Estimation of thin fracture aperture in a marble block by GPR sounding

In order to optimize the cutting of the marble blocks to be used in the restoration of the dome of the Holy Shroud Chapel of Turin cathedral, some GPR tests have been done in a laboratory to detect thin fractures in a marble block. We made some preliminary measurements to estimate the GPR wave velocity in an intact block and to calculate the permittivity of the marble. Then, the block was cut and GPR measurements with different fracture fillings and fracture apertures were performed. The results demonstrate that with at 2 GHz in antenna a fracture, a millimeter open, can be detected. An analysis carried out on the experimental reflection coefficients with the thin layer theory, allowed us to estimate the different fracture apertures. We found the possibility of estimating fracture apertures ranging from 1/10 to 1/5 of the wavelength at 2 GHz in air. The phase and the amplitude of the reflected signals with different fillings demonstrated the possibility of detecting thin fractures and discriminating between dry- or water -saturated fillings.