QUESTION: I have two raster surfaces at different depths, displayed using raster_to_geology (spatial footprint is the same for both). What is the best way to calculate the volume of material between them?

If the two rasters have the EXACT same extents AND resolutions, then it is easy.

Read both separately and connect them to combine_geology with the upper surface connected to the left input port.

The output of combine_geology goes to 3d_geology_map and then to volumetrics.

If the extents and/or resolutions of the rasters are different, you cannot use the above method.  In this case, the simplest approach is to export each surface as a gmf file using write_coordinates and then combine them in a text editor into a single file.  If it best if you crop the rasters to the regions where they overlap.

Once you have both surfaces in a single gmf file, you would just use krig_3d_geology —> 3d_geology_map —> volumetrics

Alternative approaches worth mentioning:

1) You can use interp_data to map the surface data from the lower surface onto the grid of the upper surface and then use combine_geology to merge the original upper surface and the new interpolated lower surface into a 2-surface geo model that can connect to 3d_geology_map.   This approach has the same caveats regarding surface extents, that will likely require some cropping be done on one or both surfaces in raster_to_geology.

2) The second option is less elegant but straightforward.  First you build a model using the top surface and a constant elevation specified in make_single_layer.  Then you cut that flat-bottomed model with the second surface using surf_cut.  You need to make sure the flat bottom elevation is below the lowest point on the bottom surface.