Ok, it works now

with the two surfaces

the volume between the surfaces:

with the base surface:

and the top surface:

But I can not visualize it in nviz:

- I tried with isosurfaces attributes 1 and 2 or 2 only, like the threshold values in Paraview (from r.mapcalc 'one= 1' and r.mapcalc 'two= 2' )

GRASS 6.4.2 (geol):~ > r3.univar volume20

100%

total null and non-null cells: 2410200

total null cells: 1703698

Of the non-null cells:

----------------------

n: 706502

minimum: 2

maximum: 2

range: 0

mean: 2

mean of absolute values: 2

standard deviation: 0

variance: 0

variation coefficient: 0 %

sum: 1413004

- I tried with different values associated with surfaces :

GRASS 6.4.2 (geol):~ > r.univar bxlbase

100%

total null and non-null cells: 48204

total null cells: 8133

Of the non-null cells:

----------------------

n: 40071

minimum: 463.652

maximum: 749.639

range: 285.987

mean: 577.433

mean of absolute values: 577.433

standard deviation: 87.919

variance: 7729.75

variation coefficient: 15.2258 %

sum: 23138333.880065918

GRASS 6.4.2 (geol):~ > r.univar bxlbase

100%

total null and non-null cells: 48204

total null cells: 8133

Of the non-null cells:

----------------------

n: 40071

minimum: 544.634

maximum: 1081.48

range: 536.846

mean: 842.635

mean of absolute values: 842.635

standard deviation: 126.068

variance: 15893.2

variation coefficient: 14.9612 %

sum: 33765222.471862793

But my nviz remains hopelessly empty.... (only the box outline)