Rayleigh waves are most sensitive to defects located near the surface. Which option best describes this sensitivity?

• A. Close to or on the surface
• B. 1 wavelength below the surface
• C. 3 wavelengths below the surface
• D. 6 wavelengths below the surface

Answer: (A)

Rayleigh waves are a surface-bound ultrasonic wave whose energy and particle motion are concentrated at and near the surface. Their amplitude decays with depth, so most interaction with a flaw occurs in a shallow subsurface region—roughly up to about one wavelength deep. Because the energy is strongest at the surface, flaws on or very close to the surface perturb the wave more than deeper defects. Deeper flaws, located several wavelengths below the surface, interact less with Rayleigh waves and are harder to detect with this mode. So the description that best matches their sensitivity is defects close to or on the surface.