The aging commercial aviation fleet requires new methods for detecting and characterizing corrosion. In particular, the need exists for a rapid and reliable method of nondestructively detecting and characterizing corrosion in layered aircraft skins. Aircraft skin consists of thin aluminum sheet. There are many joints at which these sheets are overlapped and attached to the airframe by fasteners that extend through the sheets and into the substructure. The overlapping aluminum sheets are generally separated by a thin gap, which may be filled with a sealant. Because of the overlapping nature of such joints, the second sheet and the substructure are not directly accessible from the surface
