Influence of desert varnish on the reflectance of gossans in the context of Landsat TM data, southern Red Sea Hills, Sudan

Laboratory spectra have been used for studying the characteristics of deser t varnish in the context of Landsat TM data in order to establish its influ ence on the reflectance of gossans and their discrimination. Supported by l imited chemical analysis, reflectance values from selected rock samples hav e been processed using statistical methods, and spectral plots produced and visually interpreted. Spectral analysis shows that paucity and Mn/Fe proportion in desert varnish , lightness or darkness of the rock, its mineral composition, and the wavel ength under consideration control the effect of desert varnish on Landsat T M data. In general, overall reflectance of rocks is reduced by up to 70% in the VIS parts and about 20% in the SWIR parts of the electromagnetic spect rum due to desert varnish. For acidic rocks this effect seems to be intense , whereas mafic rocks are less influenced. The latter generally display spe ctral crossovers partly due to Mn/Fe content. In the VIS region absorption bands turn to featureless spectra due to desert varnish, whereas in the SWI R spectra are generally subdued but preserved. This has apparently little c onsequence on the discrimination of gossans from other rock types on compos ites constituted by bands from the VIS, IR and SWIR parts.