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 metre wide lineament may extend for hundreds of metres 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.
Catching the gold fever
The key to revealing this hidden surface geology is a powerful multi-pulse airborne laser scanner, or LiDAR. It is a fast and inexpensive means of gathering topographic information critical to the success and safety of mining operations. McElhanney Consulting Services Ltd., an engineering, mapping and surveying company, has introduced two new bare-earth mapping services developed specifically for the exploration and exploitation phases of the mining industry.
The Vancouver-based 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. “Lineaments provide clues to underground geology and are a valuable aide to geological mapping — a crucial part of any gold exploration or mine engineering project,”said AzadehKoohzare, 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 prioritising exploration targets.”
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.
This LiDAR system provides the minimum 2 points per square-metre density required to generate bare-earth DEMs with the 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 metre above the sea level to collect the data. Standard processing removed the returns associated with vegetation to generate a bare-earth DEM with 10 cm vertical and 30-50 cm horizontal accuracy.
Monitoring ground subsidence
McElhanney devised its idea for ground subsidence monitoring in Saskatchewan where potash deposits are mined and used for fertiliser. Potash extraction poses a higher risk of ground subsidence than many other types of mining because the evaporated 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,” said Koohzare, adding that subsidence and uplift can cause problems for up to five kilometres 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 cost-effective way to monitor ground subsidence.
The 10-cm vertical accuracy of bare-earth DEMs routinely generated from 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 minimise the danger.
As is the case with the lineament mapping, the high-pulse 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