How to assign a 2D random function as porous matrix property (porosity)

Good morning,

I really need your help, because I can't find anything in the help files.
I have a model using the the Brinkman Equations module for 2 dimensional porous media flow.
I want the porosity of the matrix to variate around the mean value of 0.5, so I defined a 2 dimensional random function named rn1 to describe this.

Now my problem is, that I can't define the porosity in the porous matrix properties as this rn function. I suspect, there is some kind of special "term" I have to use.
It would be great if you could help me!

Thank's a lot!

Jaques