The basic tool for
geologists in all disciplines is a map depicting the distribution and identity of rock
units exposed at the earths surface. Using these maps, economic geologists search
for metal and petroleum deposits; hydrogeologists look for ground water; structural
geologists classify faults as active or inactive. With its high spatial resolution, and
bands covering a wide part of the electromagnetic spectrum, ASTER will provide data that
will greatly improve geologists abilities to produce more accurate geologic maps at
a fraction of the cost of conventional ground-based methods.
The Cuprite Mining District, located in west-central
Nevada, is one of a number of alteration centers explored for precious metals.
Cambrian sedimentary rocks and Cenozoic volcanic rocks were hydrothermally
altered by acid-sulfate solutions at shallow depth in the Miocene, forming three
mappable alteration assemblages: 1) silicified rocks containing quartz and minor
alunite and kaolinite; 2) opalized rocks containing opal, alunite, and kaolinite;
3) argillized rocks containing kaolinite and hematite. A general picture of the
alteration is shown in Figure 1, combining bands 4,6, and 8 in RGB and processed
to increase the color saturation.
1. Cuprite Mining District Nevada displayed using ASTER SWIR bands
4-6-8 as a RGB composite. Area covered is 15
x 20 km.>
Red-pink areas mark mostly
opalized rocks with kaolinite and/or alunite; the white area is Stonewall Playa;
green areas are limestones; and blue-gray areas are unaltered volcanics.
Data from the SWIR region were processed to surface reflectance by EDC
and image spectra were examined for known targets at Cuprite. Evidence of SWIR
crosstalk was apparent, making the data difficult to use for spectral analysis
using direct comparisons with library or field spectra. To reduce the crosstalk
artifacts, a spectrum of Stonewall Playa was used as a bright target, resampled
to the ASTER wavelengths, and divided into the SWIR reflectance data. Library
spectra were compiled for minerals known to occur at Cuprite; they were then
resampled to ASTER SWIR wavelengths. These spectra were used with a supervised
classification algorithm, Spectral Angle Mapper, to map similar spectral
occurrences in the SWIR data. The result of this classification is shown in
Figure 2. Spectral Angle Mapper classification of
Cuprite SWIR data. blue=kaolinite; red=alunite; light green=calcite; dark green=alunite+kaolinite;
cyan=montmorillonite; purple=unaltered; yellow=silica or dickite.
this map was compared with more detailed mineral classification produced from
AVIRIS data, the correspondence was excellent. The resampled library spectra are
shown in Figure 3 compared with ASTER image spectra extracted from 3x3 pixel
Figure 3. ASTER image spectra
(left) and library spectra(right) for minerals mapped at Cuprite.