The gridding function provides a link between the surface representation of GOCAD and the regular grids of SEPLIB. The Create Grid option is an AVS version of the GOCAD to SEP gridding program Berlioux (1993). Berlioux's generates a 3D velocity grid from a GOCAD model by shooting a ray from all the nodes on the top of the grid and locating where they intersect with surfaces in the model. A GOCAD function is then called to determine the domains that were encountered by the ray.
To define the velocity that goes in a given domain a DDL file is necessary. The DDL file defines the partition of the 3-D space. Berlioux 1993 presents an example of a GOCAD model with the corresponding partition file. Currently the creation the DDL file is done by hand, eventually the ability to create simple DDL files will be added to the module. One change was made to Berlioux's gridding algorithm, the automatic creation of a ``free surface'', normally the water surface, or ground level. If the ``freesurf'' surface does not exist in the GOCAD project, it is created temporarily during the execution of the gridding program.
Figure 2 shows the AVS user interface to the Create Grid module. The browser widget Project Name allows the user to select the GOCAD Project File which contains the model surfaces. The file browser widget DDL File allows the user to select the partition file which defines the domains of the model. Two possible outputs are allowed, SEPLIB or AVS. The two possible output format option is provided for large grids which might exceed AVS's memory limitations When SEPLIB is chosen for the output format, the file browser widget Output File allows the specification of the output file where the grid will be save to in standard SEP format. The typein widgets n1, n2, n3 correspond to the number of grid cells on axis x, y, and z. The widgets o1, o2, o3 indicate the origin of the grid and d1, d2, d3 indicate the sampling intervals on axis x, y and z. The grid limits should be smaller than the limits of the surfaces. The widget Display Border allows the creation of a geometric object delimiting the limit of the grid that will be created. The colors of the border can be changed through the Red , Blue and Green slider widgets. Finally, once the user has set all the parameters, the Write Grid boolean should be selected to create and save the Grid.