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WorldView-3 with its 8 SWIR bands becomes a boon for mineral exploration

by Pubali Mukherjee

Today, world economy is facing unprecedented crisis of resources be it food or energy and with the ever increasing population of earth we are in constant search of new oil/ gas/mineral resources. A deeper view of unexplored oil/gas resources through short wave infra-red (SWIR) of the electromagnetic spectrum will definitely give the mineral exploration industry an extra edge. Mineral identification and mapping with the help of SWIR can give mineral exploration an unprecedented accuracy. Every mineral has different spectra depending on its chemical composition and with the help of SWIR bands mineral explorers can accurately differentiate between very closely related minerals based on the subtle changes in the spectra.

DigitalGlobe`s WorldView-3 is all set to transform the Geo-Spatial industry from pixel based industry to product based industry. The satellite has 16 high resolution infrared bands (VNIR and SWIR) and has a specialized payload named CAVIS for detection of atmospheric conditions. One of the major challenges in satellite imagery information extraction is dealing with the extremely dynamic atmospheric conditions during the time of the imagery capture. Various studies have been done so far for accurate estimation of top of atmosphere radiance (TOA), thus WV-3 has an extra niche with CAVIS as one of its payload. Earlier sensors had one or two SWIR bands and so a detailed or continuous spectra of minerals was out of scope, even ASTER data which was earlier extensively used for mineral mapping has 10 bands in Very Near Infra-Red (VNIR) and its SWIR failed. So it can be said that WV-3 is first of its kind multispectral sensor with such an enhanced spatial and spectral capabilities.

SCIENCE BEHIND MINERAL IDENTIFICATION:

Interaction of the Electromagnetic energy with rocks or any other earth surface materials causes transfer of energy in the form of photons. Energy is either absorbed or excited photons release energy within specific wavelengths. Mineral identification through satellite imagery is based on identification of the subtle changes in the absorption spectra of the minerals based on their chemical composition.  As the photons interacts with the minerals, the chemical bonds within the mineral respond either by electronic absorptions or by vibrational motions at atomic level. Iron content (Fe+3) minerals are known to absorb wavelength less than 1000 nm. Vibrational movements of the chemical bonds within the rocks are known to absorb energy within 1000 nm to 2500 nm and this is diagnostic for minerals having hydroxyl group e.g.: Al-OH, Fe-OH, Si-OH and some other groups like Co3, NH4, SO4 .

SWIR BAND CAPABILITY:

SWIR region extends within 1400 um to 3000 um and major mineral absorption occurs within 2000 um to 2400 um so a deeper probe into the SWIR region will give mineral and ore identification an extra niche. Collecting satellite imagery in SWIR region has unique benefits including atmospheric transparency but since water and snow absorption generally occurs within the SWIR region so atmospheric compensation is an integral part when dealing with the SWIR range applications. Mineral exploration industry spends millions of dollars for mining operations thus WV-3 is expected to cut the cost of mineral exploration by providing an extra hand in identification of the potential mineral exploration sites especially for hydrothermal ore deposits.

Spectral comparison of WV-3 with ASTER and Landsat (Image courtesy: DigitalGlobe)

A SCIENTIFIC STUDY TO UNDERSTAND CAPABILITIES OF SWIR AND WORLDVIEW-3:

A scientific study was done by Fred a. Kruse and Sandra L. Perry to validate the efficiency of WorldView-3 data by comparison of the model simulated WorldView-3 data with AVIRIS data within the SWIR wavelength range. The study was done to evaluate the mineralogical mapping capabilities of WorldView-3 in Cuprite Nevada. This site has been used for over 35 years to evaluate spectral remote sensing data. Cuprite is a well-known example of hydrothermal system with an array of alternating mineral ores. The study was carried out before WV-3 data was available so a simulated data of the satellite was used but the results are still valid and verifiable using WorldView-3 imagery. AVIRIS (Air borne Visible/Infrared Imaging spectrometer) a hyperspectral imager was used to simulate the 7.5 m WV3 data within the SWIR spectral range as most of the mineral absorption features of minerals occur within 2.0-2.5 micrometer range. The idea of utilizing spectral signatures and hyperspectral analysis techniques for multispectral mapping is unique. The simulated results show almost accurate geological classification with high accuracy when compared to a hyperspectral imager AVIRIS. Certain minerals like Calcite, Alunite, Jarosite, and kaolinite are generally difficult to differentiate but within the SWIR range of WV -3 they can be distinctly separated.

                                                                                                         Image courtesy: DigitalGlobe

                                                  Image courtesy: DigitalGlobe

                                        Image courtesy: DigitalGlobe

                                                                        Image courtesy: DigitalGlobe

FUTURE OF WORLDVIEW-3 AND SWIR:

Before WV-3 came into existence ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data and AVIRIS(Air Borne Visible/Infrared Imaging Spectrometer) data was extensively used for mineral mapping but recent studies conclude that  WV-3 will significantly outperform ASTER in mineral mapping due to its extended SWIR bands can probe deeper into mineral spectra and thus map subtle changes in the spectra specially for hydrothermal ore deposits and help understand hydrothermal mineral alternation  pattern, moreover WV-3 has much higher spatial resolution of 7.5 meters and thus can give images of unprecedented spatial granularity of hydrothermal ore deposits compared to the 30 meters resolution of ASTER.

                                                                                                                                    Image Courtesy: DigitalGlobe

With the WV-3 hovering over earth gives geologists and scientists a great opportunity to explore the unexplored ore deposits but studies are showing that additional research is required in more challenging ore deposits to evaluate the capabilities of SWIR and WV-3.


SATPALDA is a privately owned company and a leading provider of satellite imagery and GeoSpatial services to the user community. Established in 2002, SATPALDA has successfully completed wide range of photogrammetric and Remote Sensing Projects.

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