ID-OD discrimination sensor concept for a magnetic flux leakage inspection tool

A method of determining the characteristics of the interior and exterior surfaces of a metal pipeline comprising the steps of:

• (a) moving an axially supported pig body through a pipeline;
• (b) by means of magnets carried by the pig body magnetically saturating a circumferentially zone of the pipeline that moves with the pig;
• (c) continuously measuring changes of reluctance in the moving circumferential zone to provide indications of the presence and size of anomalies in the pipeline interior or exterior surfaces;
• (d) electrically energizing a plurality of pulse coils to induce eddy currents in the internal surface of said moving circumferential zone of the pipeline;
• (e) by means of a plurality of sensing coils each paired with a said pulsed coil measuring said eddy currents to determine the presence or absence of anomalies in the pipeline interior surface;
• (f) comparing the results of steps (c) and (e) in signal processing circuitry to determine whether the anomaly detected in step (c) is on the exterior or interior surface of the pipeline;
• (g) recording the results of steps (c) and (f) to provide information as to the occurrence, size and exterior/interior locations of pipeline wall anomalies;
• and
• (h) energizing said plurality of pulse coils in step (d) only in response to signals generated by said signal processing circuitry.