3D Modeling: Key concept

This paper's primary aim is to investigate a study area by using 3D modelling in order to gain new results and to demonstrate that, in some cases, this kind of approach is the only one that can be used to represent peculiar structural settings.

3D visualization is very useful in understanding geological features. Computer modeling and visualization of geological faults in 3D allow for a more complete view of structure, geometrical relationships, cylindricity and kinematics. To create 3D models, a variety of seismic and geological data is incorporated to accurately reproduce both surface and subsurface features. Changes in topography and the geological processes can be studied by employing existing knowledge about geological faults.

Geological cross-sections and structural maps are useful in studying the geology of an area; even more so are complex tools like contour maps or allan maps, isobath and isopach maps. The problem is that all these tools (maps and sections) are two-dimensional descriptions of the structures and often geological objects must be considered in their three-dimensional complexity. Representing a geological object in only two dimensions could be misleading and imprecise if the geological contest is hardly deformed and not cylindrical. For these reasons, the three-dimensional approach has been chosen for this study as the represented context is clearly non-cylindrical and could not be completely and faithfully described by geological cross sections or maps only.

3D models have additional advantages: data are better constrained because they have to be consistent in three dimensions; 3D models often help the modeller to discriminate between alternative assumptions and/or interpretations. Even though setting up models might be time-consuming (data entry, data check, etc), they can provide for cross sections with different orientations, and contour maps as well as for volume, area and length calculations. Finally, a 3D model provides the possibility to analyse the evolution of a structure to determine whether the reconstruction is geologically valid and to perform forward analysis. It can not only describe but also allow for reconstructing the unseen parts of the system and for predicting changes of the structural development through geological time. Synthesis modeling techniques have the potential for incorporating more data, with more congruent and precise information about a study zone, as well as both increasing understanding and reducing uncertainties and risks.

The starting point of any 3D model is often a set of cross sections and geological measurements. Once designed, the model must be tested with real-world data to determine its suitability as an accurate simulation of the processes being studied. This is the approach we followed during our research and we found a good fit between the reconstructed model and the data we did not import in the reconstruction (in order to take them as an external check).

Creating 3D models is an iterative process that requires continuous checks and reinterpretations of the existing data. The construction and interactive visualisation of the conceptual model may influence the user’s understanding of the structure. This interaction continues until the user is satisfied that the computer model accurately represents his conception and reconstructions fit with available data. The actual process of building the 3D model requires assumptions about the geology of the structures that may often be untested in 2D maps or sections. Modifications to the sections are generally required to produce internally consistent 3D models, so that modelling process becomes a significant part of the whole geological interpretation and reconstruction process (Jessel, 2001). The goal of most 3D modelling software is to geometrically model complex geology from sparse data using the user’s geological interpretations. This is achieved through a combination of advanced interpolation techniques (for example linear, tessellation, b-spline and others) and 3D editing tools that allow the geologist to interactively adjust the model.

Usually 3D models developed only for visualization are made up of triangulated surfaces. Although the modelling process varies in detail, the end product is a 3D model primarily defined by a series of interlocking surfaces. The visualization is obviously important, but the real strength of this kind of tool is the parameterization of the rock volume and structures. Computerized 3D reconstruction and modelling of geological materials allows for novel approaches to data analysis and visualization, most of which do not have manual counterparts. When the structure is complex and data sparse, as is often the case in geology, interactive 3D modelling techniques should be employed that can integrate new and existing data, guided by the geological experience of the modeller (Burford et al., 2000). In particular modelling enables the user to transform a conceptual geological model, based on scientific evidence, into a well-defined 3D computer model.