Inner defect depth detection using a multifrequency alternating current potential drop technique

The alternating current potential drop technique is usually used to monitor surface defects in metal structures by taking advantage of the skin effect. However, defects often occur in the inner walls of oil or gas pipes as a result of corrosion or erosion. In this work, a multifrequency exciting current is injected into a defective pipe to obtain a series of potential drop values. The magnitude of the potential drop decreases with decreasing frequency and finally tends toward a stable value. The trend of the potential drop is related to the depth of the defect. A general solution for calculating defect depth using this multifrequency alternating current potential drop technique is given.