A form of [[radioactivity log]] used in [[production log|production logging]] for the study of tracer movements and, therefore, fluid movements in the immediate vicinity of the well bore (e.g., in casing. tubing, [[annulus]], open hole). Usually one or more [[slug|slugs]] of radioactive material are ejected into the fluid phase to be studied, and the direction and velocity of the introduced slug is monitored over different parts of the well bore. [[Tracer log|Tracer logs]] are helpful in estimating fluid flowrates, points of fluid exit or entry into the well bore, [[crossflow]], leaks, etc.