Isopach maps are often used in geology to illustrate thickness variations between two layers, such as the top and bottom of a coal seam or stratigraphic layer. If you have data defining the elevation of the top and bottom surfaces, you can use Surfer to create an isopach map. To do so, you will create a grid file defining the upper and lower surfaces, find the difference in the elevations with the Grid Math, and create a contour map. Follow the instructions below to create an isopach map of your own.
- Use the Home | Grid Data | Grid Data command to create a grid file for the upper surface and a grid file for the lower surface. Both grids must use the same XY limits and same number of rows and columns. See the below for additional information about gridding or creating grids with matching limits and rows/columns.
- Click the Grids | Calculate | Math command.
- In the Grid Math dialog, click the Add Grids button.
- In the Open Grid(s) dialog, select the grid files for the upper surface and the lower surface.
- Click Open.
- Click the Change Filename (yellow folder) icon in the Output Grid File field.
- Enter a new File name.
- Click Save.
- In the Enter a function... field, enter the formula A - B, where Variable A is the upper surface, and Variable B is the lower surface, as shown in the screenshot below.
- Click in the New layer type field and select Contour.
- Click OK, and a grid file is created along with a contour map showing the isopach lines.
Updated October 22, 2018