with a net influx from the more saline to the less. EM = K2 log10(aw/amf) where: K2 = 2.3 RT/F, where: R = ideal gas constant T = absolute temperature in kelvins F = Faraday constant aw = formation water ionic activity amf = mud filtrate ionic activity The total electrochemical potential is thus summarized as EC = EM + EJ = K log10(aw/amf) Since spontaneous potential is a measure of electrochemical potential and the ionic activity of a solution is inversely proportional to its resistivity, the above equation can be simplified as SP = EC = K log10 (Rmfe/Rwe), where Rmfe and Rwe are equivalent mud filtrate resistivity and equivalent formation water resistivity respectively. The ideal spontaneous potential across clean bed is known as Static SP (SSP), and defined as follow: SSP = −K log10 (Rmfe/Rwe) Applications in Boreholes The most useful SP component is the electrochemical potential, since it can cause a significant deflection opposite permeable beds. The magnitude of the deflection depends mainly on the salinity contrast between borehole and formation fluid, and the clay content of the permeable bed. The SP log is therefore useful in detecting permeable beds and to estimate formation water salinity and formation clay content. Due to the nature of the electric current, SP can only be recorded in conductive mud. Determination of Rw As established earlier, static SP is defined as follow: SSP = −K log (Rmfe/Rwe) Static SP (SSP) can be obtained directly from the SP curve if the bed is clean, thick, porous, permeable, and only moderately invaded. When these conditions are not met, the recorded SP will need to be corrected. Various correction charts are available for this purpose. To convert the measured mud filtrate resistivity Rmf into an equivalent mud filtrate resistivity Rmfe, the following rules are employed: If Rmf at 75 °F is greater than 0.1 Ω·m, use Rmfe = 0.85 Rmf at formation temperature. If Rmf at 75 °F is less than 0.1 Ω·m, derive Rmfe from Rmf using Schlumberger Chart SP-2 or equivalent. Schlumberger Chart SP-2 can then be used to convert Rwe to obtain Rw. Applications on the surface Electrodes can be placed on the ground surface to map relative changes in the SP value (in millivolts, or mV), typically with the goal of identifying the path of groundwater flow in the subsurface, or seepage from an earthen dam. A voltmeter measures the