with subsequent manual stitching of point clouds. T e drone was fl own in an inaccessible curved drift. For stationary scans, the drone was landed twice along its path for two minutes. T e data was downloaded from the scanner over WiFi and stitched together (Fig.2). T e fi nal point cloud was registered with the mine coordinate system using control points provided by the mine. For this, refl ective markers were placed on the control points. Since the scanner measures intensity as well, these markers turn up in diff erent colour and can therefore be easily identifi ed for registration (Fig.3). T e map in Fig.2 was confi rmed by the mine to be an exact match of the existing map that was generated by a diff erent 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 diff erent 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 fi ts well with the one obtained from the CMS unit with the added benefi t 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 fi rst stope. For this scan the drone was fi rst fl own 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. T ere 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 fl own with

the scanner mounted on top. T e wire frame obtained from the traditional system is also shown in the fi gure. It is apparent that the mesh obtained from the drone fi ts 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. T is case study clearly shows that the aerial

drone-based scanning of underground mine cavities is not only possible but also has a

Fig. 3. Top: Control point identifi cation for registration. 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 traditional CMS equipment

number of benefi ts as compared to traditional approaches. For example, the drone-based scanning is considerably faster and carries a higher level of safety for the operator. T e 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 benefi ts in terms of safety and cost savings. ●

Syed Naeem Ahmed is president of Clickmox Solutions. 15

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