Spectral reflectance (400-2500Â nm) properties of coals, adjacent sediments, metamorphic and pyrometamorphic rocks in coal-fire areas: A case study of Wuda coalfield and its surrounding areas, northern China

Although remote sensing is an effective tool extensively employed to detect and monitor coal fires, continuous spectral reflectance properties of typical land surfaces such as coals, adjacent sediments, metamorphic rocks and pyrometamorphic rocks in coal-fire regions have not been reported yet. Field- and laboratory spectral reflectance of coals (bituminous, anthracite, and coal waste), sediments rocks (sandstones, siltstone, desert sands, limestones, and shale), metamorphic and pyrometamorphic rocks located in Wuda coalfield and its surrounding areas are investigated for the reflective domain ranging from 400 nm to 2500 nm. The results indicate that coal depicts the lowest reflectance (2%-10%) and strong absorption bands between 2200 and 2500 nm resulting from organic matter. Pyrometamorphic rocks are characterized by weak absorption features at 1400 and 1900 nm due to partial dehydration, and strong ferric absorption features between 850 and 1100 nm attributed to thermal alteration and oxidation of the rocks. Sediments exhibit common absorptions features at ~ 2200 and ~ 2300 nm probably related to AlOH vibrations and carbonate degenerate CO vibrations, respectively. Spectral reflectance increases with a decrease of the coal rank and particle size. The investigations on spectral reflectance properties of typical land surfaces in coal-fire regions provide the basic understanding for applications of hyperspectral or multispectral remote sensing to classify land covers and to demarcate coal-fire risk areas in similar geological settings.