RESPONSE OF HORIZONTAL-AXIS EDDY-CURRENT COILS TO LAYERED MEDIA - A THEORETICAL AND EXPERIMENTAL-STUDY

An exact theory for the determination of the impedance change of a horizontal-axis eddy-current coil as it is brought close to a conducting homogeneous half space has been extended to consider a layered half space. The derivation of the theoretical relation for coil impedance chage is given and two test cases are considered both experimentally and theoretically. Good agreement is demonstrated between theory and experiment. The possible application of the theory to the study of electromagnetic methods for the nondestructive measurement of the thickness of a layer or coating on a base material is discussed.