Ultrasonic (UT) inspection can be used to detect a wide variety of subsurface discontinuities, such as inclusions, cracks and voids, as well as associated reactive or diffusion zones. In comparison with Eddy-Current inspection, much less work has been done on the determination of the Probability of Detection POD of UT inspections. This is because, unlike Eddy-Current inspection, it is very difficult to produce synthetic sub-surface flaws that adequately represent the acoustic properties of the naturally-occurring flaws (see Burkel et al., 1996). Here traditional methods for POD determination are difficult to apply.