A cased well in the earth is electrically energized with A.C. current. Voltages are measured from three voltage measurement electrodes in electrical contact with the interior of the casing while the casing is electrically energized. In a measurement mode, A.C. current is conducted from a first current...http://www.google.com/patents/US20020016675?utm_source=gb-gplus-sharePatent US20020016675 - Electrical voltages and resistances measured to inspect metallic cased wells and pipelines
