Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program) Imagerie sismique de la zone interne de la chaîne hercynienne sud-armoricaine (projet Armor 2/programme Géofrance 3D)

International audienceWe present results and interpretation of a 72 km long deep seismic reflection profile acquired across the internal zone of the Hercynian belt of South Brittany. The profile is of excellent quality, most of the crust being highly reflective. The “ARMOR 2 South” profile, is correlated with the “ARMOR 2 North” profile that was published in 2003. Correlation of the main subsurface reflections with surface geological and structural data provides important information about the crustal structure that resulted from thickening during Late Devonian and regional-scale extension during Late Carboniferous. In particular, seismics image shows a very high reflectivity zone, lying flat over more than 40 km at about 10–12 km depth. This zone is interpreted as a major zone of ductile crustal thinning. Nous présentons les résultats et l'interprétation d'un profil de sismique réflexion en écoute longue, de 72 km de long à travers les zones internes de la chaîne hercynienne sud-armoricaine. Le profil est d'excellente qualité, avec une forte réflectivité à travers toute la croûte. Le profil, « ARMOR 2 Sud », est corrélé avec le profil « ARMOR 2 Nord » publié en 2003. Les corrélations des réflexions les plus superficielles avec les données géologiques et structurales de surface fournissent d'importantes informations sur la structure crustale qui résulte d'un épaississement débutant au Dévonien supérieur et d'une extension régionale au Carbonifère supérieur. En particulier, la sismique met en évidence une zone sub-horizontale à très forte réflectivité, de plus de 40 km de long à environ 10–12 km de profondeur. Cette zone est interprétée comme une zone d'amincissement crustal majeur

Design, Construction and In Situ Testing of a Muon Camera for Earth Science and Civil Engineering Applications

The MUST2 (MUon Survey Tomography based on Micromegas detectors for Unreachable Sites Technology) camera is based on a thin Time Projection Chamber read by a resistive Micromegas. This innovative combination presents interesting distinctive features compared to existing muon detection technologies. It allows a wide angular acceptance of the detector with a low weight and compact volume, well adapted for confined spaces or underground operation. The current work presents the results obtained during the calibration measurements at the reference site, the Low Background Noise Laboratory (LBNL). Preliminary results from field measurement campaign carried out at the dam overlooking the village of Saint-Saturnin-les-Apt (South-East of France) are presented and discussed

Fast mapping of magnetic basement depth, structure and nature using aeromagnetic and gravity data: combined methods and their application in the Paris Basin

International audienceAssessment of deep buried basin/basement relationships using geophysical data is a challenge for the energy and mining industries as well as for geothermal or CO2 storage purposes. In deep environments, few methods can provide geological information; magnetic and gravity data remain among the most informative and cost-effective methods. Here, in order to derive fast first-order information on the basement/basin interface, we propose a combination of existing and original approaches devoted to potential field data analysis. Namely, we investigate the geometry (i.e., depth and structure) and the nature of a deep buried basement through a case study SW of the Paris Basin. Joint processing of new high-resolution magnetic data and up-to-date gravity data provides an updated overview of the deep basin. First, the main structures of the magnetic basement are highlighted using Euler deconvolution and are interpreted in a structural sketch map. The new high-resolution aeromagnetic map actually offers a continuous view of regional basement structures and reveals poorly known and complex deformation at the junction between major domains of the Variscan collision belt. Second, Werner deconvolution and an ad hoc post-processing analysis allow the extraction of a set of magnetic sources at (or close to) the basin/basement interface. Interpolation of these sources together with the magnetic structural sketch provides a Werner magnetic basement map displaying realistic 3D patterns and basement depths consistent with data available in deep petroleum boreholes. The last step of processing was designed as a way to quickly combine gravity and magnetic information and to simply visualize first-order petrophysical patterns of the basement lithology. This is achieved through unsupervised classification of suitably selected gravity and magnetic maps and, as compared to previous work, provides a realistic and updated overview of the cartographic distribution of density/magnetization of basement rocks. Altogether, the three steps of processing proposed in this paper quickly provide relevant information on a deep buried basement in terms of structure, geometry and nature (through petrophysics). Notwithstanding, limitations of the proposed procedure are raised: in the case of the Paris Basin for instance, this study does not provide proper information on Pre-Mesozoic basins, some of which have been sampled in deep boreholes

Airborne gamma-ray spectrometry to quantify chemical erosion processes

International audienceAirborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyrôme watershed (ca. 150 km2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 ± 50 kg/m2 and the chemical weathering rate of potassium at 17 ± 2 kg/km2/a

Classifying airborne radiometry data with Agglomerative Hierarchical Clustering: A tool for geological mapping in context of rainforest (French Guiana)

International audienceIn highly weathered environments, it is crucial that geological maps provide information concerning both the regolith and the bedrock, for societal needs, such as land-use, mineral or water resources management. Often, geologists are facing the challenge of upgrading existing maps, as relevant information concerning weathering processes and pedogenesis is currently missing. In rugged areas in particular, where access to the field is difficult, ground observations are sparsely available, and need therefore to be complemented using methods based on remotely sensed data.For this purpose, we discuss the use of Agglomerative Hierarchical Clustering (AHC) on eU, K and eTh airborne gamma-ray spectrometry grids. The AHC process allows primarily to segment the geophysical maps into zones having coherent U, K and Th contents. The analysis of these contents are discussed in terms of geochemical signature for lithological attribution of classes, as well as the use of a dendrogram, which gives indications on the hierarchical relations between classes.Unsupervised classification maps resulting from AHC can be considered as spatial models of the distribution of the radioelement content in surface and sub-surface formations. The source of gamma rays emanating from the ground is primarily related to the geochemistry of the bedrock and secondarily to modifications of the radioelement distribution by weathering and other secondary mechanisms, such as mobilisation by wind or water. The interpretation of the obtained predictive classified maps, their U, K, Th contents, and the dendrogram, in light of available geological knowledge, allows to separate signatures related to regolith and solid geology. Consequently, classification maps can be integrated within a GIS environment and used by the geologist as a support for mapping bedrock lithologies and their alteration.We illustrate the AHC classification method in the region of Cayenne using high-resolution airborne radiometric data acquired in 1996 across most of French Guiana. Access to the field in this region, almost entirely covered by tropical rainforest, is difficult, and therefore, use of airborne geophysical data is highly suitable to complement ground observations. Despite the vegetal cover, the U, K and Th maps, exhibit to the first order, a fairly good correlation with lithological units recognised by geologists in the field.Consequently, classification of the radiometric data is globally concordant with existing 1:100,000-scale geological map. In addition, using the radioelement contents and relations between classes at different levels of classification, additional relevant information concerning weathering effects, unexpected lithological differences or transfer by erosion, are evidenced

A multidisciplinary study on the Carboniferous (Namurian) Montmarault granitic pluton and its tectonic bearings on the activity of the Sillon Houiller Fault (Variscan Massif Central, France)

In the Variscan French Massif Central, the Sillon Houiller Fault (SHF) is a major sub meridian discontinuity which kinematics, timing and evolution remain poorly understood. The spatial relationship of this accident with the surrounding granitic plutons suggests a genetic link between the SHF activity and pluton emplacement. The SHF is classically interpreted as a Late Carboniferous left-lateral wrench fault that might have accommodated a NW-SE and NE-SW extensional tectonics to theWest and East, respectively. However, this model disagrees with structural and geochronologic data that clearly show two successive and distinct extensional strain fields. Thus, in order to better understand the faulting process, the Montmarault granitic pluton, located along the western side of the Sillon Houiller fault, has been chosen as target for a multidisciplinary study. Monazite yields a chemical U-Th/Pb age of 321± 2Ma (i.e. Namurian) interpreted as the age of pluton emplacement. Thirty-seven sites of 214 drilling cores have been collected for the study of the Anisotropy of Magnetic Susceptibility (AMS). The biotite is identified as the main magnetic susceptibility carrier with an average of 150 μSI. However, few sites show very high magnetic susceptibility carried by magnetite which is suspected to be secondary mineral because these sites are located along on the contact with three Late Carboniferous coal basins. More than 39 % sites show well defined principal anisotropic axes. To constrain the structure of the granitic massif at depth, aeromagnetic and gravity data were interpreted. A complete Bouguer anomaly map of the Montmarault granite including existing data, supplemented by 525 new gravity stations, provides an average coverage of the area with about 1 station per km2. Based on rock density and magnetic susceptibility measurements, direct 2D joint gravity and magnetic modelling has been performed along several cross-sections. These studies allow us to draw the following conclusions. The Montmarault pluton exhibits a magnetic foliation dipping at high angle towards to the fault for those sites close to the fault and flat lying or gently dipping for other sites. The majority of sampled sites shows a consistently NW-SE trending magnetic lineation which is perpendicular to the Sillon Houiller fault. The important negative gravity anomaly along the fault and a weak anomaly to the west suggest that the Montmarault pluton is rooted along the SHF and spreads out northwestwards. Therefore, for the Namurian, the SHF cannot be considered as a transfer fault during but rather as a normal fault. These geometry and kinematics comply well with the overall late-orogenic regional extensional pattern at the scale of the whole Variscan French Massif Central. A preliminary 3D modelling of the pluton combining our new structural, AMS, gravity and aeromagnetic data, is also presented. Keywords: Granitic pluton, Anisotropy of Magnetic Susceptibility (AMS), European Variscan belt, Gravity and magnetic modelling, French Massif Central

Semi-automated structural analysis of high resolution magnetic and gamma-ray spectrometry airborne surveys

International audienceA user-controlled procedure was implemented for the structural analysis of geophysical maps. Local edge segments are first extracted using a suitable edge detector function, then linked into straight discontinuities and, finally, organised in complex boundary lines best delineating geophysical features. Final boundary lines may be attributed by a geologist to lithological contacts and/or structural geological features.Tests of some edge detectors, (i) horizontal gradient magnitude (HGM), (ii) various orders of the analytic signal (An), reduced to the pole or not, (iii) enhanced horizontal derivative (EHD), (iv) composite analytic signal (CAS), were performed on synthetic magnetic data (with and without noise). As a result of these comparisons, the horizontal gradient appears to remain the best operator for the analysis of magnetic data. Computation of gradients in the frequency domain, including filtering and upward continuation of noisy data, is well-suited to the extraction of magnetic gradients associated to deep sources, while space-domain smoothing and differentiation techniques is generally preferable in the case of shallow magnetic sources, or for gamma-ray spectrometry analysis.Algorithms for edge extraction, segment linking, and line following can be controlled by choosing adequate edge detector and processing parameters which allows adaptation to a desired scale of interpretation. Tests on synthetic and real case data demonstrate the adaptability of the procedure and its ability to produce basic layer for multi-data analysis. The method was applied to the interpretation of high-resolution airborne magnetic and gamma-ray spectrometry data collected in northern Namibia. It allowed the delineation of dyke networks concealed by superficial weathering and demonstrated the presence of lithological variations in alluvial flows. The output from the structural analysis procedure are compatible with standard GIS softwares and enable the geologist to (i) compare the structural features with various data sets in a common geo-referenced frame, and (ii) take advantage of attribute selection to highlight specific structural patterns or eliminate possible artefacts

Integrated 3D geophysical and geological modelling of the Hercynian Suture Zone in the Champtoceaux area (south Brittany, France)

International audienceThis paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3D geometry of a segment of the Hercynian suture zone of western Europe in the Champtoceaux area (Brittany, France). The Champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists, with eclogite-bearing units. The exhumation of the complex, during early Carboniferous times, was accompanied by deformation during regional dextral strike–slip associated with a major Hercynian shear zone (the South Armorican Shear Zone, SASZ). Dextral shearing produced a km-scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis. Armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components. Based on the seismic constraint, direct 2D crustal-scale modelling was performed throughout the Champtoceaux fold on seven radial gravity profiles, also using geological data, and density measurements from field and drill-hole samples. The 3D integration of the cross-sections, the digitised geological map, and the structural information (foliation dips) insure the geometrical and topological consistency of all sources of data. The 2D information is interpolated to the whole 3D space using a geostatistical analysis. Finally, the 3D gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the Bouguer anomaly. The final 3D model is thus compatible with the seismic and gravity data, as well as with geological data. Main geological results derived from the modelling are (i) the overall 3D geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the Champtoceaux complex which was coeval with strike–slip along the South Armorican Shear Zone; (ii) the gravity modelling suggests the presence of a relatively dense body below the Champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the E–W trending wrench–thrust system; (iii) the northern limb of the Champtoceaux anticline is a relatively shallow feature; and (iv) Vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the SASZ and spreads away from the strike–slip zone within weak country-rocks

MUon Survey Tomography based on Micromegas detectors for Unreachable Sites Technology (MUST 2 ). Principles, experimental results and overlook.

International audienceTransmission muography is an expanding technique based on the attenuation of the natural-occurring cosmic muons' flux due to the opacity of the medium to obtain the distribution of density around the detector. The current work introduces the technology developed by the Temporal Tomography of the Densitometry by the Measurement of Muons (T2DM2) collaboration. The MUST 2 camera leans on a thin time projection chamber read by a resistive Micromegas. This new tool presents interesting distinctive features, allowing a wide angular acceptance of the detector with a low weight and volume, well adapted for confined spaces or underground operation. The results obtained from field measurement campaign carried out at the dam overlooking the village of Saint-Saturnin-les-Apt (South-East of France) are presented. The influences of (i) the host rock body of the barrage and dam's structure, (ii) the temporal water level variations of the reservoir and (iii) the effect of the temperature on the muons flux measurements are discussed The main challenge that faces the project is that the muon trajectory reconstruction algorithm cannot infer the arrival angles for a non-negligible number of detected events, with the subsequent loss of information. The data collected during the campaign of measurements, should help improving the algorithm's robustness and reconstruction efficiency. Field transportability and the capability to perform long-term out-of-lab measurements have been demonstrated. The successful proof-of-concept trial makes the MUST 2 camera a valuable candidate for transmission muography purposes, particularly in challenging available volume scenarios. The next stage of the T2DM2 project aims at imaging and monitoring the hydrodynamics across the unsaturated zone of the Fontaine-de Vaucluse aquifer. To do so, a network of 20 autonomous detectors will be constructed and deployed within the facilities of the Low Background Noise Laboratory of Rustrel (LSBB), France. The privileged emplacement of the LSBB allows the access to both the surface and to a network of 4 km of underground galleries with depths ranging from 0 to 518 m. Submission deadline: 30 Jul