Guide to petrophysical interpretation / Руководство по интерпретации петрофизических данных

The SP is a passive measurement of very small electrical voltages resulting from electrical currents in the borehole caused by the differences in the salinities (resistivities) of the formation connate water (Rw) and the drilling mud filtrate (Rmf), and by the presence of ion selective shale beds. The voltage changes are measured by a downhole electrode relative to a surface ground. Unlike other logging tools which are displayed on a specific scale with a specified reference value, the SP has no specified origin and values used for computation are referenced to deflection from the nearby shale baseline established by the interpreter. <...>