Specific evaluation of tunnel lining multi-defects by all-refined GPR simulation method using hybrid algorithm of FETD and FDTD

Irregular and complicated tunnel lining defects cannot be finely depicted with the standard finite difference simulation method based on regular mesh due to its irregular geometrical characteristics and complex physical properties of various defects. To solve this problem, a new hybrid algorithm is proposed for all-refined ground penetrating radar (GPR) simulation by combining the finite element time domain (FETD) and finite difference time domain (FDTD), which employs FETD algorithm in main region based on unstructured Delaunay mesh and Newmark-β implicit FDTD algorithm in PML region based on convolutional perfectly matched layer (CPML), respectively. The calculation efficiency and simulation accuracy of different algorithms methods are compared in detail using a representative numerical model with a water-filled void embedded. In addition, several complicated numerical models involving different typical defects and mixed case (e.g. non-compactness, voids, leaking with fractured and cracks, etc.) are also conducted, the results presented show that the proposed hybrid algorithm helps GPR practitioners to gain a deeper insight into the effects of multi-defects on the propagation characteristic of electromagnetic wave, and provides more precise technical guidance for defect interpretation. Moreover, spectrum analysis of echoes responses from defects, by using the short-time Fourier transform (STFT), is also presented, including amplitude spectrum, frequency spectrum and mode of dominant frequency.