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Quality defects, such as noncompactness, cavity, and water leakage, typically appear behind tunnel linings given the lack of standardization and loose management in tunnel construction. Thus, a method for acquiring characteristic information from tunnel lining cavity fillings using frequency band energy of wavelet packet transform (FBEWPT) was proposed in this study to master the reflection characteristics of ground–penetrating radar (GPR) signal. A forward simulation of tunnel lining cavity fillings, including air, wet clay, silt, and water, was explored through finite–difference time–domain method. The spectrum characteristics of tunnel cavity fillings were analyzed from time, frequency, and time–frequency domains. Results demonstrate that amplitude reflection clearly occurs at the inhomogeneous interface when GPR electromagnetic waves collide with the tunnel lining cavity fillings. However, the degree of attenuations is different considering the various dielectric constants of objects. These relative dielectric constants reveal the reflection interface position and other information between lining and fillings. With the increase in the difference in relative dielectric constants for lining cavity fillings, the attenuation of FBEWPT for GPR signals increasingly strengthens, and the variance and standard deviation of the FBEWPT for GPR signals constantly decrease, thereby reflecting the strong reflection and attenuation of GPR signals that appear in the propagation process. This study provides a favourable reference for obtaining GPR signal spectrum interpretation for tunnel lining cavity fillings in actual projects.

Wavelet Packet Analysis of Ground–Penetrating Radar Simulated Signal for Tunnel Cavity Fillings