The use of mobile scanning devices is increasing within the underground mining industry, specifically for geomechanics applications. These devices allow rapid data acquisition and the spatially-continuous monitoring of excavations in GPS-deprived environments. The data, when analysed, can inform us about convergence, shotcrete thickness and over-break. As with any measurement, its usefulness is greatly enhanced by estimates of accuracy and precision, both of which depend on hardware, acquisition and the subsequent alignment and comparison procedure. This paper discusses the limitations and uncertainties associated with each aspect of the hardware, acquisition, alignment, comparison and results interpretation. It will provide mining professions a greater confidence in the implementing this emerging technology for geomechnaicla applications within the mining industry.
Mobile laser scanners are a group of scanning devices that allow for the rapid generation of a point-cloud while the device is moving. In outdoor applications, the localisation often uses a global positioning system (GPS). For applications in GPS-deprived environments, the common practice, as in surveying, has been to conduct multiple static scans and to stitch the resulting point-clouds using known stationary geo-reference points. This approach is still widely practiced, and the results can be of excellent quality. The drawback, however, is typically one of time, both during the collection of data and during its integration into a single useable dataset.
In the underground mining industry, static scanning methods would require the setup, scanning and pack-down process to be repeated every 5–15 metres, depending on the range of the scanning device. Accordingly, 3D laser scans of extensive areas are seldom undertaken.
An alternative method of data collection in GPS-deprived environments became commercially available in 2012 with the introduction of the GeoSLAM Zeb-1. Localisation uses a combination of a Simultaneous Location and Mapping (SLAM) algorithm and an Inertial Measurement Unit (IMU). This allowed the device to generate a 3D point-cloud while moving through the environment. The GeoSLAM Zeb-1 has been widely applied in architecture and real estate. Other hardware devices have since come to market utilising a variety of methods for localising the resulting point-cloud.
The mining industry began to trial applications for this type of hardware soon after its release, and its acceptance as a tool for geomechanics has been rapidly increasing. One application of these developments has been the generation of spatially continuous change detection data. Geomechanical application of change detection includes, but is not limited to, convergence monitoring in underground excavations. In outline, the general steps would be:
Conduct a reference scan;
Later, conduct a secondary scan;
Compare the two scans; and
Identify, and report upon, any change.