Three-dimensional inversion of eddy current data for non-destructive evaluation of steam generator tubes

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Abstract
Three-dimensional (3D) eddy current non-destructive evaluation of steam generator tubes is cast in a wavefield integral framework and considered from an inverse problem point of view. Three inversion algorithms based on a gradient-type iterative search are studied: the modified gradient method, an inversion via the source-type integral equation, and one based on the localized nonlinear approximation. To overcome the inherent ill-posed nature of the inversion problem, a binary constraint is enforced on the contrast to be retrieved. Special attention is devoted to the evolution of the iterative processes, which is controlled by means of a tunable `cooling' parameter. The difficulty of such 3D reconstructions is discussed with emphasis on attenuation problems that are typical in eddy current analysis.

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