An SH-wave EMAT technique for gas pipeline inspection

An EMAT technique has been developed to detect corrosion defects on the outer surfaces of steel pipelines. A periodic-permanent-magnet (PPM) EMAT is placed inside the pipe, which generates and receives the SH guided waves traveling in the circumferential direction with the axial polarization. Wheels installed on the PPM-EMAT facilitate the linear movement in the axial direction and are useful for a quick inspection. At each point in scanning, the amplitude and phase shift of the round-trip signals in the SH0 and SH1 modes are measured using a superheterodyne phase-sensitive detector. They respond uniquely to surface defects and provide relevant information for locating axial positions of defects and evaluating the depth. The amplitude and phase shift of the SH0 mode are more sensitive to the presence of the defects than those of the SH0 mode. This technique is useful even if the protective resin coating is tightly glued on the pipes and lowered the signal intensity. (C) 1999 Elsevier Science Ltd. All rights reserved.