Getmapping subsidiary company Promap in South Africa has been developing a safe solution for detecting and monitoring subsidence based on its existing market leading Aerial LiDAR Survey service.
It is well known that airborne LiDAR has been utilized by coal mining operators for many years, primarily to calculate stockpile and void volumetrics. It is a service that Promap Civil Engineering Surveys, based in Pretoria and Cape Town, South Africa, have been undertaking for its numerous mining customers across sub Saharan Africa since 2013. Promap provides fast accurate volume reports, usually within 8 hours of the survey being collected.
However, it is less well known that airborne LiDAR can also be used as an effective tool in the safe identification of ground movements (subsidence) often associated with older abandoned mining operations, or in regions underlain by dolomitic rock. In the mining regions, the distribution of the older mining workings is often poorly documented. Coupled with this, underground coal fires can also be a serious problem which can lead to massive surface displacements. Accessing these areas on the ground presents a significant risk to personnel.
Promap’s airborne LiDAR Subsidence solution mitigates this risk entirely by creating subsidence maps over the entire mine area from the air.
Airborne LiDAR scanning data produces highly accurate representations of the terrain due to its ability to penetrate the gaps in the vegetation canopy and capture returns from both the vegetation and the ground itself. The vegetation can be removed by means of classification techniques to create a highly accurate terrain model of the ground. With LiDAR, typical absolute vertical accuracies in the order of 5cm to 8cm can be achieved in moderately vegetated terrain. However, when comparing surfaces for subsidence purposes, only differences of greater than 15cm are typically reported. This allows for classification variations and small spatial differences between surfaces and avoids the potential for false positives.
Without detailed mapping and monitoring of these older mining areas, small changes in surface morphology which are not easily visible to the eye or conventional aerial photography will eventually lead to cracks, fissures and sinkholes sometimes with catastrophic consequences. For example, in December 1962, a three-storey crusher plant building was swallowed into a sinkhole on the West Driefontein Mine resulting in the death of the 29 occupants.
With repeat surveys over study areas, the level of vertical subsidence can be accurately measured. Promap’s solution offers a strong technical approach in determining the location of and analysis of subsidence to ensure accurate results are obtained. For example, planning the aerial flight paths with a minimum of 50% overlap and ensuring they are perpendicular to the slope of terrain; collecting the highest point cloud density to generate higher resolution terrain models and highly accurate geodetic survey control; are all prerequisites for reliable comparative analysis.
Our subsidence report would typically reflect differences and from these positive identifiers we can undertake further investigation using ground truthing or other survey validation techniques. The main image in this article shows the results of 9 years of comparative LiDAR analysis for a mining area in Mpumalanga, South Africa.
Comparative analysis from LiDAR revealing a depression (movement) between successive aerial LiDAR surveys
The monitoring effort from aerial lidar data offers insight into:
- The severity of the depression.
- Rate of movement/growth over a period.
- Spatial location and extent when overlaid with existing underground working plans etc.
The figure above represents a typical example of a cross section of two point clouds where a depression is suspected. The image shows data from two comparative surveys that have been spatially overlaid. Points on the outer edges match within limits while there is some separation in the middle due to the vertical movement. The results of such analysis can be further supplemented with other monitoring techniques especially in areas of old mining operations.
"LiDAR data is a reliable, safe and accurate toolset for the mapping of and monitoring of subsidence. Promap have, over the last 10 years developed a proven flowline for the undertaking of subsidence detection and monitoring using aerial LiDAR for its clients, particularly in the mining sector. Please get in touch for more information on how we can help you tackle this problem."Janco van der Merwe, Technical Director, Promap