best erosion/dilation method

Hello,

I have a very large series of images (cross-section) of a column of soil (two layer of soil) with very low resolution. I have the porosity of the two layers and I used many diffrent methods and neighborhoods to do the segmentation. I have found the best methods (which gave me the true difference between porosity of two layers but the porosity of each layer is different from the experimental data. Now, I was wondering if you can have any suggestions on which dilation/erosion method might workin this case (to reduce the porosity of two layers in the same amount for example: the porosity that I got after segmentation is 0.5 and 0.55 and I need to change it to 0.35 and 0.4)

thank you very much.