[GRASS-user] Generate 3D raster from two surfaces

[grass at sundquist.imapmail.org]

I want to generate a 3D raster (and display in nviz) that is the volume
between two 2D elevation rasters.  The application is to visualize the
subsuraface area that will be subject to environmental remediation.

The values of the 3D raster will be trivial.  They will either be "1"
(or anything) or null.

The 2D rasters have elevation as their cell values.

Can r3.mapcalc be used for this?  Can I get grass to recognize the
raster value of the 2D raster as the z coordinate?  If I could do that,
it would be as simple as assigning the new 3D raster a value of 1 so
long as x and y were not null, and that z1 > z2.

Or is there some other easier way to do this?

Thanks.

J.S.