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Evaluation of remote sensing methods for the detection of hydrothermal alteration zones in Milos island (Greece).

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Data of Landsat 5 Thematic Mapper have been processed and analysed in the present paper, in order to detect the alteration zones in Milos island. These altered zones are important because they are connected with the presence of industrial mineral deposits such as bentonite and kaolinite. Two different processing and analysis methods of satellite data, Crosta technique and band ratioing, have been used aiming at the evaluation of the results of each technique separately and their comparison by taking into account the local conditions which affect significantly the final result. These two methods have been proved to be complementary.
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EVALUATION OF REMOTE SENSING METHODS FOR THE
DETECTION OF HYDROTHERMAL ALTERATION ZONES IN MILOS ISLAND (GREECE)
PARCHARIDIS IS.
1
, GARTZOS Ε.2, PSOMIADIS EM.3
ABSTRACT
Data of Landsat 5 Thematic Mapper have been processed and analysed in the
present paper, in order to detect the alteration zones in Milos island. These
altered zones are important because they are connected with the presence of
industrial mineral deposits such as bentonite and kaolinite.
Two different processing and analysis methods of satellite data, Crosta
technique and band ratioing, have been used aiming at the evaluation of the
results of each technique separately and their comparison by taking into account
the local conditions which affect significantly the final result. These two
methods have been proved to be complementary.
KEY WORDS
Remote sensing, alteration zones, PCA, band ratios, spectral reflectance,
Milos island.
INTRODUCTION
In general, the hydrothermal alteration of certain rock types leads to the
formation of industrial and/or metallic mineral deposits. The case of Milos
island is a characteristic example. Intense hydrothermal alteration of volcanic
rocks on Milos resulted in the formation of exploitable industrial mineral
deposits (bentonite and kaolinite). In addition, various hydrothermal minerals
such as barite, alunite, sulphur, galena, manganese and iron oxides, and
epithermal gold have been formed (Kelepertsis et al. 1990, Liakopoulos 1991,
Fytikas and Markopoulos 1992, Ballanti 1997).
Hydrothermal alteration mapping using earth observation satellite data is
possible because certain minerals show characteristic spectral features that
permit their remote detection (Fraser 1991, Carranza & Hale 1999).
The Thematic Mapper of Landsat 5 permits the recording of the spectral
variations of various hydrothermal alteration minerals with positive results.
Two different techniques of mapping the alteration zones were used in this
study:
a) The band ratioing technique (Goetz, 1989, Cracknell and Hayes, 1991,
Gupta, 1991, Fraser 1991, Rencz et al., 1994, Sabins, 1997, Harris et al., 1998,
Parcharidis et al., 1998)
b) The Crosta technique, which is based on the Principal Component Analysis
(Crosta and Moore 1989, Loughlin 1991, Ruiz-Armenta and Prol-Ledesma 1998).
Finally, these two techniques are evaluated by taking into account the
local conditions (atmospheric conditions, climate, topography, land cover
etc.), which influence significantly the efficiency of both techniques.
The above-mentioned methods will be applied in the case of Milos island for
the mapping of the hydrothermal alteration zones. Furthermore, the advantages
and disadvantages of the two methods will be examined, and their complementary
character will be evaluated.
GEOLOGICAL SETTING OF MILOS ISLAND
(i) Main stratigraphic units
1
Dr. Space Applications, Research Unit in Geosciences, Laboratory of
Geophysics, National and Kapodistrian University of Athens, Panepistimiopolis-
Ilissia, Athens 157 84 (GR), e-mail: parchar@geol.uoa.gr
2 Assoc. Prof., 3 Ph.D.student
Agricultural Univ. of Athens, Minerology-Geology lab, Iera Odos 75, 118 55,
Athens, e-mail: egartz@aua.gr, mpsomiadis@aua.gr.
The volcanism in Milos island is characterized by a calk-alkaline activity,
from the Upper Pliocene to the Upper Quaternary. There are five (5) volcanic
phases: The three of them are of the Upper Pliocene and are characterized by
rocks rich in SiO2 such us rhyolites, rhyolitic tuffites and dacites. The fourth
phase took place in the Lower Quaternary and gave rise to mostly basic volcanic
rocks (andesites and andesitic breccias). The last volcanic activity took place
in the Upper Pleistocene and the rhyolitic lavas and the tuffites of Fyriplaka-
Trahila were formed. The area can be characterized as volcanically active,
although it hasn’t shown such an activity the last 80.000 years. The strong
fumarolic activity is an evidence of the presence of a warm magmatic source in
depth.Four different stratigraphic units can be noticed in Milos (Fytikas et
al., 1986), (fig. 1).
- Metamorphosed alpine basement: it appears with small outcrops of strongly
deformed metamorphic rocks, mostly in the southeastern part of the island.
- Neogene sedimentary sequence: some of its parts, from the Upper Miocene to
the Lower Pliocene, appear in the southern part of the island (Fytikas, 1977).
- Volcanic sequence: volcanic and volcano-sedimentary units. This sequence can
be classified in four main units:
(a) A pyroclastic one occurring at the base (Middle-Upper Pliocene), (b) A
complex of lava flow and domes (Upper Pliocene), (c) Pyroclastics and lava
domes (Lower Pleistocene), (d) The acid complex of Fyriplaka and Trahila (Upper
Pleistocene).
- Alluvial sediments: located in the Zefiria area and consist of clay, sand
and gypsum of a total thickness of 80 m. Smaller outcrops appear around the
Adamas village.
(ii) Tectonics
Milos island has been affected by the Post-Upper Pliocene tectonics.
According to Papadopoulos (1993), the seismic activity of the island (March of
1992, Ms=5.3) is connected to structural deformations and not to the volcanic
activity. The circulation of hydrothermal fluid in the crust is due to tensional
faults (shear fractures) (Ballanti, 1997). Four main fault systems have been
noticed: (a) NW-SE system, (b) E-W, (c) N-S, (d) ENE-WSW (Fytikas 1977, Tsokas
1985).
(iii) Hydrothermal activity
Fig.1: Geological map of Milos
island (Fytikas et al., 1986).
The widespread hydrothermal activity which is evident on Milos island (fig.
2) is related to the thermal anomalies due to the occurrence of shallow magma
chambers (Vavelidis et al., 1998).
On the basis of geothermal studies (Fytikas 1977) and the presence of the
active hydrothermal system in the southeastern coasts of Milos, it is concluded
that there is a magmatic heat source, which provides the necessary thermal
energy for the function of today’s hydrothermal system. In addition, the
southeastern part of the island is dominated by mineral outcrops, which are
connected to a recent hydrothermal activity of an explosive character that
created the hydrothermal craters in this area. Finally, the presence of
hydrothermal alteration in the northwestern edge of the island, is an evidence
of hydrothermal activity in this area. In any case, the extensive hydrothermal
alteration zones occur in the eastern part of the island, north of the active
hydrothermal system.
DATA USED AND THEIR PREPROCESSING
The study area is covered by the Landsat 5 TM image, path/row: 182/035
(column/row: 6920 x 5760), 7 spectral bands, dated 23/9/1987. This image has
been chosen because of no cloud cover and of a high sun elevation angle, which
offers better signal to noise ratio helping the mapping in this specific
application. The ERDAS v.8.3.1 software has been used for the image processing
of the satellite image and the ARC-VIEW v.3.1 for the digitising and
manipulation of the spatial information.
The preprocessing of the Landsat TM data includes the radiometric and
geometric correction of the data:
a) Atmospheric correction comes under the category of radiometric
correction, which improves the ability to interpret and compare the digital
satellite images. Relevant atmospheric parameters are the vertical profile of
pressure, air temperature, humidity, ozone, aerosol type and content, which
influence the absorption and scattering properties. In order to apply
atmospheric correction the ATCOR2 for IMAGINE (Version 1.6) has been used.
b) The geometric correction, based on the EGSA87 georeference system by
selecting a set of 19 ground control points, mainly along the shoreline, has
been made. The nearest neighbor resampling method with a polynomial
transformation of second order (and a new spatial resolution of 33m/pixel after
the resampling) has been used.
SPECTRAL RECOGNITION OF THE ALTERATION MINERALS IN LANDSAT TM IMAGES
The characteristics of the spectral reflectance and emission of the rocks at
various wavelengths are the result of their physical and chemical properties
Fig.2: Map of the hydrothermal
features of eastern Milos
(Ballanti, 1997).
(Abrams et al. 1984, Goosens et al. 1994). The hydrothermal alteration minerals
have a different distribution in the various types of the hydrothermal systems,
which are characterized by groups of minerals that have spectral characteristics
in the near and middle infrared. Referring to the clay minerals (kaolinite,
smectite etc.), the spectral zone with a range from 2.1 to 2.4 μm is
characterized by a high absorption whereas the maximum reflectance (takes
places-occurs) at about 1.6 μm.
The iron oxides show a wide and strong absorption band of radiation, which
is due to the transformation of their electrons, that is located in the region
of the ultraviolet-visible blue, and it increases progressively to higher
wavelengths. In addition, a spectral absorption channel of the Ferric iron (Fe+3)
is located in the region of the near infrared (Hunt et al. 1979, White et al.
1997).
The land cover, as far as it concerns the vegetation, is a serious problem
because it presents absorption characteristics in the spectrum from 0.45 to 0.68
μm (due to the chlorophyll absorption) and a high reflectance at 1.6-2.2 μm (due
to leaf water absorption).
The Thematic Mapper (Landsat recording instrument) allows the recognition of
individual minerals, because of the recording range of its spectral bands.
Clay minerals show radiation absorption at wavelengths identical with the
spectral band TM7 in connection with the potential detection of spectral
recording of the Thematic Mapper. On the other hand, they show high reflectance
within the spectral band TM5 (fig. 3). The iron oxides show low reflectance in
the spectral band TM1 and a high one in the spectral band TM3 (fig. 4).
THE PRINCIPAL COMPONENT ANALYSIS TECHNIQUE (CROSTA TECHNIQUE)
The principal components transformation is a statistic technique of many
variables, which chooses non-correlated linear compositions (eigenvectors) of
variables in such a way that each output principal component (linear
composition) shows the minimum variance (Mather, 1991). This variable in the
multispectral images is related to the spectral response of various surfacial
characteristics.
The Crosta technique (Crosta and Moore, 1989) also known as Feature
Oriented Principal Components Selection, permits the recognition of the
principal components that give information related to the spectral responses of
specific targets. An important advantage of the technique is that it predicts
whether the surficial characteristics are enhanced by pixel with low or high
digital number in the related principal component. The Crosta technique has
been applied in the 6 bands (except the spectral band 6, thermal infrared). On
table 1 the characteristic eigenvalues (in which the reduce in the variance of
Fig.3: Spectral reflectance curves of
the main clay minerals (Sabins, 1997).
Fig.4: Spectral reflectance curves of
the main iron minerals (Sabins, 1997)
the information is shown for its component) as well as the eigenvectors, are
shown.
We notice on table 1 that the component PC1 contains the 95% (very high
rate) of the variance of the 6 spectral bands with values varying from 30.3
(TM1) to 48.5 (TM5) giving information mainly about albedo and the topography.
In the second component PC2 the information is provided by TM4 (33.3) and TM5
(44.5) and it receives a negative contribution mainly by TM1 (-71.3). The
importance of this image is that it contains the information for the vegetation
distribution with the corresponding pixels having high digital numbers.
Eigenvector matrix (%) of original bands
Input Bands
TM1
TM2
TM3
TM4
TM5
TM7
Eigenvalues (%)
PC1
30,3
35,7
38,2
46,1
48,5
43,2
95,8
PC2
-71,3
-33,7
-22,9
33,3
44,7
12,2
2,6
PC3
5,5
-21,8
11,9
-57,8
8,3
77,0
1,1
PC4
5,4
-18,5
-58,8
-2,2
51,0
25,5
0,3
PC5
-29,1
50,0
14,6
-56,3
49,0
29,0
0,1
PC7
-14,0
65,1
-64,8
15,5
-24,0
23,5
0,1
Table 1: Principal Component Analysis on six bands (eigenvector lodings in
percentages).
According to the spectral analysis curves and the range distribution of the
Landsat spectral bands the components PC5 and PC6 are those that contain the
spectral information relevant to the hydroxyd minerals and iron oxides. The
interest is focused mainly on the components PC3 and PC6. The component PC3 has
a positive contribution by TM3 (11.9), TM5 (8.3) and TM7 (77.0) in which the
iron oxides display a characteristic high reflectance and a negative or very low
positive contribution by TM2= -21.8 and TM1=5.5 in which the iron oxides show
characteristics of spectral absorption. Thus, in this image the iron oxides are
correlated to pixels with high digital numbers. The component PC6 receives a
negative contribution by TM5 (-24.0) in which the hydroxides show
characteristics of high reflectance and positive contribution by TM7 (23.5)
where the hydroxyls show characteristics of spectral absorption. According to
the spectral characteristics of the hydroxyls in PC6 image, the relative areas
appear with pixels of low digital numbers and for this reason the invert
technique has been applied.
Α False Color image has been created, with the use of the components PC2,
PC3 and PC6 (PC3, PC6, PC2 as R, G, B-fig. 5). In this image, the blue colors
are related to natural vegetation or crops (according to their characteristics),
the reddish colors are related with the areas that iron oxides dominate and the
green areas with the clay minerals. The yellow and yellow-brown areas indicate
alteration zones where clay minerals and iron oxides coexist.
Through the observation of the image we have concluded that the western part
of the island, in its major extension, is covered by vegetation. This vegetation
cover makes the mapping of the alteration zones impossible. The vegetation in
the eastern part is limited and comprises sparse bushy vegetation and crops.
Alteration zones are located, mainly in the eastern part of the island (yellow
and yellow-brown areas)and specifically in the areas Aggeries, Micro Arcontimio,
Kato Komia, Koufi, in the western part in the areas Asprovounala and Fourni and
in the middle of the northern part, along a zone with NE-SW direction from
Tsouvala up to Makrolagada. In the above-mentioned areas, expanded yellow-brown
zones appear. However, few areas along the coastal zone (extend from Alimia to
Kofto, Voudia, Kastanas and Boudari) appear on the image with similar colors and
have been described as coastal clastic deposits derived actually from the
altered rocks.
BAND RATIOING TECHNIQUE
According to what has been described about the spectral response of
hydrothermal alteration minerals and their recognition in Landsat TM images, we
have come to the conclusion that:
The ratio 5/7 of spectral images can be used for the recognition of clay
minerals and at the same time, for the distinction of altered and non-altered
rocks. These two rock types (altered and non-altered) appear to have the same
reflectance in the image of spectral band TM5 and the non-altered rocks have the
same reflectance in the image of the spectral band TM7. The ratio 5/7 gives
values higher than 1, for the altered rocks.
The iron oxides show a low reflectance in the spectral band TM1 and a high
one in the spectral band TM3. Thus, the hydrothermally altered rocks, which are
rich in iron oxides, have high values of the ratio 3/1.
The creation of Color Composite Ratios is a technique that enhances the
information through the creation of three color composite ratios and their
correspondence in colors Red, Green, Blue.
After that, three kinds of digital analysis have been made.
- Determination of the Color Composite Ratios (CCR) of the spectral bands.
- Application of the technique masking the vegetation
- Detection of the most important alteration zones.
The first step was the enhancement of the initial ratio images according to
the equation: Ratio image = atan (spectral band A/ spectral band B)
Then using these enhanced images, a false color composite image with ratios
3/1, 4/3, 5/7 (R, G, B) was created (fig. 6).
The second digital analysis concerned the presence of vegetation in the
area, because it hides possible altered areas. For this reason the image of
Normalize Difference Vegetation Index (NDVI) has been created according to the
equation: NDVI = (TM4-TM3)/ (TM4+TM3)
The value "0" has been given to the pixels correlated to the vegetation and
then the mask technique was applied to the color composite ratio image (CCR).
Finally, using the latter image, the detection of the main hydrothermal
zones has been made. In this image, the hydrothermally altered zones appear with
a magenta color. These zones are located especially in the areas Aggeries (with
ring shape), Kato Komia and Asprovounala, mostly in the eastern part of the
island. In the areas Voudia and Kastanas located along the coastal zone the
appearance of similar colors on the image, due to clastic deposits derived from
the altered areas.
CONCLUSIONS
We have observed the following after the analysis and the interpretation of
the two different final products using as guide area, Voudia location in fig. 7:
Fig.5: Color
Composite
Image of the
PC3, PC6, PC2
as R, G, B.
a. In the color composite image of the ratios, the mask technique (using and
the good knowledge of land cover of the area) minimizes the influence of
vegetation on the signal reflectance characteristics of the altered areas.
b. In Crosta technique is obvious that principal component values do not form
the ideal correlation, for the exact enhancement of the aiming characteristics
(hydrothermal alteration minerals).
Fig. 7: Color Composite Images, a) Crosta technique, b) Band ratioing
technique, for the area Aggeria-Voudia.
c. The geometry of the alteration zones is enhanced when the band ratioing
technique is used.
d. The clay minerals alteration zones appear more clearly to the interpreter
with the Crosta technique, in a quick visual interpretation.
In conclusion it can be deduced that in cases like the one of Milos
(alternating morphology, semiarid climate and differentiation in vegetation from
nude relief to dense bushy), both techniques have given good results and in many
occasions, the two techniques complement each other.
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... Milos has been extensively studied for its geothermal potential by the Greek authorities in the 1970s and 1980s, which helped develop a good geological and structural understanding of the island (Fytikas, 1989;Fytikas and Marinelli, 1976;Liakopoulos et al., 1991). Extensional tectonics in the Miocene and Pliocene produced several major fault sets across the island and created horst and grabens, which controlled the hydrothermal activity during that period (Alfieris et al., 2013;Parcharidis and Psomiadis, 2001). The two major fault trends in the eastern part of Milos are approximately NW-SE and N-S (e.g., Fytikas, 1986;Stewart and McPhie, 2006). ...
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