Using Telephoto Lens to Characterize Rock Surface Roughness in SfM Models

Abstract

The Structure from Motion (${\text{SfM}}$) photogrammetric technique has emerged as an efficient alternative for remote 3D rock characterization, compared to laser scanner (LiDAR) or stereoscopic photogrammetry, due to its economy and ease of use. This work uses a terrestrial ${\text{SfM}}$ approach, in combinations with telephoto lens, to characterize joint roughness at long range, in which photographs are taken at some distance from the slope. To that end, we employ the ${\text{SfM}}$ technique, combined with telephoto lenses ($f = 200 \;{\text{mm}}$), to quantify joint roughness in a slope. Roughness profiles are extracted from the 3D model of the slope, and their $Z_{2}$ statistical parameter is used to estimate the Joint Roughness Coefficient (${\text{JRC}}$). We also study the influence of several operational ${\text{SfM}}$ parameters—e.g. focal length, analysis distance and number of photographs—on the roughness results. Results show that the ${\text{SfM}}$ technique using photographs taken with telephoto lens can be employed to characterize joint roughness at a relatively large distance (up to 10–15 m), with ${\text{JRC}}$ estimates that are similar to those obtained at close range ($\simeq 1 \;{\text{m}}$). Furthermore, results reveal that only a few photographs are needed to generate 3D models that provide reasonable ${\text{JRC}}$ estimates, hence facilitating the applicability of the ${\text{SfM}}$ technique to quantify joint roughness in practice, since such low number of photographs is associated with short processing times ($< 5 \;{\text{min}}$) and reduced periods for data acquisition in the field ($< 15 \;{\text{min}}$).