# Improvements in Rock Mass Description for Stope Design by Geophysical and Geochemical Methods
## 5 Ground-Penetrating Radar (GPR) for Representation of Fracture Surface Geometry
### Methodology  
GPR is a non-destructive electromagnetic method that is widely used in civil engineering [32; 33; 34; 35; 36] and exploration [37]. In mining, the application of GPR has mainly been limited to hazard detection [38; 39; 40; 41] and structural mapping [42; 43; 44]. Reflections of the GPR signal from the target can provide information on rock mass integrity and geological and geometric features such as fractures, shear zones, voids, and lithological contacts. Analysis of the frequency content of a reflected GPR signal has successfully been used to determine the level of excavation damage [45].  
As a baseline for the experimental work, a 100 cm \(\times\) 90 cm \(\times\) 30 cm slab was sawcut from an intact block of Kuru grey granite (Figure 5). Single-point GPR measurements were carried out on both surfaces of a 60 cm \(\times\) 60 cm grid in order to study the propagation of the GPR signal in the rock mass [46]. A total of 63 points were measured on each side of the slab, with 250 scans per point and using a GPR antenna with a central frequency of 1600 MHz. Relative dielectric permittivity of the material was obtained based on the measured true thickness of the slab at each point. These results acted as a reference for the propagation and attenuation of the GPR signal in intact Kuru grey granite for the system used. They were used in the next phase of the study to improve the accuracy of the interpreted location and the orientation of the fracture surfaces.  
Figure 5: Slab of Kuru grey granite used in this study with the measurement grid.  
The accuracy of the GPR interpretation was tested on three slabs of Kuru grey granite of approximately 100 cm \(\times\) 100 cm \(\times\) 40 cm. An artificial fracture surface was induced in each of the slabs by carefully splitting them in the middle while avoiding excessive damage to the sides. At this point, the halves were not yet separated in order to avoid damaging the induced fracture surface. The same 1600 MHz GPR system was used to measure a grid of 9 by 9 scanlines with a 10 cm line separation from the top of each slab. Measurements were carried out in a continuous measurement mode with 200 traces per meter and 2048 samples per trace. During the measurement, a thin sheet of steel was used as a back plate under the slab to provide a strong reflection for reference. The obtained data were processed to create 2D profiles along the scan lines (Figure 6). The time-to-distance conversion was completed using the median value of the relative dielectric permittivities obtained from the measurements in phase 1. Finally, the profiles were processed to determine the profile of the fracture surface at each scan line.  
After the GPR measurements were completed, each slab was carefully opened by lifting the top half. Both halves were photographed using a Canon 5 DS R DSLR camera and a Canon 35 mm f/1.4L II USM lens. The camera was held in place using a tripod and the slab was rotated 8.4 degrees between the photographs from dip angles of 30, 45, and 60 degrees, resulting in a total of 129 photographs per half. The photogrammetric modelling and automatic scaling using a distance marker were carried out in RealityCapture 1.0.3.9696 software. Further processing and rotation and translation of the point clouds were completed using CloudCompare 2.10.1 software. The true vertical position of the fracture surface was obtained as two raster X-Y grids with unit dimensions of 1 mm \(\times\) 1 mm and 0.5 mm \(\times\) 0.5 mm. An example of the resulting photogrammetric models is shown in Figure 7 and a close-up of the models is shown in Figure 8.  
Figure 6: Example of the obtained GPR profile for slab RAKKA-1. (A) Reflection from the induced fracture surface. (B) Reflection from the back plate.