An assessment on the use of structure from motion (SFM) photogrammetry in the investigation of rock mass discontinuity orientation

The main purpose of this study is to investigate the potential restrictions of the Structure from Motion (SfM) photogrammetry method in determining the geometric properties of discontinuities for different rock masses and discontinuity conditions. For this purpose, different rock masses having different structural properties were examined through site investigations. The rock mass discontinuity properties such as orientation, persistence, weathering, aperture, filling, roughness, and waviness were measured by applying traditional scan-line surveys at first. Subsequently, photogrammetric surveys were carried out. Both traditionally measured orientations and the values obtained from the point cloud data were compared with respect to different rock masses and different discontinuity conditions. As a result, it was revealed that the use of a smartphone for image acquisition and a prismatic scale box for partial absolute orientation is sufficient to produce point cloud data with reliable precisions for the rock mass discontinuity characterizations. A new procedure namely LCP+LSPF (Least Cost Path + Least Square Plane Fitting) was suggested for the measurements of the partially closed-trace discontinuities in particular. Since the dominant discontinuity type in the sedimentary formations is the bedding plane, it was concluded that LCP+LSPF method is essential for such types of rock masses. Considering the convenience evaluations based on the GSI (Geological Strength Index) values, it was revealed that as the level of blocking in the structure increases, so as the interlocking of rock fragments decreases, the use of the LCP+LSPF method becomes essential for the representation of the rock masses.