Mode selection for corrosion detection in pipes and vessels via guided wave tomography

Guided wave tomography offers a method to accurately quantify wall thickness losses in pipes and vessels caused by corrosion, using ultrasonic waves transmitted over distances of approximately 1 to 2 m, and measured by an array of transducers. These measurements are then used to reconstruct a map of wall thickness throughout the inspected region. To achieve accurate estimations of remnant wall thickness, it is vital that a suitable Lamb mode is chosen. This paper presents a detailed evaluation of the two most suitable modes, S₀ and A₀, to compare their performance using both numerical and experimental data. The sensitivity of A₀ to thickness variations was shown to be superior to S₀; however, the attenuation from A₀ when a liquid loading was present was much higher than S₀. A0 was less sensitive to the presence of coatings on the surface than was S₀. Finally, it was shown that both modes could achieve a similar level of resolution in the plane of the plate surface.