Quality control of stone blocks during quarrying activities

Early detection of fractures and internal defects can help quarry owners in cutting quarrying costs and reducing the amount of waste products. Valuable stones used in construction industry such as marble, porphyry, granite, travertine, are usually extracted from quarries by cutting blocks whose size normally varies between 5 and 15 m3. Transportation of these blocks from quarries, often located at high altitudes in mountainous environments, down to the headquarters of the stone industry where big machines are installed to transform the big size blocks into slabs or small size stones, can be quite expensive. Money could be saved by checking on-site the big size blocks with NDT methods able to detect internal fractures and other defects (voids, clay inclusions, etc.). A detailed map of fracture extension and orientation can be also very profitable in optimizing the production line. For example, the final destination of a block (i.e., production of slabs rather than small size blocks or other) and the orientation of the cutting planes can be more properly assigned. Currently, Ground Penetrating Radar is the most promising method for this type of application. It can be applied on-site, it can generate high resolution 3D reconstructions of fractures and defects, quasi-real-time results are feasible. Thus, it can be a cost-effective tool, worth being introduced into the quarry industry. To evaluate the potential of this technology for the quarry industry we have been testing the method on marble and porphyry blocks from a couple of quarries belonging to the Brescia stone district (North Italy). Results are very encouraging: all type of defects and fractures that might have an impact on the production line can be detected by 3D surveying the block with a 2GHz GPR system. A dual-polarized antenna is a plus that makes acquisitions faster and easier. Data processing must include two-dimensional filtering to cancel the artifacts generated by side reflections and 3D migration to generate accurate reconstructions of the target geometries.