Towards underground void detection with in-pipe ground penetrating radar

Ground Penetrating Radar (GPR) is an electromagnetic inspection method that is widely used to help locate and assess conditions of sewer pipes. However, when operated from the ground surface, the method is not efficient in identifying early stages of void formation, which if untreated, can lead to the appearance of sinkholes. To remedy this issue, a few studies have introduced in-pipe GPR inspection systems to identify structural defects and the voids behind the sewer wall. Still, less research has focused on studying the implications of emission, propagation, and reception of electromagnetic waves in the enclosed environment of sewer pipes and their subsequent impact on the resulting radargrams. In this study, we address this matter by modeling and comparing equivalent planar (ground-surface) and cylindrical (in-pipe) operation environments. Our results indicate the differences between the radargrams of the cylindrical and planar topologies in terms of intensity, the slope of hyperbolas, and presence of horizontal reflection lines. These results encourage practical considerations for conducting in-pipe GPR surveys with respect to choosing antenna separation and target depth of the survey and tuning the parameters for hyperbolas in automatic object detection algorithms