You can do it using micro structure image analysis...here is link for similar work on  cheese

http://www2.units.it/ipl/research/cheese/cheese.html

Best method is x-ray tomography. A more basic method is to apply image analysis after cutting it into slices.

Image analysis is good method to check the porosity image analyzers are available in market. 

Please go through this

http://mgg.rsmas.miami.edu/students/greg/EAGE2004.pdf

thank you everybody 

If you're just looking for global porosity (not heterogeneity or local microstructure) and have a good estimate of your matrix density, also consider the simplest approach: (a) measure the volume of your sample, (b) weigh it and (c) calculate the "bulk density" by dividing mass by volume, (d) calculate porosity as 1-bulk density/matrix density.

thank you sir