Evaluation of the antenna parameters for inspection of hidden defects behind a reinforced shield tunnel using GPR

Ground penetrating radar (GPR), as a recognized non-destructive testing method, has been widely applied to detect hidden defects behind a reinforced shield tunnel. However, the scattering of electromagnetic waves caused by the dense double-layer rebar meshes embedded in a tunnel lining severely influences GPR's detection capabilities. Selecting the appropriate antenna parameters is a crucial step before conducting GPR measurements. In this paper, the penetration capability of GPR waves is evaluated using a shield tunnel model made of two shield tunnel segments. The laboratory experimental results demonstrate that an antenna with a center frequency of 400–600 MHz is appropriate for detecting hidden defects behind the concrete segment. Regarding the antenna polarization direction, it is better to set it in the longitudinal direction of the shield tunnel since the rebar spacing is slightly larger than that in the circumferential direction. The antenna height from the concrete segment should be controlled within a fifth of the wavelength in air to compromise between the data quality and detection efficiency. The results will aid in determining antenna parameters when GPR is applied to inspect hidden defects behind a reinforced shield tunnel.