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What is spectral geology?

Figure 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)

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 can be used also 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 associated products and provides MODIS (Moderate Resolution Imaging Spectroradiometer), ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), Landsat and other imagery to clients around the world.

In 2007, Geoscience Australia scientists created one of the world's largest ASTER mosaics to date, covering the Mount Isa region in northwest Queensland. The mosaic 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 state surveys to expand and refine techniques to add to the understanding of alteration and regolith expressions in the area. Geoscience Australia, along with CSIRO and its Centre for 3D Mineral Mapping, has released large ASTER mosaics and associated value-added geoscience products totalling more than 100 scenes for the Gawler-Curnamona region in South Australia, and the Mount Isa region. Data from both of these projects is available via the CSIRO FTP site.

Currently, work is starting on a mosaic around Alice Springs in the Northern Territory, A collaborative project is underway also with CSIRO, the Earth Remote Sensing Data Analysis Centre (ERSDAC) NASA Jet Propulsion Laboratory (JPL) and State and industry partners to develop an All-of-Australia ASTER mosaic and geoscience products. Geoscience Australia has run a pilot study also to evaluate the use of MODIS night-time thermal imagery to search for geothermal targets and hopes to be able to continue looking into applications from other night-time and thermal data for Australia.

Geoscience Australia uses spectral geology for detailed mineralogical and regolith mapping and the analysis of ore bodies and their surface expressions through facilities such as HyMap™ and ASTER. It also uses spectroscopy to pinpoint fine mineralogical variations associated with different fluid phases and ore petrogenesis with equipment such as a  Laser RAMAN Microprobe, Portable Infrared Mineral Analyser (PIMA), the CSIRO's automated drill core and chip logger HyLogger™ and others. 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 offshore 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?

Figure 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).

Spectral data is 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 below) (Huntington, 1996). Because they have 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 2000 and 2500 nanometres (nm) is particularly suitable for mineral mapping. The 2000-2400 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).

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.

References

Huntington, JF 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 Bock GR and Goode JA, John Wiley & Sons, Chichester, UK, pp. 214-235.

Kruse, FA 1994, Imaging spectrometer data analysis-a tutorial. Proc. Int. Symp. Spectral Sensing Res. 1, 44-50.

Rinker, JN 1994 ISSSR tutorial introduction to spectral remote sensing. Proc. Int. Symp. Spectral Sensing Res. 1, 5-19.