A tracer injection method is set forth and is practiced by using spaced end located injectors on a sonde having spaced detectors along the length of the sonde. The injectors inject radioactive isotopes which are distinguishable from one another at the detectors. The method contemplates simultaneous injection of different isotopes, making measurements to determine fluid flow velocity in the casing, and selecting peaks to obtain measurements of the fluid flow velocity in leaks through the casing into external channels along the casing.