Coupling 3D modelling and forward-inverse modelling of potential field data (gravity and magnetic data).

The 3D modeling of geological objects is often decomposed in two steps: i) delimitation of the boundaries of the units corresponding to the various geological formations or ore types; and ii) verification and estimation of these boundaries using geophysical data. A new approach using potential-field interpolators addressing the 3D modeling problem is used here (Ch.Lajaunie et al. 1997). We will discuss how we can statistically estimate the validity of such 3D model taking in account various geophysical data. This estimation can be computed by inverting complementary datasets, provided (a) the data are a function of the 3D distribution of a source, (b) the response of a given 3D source distribution can be calculated, and (c) the source distribution shows some degree of correlation with the litho-regions. Gravity and magnetic potential field data generally satisfy these criteria. Unfortunately, these data do not allow source geometry to be uniquely resolved through inversion, nor is the source geometry likely to be perfectly correlated with the litho-regions. Even allowing for these limitations, we can see through the expression for the posterior probability density function (PPD) for a Bayesian inversion procedure how uncertainty in prior geological knowledge is modified by investigating the fit to observed potential field data for various models; (1) where is a normalizing constant, is the prior probability for the property model based on geological knowledge, and is the likelihood function that reflects the agreement between the observed potential field response and the predicted response of the model. Litho-models that have reasonable probability based on prior knowledge are downgraded if the likelihood deduced from the associated potential field response is very low. To reduce the non-uniqueness, we can add to the classical data, the tensor components of the field. The main goal with gradients measurements is to improve accuracy and spatial resolution of gravity and magnetic surveys. For those reasons, we propose to build a 3D forward modelling and inversion method for tensor data

Modélisation géométrique 3D des granites stéphaniens du massif du Pelvoux (Alpes, France).

International audienceLa modélisation 3D de la géométrie des granites Stéphaniens du Massif du Pelvoux a permis de mettre en évidence le contexte cisaillant associé à leur mise en place. Dans les Massifs Cristallins Externes Français, ces cisaillements se répartissent selon deux directions, N50 et N135, respectivement dextre et sénestre. Ce système décrochant Carbonifère s'intègre dans un contexte d'extension N-S connu dans l'ensemble de la chaîne Varisque

The 3D interplay between folding and faulting in a syn-orogenic extensional system: the Simplon Fault Zone in the Central Alps (Switzerland and Italy)

Extensional low-angle detachments developed in convergent or post-collisional settings are often associated with upright folding of the exhumed footwall. The Simplon Fault Zone (SFZ) is a Miocene low-angle detachment that developed during convergence in the Central Alps (Switzerland and Italy), accommodating a large component of orogen-parallel extension. Its footwall shows complex structural relationships between large-scale backfolds, mylonites and a discrete brittle detachment and forms a 3D gneiss dome reflecting upright folding with fold axes oriented both parallel and perpendicular to the extension direction. We present a regional study that investigates the interplay between folding and faulting and its implications for the resulting exhumation pattern of the gneiss dome using 3D geometric modelling (computer software GeoModeller), together with a consideration of the chronological relationships from field relationships and 40Ar/39Ar dating. The early Simplon mylonitic fabric is clearly folded by both extension-parallel and extension-perpendicular folds, forming a doubly plunging antiform, whereas the later ductile-to-brittle fabric and the cataclastic detachment are only affected by wavy extension-parallel folds. This observation, together with the interpreted cooling pattern across the SFZ, suggests that updoming of the footwall initiated at the onset of faulting during ductile shearing around 18.5Ma, due to coeval extension and perpendicular convergence. New 40Ar/39Ar dating on micas (biotite and muscovite) from a sample affected by a strong crenulation cleavage parallel to the axial plane of the Glishorn and Berisal parasitic folds establishes that these folds formed at ca. 10Ma, broadly coeval with late movement along the more discrete detachment of the SFZ. These extension-parallel folds in the footwall of the SFZ developed due to continued convergence across the Alps, accelerating ongoing exhumation of the western Lepontine dome and promoting coeval uplift of the crystalline Aar and Gotthard massifs in the late Miocene

Emplacement in an extensional setting of the Mont Lozère-Borne granitic complex (SE France) inferred from comprehensive AMS, structural and gravity studies.

The emplacement mode and setting of the Late Hercynian Mont Lozère–Borne granitic complex (French Massif Central), which consists of several plutons, is investigated. Structural and anisotropy of magnetic susceptibility (AMS) studies have been carried out to characterize the internal fabrics of the granitic plutons. Throughout the Pont-de-Montvert–Borne pluton, an E–W-trending magnetic lineation is well developed. In the host rock and the thermal aureole, a conspicuous E–W-trending lineation is interpreted as an evidence of a late-orogenic extensional event. To the east of the pluton, the AMS fabric is characterized by values of anisotropy degree (P′) around 4–5% with a prolate ellipsoid and subsolidus structures, whereas, to the west, the P′ parameter is weaker with an oblate ellipsoid and purely magmatic microstructures. A gravity investigation allows determination of the 3D shape of the pluton. The western part of the granitic complex is thicker than the eastern one and is interpreted as the feeder zone. This suggests an eastward spreading of the magma. The consistency between regional stretching and directions of AMS lineations in the pluton and the shape of the complex deduced from gravity strongly argues that the emplacement mode of the complex was influenced by the regional extensional tectonic setting during the collapse of the Hercynian belt

From underground laser scans to 3D urban geological and geotechnical models

International audienceThe near sub-surface geology, say down to 20-30-m-depth, of many cities has been massively exploited for extracting building stones and various other industrial or agricultural materials (gypsum, lime, etc…). The long-term instability of these cavities poses a significant collapse hazard conditioned by their geometry (void location, dimensions and shape) and by their surrounding rock mechanics properties. In this presentation, we show how handheld laser scanning surveys efficiently document geometric variables and can interact with 3D geological modelling of the surrounding rocks. The construction of near-surface urban geological models can then be turned into 3D geotechnical models by attributing geotechnical parameters to rock horizons and ultimately become a key subsurface knowledge component of BIM (Building Information Model). Acquiring surface and subsurface geometry is no longer a challenge thanks to handheld laser scanners. Survey loop traverses can be pieced together to link surface and subsurface geometry with accuracies better than 1 m (an accuracy level compatible with urban risk management maps at 1/5.000) (DEWEZ et al., 2017). However, the hundreds of-millions of 3D points describing the cavity surface cannot be integrated as such into geomodeling software. Too many points with not high enough information. We suggest two different scenarios to perform their integration: (i) as independent validation of geomodeling hypotheses, or (ii) as geomodel constraints. In the first integration scenario, point cloud information is passed to the geomodeling software at a minimal level. A decimated triangular meshed model can be used to intersect the geomodel. Triangulation is performed at the point cloud processing software level (e.g. GeoSLAM desktop or Cloud Compare) and intersection is handled at the geomodelling software level with a generic query concept (here GeoModeller software with a generic query API – LOISELET et al.,2016). In this instance, cavity mesh triangular faces are refined based on the geological model queries (relying on the marching triangles algorithm) and provide geotechnical attributes based on the geological formations given by the geomodel. This scenario offers a visual display of geological properties (Fig. 1) for checking that modelled layers and structures match those observed in underground outcrops. In the second scenario, which is more integrated, higher level information is passed to the geomodelling tool. Planar surfaces of marker horizons are segmented from the point cloud either manually using Compass (THIELE et al., 2017) or semi-automatically with FACETS (DEWEZ et al., 2016) and passed as structural data objects to constrain the geomodel (Fig. 2). This data integration is demonstrated on a ca. 1 ha underground building stone quarry of the eastern suburbs of Orléans, Central France. The cavity was scanned at ca. 1pt/1cm with a Zeb-Revo (90 Mpts underground and 35 Mpts above ground). A geomodel of the subsurface area (Calcaire de Beauce, Tertiary) was created with the GeoModeller software as a tabular sub-horizontal multilayer environment. The geomodel infers rock distribution over a domain of ca. 200 x 200 m with geological and geotechnical information (e.g. limit pressure for dimensioning building foundations). Both approaches leverage a generic API query tool informing which domain surrounds a point and whether a geological contact cross-cuts a triangular face