Experiments were performed with the objective to obtain the velocity of liquids, e.g. water and oil / water mixtures flowing through a pipe. Experimentally tested flow rates were between 0.75 and 39 m<sup>3</sup>/h, corresponding to velocities of 0.023 and 1.2 m/s. Temperature disturbances were created by injecting slugs of hot water through a side inlet. The temperature was measured at regular time intervals of 1.2 s by Fibre Bragg Gratings, which were located at 40 equidistant locations at 0.5 m spacing. Hot slug movement was clearly visible in nearly all data sets. The magnitude of the temperature rise (0.1 - 0.2 °C) at that flow rate had the same magnitude as the temperature noise. Travelling temperature waves showed a relatively strong
deformation at low rates, due to relatively strong Taylor dispersion occurring at transitional flow (Reynolds number ~ 2500 at the lowest rate). Temperature disturbances travelling down the pipe were tracked by semblance processing of the temperature data taken at 34 locations after the side inlet; the data from 6 locations before the side inlet was not included. Semblance processing is a technique commonly applied in obtaining seismic wave velocities using an equidistant array of receivers, which was applied here for velocity tracking of hot liquid slugs. The best velocity estimates were obtained at flow velocities of 0.13 and 0.28 m/s, typical for a small oil well. Velocity errors were largely within a +/- 10% bandwidth.