Abstract: Twodimensional models are insufficient for capturing the three-dimensional(3D) characteristics of voids behind tunnel linings, primarily due to the interference caused by reinforcement during traditional ground penetrating radar image processing. Additionally, conventional data processing methods are computationally inefficient and have limited accuracy. To address these challenges, the authors integrate average elimination, exponential gain, and time-zero correction techniques to develop the gprMax interference elimination method. The proposed method leverages the temporal variation of radar signals within two-way travel windows and uses piecewise function calculations to enhance computational performance. The feasibility of the proposed method is validated through forward simulations. A 3D model of the voids behind tunnel linings is constructed using gprMax software based on the finite-difference time-domain method to align with real-world grouting conditions and detection processes. The gprMax interference elimination method is used to optimize the imaging results of the forward simulation under varying conditions. The analysis of the imaging characteristics yields the following findings: (1) The forward simulation results using the gprMax interference elimination method closely align with the measured results from model tests of circular cavity structures embedded in indoor concrete slabs. (2) On horizontal survey lines, the rectangular void signal appears as a "concave" shape, whereas on vertical survey lines, it resembles the characteristics of direct waves. The size and burial depth of voids can be derived from the 3D feature maps. (3) The gprMax interference elimination method significantly enhances the electromagnetic signal strength of voids at greater depths. In summary, the proposed gprMax interference elimination method improves the data processing efficiency for voids behind tunnel linings while maintaining calculation accuracy and precision.

Key words: ground penetrating radar; gprMax, tunnel lining, voids behind tunnel lining, interference elimination; forward modeling simulation