Laser Scanning methods and techniques for high-detail 3D modelling of caves

Technical advancements have widened the limits of remote sensing and Terrestrial Laser Scanning technology in studying underground cavities. Furthermore, the use of Unmanned Aerial Systems has proven to be a significant advantage in the study of caves, as under certain circumstances during the data processing, it is plausible to combine TLS and UAS data for generating a complete 3D model, representing surface and subsurface simultaneously. The use of state-of-the-art laser scanning equipment either terrestrial or handheld accompanied by total station measurements on a series of ground control points, has resulted in the scanning and detailed mapping the entire Melissani and Drogarati caves, in Cephalonia Island, in Greece, including hidden cavities. This study attempted not only to delineate a new methodology for compiling a highly detailed cave map, but also to identify structural discontinuities and faults for further investigation of the influence of rock failures in causing rock falls and further damage in the caves. Both show caves attract many visitors and since they are located at an area of very high seismicity, where large earthquakes occur very frequently, the risk is rather high.

The methodology is based on the synergy of equipment in different working levels, since the cave environment is by far one of the most difficult cases to survey, led to hazard identification in high detail and accuracy throughout the cavity. The fieldwork includes the generation of a unified point cloud for the underground cavity, generated by scanning at several bases inside the cave and by entering smaller cavities by holding the mobile scanner. The bundling of the partial point clouds is possible since the proposed methodology includes the establishment of a dense network of Ground Control Points, which are measured with Total Station equipment for gaining actual coordinates. After the merging of the partial scans were combined into a single point cloud, the methodology continues with further processing including filtering and noisy points removal. Moreover, the final product is combined with the point cloud that was generated after the photogrammetric processing and the methodology is completed with exporting the results in file formats that can be imported in several geotechnical or discontinuity recognition software for further interpretation. The results along with the produced 3D models could be utilized to determine areas susceptible to different failure types. The assessment of rock stability within a cave by combined innovative equipment, techniques, and research methods could be considered by the management authorities for the maintenance and/or re-design the tourist routes.