In non-destructive testing methods, guided wave techniques are often used for detecting damages in structures. For pipes, cylindrical Guided waves can be used for this purpose. Investigations that have been carried out at the University of Arizona to detect defects in underground pipes are reported in this paper. Cylindrical guided waves are generated by piezo-electric transducers. Guided waves are propagated through pipes that are buried in the soil by placing transmitters on one end of the pipe and the receivers on the other end. The recorded signals are then processed using 2-D Short Time Fourier Transform (STFT) or Gabor Transform. Gabor transform maps the time-amplitude signal into a two dimensional time frequency signal which provides information about both when and at what frequency a signal arrives. In other words, Gabor Transformation provides information about the group velocities of the signal. From the experiments, signals were obtained for both with flow and without How conditions for buried pipes. These signals are then processed using Gabor Transformation to identify propagating modes by comparing experimental results with theoretical group velocities. An effort has been made to show how the flow through the pipes affects the propagating modes.