Drones are a tool that can help the smart exploitation of geological resources, but they can also help public administrations to monitor compliance of regulations governing such practices.
Context: Societal and market needs
Open-pit mine is still an unavoidable activity but it can become unsustainable without the restoration of degraded sites. Monitoring site restoration after extraction has finished is a legal requirement for mine companies and public administrations in many countries, involving financial provisions for environmental liabilities. Moreover, detailed drone imagery is very useful to generate topographic and volumetric information, aiding the quarry managers to take economically wise decisions.
We use small drones (< 2 kg) drones equipped with an optical sensor, along with field measurements that are deployed to geometrically and radiometrically correct the UAS sensor data. Imagery is processed with photogrammetric, Remote Sensing and Geographical Information Systems software, resulting in accurately georeferenced spectral information, vegetation and soil indices, structural/topographic information and land cover maps.
Objectives
Spectral data aids the detection and quantification of mine waste dumping, bare soil and other land cover extension. Moreover, plant formations and vegetation development can be evaluated not only, quantitatively, but also visually, allowing an intuitive comparison with the surrounding reference systems.
Materials
Fixed wing drones are especially useful when the area to cover is greater than 50 hectares, which can be done in less than 30 minutes.
On the other hand, multicopters are especially useful when the survey area has rock faces that (cause turbulence). Between multicopters and fixed wing drones there is a wide range of models, but the pros and cons are mainly these:
For land cover mapping purposes, a multispectral sensor is ideal. This allows the retrival of more information than a RGB sensor, which can be crucial to differentiate between soil types, minerals, rocks and vegetation species.
Methodology
From drone acquired individual images (photograms) and using photogrammetric techniques (Surface from Motion, SfM), it is possible to reconstruct a tridimensional model. With this data it is possible to generate topographic maps and orthoimages of a very high detail (< 10 cm pixel), which are very useful to characterize the quarries.
Results
Volumetric measurement is relevant for the mounds of extracted stocks, as well as for the estimation of remaining fixed materials. Using drones, the volume can be calculated daily, although usually flight inspections are realised between once a month and twice a year.
Conclusion
The use of a well-defined protocol constitutes a pipeline solution intended for the implementation by public administrations and private companies. With drones, the precise evaluation of topographic and restoration dynamics is possible in an expedient manner at a very affordable budget. Furthermore, the proposed solution prevents subjective interpretations by providing objective data new technologies are integrated and put at the service of scientists, environmental managers and decision makers. Fixed wing drones can sweep an area of 50 hectares (120 acres) in less than 30 minutes.
More info: Joan-Cristian Padró, Vicenç Carabassa, Jaume Balagué, Lluís Brotons, Josep M. Alcañiz, Xavier Pons (2019). Monitoring opencast mine restorations using Unmanned Aerial System (UAS) imagery. Science of The Total Environment, Volume 657, Pp. 1602-1614, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2018.12.156.
