Abstract: An elongated body member adapted to traverse a well contains a chamber for holding a quantity of a tracer element to be injected into the well. A radially extendable arm is pivotally attached to the body member and is adapted to be extended and retracted by a first drive system within the body member. A nozzle is coupled to the arm with a path of fluid communication provided between the chamber and the nozzle such that a second drive system within the body member may cause the tracer element to traverse such path and be released into the well fluid flow column from a location adjacent to the body member and preferably to be released in a generally longitudinal direction relative to the well. One or more detectors within the body member suitable for detecting the tracer element are utilized to determine the locations and/or flow velocity of the tracer element and therefore of the well fluid.