No abstract

Ground penetrating radar is a popular approach to detect defects in tunnel lining. However, the interpretation is usually based on the original image, which is very different from the real shape of the lining defects. Full waveform inversion and reverse time migration are helpful to solve this problem. Full waveform inversion can invert the relative permittivity distribution and reverse time migration can migrate reflection events to their proper locations. Traditional full waveform inversion method is only applicable to cross‐hole ground penetrating radar data or surface multi‐offset ground penetrating radar data. We propose an improved full waveform inversion method which offers satisfactory inversion result for surface common‐offset radar. The forward modelled waveform and the objective function curve show that our new full waveform inversion method is much more accurate than traditional full waveform inversion for common‐offset radar. Traditional reverse time migration has weaker amplitude with increasing depth; we use an energy matrix to improve the imaging effect. Moreover, our reverse time migration is based on the relative permittivity distribution obtained from full waveform inversion, which provides more accurate imaging result. Through several numerical and engineering examples, we discuss the application of both methods in tunnel lining inspection. The results show that for tunnel lining without rebars, the combined methods can give satisfactory imaging results. But the image quality deteriorates rapidly when dealing with rebars.

With China's expanding economy, many tunnels are being designed and constructed. However, tunneling in hazardous geologic terrain, with faults, fractures, water-bearing openings, and other adverse geological conditions, construction safety is seriously endangered. To ensure the safety of tunnel construction, a tunnel geological prospecting method was proposed and applied at the Xiangyun Tunnel in Yunnan Province, China. In the investigation stage, the engineering geological and hydrogeological conditions were analyzed to recognize high-risk sections. In the construction stage, the “tunnel ahead” prospecting scheme was optimized based on the macroscopic geological conditions. Geological mapping of the tunnel, seismic ahead prospecting and transient electromagnetic soundings were employed as well as resistivity models to image potential adverse conditions. Horizontal drilling and tunnel excavation records verified the geophysical predictions and interpretations.