What is 3D modelling and why bother doing it?

For decades, geologists have tried to share 3-dimensional interpretations using 2-dimensional images (map, cross-sections,...). Use of block diagrams is also common but they are only made of three 2-dimensional slices, usually one horizontal and two vertical slices, but a full description of the volume is still missing. Block diagrams can even lead to misinterpretation since geologists tend to assume cylindrical structures to mentally relate structure traces on maps and cross-sections.

In hard-rock geology, full 3-dimensional information (such as 3D seismic in the petroleum industry) is usually unavailable or spatially scarce. The starting point to any 3-dimensional modelling is very often a set of cross-sections and/or a set of points associated with information such as geological measurements (foliations, lineations, thickness of a unit, etc...). For the past century, geologists have been collecting such data. Using all available information still remains a big task since different kinds of data from different sources will not be consistent once put in a 3-dimensional space. Consequently, building 3-dimensional models involves a great deal of re-interpretation of the data. As such this is a great way to validate or refine previous interpretations which are often based on 2-dimensional representations and assuming consistency in the third dimension.

In other words, 3-dimensional modelling provides the basis for:

1. visualisation of complex geological geometries,

2. refinement of previous interpretation by testing 2-dimensional interpretation against the third dimension

3. a framework for property modelling and test of the model against other datasets.

What is 3-dimensional structural geophysics ?

The research group at the Crustal Research Centre of Monash University (Melbourne, Australia) focuses on the tectonic evolution of highly mineralised or highly prospective terranes. These terranes usually have suffered a complex tectonic history and their constitutive rocks are poly-deformed and poly-metamorphosed. The methods involved are field based structural mapping, geophysical interpretation (potential field data and 2-dimensional seismic transect) and geophysical modelling (inversion and 2-dimensional forward modelling). The scale varies from the plate tectonic scale to the mine scale.

These different data are integrated using 3-dimensional models and the models tested against potential field data (magnetics and gravity). The integration of structural and geophysical information to produce 3-dimensional geometrical and petrophysical models tested against geophysical datasets correspond to 3-dimensional structural geophysics.

At the Australian Crustal Research Centre of Monash University (Melbourne, Australia), we chose to use gOcad® (Mallet, 1992; ASGA, 1999) as our 3-dimensional modelling platform because it allows for realistic modelling of geological objects. This software also allows for an easy implementation of user-developed functions.

This paper deals with and advertises new developments implemented within the gOcad® software to enable the development of 3-dimensional structural geophysics models.