A device for measuring fluid velocity in a production well. The device measures the transit time of a disturbance between two sensors separated by a fixed distance. The technology applies to multiphase flow, in which the disturbance is caused, for example, by the passage of a bubble of gas over each sensor. In practice, there will be many bubbles of gas, so it is necessary to record both sensor signals over a time window and compare, or correlate, them. The two signals will correlate best after shifting one of them by a time corresponding to the average transit time of the bubbles. Different sensors may be used, for example a measure of electrical capacitance as in a holdup meter.
The crosscorrelation flowmeter gives the velocity of the disturbance. Since this is caused by just one of the phases, it produces a type of phase velocity log.