Andrew Rodger

Satellite-derived mineral mapping and monitoring of weathering, deposition and erosion

The Earth’s surface comprises minerals diagnostic of weathering, deposition and erosion. The first continental-scale mineral maps generated from an imaging satellite with spectral bands designed to measure clays, quartz and other minerals were released in 2012 for Australia. Here we show how these satellite mineral maps improve our understanding of weathering, erosional and depositional processes in the context of changing weather, climate and tectonics. The clay composition map shows how kaolinite has developed over tectonically stable continental crust in response to deep weathering during northwardly migrating tropical conditions from 45 to 10 Ma. The same clay composition map, in combination with one sensitive to water content, enables the discrimination of illite from montmorillonite clays that typically develop in large depositional environments over thin (sinking) continental crust such as the Lake Eyre Basin. Cutting across these clay patterns are sandy deserts that developed <10 Ma and are well mapped using another satellite product sensitive to the particle size of silicate minerals. This product can also be used to measure temporal gains/losses of surface clay caused by periodic wind erosion (dust) and rainfall inundation (flood) events. The accuracy and information content of these satellite mineral maps are validated using published data.

Assessment of the stability of the Hyperion SWIR module for hyperspectral mineral mapping using multi-date images from Mount Fitton, Australia

The stability of the SWIR detector area array of the satellite-borne Hyperion hyperspectral system was investigated using six dates of cloud-free imagery collected from the Mount Fitton test site in South Australia. This test site was selected because of its mineralogical diversity, excellent geological exposure and relatively unchanging surface properties. Several aspects of Hyperions SWIR stability were examined. First, as Hyperion comprises an area array detector of 256 pixels by 242 bands, the nature of any column (along-track) striping was tested. This striping is revealed when the dominating effects of topographic illumination and surface albedo are removed and comprises a minor component of scene-independent column-striping and a major component of scene-dependent striping. Based on scene-derived pixel-band statistics, it was found that both an applied shift and gain for each detector element was required to effectively remove this striping. Scene statistics were generated using scene lengths of 185 km (6170 lines) and 42 km (1400 lines) for comparative purposes. This showed that scene heterogeneity was strongly apparent in the 42 km length scene and only weakly apparent in the 185 km length scene. However, after generating and applying scale and offset corrections to the data, the resultant pixel spectral showed minimal change though the effect of removing the line striping was dramatic in the resulting imagery. Pixel spectra and mineral maps show near identical patterns for all dates of imagery validating the success of the Hyperion system, calibration and destriping process.

Regolith-geology mapping with support vector machine: A case study over weathered Ni-bearing peridotites, New Caledonia

Abstract Accurate maps of Earth’s geology, especially its regolith, are required for managing the sustainable exploration and development of mineral resources. This paper shows how airborne imaging hyperspectral data collected over weathered peridotite rocks in vegetated, mountainous terrane in New Caledonia were processed using a combination of methods to generate a regolith-geology map that could be used for more efficiently targeting Ni exploration. The image processing combined two usual methods, which are spectral feature extraction and support vector machine (SVM). This rationale being the spectral features extraction can rapidly reduce data complexity by both targeting only the diagnostic mineral absorptions and masking those pixels complicated by vegetation, cloud and deep shade. SVM is a supervised classification method able to generate an optimal non-linear classifier with these features that generalises well even with limited training data. Key minerals targeted are serpentine, which is considered as an indicator for hydrolysed peridotitic rock, and iron oxy-hydroxides (hematite and goethite), which are considered as diagnostic of laterite development. The final classified regolith map was assessed against interpreted regolith field sites, which yielded approximately 70% similarity for all unit types, as well as against a regolith-geology map interpreted using traditional datasets (not hyperspectral imagery). Importantly, the hyperspectral derived mineral map provided much greater detail enabling a more precise understanding of the regolith-geological architecture where there are exposed soils and rocks.

Improved Aerosol Optical Thickness, Columnar Water Vapor, and Surface Reflectance Retrieval from Combined CASI and SASI Airborne Hyperspectral Sensors

An increasingly common requirement in remote sensing is the integration of hyperspectral data collected simultaneously from different sensors (and fore-optics) operating across different wavelength ranges. Data from one module are often relied on to correct information in the other, such as aerosol optical thickness (AOT) and columnar water vapor (CWV). This paper describes problems associated with this process and recommends an improved strategy for processing remote sensing data, collected from both visible to near-infrared and shortwave infrared modules, to retrieve accurate AOT, CWV, and surface reflectance values. This strategy includes a workflow for radiometric and spatial cross-calibration and a method to retrieve atmospheric parameters and surface reflectance based on a radiative transfer function. This method was tested using data collected with the Compact Airborne Spectrographic Imager (CASI) and SWIR Airborne Spectrographic Imager (SASI) from a site in Huailai County, Hebei Province, China. Various methods for retrieving AOT and CWV specific to this region were assessed. The results showed that retrieving AOT from the remote sensing data required establishing empirical relationships between 465.6 nm/659 nm and 2105 nm, augmented by ground-based reflectance validation data, and minimizing the merit function based on AOT@550 nm optimization. The paper also extends the second-order difference algorithm (SODA) method using Powell’s methods to optimize CWV retrieval. The resulting CWV image has fewer residual surface features compared with the standard methods. The derived remote sensing surface reflectance correlated significantly with the ground spectra of comparable vegetation, cement road and soil targets. Therefore, the method proposed in this paper is reliable enough for integrated atmospheric correction and surface reflectance retrieval from hyperspectral remote sensing data. This study provides a good reference for surface reflectance inversion that lacks synchronized atmospheric parameters.

Mapping of the Regolith of New Caledonia with Airborne Hyperspectral Remote Sensing

Recently, regolith mapping based on hyperspectral remote sensing has stirred up a growing interest, in particular for mining exploration purposes. For an island like New Caledonia, which nickel resources are one of the most abundant on Earth, and which economy is mainly based on nickel exploitation, regolith mapping is of first interest. In 2010 airborne hyperspectral remote sensing data were acquired for the first time over several mining sites of New Caledonia. At the same time, field spectrometradiometric measurements were made on the same sites. One site was selected to evaluate the potential of airborne hyperspectral remote sensing for the mapping of the regolith. Combining the analysis of the field measurements and the processing of the airborne data, we managed to map the regolith with great results. In the following we present broadly the geological context of New Caledonia, the dataset acquired, the method developped and the results.

Advances in hyperspectral processing for province- and continental- wide mineral mapping

Comprehensive mapping of the mineral composition within North Queensland, Australia was recently undertaken using 25, 000 km2 of airborne hyperspectral imagery. High spatial resolution, web-accessible, seamless, accurate maps of mineral abundances and physicochemistries, were delivered as part of a Queensland Geological Survey initiative. This required the development of pre-processing and mineral information extraction strategies that could be applied across a large number of individual flight lines within a diverse range of environments. This study demonstrated the successful use of spectral indices to target diagnostic reflectance absorption features associated with mineral abundances, composition and variations associated with crystalline or water bonding states. A multi-level series of masks, in a logical sequence, were applied to reduce possible ambiguities within image products. A new technique was also developed to compensate for variable vegetation cover, enabling the extraction of predominantly geological information (e.g. soil, outcrop, colluvium).