Analysis of a Defect Signal Deformations Induced by Eddy Current in RFECT System for Pipeline Inspection

For testing a conductive pipeline, remote field eddy current testing (RFECT) is used, which is composed of solenoidal exciter coil and receiving sensor coils. In RFECT, there is remote field zone where the eddy current distributions are concentrated on the external surface of the pipe wall. Therefore, it is possible to detect a defect that occurs on the external surface of the pipe wall even if the receiving coils are located inside the pipe. From the measured coil signals, it is necessary to decompose background offset and predict the pattern of defect signal in order to estimate the defect size for the maintenance of underground pipelines eventually. In the process, the amplitude variations of the induced magnetic field at sensor coils are closely related to defect depth. Therefore, it is important to analyze and eliminate distortions of measured RFECT signals. This paper analyzes the effects of the induced magnetic field on the sensing signals with respect to different depth sizes in the RFECT system. The amplitude and the distribution pattern of defect signals are computed by a 3-D finite-element method and the method to eliminate the signal distortions caused by induced magnetic fields are presented. Simulated results in this paper agreed well with measuring ones.