numerical results have been compared with the real signals recorded by the seismometers. The risks of land degradation have been evaluated afterwards.

National audience; Le projet POSA a pour but de décrire et quantifier l'énergie sismique se propageant sur les côtes lors des explosions de contre-minage menées par la Marine. En 2018, un déploiement de capteurs proches des côtes a permis d'enregistrer les champs d'ondes sur une large gamme de fréquences. Le signal lié aux explosions se caractérise par deux phases d'énergie (ondes de compression dans le milieu solide et dans l'eau). L'efficacité de la conversion eau/roche dépend fortement de la topographie et de la nature de la côte. Les stations Résif permanentes n'enregistrent que la partie basse-fréquence de ce signal.

This poster presents the POSA project (Characterization of the Propagation of Seismic Waves of Anthropic Origin, ANR ASTRID 2015-2019), which aims to describe and quantify the seismic energy propagating on the coasts during counter-mining explosions carried out by the Navy. In 2018, a deployment of sensors close to the coasts made it possible to record the wave fields over a wide range of frequencies. The signal related to the explosions is characterized by two energy phases (compressional waves in the solid medium and in the water). The efficiency of the water/rock conversion is highly dependent on the topography and nature of the coast. Permanent RESIF stations record only the low-frequency part of this signal. This poster was presented during the RESIF Scientific and Technical Meetings that took place in Biarritz in November 2019. RESIF is a national research infrastructure dedicated to the observation and understanding of the Earth's internal structure and dynamics. RESIF is based on high technology observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout the French territory. The data collected allow the study with high spatio-temporal resolution of ground deformation, surface and deep structures, local and global seismicity and natural hazards, particularly seismic, on the French territory. RESIF is integrated into the European (EPOS - European Plate Observing System) and worldwide instruments that allow to image the Earth's interior in its entirety and to study numerous natural phenomena.; Ce poster présente le projet POSA (Caractérisation de la Propagation des Ondes Sismiques d’origine Anthropique, ANR ASTRID 2015-2019), qui a pour but de décrire et quantifier l'énergie sismique se propageant sur les côtes lors des explosions de contre-minage menées par la Marine. En 2018, un déploiement de capteurs proches des côtes a permis d'enregistrer les champs d'ondes sur une large gamme de fréquences. Le signal lié aux explosions se caractérise par deux phases d'énergie (ondes de compression dans le milieu solide et dans l'eau). L'efficacité de la conversion eau/roche dépend fortement de la topographie et de la nature de la côte. Les stations RESIF permanentes n'enregistrent que la partie basse-fréquence de ce signal. Ce poster a été présenté lors des Rencontres scientifiques et techniques RESIF qui se sont déroulées à Biarritz en novembre 2019. RESIF est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. RESIF se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. RESIF s’intègre aux dispositifs européens (EPOS - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels.

International audience; The SW termination of the Gulf of Lions (GoL) is characterized by a narrowing shelf (from 80 km in the central part to ca. 20 km at the French/Spanish border), increased hydrodynamic processes and lowered sediment supply. It has been the target of a relatively large number of high-resolution seismic and acoustic investigations, as well as grab sampling, but few long cores are presently available because relatively low sedimentation rates (compared to the central GoL) did not attract the interest for paleo-environmental studies.In preparation of a map of surficial sediments in this area, a compilation of available data was undergone. In dip section, beyond the littoral prism, 4 distinct morpho-sedimentary units are at the origin of submarine landscapes. From W to E, they include: (1) between -20 and -60 m, a (2) between 60 and -75 m, a relict, 2 to 5 m-thick mud wedge, highly eroded, that can be assigned to the Younger Dryas by analogy with similar, thicker features described and well-dated further to the east; (3) between ca. -75 m and -100 m, a set of stacked thickening seaward forced-regressed Systems Tracts linked to 100 ky and 40 ky glacio-eustatic cycles. They consist of over-consolidated muds from paleo-prodeltas and lower shorefaces, and to sand bodies reworked into large dunes, drapped by a veneer of recent muds. These sands are locallycemented and form elongated reliefs at the position of paleo-shorelines (beachrocks). A characteristic of unit 3 in that part of the GoL is that, because of important reworking by hydrodynamic processes, sequences assigned to Marine Isotope Stages (MIS) 6.2, 6.4 and 7.6 are outcropping at some places, in contrast with the central GoL where they are buried underneath a 20-50 m interval of MIS 3-MIS 2 sediments. (4) between ca. -100 m and the shelf edge (that ranges between -110 and -170 m depending on the distance from canyon heads), normally consolidated offshore muds forming a wedge up to 150 m where accommodation is sufficient (infilled canyon heads).In summary, the distribution of surficial sediments in the GoL is, in general, largely controlled by the history of sea-level changes and the position of point sources. More specifically, the western termination of the GoL exemplifies the major role of hydrodynamic processes in modifying the position and water depth of boundaries between different sedimentary units and sequences.

International audience

International audience

International audience; In the last decades, sandbanks and sand dunes of various shapes and sizes have been found anddescribed on continental shelves all over the world. These structures tend to migrate and ourunderstanding of the mechanisms regulating their dynamics is limited. These sandbanks became aconcern due to the fact that they interfere with the human activities at sea. Consequently, theirpresence has economical and environmental impacts. The purpose of this study is to automaticallydetect these dunes in DTMs (Digital Terrain Models) generated from MBES data. So far, automatictechniques aiming at describing marine dune fields have always brought information such as theheight, dune spacing or the orientation at local scale but never went down to a dune scale.Currently, the dunes are still manually digitized and then form parameters are calculated. This task istedious and time-consuming. In addition, the amount of data to process being always bigger, it getsdifficult for operators to keep the pace. To address this issue, we propose a geomorphometricapproach. Attributes such as the slope or curvatures are estimated from DTMs. A combined analysisof the metrics maps enables to extract dunes from their environment. The following step is thedetermination of parameters characterizing the shape and size for each dune. This work providesknowledge about dune fields at a detailed level. Thus, the composition, spatial distribution and otheraspects of sand banks can be obtained. Further research can be conducted in order to learn moreabout the dynamics of dune fields at an individual level.

Colloque avec actes et comité de lecture. Internationale.; International audience; La prédiction du transport d'un mélange de particules de plusieurs tailles est un point sensible d'une représentation de la dynamique sédimentaire sur de nombreux faciès en rivière ou en mer. Afin de déterminer les processus critiques qui régissent la mise en mouvement de tels mélanges et de quantifier leur contributions, des essais ont été réalisés dans un petit canal à courant. Ils ont porté sur des échantillons reconstitués, formés de sables naturels lithoclastiques bien triés ou de mélanges sableux bimodaux. L'objectif est d'observer et de quantifier les différences de comportement à l'entrainement entre un sédiment unimodal et un sédiment mélangé. Les résultats obtenus sont comparés à une sélection de formulations issues de la littérature. La série de tests a permis de mettre en évidence quelques processus particuliers du début du transport sédimentaire multiclasse dans un écoulement stationnaire. La granulométrie utilisée s'étend des sables fins aux sables très grossiers. Des paramètres du mélange tels que le rapport de taille des particules, le taux de saturation des échantillons par les grains dont la taille est la plus fine ou la porosité influent sur la modulation de la contrainte de frottement critique. Les comparaisons mesures / formules ont mis en évidence la difficulté de prédire le seuil de mise en mouvement des particules d'un mélange hétérométrique. Par conséquent, l'incertitude sur la granulométrie transportée s'accroît lorsque le frottement est proche du frottement critique d'une certaine classe de taille.

Seabed of continental shelf environments is regularly covered with a multitude of bedforms formed in response to interactions between fluid dynamics and sediment. These mobile sedimentary features have been widely studied in order to prevent damages on human activities and anthropogenic structures. In coastal areas, submarine dune dynamics is mainly controlled by tidal currents and wind and wave forcings (e.g. Le Bot et al., 2004; Idier et al., 2011). However, it is generally not obvious to distinguish which forcing is predominant in dune dynamics.

Geoacoustic parameters of the seafloor are required for accurate sonar prediction and analysis of seismic reflection profiles, especially in shallow water. They are generally established by means of empirical relations. The presented work is part of CARASEDIM, an experimental project devoted to refine these geoacoustical relations in marine sediments, including coarse sands. We focus on the results of the celerimeter prototype that has been developed for that purpose. This portative device is equipped with two emitting probes and two receiving probes allowing to transmit signals between 40kHz and 400kHz. It is designed to both laboratory and in-situ measurements. We discuss about the processing techniques, the protocole of measurement and about the first results. Some laboratory results are presented in both real coarse sands and artificial glass beads. They are compared with theoretical models of sound propagation in sediments based on various assumptions (fluid, visco-elastic, porous...).

Cenozoic is marked by climatic instability and accelerating continental uplifts. Increasing sediment yield is generated, buffered and transferred by river to the deep sea. Four of the major turbidite systems in the world are located on the northern and the western margins of the Indian Ocean: the Ganges-Brahmaputra, the Indus, the Zambezi and the Tanzanian systems. This work aims to understand forcings affecting sedimentation and morphology of two main turbidite systems (Tanzanian system and Ganges-Brahmaputra system) and results have been compared with their nearest neighbors (respectively Zambezi and Indus). Based on a multiproxies approach in both area, our main results are as follows: (1) during Cenozoic, tectono-sedimentary evolution of the Tanzania margin led to the development of an atypical turbidite system wherein morphology attest of a strong structural control; (2) the Ganges-Brahmaputra system has a sedimentary activity mainly forced by sea level variations. This turbidite system records variations in many forcings impacting sedimentation from the catchment to the deep sea. Turbidite system activity and architecture revealed a strong variability mainly linked to the geodynamic and physiographic context of each studied margin. It appears necessary to integrate all forcings (affecting both the catchment and the deep sea) in the study and the understanding of sedimentary activity in a turbidite system., Le Cénozoïque est marqué par une instabilité climatique et l’accélération des surrections continentales. Ces phénomènes engendrent une augmentation de la production sédimentaire, transférée par les fleuves vers le domaine marin profond. Les façades nord et ouest de l’océan Indien voient se mettre en place quatre des plus grands systèmes sédimentaires au monde : le Gange-Brahmapoutre, l’Indus, le Zambèze et le système tanzanien. Ce travail vise à comprendre les forçages agissant sur la sédimentation et l’architecture de deux des principaux systèmes turbiditiques de l’océan Indien (système tanzanien et du Gange-Brahmapoutre), pour ensuite les comparer avec leurs plus proches voisins (respectivement le Zambèze et l’Indus). Nos principaux résultats, basés sur une approche multiproxy dans les deux zones, mettent en évidence plusieurs points : (1) l’évolution tectono-sédimentaire de la marge tanzanienne au cours du Cénozoïque a mené au développement d’un système turbiditique majeur et atypique, dont la morphologie moderne atteste de l’importance de l’activité tectonique sur sa construction ; (2) le système du Gange-Brahmapoutre enregistre une activité sédimentaire polyphasée, en lien principalement avec les variations du niveau marin. Ce système est capable d’enregistrer les variations de nombreux forçages, influençant à la fois les bassins versants et le domaine marin. Le fonctionnement sédimentaire et l’architecture des systèmes turbiditiques ont révélé une grande diversité selon le contexte géodynamique et physiographique des façades étudiées. La nécessité d’intégrer l’ensemble des forçages (agissant depuis le bassin versant jusque dans le domaine marin profond) dans l’étude de l’activité sédimentaire d’un système turbiditique apparait essentielle de nos jours.

Submarine dune dynamics is studied in the context of a macrotidal continental shelf subjected to the swell action. Off Dieppe (Eastern English Channel), the gravelly-sand seabed is moulded by dunes and sand banks, and characterized by gradients in hydrodynamics, morphologies and sediment grain-size, which are studied according to various spatial (grain, dune, gradient) and temporal scales (moment, semi-diurnal and semi-lunar tidal cycles, year, several decades). Multivaried analysis, carried out on morphological, sedimentary and hydrodynamic parameters, showed that the sediment grain-size and sediment availability are the main controlling factors of the morphology and the dynamics of the dunes. The inter-dune variability is more pronounced than the intra-dune variability, when sediment and benthic characteristics are considered. Over a 56-year period, repeated bathymetric surveys show a decrease in dune migration rates when observation periods are longer, implying a significant and frequent oscillation of dune displacements. On periods longer than one year, the dune movement concern the whole sedimentary bedform and the direction coincides with that of the residual of tide. On the semi-diurnal to the semi-lunar tidal cycle timescales, dune displacements are often confined in the crest zone. It seems that sedimentary budget is reorganized along the dune field and not at the scale of each single dune. Sediment flux calculations (bedload, non-uniform sediment) show that waves, even weak, can reverse residual sedimentary transport direction and increase the amount of transported sediments (from 10 to 100 times more for significant wave height of 1.5 m, and up to 1000 times more for annual waves). VHR seismic measurements make it possible to visualize dune internal architecture. 0.5 to 4 ° dipping discontinuities (second- order) constitute the dune master-bedding and their periodicity of formation is estimated to be inclued between 4 and 18 years. These periodicities are similar to those observed from the wavelet analysis carried out on the chronicles of tide and wave time-series recorded in the study area, and on a NAO index chronicle. Preserved second-order discontinuities are interpreted as erosive surfaces due to a temporary inversion of dune migration, caused by exceptional storm events. Their formation seems to obliterate several second-order discontinuities formed previously during weaker wave events. The medium term climatic fluctuations, which control the variability of the storm events, are presented as being the main factor influencing dune dynamics on the long term. This study of an exceptional dune field in the Eastern English Channel, according to various spatial and temporal scales, allows to put forward new interpretations as for modalities of the mobility of submarine dunes. These results can answer applied concerns, such as maritime safety, the hiding objects., La Manche orientale est une plate-forme continentale mégatidale soumise à l'action de la houle. Au large de Dieppe, les fonds marins sablo-graveleux modelés de dunes et de bancs sableux, et caractérisés par un gradient hydro - morpho - sédimentaire, sont étudiés selon plusieurs échelles spatiales (grain, dune, gradient) et temporelles emboîtées (instantané, cycles semi-diurne, et semi-lunaire, année, plusieurs décennies). Des analyses multi-variées, réalisées sur des paramètres morphologiques, sédimentaires et hydrodynamiques, ont montré que la granulométrie et le disponible sédimentaire sont les facteurs principaux de contrôle de la morphologie et de la dynamique des dunes. Une variabilité inter-dune beaucoup plus marquée que la variabilité intra-dune est mise en évidence à partir des caractéristiques granulométriques et benthiques. Sur une période de 56 ans, des levés bathymétriques répétés montrent un ralentissement des vitesses de migration des dunes lorsque la période d'observation considérée s'allonge, ce qui témoigne d'une oscillation importante et fréquente des déplacements des dunes. Sur les pas de temps supérieurs à un an, le corps dunaire dans son ensemble se déplace et la direction de migration des dunes coïncide avec celle du résiduel de marée. A l'échelle du cycle semi - diurne au cycle semi - lunaire, les déplacements sont souvent cantonnés à la zone de crête. Une réorganisation des stocks sédimentaires se fait à l'échelle du champ de dunes, et non à l'échelle de la dune. Les calculs de flux charriés en granulométrie étendue montrent qu'une houle, même faible, peut inverser le transport sédimentaire résiduel et augmenter les quantités de sédiments transportées (de 10 à 100 fois plus pour une hauteur significative de houle de 1,5 m, et jusqu'à 1000 fois plus pour des houles annuelles). Des mesures de sismique THR permettent de visualiser des discontinuités, pentées de 0,5 à 4 ° (2nd ordre), qui constituent le maître-litage des dunes et dont la périodicité de formation est estimée entre 4 et 18 ans. Ces périodicités sont similaires à celles observées à partir des analyses en ondelettes réalisées sur les chroniques de marée et de houle de la zone d'étude, et sur une chronique d'indice NAO. Les discontinuités de 2nd ordre, préservées dans l'architecture interne dunaire, sont interprétées comme des surfaces érosives dues à une inversion temporaire de migration des dunes, provoquée par des évènements de tempêtes exceptionnelles. Leur mise en place semble oblitérer plusieurs discontinuités de 2nd ordre formées précédemment pendant des épisodes de houles plus faibles. Les fluctuations climatiques à moyen terme qui contrôlent la variabilité des évènements de tempête sont présentées comme étant le facteur principal influençant la dynamique des dunes sur le long terme. Ce champ dunaire exceptionnel de la Manche orientale, permet par cette étude à échelles spatiale et temporelle emboîtées, d'avancer de nouvelles interprétations quant aux modalités de la mobilité des dunes sous-marines. Ces résultats peuvent répondre à des préoccupations appliquées telles que la sécurité maritime, l'enfouissement ou le dégagement d'objets.