Potencial espontáneo estático (SSP)
The maximum SP that would be recorded when the borehole logging sonde passes from within a very thick, porous, permeable, clean sand to a point that is well within a thick shale. The electrochemical SSP (the electrokinetic SP is often neglected) is approximately given by:
where aw is the activity of the formation water and amf that of the mud filtrate. Because of the inverse relationship between activity and equivalent resistivity, this equation can be written:
where Rmfe=equivalent mud filtrate resistivity, Rwe= equivalent water resistivity. For NaCl muds that are not too saline, Rmfe=Rmf; for other muds an activity correction should be made. In these equations where T=Fahrenheit temperature. See also SP and pseudostatic SP.