In-place detection of internal and external corrosion for underground casing pipes

Corrosion monitoring and early detection of pits and wall thinning is important to the gas and petroleum industry. A new noncontact AC electromagnetic induction system has been developed for monitoring and detecting corrosion in multistring casing configuration. The system includes a sonde which has a transmitter coil and three receiver coils, with the transmitter circuit generating three low different frequencies at the same time. The multifrequency AC current through the transmitter coil induces in the pipe wall a longitudinal alternating magnetic field which is sensed by three receiver coils located at different distances. The multiple transmitter frequencies and the multiple transmitter-receiver spacings can provide the maximum flexibility and the most extensive information in quantitatively determining the total wall thickness in single, dual, and three-string casing configuration. The transmitter circuits are designed to work at up to 175/spl deg/C without having a second breakdown. Since the dynamic range of received signal is very broad and the "road note" is severe, the high-stable high-accurate band pass filter and hard limiters are adopted in the receiver circuit and phase detect circuit. In order to discriminate internal and external corrosion of casing pipes, an "electronic caliper" is incorporated in the system to detect the average circumferential diameter of casing pipes. Therefore, the complete logging of the casing pipes can be accomplished with a single scan.<>