Depth of first detectable defect in a masonry wall using GPR

This paper focuses on the accuracy of detection of the first defect from a ground penetrating radar (GPR) signal in a masonry wall. The main conclusions are drawn from a carefully executed piece of experimental research work based upon field work on a masonry wall on the Bell Tower at Cremona. From inspection of the field GPR records, the resolution of detection of the first target or defect was found to be related to the length of the first reflection from the surface of the masonry. Thus conventional geophysics guidelines with respect to target detection related to one-tenth of a wavelength were tested against field observations and found to be inapplicable in relation to the detectability of the first defect. The shallowest detectable target proved to be at a depth of one-third the centre frequency of the antenna.