Spectral reflectance of lichens and their effects on the reflectance of rock substrates

Abstract

Rock outcrops in arid and semiarid regions frequently have abundant lichen cover. We collected lichens and their granite, slate, and hornfels substrates in the Extremadura region of Spain and measured spectral reflectance in the 400–2500 nm region in the laboratory. Lichen cover as low as 30 percent masks the spectral features of spectrally flat rocks such as slate and hornfels. The sharp spectral features of granite are obscured at 60–80 percent lichen cover. The strong reflectance peak at 2220 nm in lichens interferes with recognition of the 2200 nm absorption band in hydroxyl‐bearing minerals that is used to locate hydrothermally altered and potentially mineral‐rich areas. However, lichen spectra are characterized by distinctive absorption features that occur in few minerals and can thus be used to identify lichen presence. From computer modeling of laboratory spectra, we conclude that Landsat Thematic Mapper (TM) band ratios of 3 : 4 and 5 : 2 best minimize lichen effects in TM image analysis.