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Technical Program
Session I Intro
Session I Papers
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The Geological Reference System of France: Towards a New Generation of 3D Maps
by Jacques Vairon*, BRGM, France,
Gabriel Courrioux, BRGM, France,
Jean-Paul Chilés, BRGM, France,
Pierre Nehlig, BRGM, France
Within the framework of its program for the Geological Reference System of France, BRGM sets up databases for the national coverage of 2D geologic maps and the validated drill-holes. Another aspect of this program is to supply 3D geologic maps based on these data.
The implemented methodology includes three main phases:
(1) Geological data extraction
- oriented curves describing geologic contacts and codes of bounding formations,
- curves describing faults,
- structural data (stratification, schistosity , fold axes),
- validated drill-holes.
At this stage all data must have consistent descriptions.
(2) Preliminary data processing
- for modeling purpose, it is necessary to select and group the data points corresponding to the interfaces chosen for modeling. This is done according to a stratigraphic pile and to the properties of each interface (erosion/onlap relations),
- fault traces are over-abundant in geological maps whereas no information can be extracted about their extension at depth. Therefore, it is necessary to simplify and select the most important faults and to add relevant knowledge about their 3D geometry.
(3) 3D modeling
- in the method we use, interfaces are modeled as isosurfaces of a 3D scalar field. Data are expressed as follows :
- increments between two points belonging to a same interface are equal to 0,
- orientation data represent the gradient of this field,
- faults are introduced as discontinuous spatial functions (external drift),
- the scalar field is interpolated by cokriging,
- intersections between interfaces are managed taking into account the stratigraphic pile and the properties (erosion, onlap),
- iterations are performed in order to honor the incomplete formation intervals at both extremities of drill-holes,
- cross-sections through the model can be edited to control its validity and to add constraints in zones where initial data are insufficient.
This methodology, illustrated through the example of the "Bain de Bretagne" geological map, allows to realize a geologically realistic 3D model starting only from data stored in the geological databases. This model serves as a basis to elaborate more accurate models dedicated to water resource, landscape management or geo-engineering.
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