Measurement of sensitivity of different wave modes to subsurface defects

Excitation of acoustic wave modes in layered solid immersed in a liquid has been investigated by computing the reflection coefficient of acoustic plane waves at the liquid-layered solid interface. Then, a method to evaluate the sensitivity of various models supported in the layered structure to subsurface defects in introduced. The method involves insonification of the layered structure with a conical wave whose axis coincides with the defect. Since all the rays in the conical wavefront hit the planar interface at the same angle, it is possible to excite a single kind of mode in the layer. By adjusting the angle of inclination of the cone, it is possible to excite the modes selectively. Since the conical waves converge to a line focus at the cone axis, the excited mode will focus on the defect. By recording the reflected signal amplitude as the cone angle is varied, a curve is obtained from which it is possible to determine the sensitivity of various modes to the subsurface flaw. The results of such measurements indicate that the generalized Lamb wave modes are more sensitive to subsurface defects than the Rayleigh waves. An imaging system that makes use of focused Lamb waves has been built, and the system produces images of very small subsurface defects.