Comparison of Geological Mapping with Electrical Resistivity and Ground Penetration Radar Methods for Rock Fractured System Study

Abstract

Regional and local geological mapping play an important role in the understanding of fractured rock system. These techniques were applied in this study as well as in a comparison with active geophysical methods that respond to the physical properties of objects and materials below the earth surface, (i.e., electrical resistivity and ground penetrating radar; GPR, among others). The two geophysical methods were conducted within the tunnel outcrop. In addition, satellite imagery obtained from Google Earth was used to determine the main lineament pattern present in the area. The geological mapping that consists of the fracture system (i.e., discontinuity) mapping was used to compile the catalog of the fractured system. The electrical resistivity method that was applied alongside the tunnel was compared with the results from the ground penetrating radar method. This study, with the application of regional geological mapping using satellite imagery, was able to give a valuable comparison with in-situ geological mapping. The discontinuity data were plotted in the DIPS software to obtain the lineaments’ orientations in rose diagrams. Low resistivity values were compared with ground penetration radar results and with the lineament pattern. The existence of water-bearing structures within the fractures system inside the rock mass affects the low resistivity values, at the same time as the directions of fractures furthermore match that of the ground penetration radar results. The high resistivity values, as interpreted from the electrical resistivity data, are an indication of a lesser amount of water-bearing structures and therefore a smaller amount of damaged units. The ground penetration radar method produces similar results with the lowest amplitude radar reflections from the water free region, and stronger reflections recorded in the area saturated with water. All the parameters used in this study contribute to a Tunnel Stability Rating System (TSRS) that is in development. The Tunnel Stability Rating System could be a useful method of determining the stability of tunnels, particularly tunnels without rock exposure (e.g. that is covered by shotcrete). The overall goal is to make the evaluation process of rock tunnels able to use an alternative assessment when electrical resistivity and ground penetration radar data are accessible.