2. DATA AND PRE-PROCESSING
Data from airborne and satellite sensors have been analyzed in a
multiscale strategy. We used reflectance images of HyMAP
sensor (Cocks ef al., 1998) and the MASTER sensor (Hook et
al., 2001), acquired both the 7 March 2005 on the Turrialba
volcano and surroundings. The scene HyMAP (125 channels
between 0.4589 um and 2,491 um) has a dimension of 710 x
2415 pixels, with a spatial resolution of 15 m. The MASTER
scene is 1650 x 4466 pixels, with spatial resolution of 9 m and
radiometric resolution of 16-bit. The 50 MASTER image
channels are grouped in a port of 25 channels in the VNIR-
SWIR (0463 um - 2,427 um), and 25 channels between 3,075
um and 13 um in the MIR and TIR.
We used ASTER scenes likewise covered a period from 2002 to
2010, and a scene of Hyperion of 5™ of March 2010, acquired
through the USGS EarthExplorer
(http://earthexplorer.usgs.gov/).
An algorithm MNF (Minimum Noise Fraction) has been applied
to reduce noise of MASTER and HyMAP images. Channels
sensors have been rejected what presenting a high signal / noise
ratio, channels 62 to 65 and 125 in the case of HyMAP, and
channels 16 to 19, 25 to 41 and 50 in the image MASTER.
The airborne images have been georeferenced directly by GLT
ENVI algorithm, using the geometry calculated from position
and orientation data measured by inertial GPS/ IMU (Inertial
Measurement Unit) at the same time of acquisition over the
study area.
3. EXPLORATORY ANALYSIS
Image exploratory techniques have been applied with the aim to
contrast biophysical parameters and obtain a first approximation
of the state of vegetation and soils in the Turrialba region.
A check of radiometric and geometric corrections of images
have been performed from data measured in supervision
campaigns. In August 2010 and February 2012 has been carried
out a field survey, which allowed us samples of 29 and 20
points respectively, measured in field and laboratory with
USB400 and ASD FieldSpec 4 Hi-Res spectroradiometers. The
spectra have been used to characterize hydrothermal alteration
materials and to check HyMAP reflectance images and
MASTER by empirical linear regression.
The georeferencing of images has been tested using 50 check
points measured on the ground in projection UTM zone 17 and
WGS84 Datum by DGPS. The planimetric (X and Y) root mean
square RMS obtained were 3.2 and 1.9 pixels, respectively for
the MASTER data and the HyMAP.
Vegetation indeces and soil have been calculated for the set of
images. Three indices of vegetation from the reflectance values
of hyperspectral imagery were generated, the Normalized
Difference Vegetation Index (Rouse et al., 1974) for evaluating
the estimate of the LAI cover (Leaf Area Index), and the ratio
(Berni et al., 2010) between the rate of the Transformed
Chlorophyll Absorption in Reflectance Index-TCARI
(Haboudane et al. 2002) and the Optimized Soil-Adjusted
Vegetation Index-OSAVI (Rondeaux eft a/., 1996).
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
NDVI i= Duy 7 Pun. (1)
P unir — P un
where = near-infrared channel reflectance
UNIR
p RT red channel reflectance
Q)
TCARI / OSAVI
=3. [os T P u670 )- 0.2. (om = P usso ) (3 im = PP u670 J
(1+ 0.16): is ~ Puen Mom — P ucro + 0.16)
where
. = reflectance at wavelength
Hi 8
Figure 1. NDVI (left) abundante and TCARI/OSAVI (right)
in Turrialba volcano from HyMAP image of March 2005.
These image transformations were carried out with the intention
of assess the influence of vegetation cover in the subsequent
image analysis, and estimate the impact of volcanic activity on
the slopes and the surrounding areas. We observed in the
indices calculated from the time series of ASTER scenes, the
clear effect of acid rain on vegetation vigor at the Northwest of
the crater.
4. ANOMALY DETECTION AND HYDROTHERMAL
ALTERATION
4.4 Hydrothermal alteration in Turrialba volcano
There are some minerals that indicate hydrothermal alteration,
that we can detect remotly by their spectral responses in the
diagnostic absortion bands and reflection. These specific
spectral features can be extracted from the hyperspectral data
properly calibrated. Argillaceous minerals, such as kaolinite,
illite and alunite, have a concrete spectral characteristic
presenting a high reflectance between the wavelengths of 1.55
um and 1.75 um they also present high absorption between 2,08
um and 2,35 um. Another additional characteristic to identify
these minerals is that the rocks that have not suffered the
hydrothermal process, usually present regular values on the
wave lengths previously mentioned. The minerals with high
content in Fe present a very high contrast between the
wavelengths 0,63 pm and 0,69 pm and the wavelengths 0,45
pm to 0,52 pm.
We have chosen four variables for generating a different
Principal Component Analysis (PCA) for groups of channels,
two highly reflective and two highly absorptive for each mineral
(Crosta et al., 2004; Bataller et al, 2011). The minerals
analyzed for HyMAP case are: Illite (HyMAP channels 6, 25,
105 and 108), alunite (HyMAP channels 6, 25, 105 and 116),
kaolinite (HyMAP channels 6, 81, 116 and 108) and
