Hi, in TOFD (for that matter any UT technique) the travel time of the sound in material is of prime parameter. In TOFD, the transmitter probe induces sound wave in the material which when interfered with the defect, diffracts and is received by the Receiver probe. The travel time is basis for depth calculation for a given velocity of sound in material. Of course travel time depends on the distance of the defect. If you assume the two probes (Transmitter & Receiver) are two points along the long axis of the an ellipse and there are many locus points such that the value of sum of distances to the two these focal points is constant for every point on a curve (locus curve which is nothing but ellipse). To simplify, for given travel time of sound, there can be more than one possibility of defect position at different depths that you will see on the screen. If the defect lies at the center between the PCS the depth given by equipment would be very accurate. But also if the defect lies off the center of probes, still it will give same indication as the travel time is same. hope it helps I am sorry if i have confused you. Please refer to the PAUT notes from Olympus or other suppliers.