In geological surveys with a rock engineering focus, the investigation and description of discontinuities is an important but time-consuming and labor-intensive step. With advances in surveying and mapping technology, digital surface models (DSM) and their corresponding point clouds can reproduce the geometric properties of outcrops, and provide a useful intermediary for rock fracture investigations. Here we develop and apply a method for fracture mapping that employs a discontinuity set extractor (DSE) program to capture fracture planes from point clouds and a spherical scan approach to determine fracture intensity in a direction-dependent fashion. The proposed approach is applied to obtain the fracture intensity of six outcrops, and the spatial variation of fracture intensity and corresponding influencing factors are investigated accordingly. The fracture intensity has both heterogeneous and directional-dependent characteristics. Their variations are affected by lithology and folds.
Modern remote and sensing technologies, such as close-range photogrammetry and laser scanning, have advanced rapidly. A high-precision 3D outcrop point cloud model can be obtained by a terrain laser scanner, or be converted from the digital surface model (DSM) that can be generated using multipole photos taken from different angles. The DSM building process and corresponding point cloud model generation are now highly automated, leading these kinds of technologies are popularly carried out to help the engineering geology investigation (e.g. Xu et al. 2020 and Cirillo et al. 2022). To make good use of a point cloud model for rock outcrops, this study develops an approach for capturing geometric data of discontinuities based on the discontinuity set extractor (DSE) program (Riquelme et al. 2014). The fracture intensities of six outcrops in various rock types are then investigated, and their affecting factor are accordingly discussed hereafter.
The DJI phantom 4 RTK is used to capture multiple photos for DSM generation. To enhance the accuracy of DSM, a distance of 10–20 m between the drone and the outcrop is maintained during the image capturing. Table 1 lists the information of produced DSMs, involving one metasandstone outcrop in Provincial highway No. 2, Taiwan, two outcrops of argillite interbedded with thin metasandstone layer in No. 7 highway and three highly metamorphic outcrops composed of metasandstone and slate in No. 20 highway. Figure 1 shows the corresponding point cloud models. The density of point ranges from 250–12690 pts/m2.
