| This paper gives an eddy current inspection method for the most common defects occurring in oil pipe casing ,which are known as diameter abnormity and crack. The detection coil carrying alternating current were lowered into pipe and the diameter abnormity and crack were directly represented by the changes in distance between the defect and the detection coil. Under condition that the measurement system parameters were fixed, the alternating current coil impedance is corresponding to such distance in monotropy. Based on the assumption that the field theory can be converted to circuit theory, a theoretic formula representing the change in alternating current impedance of the coil was derived. It is shown that there is a linear relationship between the change in pipe diameter and coil impedance within a certain range by the theoretic calculation, in addition, the coil impedance is sensitive to the crack of casing. When measuring, the size of emitting coil shall be carefully selected in order to match the pipe diameter. The linear range is increased with the increase of emitting coil size, but the sensitivity will be decreased, exhibiting a contradiction between the sensitivity and the linear range; If the size of emitting coil keeps unchanged, the sensitivity of casing crack measurement is increased with the decrease in pipe diameter. The emitting coil and capacitance are connected in parallel and form a resonant amplitude-modulation circuit,which converts the impedance into output voltage. The influence factors are analyzed on the measurement such as pipe diameter, length of casing crack and thickness, excitation frequency of emitting coil, casing magnetic conductivity and emitting coil eccentricity , etc. it provids a theoretic foundation for the quantitative interpretation of casing inspection.
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