Guided wave testing, or GWT, uses low-frequency ultrasound operating between 20-150kHz compared to the MHz frequency range for conventional ultrasonics used for thickness checks. This allows the ultrasound to be broadcast away from the tool and axially along the pipe in GWT. When this broadcast ultrasound encounters a change in cross-section, the change in acoustic impedance of this region causes an echo of sound to return to the tool for detection.
Using the welds on a pipe for calibration and
Source: NDT
