Tools and Methods
Airborne imaging spectroscopy
Airborne hyperspectral imagery enables qualitative and quantitative assessment of the spatial composition of natural and man-made materials and thus their spatial pattern. It is intended to implement operational algorithms to extend relevant information products to enable decision makers and stakeholders to initiate mining impact assessment initiatives for active and abandoned mining areas. The latest developments in spectroscopic instruments (high spectral resolution along with high signal to noise ratio) in hyperspectral sensors affords the opportunity to generate quantitative information beyond the capabilities of multispectral data. Indeed, hyperspectral sensors are characterised by their high spectral resolution across a wide range of the electromagnetic spectrum, enabling the identification or characterisation of surface materials at a much finer spatial scale. This is important as the technology affords the ability not only to positively identify contaminations (and hence their potential risks) but also the potential to determine the sources and downstream impacts such as physiological impacts on the vegetation. For example, in an acid contaminated site, hyperspectral technology can measure the different secondary minerals that are produced with acid drainage.
The new Airborne Reflective Emissive Spectrometer ARES is currently being built by Integrated Spectronics, Sydney, Australia, and co-financed by DLR German Aerospace Center and the GFZ GeoResearch Center Potsdam, Germany. This instrument shall feature a high performance over the entire optical wavelength range and will be available to the scientific community from 2008 onwards. The ARES sensor will provide approx. 150 channels in the solar reflective region (0.45-2.45 μm) and the thermal region (8-12 μm). It will consist of five co-registered individual spectrometers, three of them for the reflective and one for the thermal part of the spectrum. The spectral resolution will be between 12 and 15 nm in the solar wavelength range and less then 150 nm in the thermal.
One of the major goals of ARES is to prepare the ground for the future global spaceborne hyperspectral mission EnMAP.
Airborne geophysical methods
Airborne geophysical measurements may be used for the detection, mapping and assessment of anthropogenic effects in mining areas, as well as for the definition of 3D geological structure which has an important influence on the migration of groundwater pollution plumes. Successful past implementations have included the use of the airborne radiometric method for the detection of surface plumes related to uranium mining and localised implementation of the airborne electromagnetic method in areas where saline groundwater plumes are suspected. In both cases these tend to applied on an ad-hoc basis, with limited if any integration with other remotely sensed data streams in a GIS context. The project aims:
- To utilise these methods, in conjunction with ground geophysical methods;
- To address the potential applications of radiometric surveying in non-uranium mining environments;
- To assess the applicability of airborne electromagnetic surveying in conjunction with ground-based geophysical methods for the detection and monitoring of groundwater pollution in mining nvironments;
- To develop a set of standards, indicators and standardised methods, interpretation criteria etc. for geophysical surveys in mining environments; and
- To develop methodologies for environmental risk assessment and risk reduction based on geophysical data.
The aeromagnetic method will be used to map geological structures which could have an influence on the movement of groundwater and the migration of pollution plumes. Existing data collected by the Council for Geoscience with a line spacing of 200m will be used for the purposes of this study over the South African site.
The airborne radiometric method relies on highly sensitive measurement of the radioactivity of surface deposits. In South Africa, while the coal mined in this part of the Mpumalanga Coalfield is not generally associated with highly elevated levels of radioelements, the radiometric method will be used to characterise surface geology and, where possible, map the migration of pollutants on the surface.
The airborne electromagnetic method provides a means to map the conductivity structure of the earth. A time-varying magnetic field is induced by varying the current in a transmitter loop mounted on the survey aircraft and the secondary fields due to electric currents induced in the subsurface measured from the survey aircraft. This makes it possible to produce a 3-dimensional image of the conductivity structure of the area surveyed. A major concern in the study area is the existence of saline plumes within the groundwater. These are likely to be detected as zones of high electrical conductivity, which may be detected using the airborne electromagnetic method. Additional ground-based electrical and electromagnetic surveying can be employed to verify the anomalies detected using the airborne system.