Evaluation of gpr frequency and polarisation for underground concrete tunnels assessment

This paper presents the results from a Ground Penetrating Radar experimental campaign aiming at characterising the thickness of principal concrete elements of a subway tunnel, following relevant water infiltration phenomena that might have affected their integrity and possibly have degraded their structural stability. Given the requirements of penetration and image quality, the main purpose of the study was to identify an optimal equipment design capable of accurately determine thickness variations of the subsurface structures by the joint analysis of different operational frequency, particularly a 200 MHz and a 600 MHz equipment, and antenna polarisation. While suitable depth performance has been achieved for both frequency bands, the image quality is notably higher when the interference from the metallic elements is minimised, and hence the 600 MHz equipment oriented perpendicular to the armour level has proven to be the best option. Obtained results allow a centimetric characterisation of the vertical extension of the investigated structures, and confirm their deterioration.