Weathering process effects on spectral reflectance of rocks in a semi-arid environment

Spectral properties of rocks are mainly dependent on their mineralogical composition, which produces characteristic absorption features in different wavelength regions. This can be considered as a tool to recognise and discriminate different lithological units of an area by remotely-sensed data. Nevertheless, physical and chemical natural processes produce changes that modify to a considerable extent the mineralogical composition of the rock surface (weathered surface) which mask some of the spectral properties of the original surface (fresh surface). In the present study, various rock types (gypsum, carbonate, sandstone, lamproites, phyllite, and quartzite) were selected from a semi-arid region (SE Spain), pilot zone for MEDALUS Project, and their bidirectional reflectance factors were measured under laboratory conditions over the spectral region between 400 and 2500 nm. The study reveals that reflectance differences between the fresh and weathered surfaces (in brightness and presence of characteristic absorption features) are highly significant in that spectral region, being the effect introduced by the iron oxides the most important.