Guided waves are a form of ultrasonic waves that travel through a bounded test specimen (in this case, a pipe), parallel to the boundaries (inner and outer wall), while being guided by these same boundaries. Due to the nature of the waves and the testing frequencies used, the technique can be used to scan extensive lengths of pipe from a single transducer location, thereby negating the time-consuming type of grid scans required by conventional techniques such as UT and ET. As is the case in conventional UT, the guided wave system works in the pulse-echo mode. Guided waves are generated by the transducers and propagate in both directions along the length of the pipe. Regions of change in the cross-sectional area of the pipe along the path of the guided waves cause some of the transmitted energy to be reflected. This reflected energy is then detected by the same transducers and displayed in an A-scan type graph. Changes in the cross-sectional area can be caused by the presence of features such as welds, valves, tees, elbows and etc.
