An in situ log of the electrical potential on the inner wall of a casing. The log is used to identify intervals that are susceptible to corrosion. A negative slope in the profile indicates a zone in which current is leaving the casing and therefore acting as an anode. Such zones are susceptible to corrosion. The log was first introduced in the early 1960s. Modern logs are recorded with the tool stationary, and measure the potential difference and casing resistance between several pairs of sensors pushed against the casing wall, and between sensors and surface.
The log is usually represented with casing resistance and casing axial current. Sharp increases in casing resistance can indicate corroded zones or even holes in the casing. Decreasing axial current with depth indicates a corroding region.