An industrial safety inspector uses an ultrasonic thickness gauge to monitor the remaining wall thickness of a pressurized steel pipeline prone to internal corrosion. Explain how high-frequency ultrasound pulses are used by the gauge to non-destructively determine the thickness of the steel pipeline wall.

In your answer, you should describe:

• how the ultrasound waves travel and interact with the pipeline material and its boundaries
• what physical measurements must be recorded
• how these measurements are mathematically processed to calculate the thickness.