Solution for a transmit-receive eddy current probe above a layered planar conductive structure

Exact solutions for the electromagnetic response of a transmit-receive coil pair situated above two parallel plates that are separated by a gap, have been developed. The analytical model was used to characterize the effect of variations in probe liftoff, conductor resistivity, plate wall thickness, and plate-to-plate gap. All electromagnetic coupling coefficients arising from the probe and layered plate conductors were determined and substituted into Kirchhoff's circuit equations to calculate the induced voltage in the pickup coil. Excellent agreement with experiment was observed for differing first layer plate resisitivities (174 μΩ cm and 53.9 μΩ cm), first layer plate thicknesses (3.80 mm and 3.18 mm), second layer plate resistivity (1.72-174 μΩ cm), second layer plate wall thickness (1.20 mm-4.85 mm) and probe liftoff (2.81 mm and greater), providing confidence in the general validity of the model.