A record of the velocity with which a particular phase (gas, oil or water) moves in a producing well. While most flowmeters measure some average of all the fluids, the phase-velocity log identifies one particular phase. This is particularly important in highly deviated and horizontal wells with multiphase flow, where the flow structure is complicated.

Phase-velocity measurements are made with either the crosscorrelation flowmeter, the water-flow log, or with chemical markers designed to mix specifically with one particular phase. Velocity-shot measurements, using radioactive tracers, have also been used. In a typical chemical marker technique, a gadolinium-rich marker is injected into the flow stream, dissolving in either oil or water. Gadolinium has a high capture cross section, or sigma, so that a slug of fluid with high sigma moves with the appropriate phase up the borehole. This slug can be detected by a standard pulsed-neutron capture tool, and the velocity of the phase computed from the time of flight between ejector and detector.