voltage between a fixed liquid-junction electrode and a mobile one (rover), which is moved along a dam face or over an area of investigation to collect multiple readings. Anomalies observed may indicate groundwater movement or seepage. Interpretation SP can be affected by several factors that complicates the interpretation. Beside petrochemical component, SP is also affected by electrokinetic potential and bimetallism. Besides, SP is also affected by the following factors: Bed thickness (h); Since SP is a measurement of electrical potential produced by current in the mud, its amplitude approaches the SSP value only when the resistance to current due to formation and adjacent beds is negligible compared with that of the mud. This condition is met only in thick bed. In thin beds, the SP is proportionally reduced. True resistivity (Rt) of permeable bed; As Rt/Rm increases, the SP deflection decreases, and the bed boundaries are less sharply defined. Presence of hydrocarbons also attenuates SP. Resistivity of invaded zone (Rxo) and mud resistivity (Rm); SP increases with increase of Rxo/Rm Diameter of invasion (di); SP decreases as invasion deepens Ratio of mud filtrate to formation water salinities: Rmf/Rw Neighbouring shale resistivity (Rs); SP increases with increase of Rs/Rm Hole diameter (dh); With increasing hole size, the value of SP is reduced Measurement technique Spontaneous potential can be measured by placing one probe of a voltmeter at the Earth's surface (called surface electrode) and the other probe in the borehole (called downhole electrode), where the SP is to be measured. In fact, logging tools employ exactly this method. Since this measurement is relatively simple, usually SP downhole electrode is built into other logging tools. See also Spontaneous potential logging External links Schlumberger Correction Charts for SP References M.Gondouin, M.P.Tixier, G.L.Simard, Journal of Petroleum Technology, February 1957, "An Experimental Study on the Influence of the Chemical Composition of Electrolytes on the SP curve" Guyod, H., Oil Weekly, 1944, "Electrical Potentials in Bore Holes" Pirson, S.J., The Oil and Gas Journal, 1947, "A Study of the SP Curve" Electrostatics Geophysics