A major portion of the structure of a modern transport aircraft consists of a relatively thin skin fastened to underlying elements such as stringers, frames, and ribs. All of these structural elements are critical, and flaws such as corrosion, cracks, and fastener or bond failure must be detected at an early stage. Flaws like cracks or failed fasteners in substructure (stringers, frames, etc.) are currently detected by a painstaking internal visual inspection, which requires complete teardown of the aircraft. Some flaws, such as corrosion, may manifest on the surface of the aircraft and can be detected by an external visual inspection. Even in this case, the internal extent of the flaw cannot be easily determined, and inspectors must determine whether to remove the skin for further inspection. These expensive disassembly and inspection processes create a great interest in nondestructive inspection techniques which can detect subsurface defects by observations made on the surface of the aircraft.