Eddy current testing of anomalies in conductive materials. II. Quantitative imaging via deterministic and stochastic inversion techniques

International audienceThis analysis of eddy current testing of defects in conductive materials is divided in two parts. Part I [l]was devoted to reflection-mode diffraction tomography imaging tech- niques. Part I1 discusses application of deterministic and sto- chastic quantitative inversion techniques to similar configurations, and heavily draws upon Part I for physics and calculations of the eddy currents. The defects are modeled as cy- lindrical inhomogeneities concealed within a homogeneous non- magnetic metallic half-space. Values of the conductivity within the anomaly cross section have to be retrieved from multifrequency anomalous fields observed on a line above and parallel with the surface on the damaged block when a known time-harmonic source illuminates the block. Such quantitative imaging involves the ill-posed inversion of a Fredholm first-kind integral equation with exponentially damped kernel and requires some kind of regularization. Two approaches are fo- cused upon. In a deterministic context, an iterative algorithm provides a regularized generalized inverse, with the inconvenient that the stopping parameter should be appropriately chosen. In a stochastic context, Kalman filtering rapidly yields a good estimate of the conductivities, which could be the start- up of a more precise but costlier solution