Compensation of Coil Radial Offset in Single-Coil Measurement of Metal Tube Properties

Measurements of inner radius and electromagnetic properties (magnetic permeability and electrical conductivity) of metal, ferromagnetic tubes, such as oil-well casings, are essential in the assessment of their condition or as inputs for other measurement applications, e.g. reservoir electromagnetic evaluation through casing. In our previous work, we have shown that the inner radius and the permeability-to-conductivity ratio of the tube can be obtained by measurement of the impedance of a single coil axially centered within the tube. In this paper, we propose and experimentally verify the coil impedance model that is applicable for measurement of the tube properties when the tube and the coil axes are misaligned. Correction of the measured impedance for coil radial offset improves measurement accuracy and reduces the effect of the coil wobble.