Non-destructive testing and material inspection methods like the ultrasonic methods are well established in production quality control. The checking of the thickness of metal layers, pipe walls or synthetic foils are good examples.
The different times of flight for reflected ultrasonic pulses is interpreted as material thickness - once again a precise time measurement is the key (see below)!
The velocity of sound in aluminum is 5100 m/s (at 20 °C). An ultrasonic pulse is reflected on both, the entry wall and the back wall. Take as an example a layer of 1 mm aluminum . The difference in travel time for the two reflected pulses is 2*d/c = 3,9 µs. For a needed resolution of 0.1% this means a resolution of time of 390 ps - no problem for the TDC! A TDC can measure even a layer of 10 cm aluminum with the same absolute resolution!