Three-dimensional (3D) laser scanning has become a de facto standard in underground mining operations for a variety of reasons, including accurate excavation audits, ground support inspections, rock face condition assessments and general surveys. Also, with the new push towards implementation of Internet of Things (IoT) principles and methods in the mining industry, 3D laser scanning and mapping are gaining widespread acceptance amongst technology developers and users.
Canadian company Clickmox and Swedish firm Inkonova have jointly introduced an unmanned aerial vehicle (UAV), the Tilt Ranger, which is designed for underground mines with the capabilities of scanning and mapping mine cavities in three dimensions. Here, we discuss a case study to understand the effectiveness of this system in scanning drifts and stopes in different underground mines.
The Tilt Ranger drone is based on tilting rotor technology. The combination of tilting rotors and wheels allows the system to be rolled on the ground as well. The scanner on the drone (V-Scan3D) weighs 930g and is capable of performing both stationary and mobile scans. The mobile scan processing is accomplished through a simultaneous localisation and mapping (SLAM) algorithm.
First tests were performed to see if multiple stationary scans can be done from the drone with subsequent manual stitching of point clouds. The drone was flown in an inaccessible curved drift. For stationary scans, the drone was landed twice along its path for two minutes. The data was downloaded from the scanner over WiFi and stitched together (Fig.2).
The final point cloud was registered with the mine coordinate system using control points provided by the mine. For this, reflective markers were placed on the control points. Since the scanner measures intensity as well, these markers turn up in different colour and can therefore be easily identified for registration (Fig.3*).
The map in Fig.2 was confirmed by the mine to be an exact match of the existing map that was generated by a different scanning system when the drift was accessible.
It should be noted that it is not always possible to land the drone and perform stationary scans, especially in open stopes. In such situations, mobile scanning is the only possibility. Mobile scans of stopes were performed in two different mines and the results were compared with a traditional cavity monitoring system (CMS) used by the mine sites. Since the CMS data was available only in mesh form, the point clouds from the scans were used to create meshes for comparison.
Fig.3* shows the mesh generated from a mobile scan of a stope and the mesh from the traditional CMS unit. It is apparent that the scan from the drone fits well with the one obtained from the CMS unit with the added benefit that the drone-based scan only took about three minutes, considerably less than the traditional approach.
In many mines, stopes are much larger than the one shown in Fig.3. To investigate the viability of this scanning method in such situations, two large stopes in a mine were scanned using Tilt Ranger. Fig.3 shows the mesh of the first stope. For this scan the drone was first flown with the scanner mounted on the top of the drone and then again with the scanner mounted at the bottom. Both scans were then merged together during post-processing. There was no CMS map of this stope available for comparison as the traditional scanning system could not be used due to the particular geometry of the stope.
Fig.3* also shows the mesh of the second large stope. For this the drone was flown with the scanner mounted on top. The wire frame obtained from the traditional system is also shown in the figure. It is apparent that the mesh obtained from the drone fits well with the one obtained from the traditional system. And as before, this scan took only a few minutes to accomplish as compared to more than an hour of set up and scanning time for the traditional system.
This case study clearly shows that the aerial drone-based scanning of underground mine cavities is not only possible but also has a number of benefits as compared to traditional approaches. For example, the drone-based scanning is considerably faster and carries a higher level of safety for the operator. The equipment is much lighter and easier to carry than traditional CMS systems. Only one operator can carry the equipment and perform the survey.
It is expected that the use of aerial drone-based survey systems will exponentially increase as mine operators continue to see the benefits In terms of safety and cost savings.
Fig.3. Top: Control point identification for registriation. Middle: Mesh from traditional CMS (left) and from Tilt Ranger drone (right). Bottom: Mesh of a stope obtained with the Tilt Ranger and mesh of a stope obtained from the Tilt Ranger together with wire frame obtained from the Tilt Ranger together with wire frame obtained from the traditional CMS equipment
SYED Naeem Ahmed is with Clickmox