This log measures the electrical current that occurs naturally in boreholes as a result of salinity differences between the formation water and the borehole mud filtrate (formation and surface). These logs are used as indicators of permeable beds (including determining permeable sands and impermeable shales) or for locating bed boundaries. The SP log was one of the first tools to be used to distinguish shale from sand in clastic sequences (zero matches pure shale while high SP values match sand)