Handheld Mobile Laser Scanners Zeb 1 and Zeb Revo to map an underground quarry and its above ground surroundings
Underground quarries are typically difficult environments for 3D mapping. They are dark, wet, dusty, have limited lines of sight and are full of hidden corners behind pillars. These properties call for a dense succession of active measurement (using some kind of light) stations with hardened materials. Once mapped, the next obvious question creeps up, how does this cavity relate to the above ground world? The Zebedee range of handheld mobile laser scanner was designed to address just these questions by CSIRO. We have tested a Zebedee 1 (Zeb 1) and a Zebedee Revo (Zeb Revo) in an abandoned underground quarry. The initial purpose of the work was to assess the equipment in mapping a sector of a quarry and its above ground surroundings. In a second phase, we examined how the full 3D dataset was to be processed to retrieve pertinent metrics for geotechnical purposes. The investigated site is located in the abandoned limestone quarry at Saint Maximin (Oise, Northern France) where INERIS, within the framework of the R&D part of the " Plan National Cavites " french program dedicated to cavity risk assessment, managed by the French Ministry in charge of Environment, created an underground experiment and demonstration unit. This laboratory site is dedicated to investigate underground cavity risks and operated in collaboration with Saint Maximin town council, owners, and the " Maison de la Pierre " manager of the site. The Zebedee range of handheld mobile laser scanners exists as Zeb 1 and the recent Zeb Revo. Both are time of flight line scanning lidar bundled with an inertial mapping unit so as to locate the moving lidar head in space and time. The scanning plane freely oscillates on a spring (Zeb 1) or rotates around a horizontal axis in order to scan a sphere portion surrounding the holder. A 90deg blind zone enables the holder to hide behind the scanning head and avoid being scanned inside the scene. The lidar distance meter has a nominal range of precision of 5mm 1mm/m with a maximum range of 30m. Scanning density is 40 lines per second for Zeb 1 and 100 lines per second for Zeb Revo. Both line scanners cover three quarters of a circle line made of 42,000 shots. A 3 axes accelerometer measures the attitude of the scanning head. Black box simultaneous location and mapping algorithm (SLAM) build 3D cloud from these measurements assuming that the surfaces where points rebound off are rigid and non deformable. Near neighbor point redundancy is used to make points converge towards a unique surface. Processed outputs include 3D points in LAZ format, various decimated cloud versions and 3D Zeb head trajectory.