Università degli Studi di Padova-Dipartimento di Geoscienze Dottorando: Luca Bertoldi TELERILEVAMENTO DI ROCCE GRANITOIDI IN AMBIENTE DESERTICO (ANTI-ATLANTE ORIENTALE-MAROCCO) ED ALPINO (HIMALAYA-NEPAL OCCIDENTALE): ELABORAZIONE IMMAGINI ASTER E SPETTROSCOPIA Co-tutore: Dott. Matteo Massironi SCUOLA DI DOTTORATO DI RICERCA IN SCIENZE DELLA TERRA
Della Direttore, Scuola, Ch Mo, Artioli Gilberto, Supervisore, Ch Mo, Dario Visonà, Ciclo Xxiii
2008
unpublished
RINGRAZIAMENTI Un grazie ai miei genitori Alberto e Annamaria che mi hanno consentito di vivere questo momento in ogni sua emozione. Un grazie particolare va ad Angela che in ogni momento mi fa capire quanto sia bella la vita, aiutandomi, incoraggiandomi e soprattutto volendomi bene. Un grazie a Francesco, Ermanno, Marco M., Marco B., Christian, Mauro, Daniele, Silvia, Tiziana, Katia e tutti gli amici trentini. Un grazie a tutti i compagni e colleghi del Dipartimento di Geoscienze
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... à degli studi di Padova. Un grazie al Dott. Matteo Massironi e al Prof. Dario Visonà, che mi hanno permesso di crescere, viaggiare, e soprattutto conoscere. Un grazie di tutto cuore a tutti voi e a chi non ho nominato, per avermi supportato e sopportato durante questi tre anni di ricerca. Dottorato di ricerca finanziato da CARIPARO e PRIN 2006. Abstract The aim of this research is to characterize the spectral signatures of granitoid rocks in visible and infrared wavelengths regions and find effective methodologies to discern and map granitoid plutons using ASTER (Advanced Spaceborne Thermal Emission and Reflectance Radiometer) multispectral satellite images in different environmental contexts. Despite satellite remote sensing analysis has been extensively used for geological mapping, it is not considered readily applicable to the mapping of igneous terrains, where lithological contacts are less predictable. For this reason several plutons in two different geological contexts have been taken into account: i) Anti-Atlas -Morocco Edyacarian and Cryogenian plutons (Eastern Sagrho) and ii) Himalaya -Tertiary plutons (Dolpo). Particular attention has been given to the pre-processing of the ASTER L1A and L1B images. Level-1B data have been generated performing the radiometric calibration and geometric resampling of Level-1A products. In this way radiance at sensor (W/m 2 /sr/μm) is obtained from the DN (Digital Number). Subsequently, ASTER images have been corrected for the crosstalk effect, resampled at the same spatial resolution (15m/pixel), georeferenced and orthorectified. Finally, radiance at sensor values have been converted to ground reflectance applying atmospheric correction and, where necessary, the topographic effects on spectra has been reduced performing a topographic normalization. In order to compare ASTER spectra with the minerals and rocks ones, ASTER Spectral Library version 2.0 (Baldridge et al., 2009) have been considered. This library includes contributions from the Jet Propulsion Laboratory (JPL), Johns Hopkins University (JHU) and the United States Geological Survey (USGS) (Christensen et al., 2000, Clark et al., 1993, 2007 ). In addition, forty-five samples of granitoid rocks collected in the field, were analyzed in the VIS/SWIR spectral range at the DEI (Department of Information Engineering -Università di Padova) using the VARIAN-CARY5000 R spectrophotometer in a wavelength range spanning from 0.35µm to 2.5µm at a 1nm sampling step. High-resolution spectral signatures were resampled simulating the ASTER bands filter. This transformation drastically decrease the discerning capability of spectral signatures since the 2150 laboratory "bands" are reduced to the 9 bands obtained by the ASTER filters in the VIS/SWIR range. The remote sensing elaboration in Morocco was challenging in discriminating Ediacarian granitoid bodies, that are characterized by very similar compositions and a widespread desert varnish coating. Using false color composites, band ratios, spectral angle mapper (SAM) and supervised maximum-likelihood (MLL) classifications on ASTER bands, we were able to discriminate and map four calc-alkaline plutons. The analytical comparison of ASTER TIR (Thermal Infrared Region) and VNIR/SWIR (Visible Near Infra Red Region, Short Wave Infrared Region) data has demonstrated that the latter are very effective in the distinction of granitoids, although with very similar silica content. This because secondary effects like hydrothermal and surface alterations which may depend respectively on the magmatic evolution and on the texture and modal composition of the plutonic body, can be quite easily recognized. The remote sensing elaborations in Himalayan case study, point out that detailed lithological discrimination of heavily vegetated and topographically rough areas, requires advanced digital image processing techniques. A good approach consists on masking no "bedrock" pixels through a classification based filter which excludes water, snow, vegetation and clouds. Subsequently we were able to cluster "rocky" pixel reflectance values into a few end-members, each with a specific mean spectral signature. Indeed, these "rocky" pixels have shown signatures constituted by a non-linear mixing of rocks and lichens. In particular, granitoid rocks of the Higher Himalayan Granitoid form an acid substrate influencing the distribution of acidophilic lichens species which can be diagnostic of such rocks. Consequently, the effect of these lichens on rock spectra along with muscovite absorption bands can be used as a proxy for the presence of leucogranitic rocks in mid latitude alpine environment. The image analysis were carried out on SWIR wavelengths using false color composites of band ratios and PCAs (Principal Component Analysis) studied under the light of this new finding. In this way the image analysis provided the detection and the geological map of a new 110 km 2 granitoid body (Buraburi Granite-BG) in the Dolpo region. Results point out that the pre-processing and processing steps necessary to geological remote sensing application must be chosen in function of the specific scene characteristics (season and the environments). Moreover, has been highlighted the important role of the indirect proxies, such as desert varnish and acidophilic lichens, in the granitoid rocks spectral discrimination, based on satellite signature.
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