A record of the presence of tracer material placed in or around the borehole to measure fluid movement in injection wells. There are two traditional techniques for recording radioactive-tracer logs: the tracer-loss measurement, in which a tracer material is added to the completion fluid and its progress monitored with a gamma ray tool; and the velocity-shot measurement, in which the tracer is ejected from one part of a production logging tool and its progress monitored by one or more gamma ray detectors farther down the tool.

Radioactive-tracer logs are used to determine injection-flow profiles and detect channels or leaks. They may also be used in production wells, but care must be taken to isolate the fluids on surface until the tracer has decayed to safe levels. The tracer is a radioactive isotope that is soluble in water, oil or gas, or else insoluble, as in the bead tracer. Different radioactive elements with distinct energies and lifetimes may be used. Today, for water injection, the most common is a water-soluble iodine tracer that has a half-life of 8. 1 days, while for steam injection a gas with krypton is used.