Mapping the wavelength position of deepest absorption features to explore mineral diversity in hyperspectral images

A new method is presented for the exploratory analysis of hyperspectral OMEGA imagery of Mars. It involves mapping the wavelength position and depth of the deepest absorption feature in the range between 2.1 and 2.4 µm, where reflectance spectra of minerals such as phyllosilicates, carbonates and sulphates contain diagnostic absorption features. For each pixel of the image, the wavelength position maps display the wavelength position of the deepest absorption feature in color and its depth in intensity. This can be correlated with (groups of) minerals and their occurrences. To test the validity of the method, comparisons were made between wavelength position maps calculated from OMEGA images of the Nili Fossae area at two different spatial resolutions, of 0.95 and 2.2 km, and five CRISM images in targeted mode, at 18 m spatial resolution. The wavelength positions and their spatial patterns in the two OMEGA images were generally similar, except that the higher spatial resolution OMEGA image showed a larger diversity of wavelength positions and more spatial detail than the lower resolution OMEGA image. Patterns formed by groups of pixels with relatively deep absorption features between 2.250 and 2.350 µm in the OMEGA imagery were in agreement with the patterns calculated from the CRISM imagery. The wavelength positions of clusters of similar pixels in the wavelength position maps are consistent with groups of minerals that have been described elsewhere in the literature. We conclude that mapping the wavelength position of the deepest absorption features between 2.1 and 2.4 µm provides a useful method for exploratory analysis of the surface mineralogy of Mars with hyperspectral OMEGA imagery. The method provides a synoptic spatial view of the spectral diversity in one single image. It is complementary to the use of summary products, which many researchers have been using for assessment of the information content of OMEGA imagery. The results of the exploratory analysis can be used as input for the construction of surface mineralogical maps. The wavelength position mapping method itself is equally applicable to other terrestrial and planetary data sets and will be particular useful in areas where field validation is sparse and with imagery containing shallow spectral features

Multi- and hyperspectral geologic remote sensing: A review

Geologists have used remote sensing data since the advent of the technology for regional mapping, structural interpretation and to aid in prospecting for ores and hydrocarbons. This paper provides a review of multispectral and hyperspectral remote sensing data, products and applications in geology. During the early days of Landsat Multispectral scanner and Thematic Mapper, geologists developed band ratio techniques and selective principal component analysis to produce iron oxide and hydroxyl images that could be related to hydrothermal alteration. The advent of the Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) with six channels in the shortwave infrared and five channels in the thermal region allowed to produce qualitative surface mineral maps of clay minerals (kaolinite, illite), sulfate minerals (alunite), carbonate minerals (calcite, dolomite), iron oxides (hematite, goethite), and silica (quartz) which allowed to map alteration facies (propylitic, argillic etc.). The step toward quantitative and validated (subpixel) surface mineralogic mapping was made with the advent of high spectral resolution hyperspectral remote sensing. This led to a wealth of techniques to match image pixel spectra to library and field spectra and to unravel mixed pixel spectra to pure endmember spectra to derive subpixel surface compositional information. These products have found their way to the mining industry and are to a lesser extent taken up by the oil and gas sector. The main threat for geologic remote sensing lies in the lack of (satellite) data continuity. There is however a unique opportunity to develop standardized protocols leading to validated and reproducible products from satellite remote sensing for the geology community. By focusing on geologic mapping products such as mineral and lithologic maps, geochemistry, P-T paths, fluid pathways etc. the geologic remote sensing community can bridge the gap with the geosciences community. Increasingly workflows should be multidisciplinary and remote sensing data should be integrated with field observations and subsurface geophysical data to monitor and understand geologic processes