Determining the Step-change Conductivity Profile within Layered Metal Structures Using Inductance Spectroscopy

This paper presents an inverse method for determining the conductivity distribution of a flat, layered conductor using a multi-frequency electromagnetic sensor. Eddy current sensors are used in a wide range of non-destructive testing (NDT) applications. Single frequency sensors are very common, however, the potential of an eddy current sensor with spectroscopic techniques offer the ability to extract depth profiles and examine more fully the internal structure of the test piece. In this paper, the complete forward solution for a small right-cylindrical air-cored coil placed next to a layered conductor is based on the analytic solution provided by the transfer matrix approach. A simplified model is derived from the complete forward model to estimate the phase signature of the coil. This simplified model reduces the computation time by many fold compared to the complete analytic solution. For inverse solution, a simple search method was used to find the conductivity profile to fit a set of multi-frequency phase values in a least-squared sense. Numerical results of the forward solution are provided for cases of step-change conductivity profiles. Good estimates for the conductivity profile were obtained. Experimental eddy-current tests are performed by taking the difference in inductance of the coil when placed in free space and next to a layered conductor over the range 100Hz-1MHz. Inverse results based on experimental and simulated data verified this method