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Correction of Mobile TLS Intensity Data for Water Leakage Spots Detection in Metro Tunnels

Terrestrial laser scanning (TLS) can accurately and thoroughly document the actual geometrical deformations and surface conditions of metro tunnel structures using high-density point clouds. In addition, TLS can record the backscattered intensity values of each point simultaneously. Backscattered intensity values are significant measures of the spectral property of scanned objects in the near-infrared region of the electromagnetic spectrum. The intensity values of water leakage regions are theoretically lower than those of the background because of the high water absorption coefficient in the near-infrared spectrum. Thus, the intensity data can be used in water leakage detection and quantitative analysis. This letter investigates the feasibility of a new mobile system called Faro Focus3D X330 for data collection, processing, and intensity correction to detect and extract water leakage spots in metro tunnels. A case study of the Shanghai Metro was conducted. Results show that the corrected intensity value is an effective physical criterion to detect water leakages in metro tunnels. By combing the 3-D point cloud, the location, area, and even water content of the leakage regions can be accurately determined.