HYPERSPECTRAL REMOTE SENSING
Spaceborne optical imagers are currently either panchromatic or multispectral, providing just a few spectral bands and limited resolving power. Hyperspectral imagers typically collect data in numerous (sometimes several hundred) contiguous narrow bands spanning a vast region of the electromagnetic spectrum ranging from .001µm to 14.0 µm. Hyperspectral imagers produce vast quantities of data because of the number of bands simultaneously imaged.
One of the first operational applications of remotely sensed earth observation data in the 1980s was in the field of mineral exploration. Investigations made in the last 20 years indicate that hyperspectral remote sensing can significantly contribute to geological investigations, especially in the identification and mapping of minerals and lithology in arid (non-vegetated) environments. This is possible because hyperspectral sensors, in contrast to other existing broad-band multi-spectral scanners, are able to better resolve absorption features unique to specific mineral species.
Analysis of hyperspectral data involves using the reflectance of each pixel in each of the hundreds of bands, and and representing this data as ‘spectral reflectance curves’. The identity of material constituting the target is determined by comparison of its spectral reflectance curve with 'library' spectra of known materials measured in the field or in the laboratory. It is envisaged that hyperspectral data will enable the identification of terrestrial features with greater accuracy.
The NASA EO-1 Hyperion sensor was the first satellite to collect hyperspectral data from space (November 2000). The spaceborne hyperspectral imagers to date have been technology demonstrators. There are currently no commercial spaceborne hyperspectral sensors in orbit, although some are planned.
In addition to these benefits, hyperspectral imaging has the potential to augment expensive surveying methods. The cost of hyperspectral surveys is less than 10% as compared to that of traditional surveying techniques.
Applications of Hyperspectral Imaging:
Spaceborne multi-spectral imaging provides gross lithologic information or identification of stressed vegetation. It has severe limitations in so far as detailed mineralogy or geobotany of importance for mineral exploration is concerned. Specific mineralogical and geobotanical information is currently provided by field crews. This is expensive and time-consuming. Hyperspectral remote sensing has the potential of highlighting significant mineralogical and geobotanical anomalies or trends.In mineral exploration, hyperspectral data finds two major applications:
1. Lithologic mapping
2. Geobotanical mapping
Lithologic Mapping: The key elements in planning a mineral exploration program prior to undertaking intensive field exploration activities are: 1) to obtain a preliminary understanding of a geographic area through lithological mapping, and 2) to help in the identification of potential exploration targets. Bedrock mapping and identification of the presence and abundance of particular minerals are facilitated by hyperspectral data. Lithologic maps help geologists decipher the lithologic and structural history of a region. This is particularly valuable for areas for which no maps or very generalized maps exist.
Minerals that can be successfully identified with hyperspectral imaging are: OH-bearing minerals, carbonates, sulfates, olivines, pyroxenes, iron oxides and hydroxides. The identification of these minerals and mapping their distribution provides a framework for exploration of precious and base metals, diamonds, etc. Whereas hyperspectral products can be used for characterization of lithology in arid (non-vegetated) environments, basic research is required to find ways to utilize these in vegetated terrains.
Geobotanical Mapping: Lithologic mapping through hyperspectral data is effective only in arid regions. Surface geology is obscured to varying degrees by vegetation in most areas of the world. Specific element associated spectral changes in vegetation, which in turn are related to lithology and soil chemistry, help in the identification, distribution and spatial relationships of anomalous zones. Geobotanical mapping thus holds promise in mineral exploration activity. This approach makes use of the fact that the spectral reflectance of vegetation is affected in the presence of heavy metals or alteration zones. For example, accumulation of heavy metals induces stress on the vegetation causing a shift of the red-edge (680 nm - 800 nm). Such a shift is only detectable with a hyperspectral imager. Geobotanical anomalies associated with ore bodies may sometimes be evident as abrupt changes in plant species, being thereby indicative of lithological changes rather than stress induced physiological changes. The ability of acieving element specific geobotanical products is not yet well developed, and considerable research is required before geobotanical mapping can be effectively utilized in mineral exploration.
Although the use of geobotanical mapping is very promising, the ability of achieving element specific geobotanical products is not yet well developed and requires further basic research and development before it achieves operational status.
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