An Improved Entropy Regularization Method for Defect Detection in Multi-Layer Composites

Microwave time reversal (MTR) as a microwave imaging method can provide auto-focus and super-resolution with real-time, efficient, high contrast, and low cost properties. It is applied here for non-destructive testing (NDT) of small defect buried in multi-layer composites backed by metal. It is critical to estimate the focusing time with localization methods in MTR. The traditional entropy regularization method (ERM) may give a wrong focusing time estimation for the strong reflection from background metal. This leads to a wrong localization of defect. An improved ERM (IERM) is proposed here by properly combining ERM and target initial reflection method (TIRM) to ensure a correct localization. MTR based on IERM (IERM-MTR) is then applied to detect the defect in multi-layer composites when the permittivities of each layer are known. Its stability, accuracy, and noise tolerance are proved by multiple numerical investigations. An iterative strategy based on IERM-MTR and particle swarm optimization (PSO) method is also proposed here for detecting the defect in multi-layer composites when the permittivities of each layer are unknown. Numerical investigations are carried out to prove its effectiveness. Small defects are easily detected in multi-layer composites applied here.