Estudo GPR de rochas carbonáticas fraturadas e carstificadas da Formação Salitre no município de Morro do Chapéu/BA

Abstract

This study has the purpose imaging and interpreting features in fractured and karstified carbonate rocks using the GPR method. Based on GPR data, it was possible to characterize the process of carstification of the studied site. The area is located at Fazenda Mamonas, municipality of Morro do Chapéu/BA, and from the geological point of view, is inserted in the Irecê Basin, Salitre Formation (Nova América unit), located very close to an important shear zone that limits the basin. The local geology is dominated by solid dolomitized orange carbonates, intensevely fractured and filled by hydraulic breccia and cut by veins composed of barite, galena and quartz. The carbonate rock is weathered and karstified preferentially along subvertical faults and fractures and is partially covered by reddish clay soil. A total of 9 GPR sections were surveyed, around a pit for the extraction of galena and barite ores, using 400 MHz antennas. In addition, a processing flow and the LnEnergy attribute were executed to improve the visualization of internal carbonate features in depth. Two distinct radarfacies have been identified, one of which contains reflections of low to medium amplitude, with continuous and plane parallel reflectors. The other radarfacies has high vertical amplitude zones containing discontinuous reflectors and laterally truncated by the local fault and fracture system. This represents the least altered carbonate rock, forming underground karst towers, more resistant to carbonate dissolution processes. More over, 6 GPR sequences were interpreted according to the differentiation of the GPR signature along the internal features of the carbonate rocks in depth. These were individualized based on the lateral continuity of more pronounced reflectors and their superposition in the form of plane-parallel strata, sometimes inclined and truncated by faults and fractures. In addition, it was possible to observe the behavior in depth, in the GPR sections, of some fractures observed in a Digital Surface Model (DSM), extracted from a drone image of the area, with this, a virtual 3D interface was created to better visualize these fractures in the subsurface. The data integration allowed the construction of a diagram block representative of the study area explaining the influence of shear zones and hydrothermal fluids for the formation of breccias and the characterization of the carstification and fracturing processes.