Mapping Minerals with Satellite Imagery

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Mapping minerals from space was one of the main motivating reasons for creating the earliest multispectral satellite systems such as the Landsat satellites when they were launched in 1972. More modern systems, including the recent Landsat 8, Sentinel-2, and ASTER, have improved upon this endeavor by giving more accurate readings at higher resolutions.

Using spectral response functions along with pixel aggregate has shown that the Sentinel-2 satellite compares favorably to maps created by ASTER, which since the early 2000s had become among the leading multispectral satellite systems for geological mapping. In particular, shortwave infrared (SWIR; 0.9-1.7 microns for wavelength) and visible and near-infrared bands (VNIR; 0.36-1.05 microns for wavelength) show comparable results in detecting iron.[1]

In 2000, NASA had launched its Hyperion system, which covers 0.4-2.5/spi mu/m spectral range with 242 bands. While hyperspectral imagery has been established with aircraft systems, where the wide band coverage means that many types of minerals could potentially be differentiated more easily, this type of system was not used on satellites. Hyperion was NASA’s way of testing the utility of hyperspectral satellite imagery for various applications. Because of the increased band coverage, what has been useful about Hyperion is that it does provide capabilities to detect a wide variety of minerals, comparable to some extent to some aircraft systems. Minerals such as carbonates, chlorite, epidote, kaolinite, alunite, buddingtonite, muscovite, hydrothermal silica, and zeolite are evident using different bands from the system.[2]

In many regions, the most common surface minerals are found within clays. This has required new techniques to be made to utilize satellite systems to better map types of clays and minerals found within. Using SWIR and thermal infrared (TIR) bands, it has become possible to differentiate clay types that include illite from montmorillonite clays. This can have important implications for agricultural areas or areas that have greater or worse water retention, including groundwater deposits, due to clay and mineral types found affecting water deposits. Variation in the clay signatures could also be potentially used to monitor erosion activities, including wind and water erosion that affects surfaces.[3]

Data from the Copernicus Sentinel-1 and Sentinel-2 missions, as well as data from other satellites such as NASA’s Shuttle Radar Topography Mission and the US WorldView-3 mission were used to map Namibia’s geology and mineral resources.

Data from the Copernicus Sentinel-1 and Sentinel-2 missions, as well as data from other satellites such as NASA’s Shuttle Radar Topography Mission and the US WorldView-3 mission were used to map Namibia’s geology and mineral resources. Image: GAF via ESA

In some regions, particularly tropical areas, the presence of forested canopies prevent easy mapping of surface minerals. Combing multispectral satellites such as ASTER with radar satellite sensors, such as Phased Array type L-band Synthetic Aperture Radar (PALSAR), allow structural geological elements to be more detectable, including faults, using radar, while ASTER’s SWIR bands helps identify clay areas. Using the properties of the two systems allow an index to be created to better understand the types of minerals that are present in a region. Thus, one system by itself may not be able to easily detect many minerals but combined radar and multispectral imagery allow a better understanding of the structural and surface elements.[4]

Recent work has also shown the importance of mapping mineral dust deposits driven by wind. Such dust deposits could darken snow fields and glaciers, changing reflectance properties for snow, which has the effect of changing surface temperatures and can accelerate melting. The work shows that satellite monitoring could be also useful for understanding variation in mineral deposits in how they affect larger global warming factors such as surface reflectance.[5]

We generally see mineral deposits as static features that have been known from geological mapping. Satellite systems have long been used to investigate mineral deposits. While work has been conducted in mapping remote deposits or types of minerals that are harder to quantify, such as from clays, recent work has also shown that minerals that move around, due to wind erosion, could have also important consequence for the planet. Remote sensing might then be critical not only for commercial interests for minerals but also for global change.

References

[1]    For more on the use of Sentinel-2 for detecting surface or near surface elements, see:  van der Meer, F. D., van der Werff, H. M. A., & van Ruitenbeek, F. J. A. (2014). Potential of ESA’s Sentinel-2 for geological applications. Remote Sensing of Environment, 148, 124–133. https://doi.org/10.1016/j.rse.2014.03.022.

[2]    For more on Hyperion, see:  Kruse, F. A., Boardman, J. W., & Huntington, J. F. (2003). Comparison of airborne hyperspectral data and eo-1 hyperion for mineral mapping. IEEE Transactions on Geoscience and Remote Sensing, 41(6), 1388–1400. https://doi.org/10.1109/TGRS.2003.812908.

[3]    For more on clay mineral mapping, see:  Cudahy, T., Caccetta, M., Thomas, M., Hewson, R., Abrams, M., Kato, M., … Mitchell, R. (2016). Satellite-derived mineral mapping and monitoring of weathering, deposition and erosion. Scientific Reports, 6(1). https://doi.org/10.1038/srep23702.

[4]    For more mapping of mineral features in tropical zones, see:  Pour, A. B., & Hashim, M. (2015). Integrating PALSAR and ASTER data for mineral deposits exploration in tropical environments: a case study from Central Belt, Peninsular Malaysia. International Journal of Image and Data Fusion, 6(2), 170–188. https://doi.org/10.1080/19479832.2014.985619.

[5]    For more on wind deposited minerals, see:  Di Mauro, B., Fava, F., Ferrero, L., Garzonio, R., Baccolo, G., Delmonte, B., & Colombo, R. (2015). Mineral dust impact on snow radiative properties in the European Alps combining ground, UAV, and satellite observations: MINERAL DUST ON SNOW IN THE ALPS. Journal of Geophysical Research: Atmospheres, 120(12), 6080–6097. https://doi.org/10.1002/2015JD023287.


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