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Updated:
22 February 2008
Spectral geology
What is spectral geology?Spectral geology is the measurement and analysis of portions of the electromagnetic spectrum to identify spectrally distinct and physically significant features of different rock types and surface materials, their mineralogy and their alteration signatures. Spectral techniques also can be used to detect hydrocarbons present in the water column and on the sea surface as well as dissolved organic matter and living micro-organisms. At Geoscience Australia, there are numerous projects across various disciplines which use spectral data to help understand and map geological parameters. How does Geoscience Australia use spectral data?Geoscience Australia is a supplier, archiver, and user of many types of Spectral data and their related products and provides Moderate Resolution Imaging Spectroradiometer (MODIS), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Landsat and other imagery to clients around the world. Currently, Geoscience Australia is undertaking a pilot project to evaluate the use of MODIS night-time thermal imagery to look for geothermal targets. In 2007, Geoscience Australia scientists created the world's largest ASTER mosaic which covers the Mount Isa region and includes more than 15 geoscience products designed to help explorers examine hydrothermal systems and their surface expressions. This work is continuing with collaborators from the CSIRO and the Geological Survey of Queensland to expand and refine techniques and add to the understanding of the regolith in the area. Geoscience Australia uses spectral geology for detailed mineralogical and regolith mapping and the analysis of ore bodies and their surface expressions, eg. HyMap™ and ASTER. It also uses spectroscopy to pinpoint fine mineralogical variations associated with different fluid phases and ore petrogenesis, eg. Laser RAMAN Microprobe, Portable Infrared Mineral Analyser (PIMA), the CSIRO's automated drill core and chip logger HyLogger™ , etc. Geoscience Australia's projects also receive a range of satellite and remotely sensed imagery and value-added spectral products from services such as ASTER, EO-1 Hyperion and ALI. As part of its program to detect traces of hydrocarbons present on the continental shelf and in coastal waters, Geoscience Australia has undertaken a scoping project to asses the viability of using HyMap™ hyperspectral imagery to detect off shore oil seeps in the Timor Sea. This work was undertaken over known areas of natural hydrocarbon seepage and production areas with anthropogenic hydrocarbon slicks. What are spectral data and what can they be used for?Spectral data are measured using spectral sensors, which record either solar or artificially provided radiation reflected from the surface of materials. Because many materials absorb radiation at specific wavelengths, it is possible to identify them by their characteristic absorption features, which appear as troughs in a spectral curve (Kruse, 1994). Wavelength ranges most suitable for the discrimination of geological materials and oil slicks include the visible and near-infrared (VNIR), short-wavelength infrared (SWIR), and the mid or thermal infrared (TIR), while the characteristic fluorescence of hydrocarbons occurs in the ultra-violet (UV) spectral region. Spectral variation is the result of different compositions, the degree of ordering, mixtures and the grain size of different rocks and minerals (Table 1) (Huntington, 1996). Owing to their multiple valence states, transition elements such as iron (Fe), copper (Cu), nickel (Ni), chromium (Cr), cobalt (Co), manganese (Mn), vanadium (V), titanium (Ti) and scandium (Sc) display the most prominent spectral features in the VNIR wavelength range (Kruse, 1994). The SWIR wavelength region between 2,000 and 2,500 nm is particularly suitable for mineral mapping. The 2,000-2,400 nm wavelength region can show many absorption features characteristic of certain hydroxyl and carbonate bearing minerals and mineral groups which are characteristic of hydrothermal alteration. These mineral groups may include pyrophyllite, kaolinite, dickite, micas, chlorites, smectite clays, alunite, jarosite, calcite, dolomite, and ankerite. Using spectroscopy, particularly hyperspectral imaging technology, it is possible to make accurate maps of surface mineralogy, including boundaries, relative abundances and mineral assemblages. Hyperspectral mapping techniques can identify individual species of iron and clay minerals, which can provide detailed information about hydrothermal mineralisation and alteration zones (Thomas and Walter, 2002). Fig. 1 Schematic diagram of the imaging spectrometry concept. Images of up to several hundred narrow spectral bands are acquired simultaneously, providing a complete reflectance spectrum for every pixel in the imaging spectrometer scene. (Image courtesy NASA) Advances in technology also have led to the development of highly accurate, high-resolution field spectrometers. They include the Australian-designed Portable Infrared Mineral Analyser (PIMA) and drill core and chips hyperspectral scanners such as CSIRO's HyLogger™ which Geoscience Australia's Gawler project worked on in collaboration with Primary Industries and Resources South Australia (PIRSA) and the Cooperative Research Centre for Landscape Environment and Mineral Exploration (CRCLEME) on the Mineral Mapping South Australia project and with the CSIRO Mineral Mapping Technologies Group. Fig 2. Electromagnetic spectrum. The top line in the diagram locates various bands in a relative sense while the next line is an expansion of the AB portion. The bottom lines show approximate band locations for some of the operational multispectral and hyperspectral systems (modified from Rinker, 1994). TABLE 1. GEOLOGICALLY SIGNIFICANT REGIONS OF THE ELECTROMAGNETIC SPECTRUM
ReferencesHuntington, J.F. (1996) The role of remote sensing in finding hydrothermal mineral deposits on Earth. In: Evolution of Hydrothermal Ecosystems on Earth (and Mars?), Ciba Foundation Symposium 202, edited by G.R. Bock and J.A. Goode, John Wiley & Sons, Chichester, UK, pp. 214-235. Kruse, F.A. (1994) Imaging spectrometer data analysis-a tutorial. Proc. Int. Symp. Spectral Sensing Res. 1, 44-50. Rinker, J.N. (1994) ISSSR tutorial introduction to spectral remote sensing. Proc. Int. Symp. Spectral Sensing Res. 1, 5-19. Thomas, M. and Walter, M.R., (2002). Application of Hyperspectral Infrared Analysis of Hydrothermal Alteration on Earth and Mars. Astrobiology, Vol. 2, (3), pp. 335-351. Related linksSpectral geology research and instruments at Geoscience Australia:
Related external collaborators and researchers:
Australian Hyperspectral Instrument manufacturers and contractors: Other links: |
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