Unmixing the effects of vegetation in airborne hyperspectral mineral maps over the Rocklea Dome iron-rich palaeochannel system (Western Australia)
Highlights
► Generic approach to compensate quantitative mineral maps for vegetation cover ► Validation of unmixed airborne mineral abundance maps against ground data ► Application of quantitative mineral abundance maps to geological mapping ► Application of quantitative mineral abundance maps to channel iron ore exploration
Introduction
Airborne or satellite based spectroscopic sensing at visible to infrared wavelengths has proven a valuable tool, not only for mapping surface soils and geology, but also for recognising alteration haloes around mineralised systems (Cudahy and Barry, 2002, Hewson et al., 2005, Kruse et al., 1990, van der Meer et al., 2012, van Ruitenbeek et al., 2006 and references therein). The maps produced in these studies provided detailed information on relative mineral abundances and compositions.
Various workers have shown that quantitative mineralogy (and geochemistry) is achievable from proximal reflectance data collected at visible-near to shortwave infrared (VNIR–SWIR) wavelengths (Clark and Roush, 1984, Fredericks et al., 1985, Haest et al., 2011, Haest et al., 2012a, Zhang et al., 2001; VNIR–SWIR reflectance spectroscopic data will from here on be referred to as infrared spectroscopic data (VIRS data)). It should be possible to produce quantitative mineral maps from a remote sensing platform, if the effect of vegetation cover is compensated.
Several authors have successfully quantified the fractions of bare soil and green vegetation and dry vegetation on a per pixel basis in remotely sensed hyperspectral images, by modelling the remotely measured spectra as linear combinations of end member spectra of bare soil and green vegetation and dry vegetation (Asner and Heidebrecht, 2002, Roberts et al., 1998). Rodger and Cudahy (2009) demonstrated that it is not only possible to calculate the fraction of bare soil in the pixel of interest, but also its components, like for example the AlOH-clay abundance. Once the AlOH-clay abundance in the bare soil is known, its quantity can be adjusted to more accurately measure the AlOH-clay abundance of a given pixel through compensation of the diluting (mixing) effects of green vegetation and dry vegetation. Rodger and Cudahy (2009) first estimated the contributions of clay, green vegetation and dry vegetation at the pixel level using continuum-removed absorption depths, namely: (1) combination of Al-OH bending and OH stretching vibrations at ~ 2200 nm to estimate the AlOH-clay abundance; (2) chlorophyll absorption at 670 nm related to green vegetation; and (3) cellulose–lignin absorption at ~ 2100 nm associated with dry vegetation. From these estimates and associated modelled mixing relationships, vegetation-free AlOH-clay abundances were retrieved for pixels with up to 60% vegetation cover. The resulting vegetation unmixed AlOH-clay abundance map was validated: (1) by comparing ground and airborne AlOH-clay abundance measurements (n = 30) and (2) by visually confirming that areas with and without vegetation did not correspond to breaks in mineral abundances, except if they corresponded to geological contacts.
The procedure presented by Rodger and Cudahy (2009) differs significantly from the approach taken in previous studies that unmixed the effect of vegetation. Rodger and Cudahy (2009) did not use spectral end members to model the fractions of soil, green vegetation and dry vegetation on a per pixel basis (e.g. Asner and Heidebrecht, 2002, Roberts et al., 1998). They rather quantified these fractions using the continuum-removed depths of the diagnostic absorption features. However, they quantified only one mineral component of the soil, namely the AlOH-clay abundance, instead of calculating the more general ‘fraction of soil not covered by vegetation’.
The mineralogy exposed in a given pixel typically comprises more than just AlOH-clays, such as kaolinite, illite and montmorillonite. Also Fe-(oxyhydr-)oxides (Fe-Ox), carbonates, chlorites, amphiboles, etc., can commonly be observed at the surface. Thus accurate mapping of mineral contents would also improve geological mapping and mineral exploration, especially if quantitative measures are required to model specific geological, metamorphic and regolith units. For example, Anand and Butt (2010) developed models based on specific mineral percentages that characterise various lateritic horizons developed over different parent rocks. To use these types of quantitative models for geological mapping requires the extraction of mineral percentages from the spectral imagery, which can only be obtained when the obscuring effects of both green vegetation and dry vegetation are removed. Armed with both spatially comprehensive mineral percentages and appropriate quantitative mineral–geological models, geoscientists will be able to not just recognise the footprints to new mineral deposits, but also a 1st pass assessment of the size and quality of any ore resource exposed at the surface.
In this study, we build on the methodology of Rodger and Cudahy (2009), by first establishing a generic procedure for estimating the per-pixel contribution of both green vegetation and dry vegetation that is then used to unmix their contribution from a range of other spectrally-derived mineral products. This includes for example the contents of iron (oxyhydr-)oxides, carbonates, as well as clays, which are the major mineralogical components of soils and rocks exposed in the “target” palaeochannels, hosting channel iron ore deposits (CID), in the Rocklea Dome area in the Hamersley Basin of Western Australia. Airborne IRS data were collected from over this test site with the derived vegetation-unmixed mineral contents validated against ground traverses/samples of field spectra and portable X-Ray Fluorescence (PXRF) measurements.
Section snippets
Geological setting
The Rocklea Dome area comprises a wide range of geological and regolith units, variably covered by green vegetation and dry vegetation (mostly Spinifex grass and bushes). The bedrock geology comprises an exposed double, shallow plunging anticline with a hinge zone trending approximately WNW-ESE. The exposed core of the Rocklea Dome consists of an Archaean age monzogranite pluton and minor cross-cutting mafic and ultramafic intrusives that form part of the Pilbara Craton, underlying the
Methods
The methodology comprised several steps, namely: (1) reduction of the airborne AMS data to apparent reflectance through calibration and radiative transfer correction; (2) processing of the AMS apparent reflectance data to retrieve the vegetation unmixed mineral products; (3) ground data collection of both PXRF and IRS data; (4) processing of the ground IRS data to spectrally simulate the AMS data and then using the same mineral content extraction methods, i.e. continuum depths of diagnostic
Vegetation removal from remote mineral abundance estimates
The Fe-Ox abundance, AlOH-clay abundance and carbonate abundance estimates for all vegetation free ASD spectra collected along the transects are plotted as a function of the green and dry vegetation cover parameters (Fig. 3A–F). The NDVI and 2100D parameters for the ASD data average 0.047 and 0.0034, respectively, and these values are chosen to represent 0% green or dry vegetation cover.
Green vegetation spectra have been selected from an unpublished green vegetation spectral library compiled by
Applications
The vegetation unmixed mineral abundance images improve mapping of basement units (granite, chert and schist in AlOH-clay abundance image; Fig. 10) and the palaeochannel, if CID is outcropping at the surface (Fe-Ox abundance image; Fig. 11), which has applications for geological mapping and CID characterisation/exploration.
The AlOH-clay abundance map without vegetation unmixing (Fig. 10A) shows increased but variable AlOH-clay abundances. The granite is highlighted with pink polygons and
Summary and conclusion
Maps of Fe-(oxyhydr-)oxide, AlOH-clay and carbonate contents were generated by unmixing the effects of both green and dry vegetation cover at the pixel-level in airborne hyperspectral VNIR–SWIR imagery collected over the Rocklea Dome in Western Australia. The accuracy of these mineral content maps was validated using ~ 5500 field ASD VNIR–SWIR spectra and 300 field PXRF geochemical measurements. Estimates (0–100%) of the green and dry vegetation components at the pixel-level were gauged using
Acknowledgements
We would like to express our sincere gratitude to the Western Australian Department of Commerce and the CSIRO Minerals Down Under Flagship for financial support to this research project. Murchison Metals Limited is acknowledged for providing the raw AMS data. Constructive comments from an anonymous reviewer of RSE have improved this paper and are appreciated.
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