Model of magnetic dipole established in weak magnetic field

The magnetic leakage field distribution above linear defects of a tube sample under the geomagnetic field was modeled according to magnetic dipole theory. The formula to compute the normal component of the weak magnetic field above the defects was derived based on the spatial distribution of magnetic dipole. The shape and character of the zero line, an important criterion for magnetic memory testing of the normal field is analyzed under different longitudinal magnetization. The results showed that the character of the zero line should be considered when using metal magnetic memory testing method to find and locate the defects.