基于gprMax的衬砌脱空三维正演分析

3D forward modeling analysis of lining void detection based on gprMax

衬砌脱空是一种常见的施工缺陷,会导致结构的劣化,降低隧道的安全性,需要在施工及运营阶段对隧道衬砌进行检测.利用基于时域有限差分法的gprMax软件,建立含脱空的隧道衬砌模型,对三维探地雷达的衬砌检测过程进行正演模拟,获取脱空的三维数据体.对脱空的响应特征进行分析,讨论了脱空对电磁波波形及波场分布的影响,结合工程实例验证了正演结果的可靠性.结果表明,脱空区电场强度随测线靠近中心而增强,依据这种变化可实现对脱空三维尺寸的探测;电磁波在脱空内部产生多次反射,引起围岩层中电磁波信号的多次震荡;金属部件会对脱空区信号产生较强的干扰,电场极值从大到小依次为无金属部件、单层钢筋网和双层钢筋网.研究结果对于推动三维探地雷达在隧道衬砌脱空检测中的应用,提高脱空检测的准确性具有参考价值.

Lining void is a common defect in tunnel construction,which might lead to the deterioration of tunnel structure and reduce tunnel safety.It is necessary to detect the tunnel lining in the stages of construction and operation.In this study,a three dimensional(3D)model of tunnel lining and surrounding rock is established based on finite difference time domain(FDTD)method,and the application of 3D GPR(ground penetrating radar)in lining detection is simulated forward to obtaine the 3D GPR data of void.The response characteristics of void in different measuring lines are analyzed,the influence of lining void on electromagnetic waveform and wave field distribution are discussed.Finally,the reliability of the forward modeling result is verified by a tunnel project.The results show that the electric intensity of void area increases as the measuring line is close to the center,and the three dimensions of void can be detected according to this change.In addition,the electromagnetic wave is reflected many times in the void,causing the concussion in the surrounding rock layer.Besides,the metal parts will cause strong interference to the signal of the void,and the extreme values of the electric field are metal-free parts,single-layer steel and doublelayer steel in order from large to small.This study will provide valuable reference for promoting the application of 3D GPR in lining void detection and improving the accuracy of detection.