MAPPING OF HYDROTHERMALLY ALTERED AREAS IN VEGETATED TERRAIN, USING
MULTISOURCE DATA INTEGRATION AND SEGMENTATION TECHNIQUES
Raimundo Almeida-Filho
Icaro Vitorello
National Institute for Space Research - INPE
C.P. 515 - Säo José dos Campos/SP/Brazil
e-mail: rai@ltid.inpe.br
Commission VII, Working Group 4
KEY WORDS: Landsat images, aerial photographs, geology, thematic mapping,
ABSTRACT
Abstract Field spectra data were used to guide the selection of the best Landsat-TM bands to map areas of
hydrothermally altered materials in a savanna vegetated terrain. Landsat images were merged with a digitalized aerial
photograph via IHS techniques. The resulting hybrid products kept the high spatial resolution of the aerial photograph
and the spectral information derived from the Landsat images. A supervised region classifier algorithm was applied to
a segmented image, obtained from the hybrid product. The produced region classified image permitted the accurate
mapping of the target areas, even in areas where vegetation (mainly herbaceous plants) covered up to 60% of the
terrain surface.
1. INTRODUCTION
Several studies using airborne or orbital images
have indicated the feasibility of mapping
hydrothermally altered areas, which are potential
sites for mineral deposits (Abrams et al., 1977;
Prost, 1980; Rowan and Kahle, 1982; Podwysocki
et al., 1983). These studies are based on the fact
that diagnostic minerals associated with
hydrothermal processes, such as iron-bearing
minerals (limonite) and hydroxyl-bearing
minerals (clays, K-micas etc), show diagnostic
spectral features that allow their identification by
remote sensing techniques. However, the use of
these techniques has been constrained to arid and
semi-arid environments with sparse vegetation
cover that permit the spectral information to be
collected directly from the rock-soil assemblage.
Furthermore, in the tropics, the diagnostic
minerals may also be ubiquitous products from
weathering processes. On the other hand, strong
leaching might cause the removal of the
hydrothermal-derived products from alteration
zones. Such environmental conditions drastically
reduce the chance of successful use of these
techniques in the tropics. So, the use of remote
sensing techniques for mapping hydrothermally
altered materials in the tropics requires judicious
analysis of the role of the different environmental
factors that contribute to the information
registered by the sensor systems. In this study we
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evaluate the feasibility of spectral discrimination
of hydrothermally altered materials in a vegetated
terrain, using the Serra do Mendes granitoid as a
case-study area.
2. STUDY AREA
The study area is located in central Brazil, a
region of tropical savanna climate, with a well-
defined rainy summer and a dry winter.
Weathering processes have created poor soils,
with high concentration of iron and aluminum
oxides. The vegetation is a savanna-like
vegetation cover constituted by sparse small trees
and interspersed shrubs and herbaceous plants.
The Serra do Mendes massif is one of the more
than twenty granitoid bodies of middle-to-low
Proterozoic age (Araüjo and Alves, 1979; Marini
and Botelho, 1986) of the Tin Province of Goiás.
The granitoid is a 22 km long by 10 km wide
dome that rises up to 400 meters above the
surrounding gneisses and migmatites of the
Central Brazilian Shield of Archean age
(Almeida et al., 1981). It is constituted mainly of
dark-grey biotite-granitoids with medium-to-
coarse hypiodiomorphic granular texture. Facies
of hydrothermal alteration, composed of
greisenized and albitized muscovite-granitoids
with cassiterite, have been found in the central
part of the Serra do Mendes massif (Padilha and
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
