391 
, chlorite, 
cies show 
he interval 
principal components. In consequence, the 
obtained principal components will be 
influenced by the spatial abundance of diverse 
materials and by the statistics of the image. 
In 1989, CROSTA and MOORE proposed a 
method for the mapping of residual soils with 
TM images. The technique was described as 
“Feature Oriented Principal Component 
Selection (FPCS)”. The FPCS method is based 
on the study of the weight of the obtained 
eigenvectors, obtained from the analysis of the 
standard principal components. This is used to 
decide which of the principal components of the 
images are those, which concentrate the related 
information directly with the theoretical spectral 
signature of a specific substance. 
A version of the technique by CROSTA and 
MOORE (1989) is to take the maximal 
differences of the covariances of the 
eigenvectors obtained by the CP analysis instead 
of using their weight. This version converts the 
CP technique into an interactive tool, where the 
questions are planed in function of field 
knowledges of the study zone as well as 
mineralogical evidences, experimental 
determinations of rocks and minerals, and 
inclusive radiometrical measurements at the 
surface. 
ìe 2.2 
ns of the 
H bonds 
nguish the 
îydroxides 
such as in 
cates and 
¡presents a 
*y. As an 
ctrums for 
ote the 
.4, 1.9 and 
principal 
technique 
leralogical 
ìe spectral 
vhich are 
obtain its 
CAOLINITE WAVELENGTH LM™) 
0 4 0.5 0 6 0.7 0.8 0.9 1.0 1.1 0.8 0.9 10 1.1 1.2 1.3 1.4 1.5 16 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 
MUSCOVITE WAVELENGTH (>4m> 
Figure 3 Reflectance spectra of caolinite and muscovite. (From Hunt and Salisbury. 1970). 
In the study area, evidences of active geothermal 
activity are encountered, characterized by the 
presence of discharging thermal water, gas 
exhalations and wide areas of hydrothermal 
alteration. The mineralogical consequence of the 
anteriour is the abundance of clay zones with the 
illite type, caolinite and the formation of 
oxydation zones with the presence of limonite 
soils. 
For the zone of Acoculco, Puebla, the 
elaboration of an hydrothermal alteration map 
initiated with the mapping of the hydroxide 
minerals (caolinite, illite) and the minerals with 
iron oxidation (limonite). This first stage did not 
consider the effect of hematite. 
The analysis process of the images is as follows: 
a. Pre-processing: atmospheric correction