A method for measuring the thickness of a pipe wall is provided. The method includes the steps of: 1) emitting ultrasonic signals into the pipe wall at one or more frequencies below 1 MHz and receiving reflected ultrasonic signals, and providing a sensor signal representative of the received reflected signals, including a first received reflected signal and a second received reflected signal； 2) providing a first model signal representative of the first received reflected signal and a second model signal representative of the second received reflected signal； 3) correlating the received reflected signals to the model signals, including varying a time delay between the model signals, until the model signals separated by the time delay substantially agree with the received reflected signals； and 4) determining the thickness of the pipe wall using the time delay.
