The use of eddy currents to measure the depth of surface modified layers in ferromagnetic materials has been the subject of numerous studies which are generally based on changes in impedance associated with differences in permeability (and to a lesser extent resistivity) in the surface modified layer compared to the core material (see for example reference 1). By changing the frequency, the material can be probed at different depths. Recently a different approach has been studied by Theiner et al[2] and others[3, 4] based on using eddy currents of different frequencies to probe spatial distributions of magnetic coercivity. This is obtained by measuring eddy current response while simultaneously cycling an externally applied magnetic field to near saturation. For a uniform material the impedance of the eddy current coil reaches a maximum at a field equal to the coercivity of the material, Hc.