4. GEOVIEW SYSTEM
The presented system aims at 3D subsurface modeling to
improve oil, gas, and mining explorations. Subsurface data are
loaded into the system as 3D multi-layers. For each grid point
on the x-y plane, depth values of top and bottom surfaces of
each layer are registered in the z direction to form the
geometric description of the subsurface layers to which layer
attributes are attached. Efficient data structures and modeling
tools are provided for visualising, analysing, and managing the
3D geological subsurface data. The system is developed in C
programming language and based on a strong Graphics Library
package which takes advantages of hardware of Silicon
Graphics.
Two modelers coexist in the system: a surface modeler and an
octree modeler. The surface modeler is used to convert the
input data into a surface model according to the top-bottom
layer-surface information; while the octree modeler transforms
the same data into an octree model if necessary. Since the
surface model and the octree model depict the same objects by
using surface and solid geometric information respectively, the
efficiency of modeling functions based on these two models is
also different (L1 1994). For example, the surface model gives
a relatively realistic shaded surface for visualisation because
subtle normal vector changes of the surfaces can be
represented. This is especially important when the lighting
function is used. On the other hand, octants have six faces
which, in turn, give only six normal vectors parallel to three
principal axes. If the resolution of the octree model is set as the
same as that of the input data, there is no loss of geometric
information in the resulting octree model. However, the
graphic quality of the octree model display is not comparative
to the realistically shaded surface model because of the
restriction of normal vector directions. Furthermore, layer-
related topology can be constructed in surface models.
One of the major advantages of octree models is efficient
Boolean operations because of the simple geometry and
topology of octants. If encoded by Peano keys (Laurini and
Thompson 1992), some spatial operations can be carried out at
the bit level. In this system, octree models are, therefore, used
to perform 3D spatial operations for analysis and simulations.
Consequently, the system maintains two kinds of models for
the same loaded object, namely the surface model for
visualisation and the octree model for spatial operations.
Since most spatial operations are based on the octree
representation the efficiency of octree operations often
determines system responses to users requests. In special cases
of geological subsurface modeling with large layer datasets,
this is especially true. Among others, one of the critical and
frequently used basic spatial operations is finding neighbour
octants for a given octant. An application of this basic function
in subsurface modeling could be to find all octants on the
surface, for example, for a conversion from an octree to a
surface model. Surface boundary lines can then be extracted
from the boundary octants. The same basic function is also
used in the operations to cut a subsurface model by a plane or a
half cylinder face so that the intersection profile of the solid
surface model is exposed for material queries and geological
interpretations (Li and Xu, 1995). Figure 4 shows one of the
examples in geological subsurface modeling. "Fences" are
interactively defined on a 3D subsurface model. Spatial
operations are required to perform the intersection between the
"fence" (multi-planes) and the model. The front part of the
model is then removed so that the defined profile can be
visualised. In light of the above facts, it is necessary to develop
an efficient algorithm for finding boundary octants in order to
support quick system responses to users' octree-based requests.
5. ACKNOWLEDGMENTS
This project has been supported by the Geological Survey of
Canada and the National Sciences and Engineering Research
Council of Canada (NSERC).
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