La connaissance du littoral, sa protection, son aménagement nécessitent un suivi des changements qui s'y produisent. Toutefois, en France, devant l'étendue des façades maritimes à couvrir, les techniques de mesures in situ ne peuvent être appliquées de façon systématique. Dans ce cas, les techniques de télédétection spatiale ou aérienne sont des approches complémentaires. A partir de ces plates-formes, des images stéréoscopiques multi-temporelles sont, après une série de traitements photogrammétriques appropriés, directement exploitables sous formes d'orthophotographies et de Modèles Numériques de Terrain (MNT). Ce type de produits permet notamment la quantification des changements morphosédimentaires à l'interface Terre-Mer (transport transversal et longitudinal de sédiments, érosion,...). Actuellement, la résolution spatiale des MNT générés à partir d'images aériennes ou satellitaires (<50 cm) est insuffisante pour la plupart des applications sur la frange littorale. En outre, dans le cas d'interventions consécutives à un événement extrême (tempête, raz de marée, pollution…), ces systèmes manquent de souplesse (trajectoires prédéfinies, contraintes d'altitude, de vitesse, coût de mise en oeuvre...) L'utilisation de drones constitue donc une alternative intéressante pour des suivis de précision ou des interventions rapides.

Very high spatial resolution remote sensing images and Digital Elevation Models (DEM) are widely used in coastal management applications. For example, they are used for the quantification of morphosedimentary changes of the coastal fringe, including cross-shore and longshore sediment transport. They are also used as input in hydrodynamics numerical modelling . Spatial resolution, precision and accuracy are critical parameters of the DEM. Presently, most of DEM are built using aerial or satellite images with a spatial resolution coarser than 50cm is not accurate enough for most of applications. An unmanned photogrammetric helicopter (DRELIO) has been developed. It is equipped with an autopilot system. After loading the flight plan, no ground communications are needed from take off to landing. The fly altitude can reach 100m above the ground. DRELIO can operate in windy conditions up to 50km/h and it is able to make stationary flights. A reflex camera with high quality interchangeable optics is onboard. Depending on the focal length and flying altitude, the resolution of the images varies from 1 to 5cm with a ground coverage of 50 by 75m up to 250 by 375m. Due to specific flight conditions and image acquisitions, a photogrammetric toolbox has been developed. Using stereoscopic images and GPS positioning of reference points on the images, it allows building DEM and an orthorectified image with a spatial resolution better than 5cm. In this study, we present an example of an acquisition realzed on the beach of Porsmillin (French Brittany) and we discuss the precision and accuracy obtained by this method. The DRELIO system, which produces DEM concurrent to LIDARs, appears now to be more flexible and efficient than UAV (Unmanned Aerial Vehicle) helicopters equipped with electric engines, UAV planes and less expensive than LIDAR.

In the Chelif basin, the geochemical characterization reveals that the Upper Cretaceous and Messinian shales have a high generation potential. The former exhibits fair to good TOC values ranging from 0.5 to 1.2% with a max. of 7%. The Messinian series show TOC values comprised between 0.5 and 2.3% and a high hydrogen index (HI) with values up to 566 mg HC/g TOC. Based on petroleum geochemistry (CPLC and CPGC) technics, the oil-to source correlation shows that the oil of the Tliouanet field display the same signature as extracts from the Upper Cretaceous source rocks (Cenomanian to Campanian). In contrast, oil from the Ain Zeft field contains oleanane, and could thus have been sourced by the Mes-sinian black shale or older Cenozoic series. Two petroleum systems are distinguished: Cretaceous (source rock) e middle to upper Miocene (reservoirs) and Messinian (source rock)/Messinian (reservoirs). Overall, the distribution of Cretaceous-sourced oil in the south, directly connected with the surface trace of the main border fault of the Neogene pull-apart basin, rather suggests a dismigration from deeper reservoirs located in the parautochthonous subthrust units or in the underthrust foreland, rather than from the Tellian allochthon itself (the latter being mainly made up of tectonic m elange at the base, reworking blocks and slivers of Upper Cretaceous black shale and Lower Miocene clastics). Conversely, the occurrence of Cenozoic-sourced oils in the north suggests that the Neogene depocenters of the Chelif thrust-top pull-apart basin reached locally the oil window, and therefore account for a local oil kitchen zone. In spite of their limited extension, allochthonous Upper cretaceous Tellian formations still conceal potential source rock layers, particularly around the Dahra Mountains and the Tliouanet field. Additionally they are also recognized by the W11 well in the western part of the basin (Tahamda). The results of the thermal modelling of the same well shows that there is generation and migration of oil from this source rock level even at recent times (since 8 Ma), coevally with the Plio-Quaternary traps formation. Therefore, there is a possibility of an in-situ oil migration and accumulation, even from Tellian Cretaceous units, to the recent structures, like in the Sedra structure. However, the oil remigration from deep early accumulations into the Miocene reservoirs is the most favourable case in terms of hydrocarbon potential of the Chelif basin.

In the eastern Algerian offshore basin, 3D basin modeling applied for facies prediction and petroleum potential assessment shows that most favorable zones for reservoir development and hydrocarbon occurrence are located at a maximum distance of 60 km from the coastline. The lack of well data in this area is partly compensated by a large data set of geophysical and geological (G&G) data such as multichannel seismic (MCS), magnetism, wide angle velocity models, and geological outcrops; they represent important constraints for 3D dynamic modeling. Facies distribution model is constrained by sequence and environment evolution through time that is defined from onshore outcrops. 2D structural re-constitution and thermal modeling were also undertaken with ArcTem software. The structural interpretations highlight the occurrence of north-verging ramps during the Quaternary which played an important role in HC generation and migration. Three source rocks have been considered for maturation modeling with Temis Flow software, Burdigalian, Langhian, and Tortonian. They are found to be in gas window in the deep areas and locally in oil window at shallower structured zones. The modeling results indicate that the main fluid discharge was focused toward the southern border of the offshore basin where recent thrust faults (parallel to the margin) are located. In order to test the role of these faults in terms of hydrocarbon migration and trapping, two scenarios are considered according to whether they were sealed or not. In both cases, the 2D/ 3D simulations depict overpressures (2,000-4,000 psi) in the pre-salt sedimentary package. However, the hydrocarbon charge is most efficient with the sealing faults for the lower Langhian and lower Messinian reservoirs. The hydrocarbon potential depends mostly on the lateral extension of seals, their sealing capacity and the organic carbon (TOC) content of potential source rocks. Besides, the sea drop of about 1, 000 m during the Messinian salinity crisis has induced depres-surization that caused oil and gas seepage from Miocene reservoirs.

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a t...

We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore-offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ∼5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 and 6.0 km s −1 , from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 and 7.1 km s −1. The Poisson ratio in the lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 and 7.0 km s −1 , from the top to the bottom, shows a gradual seaward thinning of ∼15 km over an ∼35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ∼2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene emplacement of magmatic intrusions in both the deep basin and the continental margin. Based on the crustal structure, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW-SE elongated spreading centre that ran perpendicular to the subduction trend. Such an atypical geometry is explained by the diverging directions of the subduction rollback during the backarc opening: eastward for the Corsica-Sardinia block, and southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the surface expression of a slab tear at depth, which is responsible for atypical magmatism in the overlying backarc oceanic basin.