Application of GPR reverse time migration in tunnel lining cavity imaging

Correctly locating the tunnel lining cavity is extremely important tunnel quality inspection. High-accuracy imaging results are hard to obtain because conventional one-way wave migration is greatly affected by lateral velocity change and inclination limitation and because the diffracted wave cannot be accurately returned to the real spatial position of the lining cavity. This paper presents a tunnel lining cavity imaging method based on the ground-penetrating radar (GPR) reverse-time migration (RTM) algorithm. The principle of GPR RTM is described in detail using the electromagnetic wave equation. The finite-difference timedomain method is employed to calculate the backward extrapolation electromagnetic fields, and the zero-time imaging condition based on the exploding-reflector concept is used to obtain the RTM results. On this basis, the GPR RTM program is compiled and applied to the simulated and observed GPR data of a typical tunnel lining cavity GPR model and a physical lining cavity model. Comparison of RTM and Kirchhoff migration results reveals that the RTM can better converge the diffracted waves of steel bar and cavity to their true position and have higher resolution and better suppress the effect of multiple interference and clutter scattering waves. In addition, comparison of RTM results of different degrees of noise shows that RTM has strong anti-interference ability and can be used for the accurate interpretation of radar profile in a strong interference environment.