From CARG Database to 3D modeling: a methodological approach for 3D reconstruction of subsurface geological units

This study is intended to show a methodological approach for the 3D reconstruction, visualisation and modeling of buried deep and shallow subsurface geological bodies by means of GOCAD®. Using this procedure it has been possible to visualise in the 3D space the information originally "stored" within a 2D geological map and to establish topological relationships among the analysed objects. A simple link among different software is initially established through a set of conversion programs that allow to couple the data processing capabilities of GIS (ArcINFO®, ArcView®), where data are stored, with 3D modeling operations in GOCAD® environment, where data are modelled. The use of GOCAD® involves the discrete modeling of natural objects, basing on the following assumptions: o the geometry of any object can be defined by a discrete set of nodes (points) in the 3D space; o the topology can be modelled by links among these nodes; o the physical properties can be modelled as values attached to these nodes. Such a discrete approach is possible by means of a powerful mathematical tool, the Discrete Smooth Interpolator (DSI), able to interpolate both the physical properties and the location (x, y, z) of each node defining the geological objects in the 3D space. The DSI has been especially designed for modeling natural and complex sub-surface geological structures, taking into account a wide range of data as well as their complexity and variability. Following this discrete approach, all the data related to geometry, topology and properties of the geological objects stored in the GIS database can be retrieved and used for 3D representation purposes. Three basic GOCAD® objects have been created with data exported from the GIS: 3D points (VSet) with elevation values, 3D lines (PLine) including all the linear elements with elevation value, and the down-dip projection of bedding, fault planes, and eventually fold axes, and 3D volumes (Voxet and SGrid) characterized by regular or irregular 3D cells. Further information derived from geological cross-sections are directly added in GOCAD®. In this study four main sources of information have been used: 1 - topographic data, represented by contour lines and quoted points; 2 - geological, geomorphological and tectonic boundaries, consisting of 2D linear elements; 3 - mesoscopic structural measurements, including attitude of planar and linear elements (bedding, thrusts, strikeslip, normal faults, lineations, etc.); 4 - geological cross-sections, reconstructed through the analysis of surface geological data. All these information are originally stored in the CARG Project database, realized by the Italian National Geological Survey (SGN) in collaboration with the Italian National Research Council (CNR). The study area is located within the geological sheet 284 - Rosignano Marittimo (Central Apennines, Tuscany, Italy) of the new Geological Map of Italy (1:50.000) where a complex nappe structure, forming during the Apennine mountain building, is deeply cross-cut by extensional faults, related to the Thyrrenian sea opening. In this work specific emphasis has been given to the reconstruction of the geometrical stratigraphic features of the Neogene extensional basin which has been modelled through the construction of 3D grids. The geometric features of the reconstructed geological bodies can be used to design preliminary monitoring plans or subsurface investigations through seismic surveys and drilling. The characterisation of the shallow subsurface is important for civil engineering and environmental applications that depend upon precise definitions of the geometrical, geomechanical and hydrological properties of rock bodies.