A method of producing a radioactive-tracer log, in which a slug of radioactive material is injected into the flow stream of a production or injection well from one section of a logging tool and observed as it passes one or more gamma ray detectors in another section. The slug, or shot, causes a peak in the gamma ray reading as it passes a detector. The flow velocity is determined from the difference in the time of arrival of the slug at the two detectors, or between ejector and detector. This technique has been applied for many years using radioactive tracers such as iodine. Water-soluble tracers are the most common, but oil- and gas-soluble tracers are also used.

Velocity-shot measurements are recorded with the tool stationary. They are more accurate than flowmeters at low flow rates, below approximately 100 B/D (16m³/d). They are not usually run in production wells because of problems of tracer disposal. In multiphase flow, the tracer most often travels with the continuous phase, thereby giving a type of phase-velocity log.