Abstract: A method and apparatus for inspecting a wall of a mechanical structure including obtaining an infinite response from a sample material having a front face, a thickness, and a back wall, by transmitting into a sample material an ultrasonic wave having a frequency and duration and being generated by one or more transducers, wherein the thickness of the sample material is sufficiently great that only a wave corresponding to the front face of the sample is received back; transmitting an ultrasonic wave generated by one or more transducers into the wall of a mechanical structure to be inspected at a time (T), wherein the ultrasonic wave has the same frequency and duration as the ultrasonic wave transmitted into the sample material; receiving a response signal back from the wall to be inspected; and correlating the response signal to the infinite response, thereby creating correlated data.