Abstract
Abstract. In the present study, different temperature-emissivity separation algorithms were used to derive emissivity images based on processing of ASTER( Advanced spaceborne thermal emission and reflection radiometer) thermal bands. These emissivity images have been compared with each other in terms of geological information for mapping of major rock types in Hutti Maski schist Belt and its associated granitoids. Thermal emissivity images are analyzed conjugately with thermal radiance image, radiant temperature image and albedo image of ASTER bands to understand the potential of thermal emissivity in delineating different rock types of Archaean Greenstone belt. The emissivity images derived using different emissivity extraction algorithms are characterised with poor data dimensionality and signal to noise ratio. Therefore, Inverse MNF false-colour composites(FCC) are derived using bands having better signal to noise(SNR)ratio to enhance the contrast in emissivity. It has been observed that inverse-MNF of emissivity image; which is derived using emissivity-normalisation method is suitable for delineating silica variations in granite and granodioritic gneiss in comparison to other inverse- MNF-emissivity composites derived using other emissivity extraction algorithms(reference channel and alpha residual method). Based on the analysis of ASTER derived emissivity spectra of each rocks, band ratios are derived(band 14/12,band 10/12) and these ratios are used to delineate the rock types based on index based FCC image. This FCC image can be used to delineate granitoids with different silica content. The geological information derived based on processing of ASTER thermal images are further compared with the image analysis products derived using ASTER visible-near-infrared(VNIR) and shortwave infrared(SWIR) bands. It has been observed that delineation of different mafic rocks or greenstone rocks(i.e. separation between chlorite schist and metabasalt) are better in SWIR composites and these composites also provide comparable results with thermal bands in terms of delineation of different types of granitoids.
Highlights
The emissivity of an isothermal, homogeneous emitter is defined as the ratio of the actual emitted radiance to the radiance emitted from a black body at the same thermodynamic temperature(Norman, 1995)
Older granodiorite gneiss is regarded as distinct unit in comparison to younger alkali rich granites as albedo of granodiorite is high and slightly higher at places but broadly similar to the albedo of younger granite( which has good exposures reflecting incident energy well) as exposures of these rocks are massive and rich in high albedo bearing Calcium rich feldspar with low radiant temperature( please refer enlarged chips in the figure 5 where contrast in colour in radiance image is shown with contrast in albedo and radiant temperature image).In day time data; radiant temperature of granodiorite is very loe; even slightly lower than granite
Black soil(BS),amphibolite(AMP) and basalt(MBL) are indistinguishable with their bright green colour in the radiance composites. These elements are all characterised with low albedo and high radiant temperate(Fig.5). reddish tint observed below low albedo and high temperature bearing black soil( at the south-east part of the study area (Fig. 5.a) indicate the presence silica rich rock underneath(soil is developed above granodiorite at the south-east part of the study area).This indicates that the transported nature of this soil
Summary
The emissivity of an isothermal, homogeneous emitter is defined as the ratio of the actual emitted radiance to the radiance emitted from a black body at the same thermodynamic temperature(Norman, 1995) It is an important geophysical parameter of terrain elements for the studies related to climatology, hydrology and used in modelling of the greenhouse effect (Hulley, 2009). Spectral delineation of different igneous rocks having varying silica content can be attempted using emissivity features based on processing of multiband thermal sensor data like ASTER. The differences in thermal properties( based on derivation of thermal inertia and variation in temperature) have been used to identify geological units(Abrams et al, 1984; Kahle and Rowan, 1980; Lyon, 1972).For example, important igneous rocks like basalt, gabbro, granites etc are delineated from the apparent thermal inertia images (ATI). In recent times different thermal composites and thermal ratio images have been used to delineate rock types
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