1. n. [Formation Evaluation]
The electromagnetic force generated by a boundary between solutions of high salinity and low salinity. In a permeable formation, a liquid-junction potential is generated between the invaded zone and the undisturbed zone when the mud filtrate and the formation water have different salinities. This potential is one component of the electrochemical potential, from which the spontaneous potential log is derived. The other, much larger component is the membrane potential at a shale boundary. The liquid-junction potential is reduced if there is clay in the permeable formation, since this generates another, local membrane potential with the opposite polarity to the liquid-junction potential.