By comparison with the detection of the presence of a discontinuity, the inverse process of attempting to define the characteristics of the discontinuity which led to that indication is fraught with difficulty. Detection requires only that the ultrasonic indication is clearly distinguishable from other signals attributable to electronic or material “noise”. That is, it is confined to a relatively short time period, and exhibits a signal-to-noise ratio significantly larger than unity. However, both the signal and the noise are functions of numerous parameters, and, for inversion to be accurate and unambiguous, the inspection process would have to provide as many independent pieces of data as there are independent factors contributing to the signal (and to the noise, too, if its nature is such that it is added coherently to the signal from the discontinuity).