The Black Top Hat function applied to a DEM: A tool to estimate recent incision in a mountainous watershed (Estibère Watershed, Central Pyrenees)

International audienceThe Top Hat Transform function is a grey-level image analysis tool that allows extracting peaks and valleys in a non-uniform background. This function can be applied onto a grey-level Digital Elevation Model (DEM). It is herein applied to quantify the volume of recent incised material in a mountainous Pyrenean watershed. Grey-level Closing operation applied to the Present-Day DEM gives a new image called ''paleo'' DEM. The Black Top Hat function consists in the subtraction of the ''paleo'' DEM with the Present-Day DEM. It gives a new DEM representing all valleys whose sizes range between the size of the structuring element and the null value as no threshold is used. The calculation of the incised volume is directly derived from the subtraction between the two DEM's. The geological significance of the quantitative results is discussed

Un risque d'origine anthropique : la contamination chronique par les métaux lourds à proximité d'anciens sites miniers. Le cas de la mine (plomb, zinc, cadmium) de Jebel Ressas (Tunisie nord-orientale)

L'analyse des risques en relation avec l'environnement s'intéresse surtout aux catastrophes naturelles telles que les éruptions volcaniques, les inondations, les séismes. Or, la société actuelle étant fortement marquée par le développement industriel et technologique, les risques d'origine anthropique deviennent de plus en plus prégnants.L'objectif de l'étude entreprise est de proposer une nouvelle méthode d'évaluation et de cartographie du risque de contamination métallique sur et autour d'un ancien site minier situé à proximité de la ville de Tunis. Ce type de risque que les sociologues dénomment " risques diffus " (Beck, 1992 ; Peretti-Watel, 2000), appelle une approche interdisciplinaire couvrant un large spectre depuis la géochimie jusqu'aux données sociétales afin d'intégrer les données quantitatives et qualitatives dans un même modèle. Les résultats préliminaires exposés dans cet article ont été obtenus en adoptant une démarche d'analyse du risque basée sur le couplage entre une analyse de l'intensité de l'aléa et de l'estimation du degré de vulnérabilité des enjeux, adaptés à une contamination chronique d'un écosystème

Obtention d'une image par numérisation d'empreinte : une nouvelle méthode d'analyse des faciès carbonatés. Acetate peels digitalization : a new method for carbonate analysis

This paper deals with a new carbonate analysis approach consisting in digitalization of acetate peels. This analytical procedure provides an image which is significant of mineralogy and textures of the different carbonate phases. Image analysis processings (mathematical morphology, local statistic) appear to be very helpful for facies visualization, quantification of usual descriptive parameters and new parameters inherent in the method. Carbonate analysis may evolve due to the new concepts proceeding with the acetate peels digitalization.L'approche nouvelle consistant à numériser des empreintes de roches carbonatées, réalisées selon la technique classique des empreintes à l'acétate («acetate peels »), permet d'obtenir une image en tons de gris significative de la nature minéralogique et des textures de leurs différentes phases. Les traitements (morphologie mathématique, statistique locale) effectués sur l'image, d'une part apportent une aide à la visualisation du faciès, et d'autre part permettent la quantification de paramètres classiques (pourcentages, ...) et de paramètres descriptifs nouveaux inhérents à la méthode présentée (indice de compacité morphologique, ...). Les apports de cette méthode permettent d'envisager une évolution future des concepts de classification des carbonates.Bourdeau Eric, Courjault-Radé Pierre. Obtention d'une image par numérisation d'empreinte : une nouvelle méthode d'analyse des faciès carbonatés. Acetate peels digitalization : a new method for carbonate analysis. In: Sciences Géologiques. Bulletin, tome 41, n°2, 1988. Traitements d'images et objets géologiques. pp. 175-191

Geological caracterisation of plots belonging to the Gaillac vineyard (Tarn, Midi-Pyrénées) - consequences on the determination of Basic Terroir Units (BTU) and the choice of vegetative material

Detailed geological analyses of plots belonging to the «AOC Gaillac» area have been carried out in order to adress one of the main natural component ruling the terroir effect process. These plots belong to terraces of the left bank of the Tarn river which coincides with one of the three main terroirs of the AOC area. Precisely, the analysed plots are localised on the rissian-aged (≈ 200 000 yrs B.P.) terrace composed of alluvial shelves crosscut by small valleys where the Oligocen (ca. 28 My) marly molassic basement outcrops. Three different Basic Terroir Units (BTU) have been identified : terrace shelf, terrace slope and comb. Each of them has specific viticultural potentialities related to its topographical, geological and pedological characteristics. Representative profiles have been analysed in each BTU. Field analysis has evidenced that all rocks material have derived from Rissian alluvial deposits due to solifluxion processes when part of the alluvial material deposited on the terrace shelf has slept onto the slope overlying the marly Oligocen molassic basement. This solifluxion phase has taken place during the late-glacial Würmian climatic oscillations interval (Böllering-Alerod episode ca. 12,000 years BP). Afterwards, during the Holocene period (i.e. the last 10,000 years) the alluvial-derived material has suffered pedogenetic alteration. The nature of the resulting alterites depends on the initial topographic situation inherited from the late-Würmian solifluxion phase. On to the terrace shelf the soil sequence begins by a reddish clayey horizon (up to 0, 6m) because of the erosion of the eluvial horizon during the last 10,000 years. It is followed by a thick (≈ 1m average) reddish coarse-pebble horizon rich in clays and iron oxydes. On the terrace slope, characteristics luvisols have developped composed by an eluvial silty-sandy horizon (up to 0.60 m) overlying an illuvial pebble-sand level (up to 3m) where clays and ferrous oxydes are moderatly accumulated. Finaly, the thick (> 2m) dark silts and clays sequence (with scattered gravels and small pebbles) of the comb derive from the deposition of eroded soil material of the above terrace shelf and slope units (colluvium). On the basis of the role of high qualitative limiting factor played by the water stress parameter such as quality of drainage, permeability of soils, the down-side slope terrace unit appears as the most appropriate unit because of its slope gradient combined with the occurrence of a thick permeable pebble-sand sequence. Finally, combination of physical and chemical results - acidic pH and very low CEC - permits to recommand the Gravesac rootstock adapted to well-drained acidic soils and Syrah/Fer Servadou climatic-adapted grapevine varieties as the most suitable vegetative material. In addition, the knowledge of the geological component at the scale ot the basic units allows for the adaptation of some cultural practices in order to enhance the viticultural potentialities of the plots. In order to encourage the vine's roots to dig deep and reach the sandy-pebbles horizon, two cultural possibilities are proposed: inter-row grassing associated with the « inter-plant » method or earthing down under the row associated with inter-row ploughing. The choice will depends on the soil erosion amplitude if the inter-row ploughing method is used. The analysis demonstrate the efficiency of detailed geological survey using BTU concept as an operational tool. Further, it enhances that the geological component can be regarded as an amplification point of the terroir system as any alteration - even of minor importance - of the geological parameters, may have noticeable consequences on the resulting terroir effect

= 5.1 1980 Arudy earthquake sequence (western Pyrenees, France): a revisited multi-scale integrated seismologic, geomorphologic and tectonic investigation

International audienceThis paper presents a combination of seismic imaging, geomorphologic, and tectonic data and an interpretation of the M = 5.1 1980 Arudy earthquake sequence putting in relation the seismicity, the inherited faults, and the geomorphologic (Würm and postwürm) markers in this region of the Pyrenees. Since the anticlockwise rotation of the regional compression axis in Oligocene time, western Pyrenees are under a dextral regime and the resulting motion is accommodated along major inherited E?W dextral strike-slip faults. The Arudy aftershocks sequence is controlled by antecedent horsetail splay faults built at the boundary between two shallow Mesozoic crustal blocks most probably due to their differing rheology. This boundary has played the role of a seismic barrier stopping the E?W slip motion. The Arudy earthquake has reactivated the eastern segment of the main E?W strike-slip fault, while the post-seismic aftershocks correspond to local relaxation processes in normal tectonic behavior

Structural analysis of hypocentral distribution of an earthquake sequence using anisotropic wavelets: Method and application

International audienceSpatial organization of earthquake sequences is investigated to localize active rupture planes and to reconstruct the geometry of the inferred rupture zone. We have developed a new approach, the Normalized Optimized Anisotropic Wavelet Coefficient (NOAWC) method, to extract from a set of hypocenters the active ruptures planes. Our approach permits the detection of organized structures within a plane regardless of its size, location, shape anisotropy, and orientation. It includes the determination of a system of three perpendicular sections, minimizing the effects of projections of the hypocenters, and intrinsically accounts for uncertainty in the location of the seismic events. The accuracy and the effectiveness of the NOAWC procedure are illustrated on both synthetic and real data. An application to the M = 5.1 Arudy (French Pyrenees) aftershock sequence shows how a combination of the possible mathematically reconstructed geometries can be combined with the available fault plane solutions and geomorphological markers to determine the active rupture planes and to propose and validate a local tectonic model. This new multitool approach lends itself to quantitative and computer-assisted analyses of large data sets