<p><em>Abstract&#8212;</em>Metro traffic in subsurface tunnels is under a rapid development in many cities in the recent decades. However, the voids and other concealed defects inside and/or behind the tunnel lining pose critical threat to the safety of the operating metro tunnels. Ground penetrating radar (GPR) is a non-destructive geophysical technique by transmitting electromagnetic (EM) waves and receiving the reflected signals. GPR has proved its capability in the detection of the existence of tunnel structural defects and anomalies. However, the voids are still hard to be recognized in a GPR image due to the strong scattering clutter caused by the dense steel bars reinforced inside the concrete lining [1]. In this paper, we analyze the propagations of EM waves through reinforce concrete segments of shield tunnels by finite difference time domain (FDTD) simulations and model test. &#160;Firstly, a series of simulations results we have done, indicates that the center frequency of GPR ranges from 400 MHz to 600 MHz has a good penetration through the densely reinforced concrete lining. And the distance between the antennas and the surface of shield tunnel segments should be less than 0.2 m to ensure a good coupling of incident electromagnetic energy into the concrete structure. Then, to image the geometric features of the void behind the segment, reverse-time migration method is applied to the simulated GPR B-scan profile, which presents higher resolution results than the results by using the traditional diffraction stack migration (Figure 1) [2]. Finally, the field experiment results prove that a commercial GPR system operating at a center frequency of 600 MHz do detect a void behind the shield tunnel (Figure 2). The reflection from the void, which starts from the back interface of the segments and lasts over 20 ns, are significantly different from the reflections from the rebars (Figure 3). In summary, GPR has potential in the detection of voids behind the shield tunnel segment. More simulations and field experiments will be performed in the future.</p><p>Keywords&#8212;ground penetrating radar (GPR); shield tunnel; voids; reverse time migration (RTM)</p><p>Acknowledgement&#8212;this work was supported by Shenzhen Science and Technology program (grant number:KQTD20180412181337494).</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.5ecbddc6f70069664311161/sdaolpUECMynit/12UGE&app=m&a=0&c=eb6a5ae55b4b24b5585021db0e5ca760&ct=x&pn=gnp.elif&d=1" alt=""></p><p>Fig. 1 Numerical simulation of two segments of 2D shield tunnel. (a) numerical model, (b) simulated GPR B-scan profile, (c) migrated profile by using diffraction stack migration and (d) migrated profile by using reverse-time migration.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.659ecbe6f70069964311161/sdaolpUECMynit/12UGE&app=m&a=0&c=07531c033a4f74c3a8e3ac1f5f47316c&ct=x&pn=gnp.elif&d=1" alt=""></p><p>Fig. 2 One photo of the field experiment.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.d7c12807f70068274311161/sdaolpUECMynit/12UGE&app=m&a=0&c=dd5c073fd4c06a9fd77db502c1d017f2&ct=x&pn=gnp.elif&d=1" alt=""></p><p>Fig. 3 GPR reflections from a void behind the segment of a subway tunnel</p><p>References</p><p>[1]&#160;&#160;&#160;&#160; H. Liu, H. Lu, J. Lin, F. Han, C. Liu, J. Cui, B. F. Spencer, &#8220;Penetration Properties of Ground Penetrating Radar Waves through Rebar Grids&#8221; , IEEE Geoscience and Remote Sensing Letters ( <strong>DOI:</strong> 10.1109/LGRS.2020.2995670)</p><p>[2]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; H. Liu, Z. Long, F. Han, G. Fang, Q. H. Liu, &#8220;Frequency-Domain Reverse-Time Migration of Ground Penetrating Radar Based on Layered Medium Green's Functions&#8221;, IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 11, no. 08, pp. 2957-2965, 2018.</p><p>&#160;</p>
Reverse-Time Migration Imaging of Ground-Penetrating Radar in NDT of Reinforced Concrete Structures