How do I add apparent resistivity with comsol？

Hello,

I recently began using COMSOL and I have some questions. I created a simple ground model (2D), with resistivity 200 ohmm and a small parallelepiped in its center with resistivity 10 ohmm. I created a polygon of 25 points (on surface) that represents my electrodes: the electric current is injected through 2 of them，and there are two measuring electrodes in the middle. I used AC/DC module, electric currents interface. I want to draw a resistivity profile, but I don't know how to add the apparent resistivity formula, I measure it with a quadrupole symmetric device. If anyone has an existing model can you send it to me for reference? Thank You very much!!!