Which of the following is a potential cause for non-repeatable A-scan patterns across scans?

• A. Electrical interference
• B. An irregular crack
• C. Porosity
• D. Blow hole

Answer: (A)

Repeatability in A-scan patterns hinges on stable signal integrity; electrical interference introduces random noise that changes from scan to scan, so the waveform looks different each time. A-scan displays are time-based responses of reflections from interfaces, and any wandering baseline, jitter in timing, or added noise can shift peak amplitudes or even hide subtle reflections. When the system picks up stray signals from power lines, other equipment, or improperly shielded cables, the resulting noise overlays the true echoes, making patterns non-repeatable across scans.

In contrast, an irregular crack, porosity, or a blow hole are physical features that, if you scan the same spot with the same coupling and orientation, tend to produce reflections at roughly the same depths with similar amplitudes. They represent real features rather than random electronic noise, so they don’t inherently cause non-repeatable patterns across scans. If those features vary, it’s usually due to changes in geometry, scan angle, or coupling rather than the fundamental cause of non-repeatability.