Inversion of Eddy-Current Data and the Reconstruction of Flaws Using Multifrequencies
Sabbagh, L. David
We describe a model, together with the results of numerical experiments, that uses multifrequencies to acquire and invert eddy-current data for reconstructing flaws in tube walls. The model that we describe here uses sixty frequencies, from 200 kHz to 16 mHz (though more or fewer frequencies can be used, spanning a greater or smaller spectrum), and allows the reconstruction of flaws on a grid whose cells measure 0.002” by 0.005”. A single coil wound on a ferrite core is simulated for excitation and detection; thus the system is monostatic (the ferrite core is used to achieve satisfactory field concentration). The method of solution is based on minimizing the squared error between the measured data and the model data. The mathematical algorithm that is used for inversion is a constrained least-squares technique using a Levenberg-Marquardt parameter for smoothing. The numerical experiments indicate that the model performs satisfactorily in reconstructing simulated ‘high’ and ‘low’ contrast flaws in the presence of data uncertainty. The grid consists of a single column of twenty-five cells spanning the wall thickness of the tube.