Detection and imaging of city's underground void by GPR

Effective ways of underground void detection and imaging by ground penetrating radar (GPR) in urban environment were studied in this paper. Through indoor laboratory validation experiments, two criterions were suggested and validated as the tell-tale sign of underground void on GPR 2D and 3D imaging. These two criterions are (1) non-continuous strong reflections in 3D slice scans, and (2) reverberation patterns with decaying amplitude in later time windows in 2D radargrams. The lab validations were further proved in line with two field studies in Hong Kong where the overlaid pavements were made of asphalt and concrete, and actual positions of voids were confirmed by subsequent ground truthing. Sources of underground noise, levels of difficulties due to different types of overlaid pavement, reasons and solutions were summarized based on the lab and field studies reported in this paper.