Not really, thanks anyway, I found out how to do it:
sum(2*3.14*porosity(i,j,k)*r(i)*dr(i)*dz(k)*density)
where (i,j,k) are the coordinates of a point,
r(i) the radius,
dr(i) the radius variation,
dz(k) the height variation.
:smile: thanks 4 everybody,