Comparing Ultrasonic Techniques for Pipeline Crack Detection: Shear Waves vs. Lamb Waves
A new study compares shear wave and Lamb wave ultrasonic techniques for crack detection in gas pipelines, highlighting performance trade-offs and advances in inline inspection technology.
R. GUAJARDO and M. HAAS, NDT Global
(P&GJ) — Ultrasonic testing (UT) has long been the backbone of in-line inspection (ILI) for crack detection in liquid pipelines but applying that technology to gas pipelines is a fundamentally different challenge. A comparative study presented at the 2026 Pipeline Technology Conference in Berlin examines how shear wave and Lamb wave techniques stack up against each other, and what the industry can expect as gas pipeline inspection technology matures.
The liquid pipeline standard. In liquid pipelines, UT crack detection is well-established. Shear wave tools transmit pulses through the liquid couplant at a defined angle, generating a refracted 45° wave that travels in a zig-zag path through the pipe wall. When that wave hits a crack, it reflects energy back to the probe. Using pulse-echo (PE) and pitch-and-catch (P&C) configurations, high-resolution tools achieve circumferential resolution down to 5 millimeter (mm), measurement repeatability of ±0.5 mm and can size features across the full wall thickness. The technology is mature, validated and well-understood by operators and regulators alike.
The gas pipeline problem. Gas pipelines present a fundamental physics challenge. The low acoustic impedance of gas makes it extremely difficult for shear waves to transfer energy into the pipe wall, and the narrow incidence angle tolerance required for proper shear wave generation becomes essentially unworkable in the dynamic environment inside a gas pipeline—with vibrations, speed fluctuations, welds and bends all in play.
The solution is lamb waves. Unlike shear waves, which propagate through the full wall thickness, lamb waves travel along the pipe wall itself with mixed particle motion. Critically, they tolerate a much wider incidence angle, making them naturally suited to gas as a couplant medium.
How lamb waves perform. The trade-offs are real but manageable. Lamb wave tools currently achieve 7 mm axial resolution compared to 3 mm for high-resolution shear wave tools, and the minimum probability of detection (POD) threshold is 40 mm in length by 1.5 mm in depth vs. 25 mm by 1.0 mm for shear wave systems. Depth sizing is currently capped at 6 mm, though discrete sizing is possible up to 60% of wall thickness in 9.5 mm pipe.
Where lamb waves gain a meaningful advantage is in feature discrimination. Shear wave tools are sensitive to feature geometry, meaning corrosion and laminations frequently appear in inspection data alongside actual cracks, creating classification burdens for analysts. Lamb waves are largely insensitive to non-linear features like general corrosion, reducing false positives and simplifying the identification workflow. Testing in natural gas environments has confirmed that lamb wave PE reliably detects crack-like features while largely ignoring corrosion.
The bottom line. Both technologies rely on PE and P&C data in tandem for feature identification, and both achieve PODs above 80% for linear crack indications. Shear wave technology’s maturity gives it an edge in sizing precision and feature discrimination today. Lamb wave technology, still developing, is on a clear trajectory, drawing directly on lessons from the shear wave world to accelerate its growth. For pipeline operators running gas systems, that maturity curve is moving in the right direction and quickly.
This paper was presented at the 21st Pipeline Technology Conference, Berlin, April 27-30, 2026, Berlin.
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