Three dimensional geometries are often derived from two dimensional data such as surface mapping, interpretative cross-sections and kinematic interpretation of micro- to mesoscopic structures. To provide further constraints during 3-dimensional modelling, forward geophysical modelling capabilities have been built in gOcad® using Noddy (Jessell and Valenta, 1996).
Noddy is a 3-dimensional forward modelling package enabling computation of potential field data for 3-dimensional geometries. It has been developed by Dr Mark Jessell at Monash University. The software works from a set of kinematic events that defines a history applied on an initial layer-cake stratigraphy. The software computes the resulting 3-dimensional geometry using space-filling equations on a 3-dimensional regular and cubic grid which the exact analog of a gOcad's voxet. Once the 3-dimensional geometry is created, gravimetric and magnetic fields are calculated using interactively set values for density and magnetic susceptibility.
Once a surface model is created in gOcad®, the cells of the voxet (3-dimensional regular grid) are divided into sets (or gOcad's regions) according to their relative position with respect to the volumes bounded by the initial surfaces. In other words, all the cells falling within the same volume bounded by a set of surfaces belong to the region defined by these surfaces. In Fig. 6a, the green unit was defined by (1) its lithologic boundaries (with the blue and light-brown units) and (2) the thrust on the left hand side separating the dark-grey unit from the above lithologic package.
Once the gOcad voxet is divided into regions, properties such as density and magnetic susceptibility are assigned to each of the regions. This is done either by assigning a constant value throughout a whole region or using the geostatistical tools built-in gOcad. The newly created geophysical model is exported to Noddy and the geophysical response in calculated running Noddy in batch mode. The calculated result is consequently re-imported into gOcad as a 2-dimensional raster (Fig. 7). This link shows the full process from surfaces to a potential field response.
A full set of functions allowing a complete two-way exchange of data/models between Noddy and gOcad® has been developed (Fig. 8). These functions allow the user to:
(1) export a gOcad voxet model as a Noddy Geological Block (Fig. 6),
(2) export a gOcad voxet model as a Noddy Geophysical Block (Fig. 9),
(3) compute the geophysical response of a model previously exported (3),
(4) load the raster images corresponding to a model,
(5) import a Noddy Geological Block as a gOcad voxet,
(6) perform a calculation of the geophysical response of a voxet model and load its raster images (Fig. 7).