Spectral data in geological mapping

 

Multiscale hyperspectral analysis of lithologies in the Arctic

Regional geological mapping in northern regions is time consuming and costly, primarily owing to poor accessibility and a short field season caused by snow cover. Along with established geophysical exploration technologies (e.g. magnetic and radiometric surveys), hyperspectral imaging can facilitate detailed continuous regional mapping in the arctic and subarctic owing to low vegetative cover, although issues such as low illumination and pervasive rock encrusting lichens remain a challenge. Our focus is thus integration of spatial and spectral information for lithological identification and mineral mapping from different scale (satellite-based, airborne, ship-based, drone-based, terrestrial). We apply and develop automatic methodologies and investigate their efficiency in dealing with challenges that may rise in arctic areas, such as lichens, low sun angle, shadows, snow and ice, etc.


Hyperspectral Analysis of Lithologies in Presence of Abundant Lichens

Airborne hyperspectral (HyMap) data was chosen to investigate the potential of hyperspectral sensors for detailed lithological mapping in central West Greenland (Innarsuaq), where an ultramafic rock unit is exposed with the presence of lichen coatings (Figure 1).

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Figure 1. Central West Greenland (Innarsuaq), where ultramafic rock units are exposed b) Coverage of the airborne hyperspectral data is marked by red frame, c) the CIR color composition of HyMap data, highlighting abundant vegetation coverage, (Salehi et al. 2017a)

Spatial–spectral endmember extraction (SSEE) method (Rogge et al. 2012; Rogge et al. 2007) is used for the selection of optimal endmembers and assessment of subtle lithological variability across the given study area. The amphibole minerals as exemplified by the hornblende, actinolite and anthophyllite in Figure 2 dictate the SWIR spectral characteristics of ultramafic rocks. The absorption features in the SWIR region are at 2.32 and 2.38 μm, respectively, and both features are of the same order of magnitude. The SWIR spectrum of the olivine rich rocks clearly reflects the mixture of antigorite, serpentine with the characteristic stronger absorption feature at 2.32 μm. A less distinct absorption feature at 2.31 μm is present for rocks enriched in talc.