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The inference of an object's DC electrical conductivity from measurements exterior to that object is an unsolved inverse problem. The authors provide an exact solution for one-dimensional surface layers of arbitrary conductivity on a non-magnetic half-space of otherwise constant conductivity. The input data are impedances (voltages) measured at an applied periodic current sheet (eddy current probe) located just above the sample's surface.
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Rose et al. (1991) studied this question.
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