Tailings storage facilities (TSFs) and other waste dumps are often a high profile operating issue for a mine’s management team, who need to manage tailings and meet regulatory requirements for a facility with many “moving parts” and performance uncertainties.
A lot of a mine’s liability is associated with the tailings facility. Good risk management means keeping a close eye on the tailings and optimizing or providing contingencies where appropriate. In particular, the tailings facility can certainly affect mine production if the capacity does not match the scheduled containment expansion works.
Yet despite their importance, TSFs can be difficult to monitor with any precision. They tend to be geographically large, with surfaces that are not generally trafficable. The boots-on-the-ground approach also has its limitations because TSF beach surfaces can be highly variable, which is not always visible to someone standing on the edge.
Having detailed information about a tailings surface, or changes to a surface over time can help identify measures to optimize management of the tailings. Better information can also help to avoid relatively expensive emergency measures to fast-track a dyke raise or an unscheduled spigot move.
Ground-based LIDAR is seeing increased use for monitoring TSFs, but this technology requires a direct line-of-site from the ground, which may risk missing important surface features such as large cracks or gullies.
Monitoring by aircraft can be expensive or unnecessarily dangerous, depending on the location.
Satellites offer a variety of services from imagery to rough scale topography, and can provide a reasonable solution in many cases. They can, however, be limited in terms of accuracy and in areas with persistent cloud cover.
Another new tool in the miner’s toolbox, especially for keeping an eye on TSFs and other mine disturbed areas, are unmanned aerial vehicles (UAVs), sometimes referred to as unmanned aerial systems or drones.
These small “aircraft” are seeing increased use in mining. UAVs provide ultra-high resolution imagery of the ground, and the images collected can be used to interpret accurate topography equivalent to survey points every few inches. They can also carry a variety of payloads that include thermal imaging and multi-spectral cameras.
Survey-grade UAVs, such as the fixed-wing senseFly eBee, are designed for accurate topographic surveys using photogrammetry techniques to interpret overlapping photographs. The eBee, in particular, is the equivalent size of a small bird of prey. It has a wingspan of about 100cm and weighs under a kilogram, camera included. This style of instrument is fast becoming standard survey equipment for many applications.
Getting best use of UAVs for tailings management
As a new tool in the toolbox of mining companies, UAVs can be quite versatile in their application. UAVs are being used to:
- Measure the remaining containment capacity of the TSF;
- Measure the change in containment capacity of the TSF over time, via multiple surveys, to help interpret volumetric changes and moisture content;
- Measure the beach slope to plan for future spigotting arrangements;
- Characterize the surface cracking and other features to improve future evaporation estimates;
- Measure waste dump or material stockpile volumetrics;
- Plan logistics for various activities using the high resolution imagery;
- Characterize spills, slides, or slope failures; and,
- Identify leaks or inventory wildlife via thermal imaging document pre-disturbance or post-reclamation conditions.
Many of these tasks have previously been difficult to accomplish with any precision because the tailings landscape is often untrafficable to traditional survey methods.
The overhead photogrammetry methods have been proven to be sufficiently accurate for these purposes (e.g. Vallet et. al. 2011), and with some key advantages in some cases. UAVs can:
- Be more cost-effective for small areas, as compared to aircraft;
- Provide ultra-high resolution, as needed;
- Help to visualize micro-topography, as needed; and,
- Be operated in areas with persistent cloud cover.
Other mining applications include the use of hovering vehicles (i.e. quadcopter) to map pit walls for a variety of assessment purposes. It’s difficult to imagine the extent of possible applications. One of our clients actually uses our UAVs to test their RADAR bird-deterrent systems – something that can be done because a UAV has the equivalent RADAR signature of a bird.
Like any tool, UAVs have their limitations, and mining companies need to be aware of these. For example, weather conditions can “ground” a UAV, particularly strong or gusty winds.
Photographing surfaces that are entirely homogenous (such as snow cover) can lead to limited results because the photogrammetry techniques rely on image texture or unique features to automatically detect tie points for comparing images. The colour variation and cracking patterns found on most tailings beaches are usually sufficient to generate the required tie points. Surveys near water can also be tricky, because waves will shift between photographs and confuse the software as it looks for tie points.
Expectations need to be managed for UAV surveys of the ground surface in forested areas, as the photogrammetry techniques will deliver the equivalent of topography for the canopy cover instead of “bare earth.”
Many UAVs also require the use of surveyed Ground Control Points (GCPs) to calibrate the topography products to real-world elevations, although some UAVs are now equipped with Real Time Kinematic (RTK) technology to reduce the need for GCPs. To avoid creating bias in the calculated surfaces these GCPs must be arranged properly, usually by forming large-scale triangles on the ground. If used properly, the GCPs can help to develop an accurate 3D surface equivalent to LiDAR for most mine-disturbed areas.
The responsible use of UAVs includes proper training and regulatory approvals from transportation authorities, who in some countries treat UAVs similar to other manned aircraft – with corresponding safety requirements.
To obtain the most effective use of UAVs for tailings facility planning, mining companies should be sure that they have access to personnel (either on staff or through external counsel) who are familiar with the application of these tools for mine-site geotechnical or tailings management purposes.
Michael Bender (PhD. PEng.) is the director of UAV Services with Golder Associates Ltd., in Calgary.