the method of inspecting a degraded area of a metal structure covered by a composite repair generally comprises operating a compton scattering inspection device onto the degraded area , including emitting a beam of radiation particles directed towards and across the composite repair , detecting at least some backscattered photons scattered back from the metal structure , and acquiring compton scattering data from the detected backscattered photons , the compton scattering data being indicative of remaining wall thickness of the degraded area .