Draw the polygons as polyline links on a line layer; cleanup the drawing; make a polygon topology on a topology layer; and create a polygon set on a polygon layer. Identify the polygons either by Z value (use the CHANGE properties command), or colour (use the Modify Properties command) or place each set of rock unit polygons on differently named layers.
Enter the DXFOUT command on the command line. In the Create DXF File dialog box enter a file name, and select AUTOCAD r12 as the ‘Save as Type’option. Click the Options button, and in the Export options dialogue box turn on the Select Objects button. The default format should be ASCII, and the default decimal places ‘6’. Click the OK button followed by Save. In response to the Select objects request, type ‘filter (i.e. the quote symbol followed by the word “filter”) to get the Object Selection Filters box. In the Select Filter scroll box, select Layer, and in the Select options box select Polygons as the layer to be exported. Click OK and the Add to List button. Click the Apply button and in response to the Select objects query type ‘all’ and press the ENTER key twice. The DXF file will be saved with only the polygon data.