Understanding the geometry of structures and relationships among stratigraphic and tectonic units at the depth is one of the main challenges faced by geologists. This aim is traditionally pursued through interpreting geological and geophysical data in one or two dimensions (logs of deep wells, stratigraphic columns, cross-sections, structural maps) and performing structural analysis (cut-off lines maps, branch lines maps, restoration and balancing of cross-sections). The recent development of software devoted to geological modelling in three-dimensions represents a significant resource for geologists for reconstructing and visualizing geological bodies at depth (SLAT et alii, 1996; De Donatis, 2001). Geological models in three-dimensions integrate different kinds of data (cross-sections, seismic profiles, logs of deep wells) more easily than traditional techniques and allow a more coherent and exhaustive representation of the reality. Furthermore, progressive insertion of data in the model makes the geologist able to verify step by step the quality of his previous interpretations and to reach better results through iterative corrections. The aim of this prototype study project is to define and test a methodology for building three-dimensional geological models based on maps on the scale of 1:50000, from the new Italian geological mapping program. Another target of this work is to test the suitability of the national geological data base for 3D reconstruction purpose (De Donatis et alii, 2002). The recently developed three-dimensional visualization technique is used to build a structural model of sheet 280-Fossombrone, an external area of northern Apennines, central Italy. This area is characterized by faulted anticlines and broad synclines, involving a Triassic-Paleogene succession that is detached from the underlying basement. The exhaustive knowledge of the regional and local geology together with the availability of subsurface (well and seismic) data makes this area a good test to define a methodology for building a 3D geological map. This model has been built in two main steps. In the first step, a 2.5 dimensional geological model has been built using a digital elevation model and the new geological map of the study area at the scale of 1:50000. This 2.5 model better shows the relationships among topography, geology and main structures better than traditional 2D geological maps. During the second phase a deep model was built, integrating a large amount of sub-surface data, as commercial seismic profiles (arising from hydrocarbon explorations made by Eni-div. Agip during past decades) and deep well logs, with field data resulting from the recent mapping project. The whole 3D geological model of Sheet 280-Fossombrone allowed a better understanding of the geometry of the deep structures and the kinematics of this external part of northern Apennines. Structural and geomorphological analysis are performed on the model to improve the knowledge of several aspects of the study area.

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