I. Introduction
The magnetic flux leakage (MFL) method for nondestructive testing (NDT) has been applied for the inspection of defects in ferromagnetic materials such as natural gas pipelines [1]. The basic principle of the MFL method is that a strong magnetic field produced by the permanent magnet saturates the ferromagnetic pipeline to leak the magnetic field around the defect. Hall sensors equipped along the pipeline measure the magnetic leakage fields; then, the size of defects could be estimated by the measured signals [2], [3].