A distortion of the magnetic flux has been introduced into the casing by a low frequency electromagnetic or permanent magnet. The principle of flow dispersion is used to detect corrosion in the casing, as the leakage flux is caused by rapid changes in the thickness and through holes in the wall either internal or external. This flow distorts the magnetic flux lines and induces a signal into an electrical coil. On site, the dispersion flow measurements make use of this effect by placing coils on or near the casing wall. The results are usually combined with high frequency.
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