Going for gold with LiDAR
Airborne LiDAR is a fast and relatively inexpensive means of gathering topographic information critical to the success and safety of mining operations. McElhanney Consulting Services Ltd. of Vancouver, B.C., Canada, has introduced two new bare-earth mapping services developed specifically for the exploration and exploitation phases of the mining industry. In the first application, McElhanney used LiDAR to find surface structures and lineaments that have been missed by aerial photography and satellite imaging because of dense vegetative cover. McElhanney, an engineering, mapping, and surveying company, verified the use of LiDAR bare-earth digital elevation models (DEMs) for lineament and fault identification in a gold mining district of British Columbia.
Surface lineaments are linear ground features associated with complex subsurface geological structures, including faults, fractures, and other features such as contacts between different rock types. Sometimes just a half meter wide, lineaments may extend for hundreds of meters in length. Due to their large scale, these features can be difficult to spot from ground level, and they can be even harder to see in most remotely sensed imagery if obscured by vegetation or loose sediment.
“Lineaments provide clues to underground geology and are a valuable aid to geological mapping – a crucial part of any gold exploration or mine engineering project,” says Azadeh Koohzare, Ph.D., P.Eng. “Geologists can interpret the pattern and direction of these surface features and, as many gold deposits are associated with geological structures, use this information when selecting and prioritizing exploration targets.”
High Point Density Required
The key to revealing the hidden surface geology is a powerful multipulse airborne laser scanner, or LiDAR, explains Koohzare. McElhanney, which owns three Leica Geosystems LiDAR scanners and two Leica Geosystems ADS digital cameras, initiated the lineament mapping project using the Leica ALS60 and is upgrading to the more powerful 500 kHz ALS70-HP system.
These LiDAR systems provide the minimum two points per square-meter (two points per 2.4 square yards) density required to generate bare-earth DEMs with an accuracy and resolution sufficient for revealing the narrow linear surface features. In the British Columbia pilot project, McElhanney operated the LiDAR at an altitude of 2,500 – 3,000 m (8,200 – 9,800 ft) above mean sea level to collect the data set. Standard processing removed the returns associated with vegetation to generate a bare-earth DEM with 10 cm (4 in) vertical and 30 to 50 cm (12 to 20 in) horizontal accuracy.
“The Leica Geosystems ALS LiDAR operates with a high pulse rate to ensure the vegetation is penetrated with a point density that is sufficient to find surface lineaments measuring just 50 cm (20 in) in width,” says Koohzare. “And the high power of the unit means the dense point data can be captured at high aircraft speed, which saves time and money.”
McElhanney devised its idea for ground subsidence monitoring in Saskatchewan where potash deposits are mined and used for fertilizer. Potash extraction poses a higher risk of ground subsidence than many other types of mining because the evaporate deposits are found in soft rock formations that are structurally less than ideal for tunneling. As a result, potash mines must be continually monitored for subsidence or sinking of ground above and around the excavation site.
“Subsidence above the mine gives advance warning that personnel inside may be at risk of a cave-in or collapse,” says Koohzare, adding that subsidence and uplift can cause problems for up to 5 km (3 mi) in any direction from the mine site. In addition to dangers inside the mine, the ground movement can also sever pipelines, damage roads, and crack building foundations in the affected region.
Monitoring subsidence around potash mines – and other mineral extraction projects – is typically carried out using traditional ground survey techniques, which are expensive and time consuming. Based on LiDAR operations in hundreds of projects, many involving energy and mining clients, McElhanney says that airborne LiDAR is the fastest and most costeffective way to monitor ground subsidence.
The 10 cm (4 in) vertical accuracy of bare-earth DEMs routinely generated from the firm’s Leica ALS60 and ALS70 laser scanners can identify significant shifts in the ground surface – either up or down – that may signal dangerous conditions in the mine. McElhanney recommends collecting an initial baseline data set above each mine site and then continuing to collect new data every year. Once subsidence is revealed, monitoring flights should be repeated while steps are taken inside the mine to minimize the danger.
As is the case with the lineament mapping, the highpulse rate of the LiDAR sensor is crucial to penetrating the vegetative canopy around the mine site to get extremely accurate elevation measurements of the ground surface, or bare earth, according to Koohzare.
The Leica ALS70 is one of the few airborne laser scanners with the power and multi-pulse capability able to provide the quality of bare-earth DEM required for these mining applications.
Written by Kevin P. Corbley