Least-squares reverse time migration (LSRTM) for damage imaging using Lamb waves

Large-area monitoring and accurate damage quantification are two primary goals of ultrasonic, guided wave-based structural health monitoring (SHM). Reverse-time migration (RTM) is an effective damage imaging technique for both metallic and composite plates. In geophysics, incorporating least-squares inversion into migration can generate images with higher resolution and suppressed artifacts in comparison with conventional RTM. Development of a least-squares reverse time migration (LSRTM) technique is promising for SHM since it could expand the imaging area for a given sensor array while maintaining a relatively high resolution. An LSRTM technique is introduced in this research for damage imaging in an isotropic plate using A ₀ mode Lamb waves. A finite difference algorithm based on the Mindlin plate theory was used to simulate the flexural wave propagation. To form the theoretical foundations for guided wave-based LSRTM, a forward modeling operator and its adjoint are defined. The damage images from both numerical simulations and experiments show that LSRTM can enhance imaging resolution and reduce artifacts.