The effective resistivity of the formation water or the mud filtrate, as it affects the electrochemical potential. The electrochemical potential is the main source of the spontaneous potential log. The equivalent water resistivity, or equivalent resistivity, is defined as the reciprocal of the activity of a solution, so that the formula for the electrochemical potential, E_c, can be written as:

E_c = - K log₁₀ (R_mfe / R_we)

where K is a coefficient, and R_mfe and R_we are the equivalent water resistivities. The importance of R_mfe and R_we is that they are equal to the actual water resistivities for NaCl solutions above about 0. 1 ohm-m. In salty waters below 0. 1ohm-m, the equivalent resistivity is lower by an amount that depends on temperature and salinity. In fresh waters, divalent ions such as Ca⁺⁺ and Mg⁺⁺ have a strong effect on E_c. R_we is then related to R_w by an empirical transform that assumes average concentrations of these ions. For fresh mud filtrates, it has been common practice to set R_mfe = 0. 85 * R_mf.