IEEE Geoscience and Remote Sensing Magazine - June 2019 - 64

TABLE 4. THE ABSORPTION FEATURE CENTER WAVELENGTH OF SELECTED REFLECTANCE SPECTRA OF THE USGS SPECTRAL
LIBRARY [27].
SWIR (2-2.5 nm)

VNIR (0.7-1.3 nm)

MINERAL

AVIRIS-NG (nm)

ASD (nm)

MINERAL

AVIRIS-NG (nm)

ASD (nm)

Alunite (AL706 Na)

2,174.46

2,175

Goethite (WS210)

947.34

949

Buddingtonite (GDS85)

2,119.37

2,115

Goethite (WS222)

947.34

949

Calcite (WS272)

2,334.74

2,335

Hematite (GDS27)

902.26

904

Chlorite (HS179.3B)

2,324.72

2,325

Hematite (WS161)

882.23

883

Iron-rich chlorite (GDS157.b)

2,344.76

2,345

Jarosite (GDS24 Na)

937.32

941

Dickite (NMNH106242)

2,174.46

2,175

Jarosite (WS368)

947.34

949

Halloysite (CM13)

2,204.52

2,205

Illite (IL101)

2,194.5

2,195

Kaolinite (CM9)

2,204.52

2,205

Montmorillonite (CM27)

2,204.52

2,205

Muscovite (GDS107)

2,204.52

2,205

Nontronite (GDS41)

2,354.78

2,355

Note: The table shows results for the original spectral resolution and when resampled to the bands of the AVIRIS-NG airborne imaging spectrometer. We used a permitted feature
depth of 1-100% over the full wavelength range, without average smoothing. The shortest and longest values in each category are indicated in bold.

TABLE 5. THE MINIMUM WAVELENGTH PRODUCTS PRODUCED AND THE PARAMETERS USED.
PRODUCT

STEP 1 SPECTRAL SUBSET (NM)

STEP 2 SPECTRAL SUBSET (NM)

STEP 2 DEPTH STRETCH (%)

SWIR overview

2,000-2,400

2,100-2,400

5-20

SWIR 2,200

2,000-2,400

2,150-2,250

5-20

SWIR 2,300

2,000-2,400

2,280-2,365

0-20

VNIR overview

750-1,300

880-1,180

2-11

features in the entire SWIR region. By choosing the step 1
subset of 2,000-2,400 nm, we stay away from the edges of
the water absorption features around 1,900 and beyond
2,500 nm. The step 2 subset is set to 2,100-2,400 nm, as
we do not expect any minima below 2,100 nm. To show
more details in the absorption feature wavelengths of individual groups, the SWIR 2,200 map is calculated with a
tighter step 2 subset to show the differences among minerals with Al-OH features around 2,200 nm, such as alunite, kaolinite, and white mica. The SWIR 2,300 map is
the equivalent for the 2,300-nm region that shows small
wavelength differences among minerals with iron/Mg-OH
groups, such as epidote and chlorite, and carbonates, such
as calcite.
In the VNIR overview product, the Wavelength Mapper
is applied to the broader electronic features in the VNIR region. To prevent band-to-band noise from disturbing the
wavelength product, a 3 × 3 × 3 mean filter (a 3D spatial-
spectral smoothing kernel) is applied prior to calculating
the minimum wavelength. The step 1 range is chosen to
contain the shoulders of the broad electronic features of
hematite, goethite, and jarosite (see Figure 1 for examples)
while the step 2 range is kept narrower to give better color
separation among the expected feature positions of the
three minerals.
64

RESULTS
The results presented here in color figures are best viewed
in digital format on a computer screen, as small differences
in hue and intensity are better preserved for the interpreter
than in a printed format.
The SWIR overview map (Figure 12) clearly shows
which parts of the study area contain minerals with absorption features in the 2,100-2,400-nm range. Alunite, with
an absorption wavelength of around 2,175 nm, displays
in cyan in the western and eastern center. Minerals with
an absorption wavelength of around 2,200 nm (e.g., white
mica and kaolinite) show in hues of green but are difficult
to differentiate from one another. They also occur in both
centers and in the talus below the outcrops that contain
these minerals. Minerals with a 2,300-nm feature (e.g.,
calcite and chlorite) are clearly apparent in colors of burgundy red, but, again, differentiations within this group
are difficult in this map. They occur in the central part of
the eastern center and in several positions in the western
center (related to original lithology and to alteration). In
the central part of the western center (see arrow), a number of pixels occur with an intermediate wavelength range
(roughly 2,270 nm; yellow hue). This wavelength is diagnostic for jarosite in the SWIR. At the western edge of the
eastern center, several outcrops containing buddingtonite
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

JUNE 2019
IEEE Geoscience and Remote Sensing Magazine - June 2019

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - June 2019
