1. n. [Production Logging]
A distortion of the magnetic flux that has been introduced into a casing by a low-frequency electromagnet or permanent magnet. The principle of flux leakage is used to detect casing corrosion, since flux leakage is caused by rapid changes in the thickness of the casing and by pits and holes in either the internal or external wall. Flux leakage distorts the magnetic-flux lines and induces a signal into an electric coil moving past it. In-situ flux-leakage measurements make use of this effect by placing coils on or close to the casing wall, azimuthally distributed to cover the entire wall. The results are often combined with a high-frequency, eddy-current measurement, designed to detect flaws only on the inner wall.