Internal defects such as concrete lining empties have always been associated diseases of hydraulic tunnels,which have complex impacts on the safe operation of the project.Ground penetrating radar(GPR)has been widely used to detect internal defects of structures because of its high efficiency and non-destructive characteristics.Due to the complexity of internal defects,the formation mechanism of different shapes and types of defects and detection radar map features is still unclear,which is easy to cause misjudgment.The forward simulation of air and water containing defects in regular and irregular cavitation is carried out based on finite-difference time-domain method.The response characteristics in time domain and frequency domain are analyzed,and the response mechanism is analyzed according to the radar wave transmission process.The results show that the irregular defects make the radar signal echo shape distorted and appear "multi-arc"characteristics.Compared with air-containing defect radar,the phase inversion oc-curs,the time interval of the upper and lower boundary reflection signal is longer,and the main frequency shifts to the low frequency.The above rules are applied to the detection and interpretation of a real tunnel project,and the defect type and size are accurately identified.
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