GPR investigations of rock fractures: Considerations on thin beds

Detection of fractures within resistive rock masses is at present a very Common application of Ground Penetrating Radar (GPR) technique. GPR investigations have been performed in several mines and quarries, as well as over unstable rock slopes. The detection and location of fractures is obviously of great importance for safety reasons but can also be used, for instance by the quarrying industry, to improve the production of ornamental rock slabs. According to the desired trade-off between resolution and penetration depth, the full frequency range of Commercial GPR systems (from tens of MHz to few GHz) has been employed in these investigations. In most cases rock fractures can be considered as beds whose thickness is smaller than the resolution limit (i.e., a thin bed), and can be envisaged as layers embedded in a homogeneous formation, giving rise to reflected signals from the top and the bottom of the bed having opposite polarities. Studies on reflections from thin beds have been carried out by the seismic industry for nearly sixty years now and new developments are still underway. In the last decade, the GPR Community has focused on thin beds response to determine rock fracture features (aperture and filling material). In our work we attempt to review the fundamentals of thin bed response, mainly addressing the relationship between the thickness of the bed and the time and frequency characteristics of the reflected wavelet. Then we consider GPR numerical simulations and experiments performed with a high frequency antenna on quarried marble blocks to compare analytical expressions of thin bed response to synthetic and real GPR datasets.