GPR Study of a Thrust-fault in an Active Limestone Quarry (SW Slovenia)

After conducting a successful GPR pilot study in a tectonically complex area of the Karst Thrust Edge, a follow-up study was carried out. In the pilot study, the geometry and spatial extent of the Socerb thrust-fault, which separates limestone above from flysch below, were investigated using a 50 MHz antenna in the Crnotice quarry. After one part of the quarry was deepened and widened, new flysch outcrops were exposed and GPR profiling was made possible in areas where the thrust contact could not be reached before. By comparing the first GPR results with the locations of new flysch outcrops, we found it reached the surface almost exactly where we had predicted. In order to gain new information about the location of the thrust contact, nine new profiles were recorded. In this newly deepened area, the thrust contact runs close to the surface, therefore a 250 MHz antenna was used in addition to the 50 MHz antenna in order to ensure an adequate level of resolution. This combination of antennas provided the depth penetration of 34 m as well as resolution high enough to obtain accurate information from the shallowest parts. GPR profiles were used to create a 3D model of the thrust-fault plane, providing information about the spatial position and lateral undulations of the thrust contact. The results provided new data about the geometry of the Socerb thrust-fault and correlated well with existing borehole data. The results also showed the importance of using appropriate antenna frequencies as other features capable of becoming hazardous during excavation (air-filled and sediment-filled karst caves) could otherwise be misinterpreted or overlooked. The follow-up research not only provides new data important for understanding the tectonic setting of the area, but also information needed for calculating exploitable material quantities and for planning safe excavation processes.