Cartography of the Ronda peridotite (Spain) by hyperspectral remote sensing

The Ronda Peridotite, south of Andalusia (Spain), was imaged by AVIRIS in 1991 and partially sampled by us in the field with a GER 3700 spectrometer in 1997 in order to get experience in processing hyperpectral images of planetary surfaces with probes such as ISM Phobos (1989), OMEGA Mars Express (2003) and VIMS Cassini (2004). The high spectral resolution of the images (224 channels from 400 to 2455 nm) is necessary to conduct geological analysis with remote petrological determinations of rock types. On Earth, it is also necessary to determine species of vegetation because of their strong influence in mapping lithology, even in dry areas like the Ronda peridotite. The Ronda AVIRIS image was first processed to infer geological features using photo-interpretation of colour composite images extracted from 150 useful channels compared to geological maps and checked on the field during the campaign of July 97. This allows us to distinguish easily the peridotite massif from its surrounding rocks and its own serpentine zoning. Since this work followed the work of Chabrillat et al. [2000] we chose to explore the AVIRIS data with other techniques. We chose to remove the contribution of the atmosphere with spectra collected in the field on a white target at various altitudes and to remove the main vegetation with spectra of the most characteristic vegetation of the peridotite. In both cases we first estimated the amount of atmosphere and vegetation with band ratios and remove them with two similar empiric corrections of the reflectance. From the spectroscopy data, after removal of the atmosphere and some vegetation signal, we were able to clearly distinguish the crustal rocks from the mantle ones, as well as compositional variations due to pyroxene and mostly serpentine abundance within the peridotites. Hyperspectral infrared spectrometry will provide good geological mapping of the main rocks on planetary surfaces, if images can also be calibrated with in situ field measurements which will not miss any unexpected component. However, some ambiguities remain between certain types of rock which have close mineralogical composition (e.g. harzburgite compared to lherzolite) or which have resulting spectra very similar to each other (plagioclase and lizardite in peridotites). Some other ambiguities between spectra are also introduced by techniques of analysis based on relative reflectance. By not taking into account absolute intensity of the reflectance, because of roughness and topographic shading effects, small mineral variations are not always visible.