Analysis of GPR response to thin layers

Detection and location of discontinuities like fractures within resistive rock masses by means of Ground Penetrating Radar (GPR) is an effective geophysical method. Investigation with GPR is based on the propagation and reflection of electromagnetic waves. Thanks to the high resolution of this investigation technique, it is possible to detect fractures in mines and over unstable rock slopes. It may also help to improve the production of ornamental rock slabs in the quarrying industry. In most cases, rock fractures could be considered as beds whose thickness is smaller than the resolution limits (i.e., a thin bed) which are embedded in a homogeneous medium. In the last decade, preliminary studies on GPR response to thin beds have been carried out to determine aperture and filling material of rock fractures. In this work, GPR experiments and numerical simulations were performed with a high frequency antenna on quarried blocks to compare synthetic and real data. Two rock blocks were placed one in front of the other, to simulate the presence of an air-filled rock fracture. Simulations were performed separately with sandstone blocks (Pietra Giuggiulena, Sicily, Italy) and marble blocks (Carrara, Massa-Carrara, Italy). The results suggest that, for apertures smaller than one eighth of the dominant wavelength (λd/8), a quasi-linear relationship exists between thickness and reflection amplitude. It was found that below this thickness, the information is encoded in the amplitude of the thin-bed response and may be used to determine bed thickness. In addition, the trend of the reflection coefficient as a function of bed thickness was compared with Chung and Lawton (1995) equation, and a good match was observed for thicknesses below λd/8.