Porosity is the number of pores per unit of volume in soil. Porosity may decrease through packing or compaction, increase through dilation during shear, or change when fines are transported from one area to another. Materials with higher porosity have a higher storage capacity to hold fluids within the void space. Porosity, in conjunction with the pore fluid properties, directly impacts resistivity.
Permeability is the capacity of a material to conduct a fluid. High permeability materials are less resistant to fluid flow and require less pressure to force the fluid through than lower permeability materials. Permeability is dependent upon the geometry of fissures, pores, and cracks.
Relationship with Geological Materials
Porosity and permeability are highest in coarse-grained poorly-consolidated sedimentary rocks, with decreasing porosity with finer grained and more compacted materials. Igneous rocks typically have low porosity, excepting extrusive tuffs and pumice. Metamorphic rocks usually have intermediate porosity, where compaction and recrystallization infill pore spaces. Weathering increases porosity and permeability, where increasing cracks, fractures, and void spaces all allow for greater pore fluid mobility.