Forward modelling on GPR responses of subsurface air voids

Along with the development of urbanization, an increasing number of subsurface voids in urban cities can have serious effects on the expected functionality of key infrastructure elements such are roads. Ground penetrating radar is an effective technique for imaging the subsurface and detecting such air voids. The GPR responses of air voids with different horizontal width were quantitively investigated in this study using forward modelling: empirical experiments in both laboratory and site; as well as numerical simulations with FDTD. Two typical road constructions were modelled: reinforced concrete pavements and bituminous pavements, where the majority of underground utility networks are buried underneath. Pulse GPR signals having two centre frequencies: 600 MHz and 200 MHz were simulated. It was observed that the ratio of void size and GPR signal wavelength would result in various patterns in the GPR B-scans: they can be a hyperbola, a cross pattern, a bowl shape pattern, and a plain reverberation pattern. And GPR responses of voids under road structures were distorted by reinforcement. With signatures of air voids appeared in GPR B-scans through validation by lab experiments and ground truthing, confidence of void identification will be enhanced.