Effect of Thickness of an Air Cavity behind Concrete Tunnel Linings on 400 MHz GPR Time Signals: Numerical Simulation and Analysis

This study proposes a practical method for evaluating the thickness of an air cavity (H-cavity) using low-frequency GPR. The interaction between low-frequency GPR signals with a center frequency of 400 MHz and air cavities with various thicknesses, which are formed behind tunnel concrete linings, is investigated using numerical analyses. The following three steps are performed: First, a numerical simulation model is developed to simulate transient electromagnetic wave propagations in an air cavity behind a tunnel concrete lining using open source software (gprMax). Second, based on the analytical results, a practical equation that relates the apparent time difference of EM reflections in the an air cavity (Delta(t) over bar) and its thickness (H-cavity) is established. The results indicate that the Delta(t) over bar -H-cavity relation is effective for evaluating the thickness of the air cavity H-cavity) with an error margin of 20% even for air cavities that are relatively thinner than the wavelength of primary GPR signals, lambda(GPR) (H-cavity/lambda(GPR)similar to 1/8). Finally, the obtained analytical results are validated through comparison with the experimental results of previously reported studies.