Abstract
STRUCTURAL MODEL The evolution of time to depth conversion methods over the history of geophysical exploration has crossed through a very important but cumbersome field: Seismic Tomography. Having its origin in the medical profession, transmission tomography in geophysics has come to symbolize almost the same thing: A graphical slice (tomos) of the earth subsurface in terms of velocity and reflector depth by means of travel time information. We present here a new reflection tomography methodology developed under the project SISTRE using the GOCAD 3D modeler as the main structural modeling facility. A high level performance ray tracer runs over this modeler and can account for various complexities within the model. The method can deal with inversion of prestack multi 2D and post stack 3D data for macro model estimation. Velocity fields can vary linearly in the vertical and lateral directions or have a general lateral variation. Results on synthetic data to date have validated the effectiveness of the method to retrieve velocity and srtucture information in relatively complex geological environments. Most of the 3D modelers used so far in the field of earth sciences are based on spline functions. This is possibly related to the fact that no specific modeler was ever developed for geological applications. Consequently, the 3D modelers available on the market are very much driven by the CAD/CAM industry needs. These CAD/CAM tools are all based on spline interpolation techniques (B-splines, Bezier, etc). Spline surfaces, such as B-splines, have a number of properties which make them handy for many 3D modeling needs. They are mathematically well defined, so that we can have good control on their smoothness, their derivability, their adjustment to some constraints, etc. Bsplines surfaces are appropriate for parametric and model smooth geological interfaces. Nevertheless, difficulties arise when geological features such as the following have to be handled:
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