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 or isochore map. Both maps show thickness between two surfaces.

An isopach map is also referred to as True Stratigraphic Thickness (TST) map. This type of map represents the true thickness of a stratigraphic unit, even when tilted. An isochore map, also known as True Vertical Thickness (TVT), shows the vertical thickness of a unit. If the unit was horizontal, this would be the same as TST, but if units have a dip or incline, the vertical thickness is different from the stratigraphic thickness. 

Isopach maps measure True Stratigraphic Thickness, and isochore maps measure True Vertical Thickness.

To create an isopach or isochore map in Surfer, follow these instructions:

1. Create grid files of the upper and lower surfaces (for example, grid files that define the elevation of the top and bottom of a coal seam). 
2. Click the Grids | Calculate | Isopach command.
   
   
3. In the Grid Isopach dialog, click the Browse button next to Upper Surface. In the Open Grid dialog, select the grid file for the upper surface and click Open or click the Download button to download a grid file. 
4. Click the Browse button next to Lower Surface. In the Open Grid dialog, select the grid file for the lower surface and click Open or click the Download button to download a grid file. 
5. Based on the type of map that you want, click Isopach or Isochore for the Calculation Type. 
6. Click the open folder button in the Output Grid section, enter the name for the thickness grid file in the Save Grid As dialog, and click Save. 
7. Check the Add grid as layer to check box to create the grid file and display the grid as a map layer after it is created, or leave unchecked to just create the grid file only. 
8. If adding the grid as a layer after it is created, select the layer type for the new grid from the New layer type list.
9. Click OK to generate the selected grid. 

Create an isopach grid and display the results to create an isopach map.

For Surfer versions 21 and previous, see the instructions below 

If using Surfer 21 or a previous version, you can create an isochore map (True Vertical Thickness) in just a few steps by using Grid Math to subtract the bottom surface from the top to obtain the difference: 

1. 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.
   1. I get the error "Grids must be the same size" when using Grid Math or Grid Volume. What does that mean?
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2. Click the Grids | Calculate | Math command.
3. In the Grid Math dialog, click the Add Grids button.
4. In the Open Grid(s) dialog, select the grid files for the upper surface and the lower surface.
5. Click Open.
6. Click the Change Filename (yellow folder) icon in the Output Grid File field.
7. Enter a new File name.
8. Click Save.
9. 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.
   
10. Click in the New layer type field and select Contour.
11. Click OK, and a grid file is created along with a contour map showing the isochore lines.

Video Tutorial: