# Dictionary:Static self-potential (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 *a*_{w} is the activity of the formation water and *a*_{mf} that of the mud filtrate. Because of the inverse relationship between activity and equivalent resistivity, this equation can be written:

where *R*_{mfe}=equivalent mud filtrate resistivity, *R*_{we}= equivalent water resistivity. For NaCl muds that are not too saline, *R*_{mfe}=*R*_{mf}; for other muds an activity correction should be made. In these equations where *T*=Fahrenheit temperature. See also *SP* and *pseudostatic SP*.