Abstract: Fluid injectivity within an interval in a well bore is determined by injecting into the well two fluid streams, one of which flows down the tubing and one of which flows down the annulus, one of said fluid streams containing a radioactive tracer. The sum of the two fluid flow rates is held constant while each flow rate is varied against the other. At each different pair of flow rates, a stable interface is formed between the fluid containing the radioactive tracer and the fluid without it. The position of this stable interface at each different set of fluid flow rates is measured by a conventional gamma ray well logging tool, and from the series of such measurements an injectivity log over the measured interval can be determined. The injectivity profiles for the two components of a two phase fluid flow system are determined by conducting the survey with a radioactive tracer soluble in the gas phase and repeating the survey with a radioactive tracer soluble in the liquid phase.