To interrogate the internal structure of a part using conventional ultrasonic testing, pulsed elastic waves are induced in the part and reflect off internal discontinuities. The amplitude and time of flight of the reflected wave provide information about the discontinuity. There is a direct correlation between the location of a reflection and the round trip time. Reflections emanating from deep in the part require more time to return to the ultrasonic probe than do shallow reflections. This makes possible the use of time based “windows” or “gates” to acquire data corresponding to a specific depth of interest. Although complex and expensive electronic circuitry is required, under most circumstances time based data acquisition works well. However, there are occasions in ultrasonic testing where time based data acquisition is inadequate and where true spatial resolution is desired.
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