USGS Digital Spectral Library and convolved to AVIRIS
bandwidths prior to running the SAM classification.
In the SWIR, 46 AVIRIS bands were used with 15 reference
spectra of common hydroxyl-bearing minerals. SAM classified
most of the alteration at Bodie and Paramount as Na-
montmorillonite, Ca-montmorillonite and a mixture of
kaolinite+smectite. This mixture was concentrated
predominantly at Paramount, with smaller occurrences at three
locations at Bodie: Silver Hill, near the center of district, and
Bodie Bluff. At Paramount, a clear zonation was revealed with
most of the western part classified as a mixture of
kaolinite+smectite, surrounded by pixels classified as Na-
montmorillonite, and the eastern part classified as Na- and Ca-
montmorillonite. SAM identified the two types of kaolinite
used as references (well and poorly crystalline) in a very small
number of pixels. Illite and Ca-montmorillonite were mapped
mostly as isolated pixels scattered throughout the scene with no
meaningful pattern. We treated these as misclassifications and
removed them from the final result.
Computation time for the SWIR 15-mineral classification using
46 AVIRIS bands on a low-end Unix workstation (Sun IPX)
was 8 minutes.
In the VNIR, 94 AVIRIS bands were used with 9 reference
spectra of common Fe”*- and Fe” -bearing minerals. Initial
results were not satisfactory because it classified most pixels in
the entire scene as a mixture of hematite and quartz. Even after
experimenting with different values for the angle threshold and
removing the hematite+quartz spectrum from the reference
spectra, SAM classified most of the pixels in the scene as a
single reference mineral. We then decided to use only three
reference spectra: hematite, goethite and jarosite. Changes in
the value of the threshold produced significant changes in the
number of pixels classified as one of the reference minerals.
The results showed goethite in altered areas at Bodie Bluff and
Silver Hill together with jarosite in mine dumps and in a
tailings pond. A few pixels were mapped as hematite at Silver
Hill and in an area to the north of the major alteration. SAM
also mapped goethite in the Paramount district as well as
jarosite which also appeared scattered throughout the western
part of the scene and along dirt roads.
It took 15 minutes to run SAM using 94 AVIRIS bands and 3
reference spectra on a low-end Unix workstation (Sun IPX).
7.2 Tricorder Results
Tricorder is run through command files which are pre-set to
analyze specific groups of minerals. The command file used in
this study included 126 materials (minerals, vegetation and ice).
Among the minerals analyzed were some of the most common
alteration minerals. All the minerals analyzed using SAM were
included in Tricorder's command file.
Tricorder also analyzes separately for multiple minerals in the
VNIR and SWIR spectral regions. Therefore, the analysis of the
whole spectrum for a single pixel may find two different
minerals, one for each spectral region, showing that the pixel
contains diagnostic features for two minerals in different
spectral regions.
164
The output from Tricorder are individual “mineral maps” for
the SWIR and VNIR respectively. Compared to results from
SAM, these maps were found to be more detailed resulting in
greater number of spectral classes relatively to mineral species.
In the SWIR region, Tricorder distinguished Na- and Ca-
montmorillonite in most of the Bodie district and in the eastern
part of Paramount, two types of kaolinite (well- and poorly-
crystalline) in the western part of Paramount, a mixture of
kaolinite*smectite in the central part of the Bodie district,
halloysite in the western part of Paramount, plus a few pixels as
Al-muscovite, mostly at Silver Hill and the center of the Bodie
district, and illite in mine tailings at Silver Hill. Dolomite was
given a good fit by Tricorder, and a fair number of pixels were
mapped as such, but these pixels were isolated and scattered all
over the image without any meaningful geological pattern. We
treated these as misclassifications and removed them from the
final result, together with a other minerals with low fits and
small number of pixels: dolomite, phlogopite, hectorite,
diaspore, clintonite, palygorskite, elbaite and nontronite.
In the VNIR, Tricorder mapped two different types of hematite.
The first one, a fine-grained variety, was mapped scattered
around hydrothermally altered areas in Bodie and Paramount
and the other, a mixture of hematite and quartz, was
distinctively associated with the alteration at Bodie Bluff, plus a
few pixels in the western part of Paramount. It also mapped
goethite in the center of the Bodie district and in a tailing pond
nearby. A few pixels scattered across the Bodie district were
mapped as K-jarosite, while some others were classified as
Fe*'-bearing minerals (mostly cummingtonite) in the center of
district and at Silver Hill.
Tricorder took around 4 hours to process the 141 MBytes scene
for 126 minerals in the VNIR and SWIR running on a medium-
performance Unix workstation (HP 9000) at the USGS
Spectroscopy Laboratory. Since then the hardware at the
Laboratory has been upgraded to more powerful workstations
and it is now taking 2 hours to process a similar scene.
7.3 Comparison of results obtained by SAM and Tricorder
A spatial comparison between the areas of major alteration
identified by both methods reveals that Na-montmorillonite was
identified by both methods at approximately the same locations
in Bodie and the eastern part of Paramount. SAM, however,
mapped a larger number of pixels as Ca-montmorillonite in
both districts, particularly at eastern Paramount, in areas which
Tricorder assigned to halloysite and both types of kaolinite.
Also, an area in the northern extreme of Paramount located at
the northern corner of the scene, which Tricorder did not map
as any mineral, was assigned by SAM to Ca-montmorillonite.
The mixture of kaolinite+smectite shows good correlation
between both methods only at the center of the Bodie district,
whereas another major area of occurrence for this mineral
mapped by SAM at the western part of Paramount was mapped
by Tricorder as both types of kaolinite and halloysite. The
separation achieved by Tricorder between Na-montmorillonite,
halloysite and poorly-crystalline kaolinite in the eastern part of
Paramount, did not appear in the SAM results.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
