An empirical law which relates, for a clay-free sediment, the electrical resistivity ρ‎ of a porous rock containing water and cement to the fraction of the pore space that is filled with water. ρ‎ = ρ‎o^−ms⁻ⁿ, where ρ‎o is the resistivity of the water and s is the fraction of pore space filled with water. The exponent n is usually about 2.0 if one-third of the pore space is filled with water, and exponent m usually varies between 1.3 for unconsolidated sediments to approaching 2.0 for a well-cemented sediment. In the oil-bearing rocks, the remaining pore space is considered to be filled with either oil or gas and the law is usually expressed as S_w = (R_w/R_t)^0.5ϕ‎, where S_w is the fraction of pore space filled by water with a resistivity R_w in a rock with a true resistivity of R_t and a porosity ϕ‎.