DIVISION OF EXPLORATION AND MINING
EXPLORATION AND MINING REPORT 668F
An appraisal of the hyperspectral thermal-infrared SEBASS data recorded from
Oatman, Arizona and a comparison of their unmixed results with AVIRIS.
Robert Hewson1, Peter Hausknecht2, Thomas Cudahy1, Jon Huntington3,
Peter Mason3, John Hackwell4, John Nikitas4, and Kazuya Okada5
May 2000
OPEN FILE
Copyright  CSIRO 2000
1 CSIRO Exploration and Mining, Floreat Park, Western Australia
2 FUGRO Airborne Surveys Pty Ltd, Floreat Park, Western Australia
3 CSIRO Exploration and Mining, North Ryde, NSW, Australia
4 Aerospace Corporation, Los Angeles, USA
5 Sumitomo Metal Mining Ltd. Tokyo, Japan
CSIRO Exploration and Mining
Private Bag, PO Wembley, Western Australia 6014
CONTENTS
SUMMARY ....................................................................................................................... 1
1.0 INTRODUCTION..................................................................................................... 3
2.0 PROCESSING PROCEDURES .............................................................................. 3
2.1 SEBASS data ............................................................................................................ 3
2.2 Endmember extraction. ............................................................................................. 4
3.0 ANALYSIS OF SPATIAL NOISE WITHIN SEBASS DATA. .............................. 6
3.1 MNF transformations ................................................................................................ 6
3.2 Statistics .................................................................................................................... 7
3.3 Atmospheric spectral line features ............................................................................ 8
4.0 RESULTS .................................................................................................................... 8
4.1 Endmember spectra extracted ................................................................................... 8
4.2 Imagery of SEBASS endmembers ............................................................................ 9
5.0 DISCUSSION .......................................................................................................... 10
6.0 REFERENCES ........................................................................................................ 11
ii
FIGURES
Figure 1 Spectral resolution of six-band TIMS scanner (FWHM) compared to 128 band
SEBASS system between 7.5 and 13.5 m ...............Error! Bookmark not defined.
Figure 2 Spectral resolution and range of airborne MIRACO2LAS line profile
pectrometer compared to SEBASS. The SEBASS exhibits an improved continuous
....................................................................................Error! Bookmark not defined.
Figure 3 SPOT panchromatic of Oatman mining district with RGB AVIRIS endmembers
alunite, kaolinite and muscovite respectively ........................................................... 14
Figure 4 Observed SEBASS at (21,708) Run3 raw radiance, atmosphere corrected and
derived emissivity data.............................................................................................. 15
Figure 5 Extracted SEBASS emissivity for (21,708) Run3 showing quartz reststrahlen
feature with diagnostic 8.625 m peak. .................................................................... 15
Figure 6 MNF bands 1-5 derived from SEBASS bands 4-124 (7.628-13.438 m) for raw
data (lines 1868-2412). ............................................................................................. 16
Figure 7 MNF bands 1-5 derived from SEBASS bands 10-112 (8.0-13.0m) for raw data
(lines 1868-2412). ..................................................................................................... 17
Figure 8 MNF bands 1-5 derived from atmosphere-corrected SEBASS bands 4-124
(7.628-13.438 m) for lines 1868-2412. ................................................................... 18
Figure 9 MNF bands 1-5 derived from atmosphere-corrected SEBASS bands 10-112 (8.013.0 m) for lines 1868-2412. .................................................................................. 19
Figure 10 MNF bands 1-5 derived from derived emissivity SEBASS bands 1-124 (7.42313.438 m) for lines 1868-2412. .............................................................................. 20
Figure 11 MNF bands 1-5 derived from derived emissivity SEBASS bands 10-112 (8.013.0 m) for lines 1868-2412. .................................................................................. 21
Figure 12 Statistical mean of raw and atmosphere corrected SEBASS Run3 showing the
removal of atmospheric gas absorption line features from the raw data................... 22
Figure 13 Ratio of mean/maximum SEBASS atmosphere corrected data (Run 3) and the
theoretical Planck blackbody response (LBB) for temperature of 297 K. ................ 22
Figure 14 Comparison of statistics for raw and atmosphere corrected SEBASS data
(Run3) ....................................................................................................................... 23
Figure 15 Ratio of standard deviation to mean for raw, atmosphere corrected and derived
emissivity SEBASS Run3 data (Run 3). ................................................................... 23
Figure 16 Transmittance of H2O, CO2 and O3 at 1cm-1 resolution (MODTRAN) and
resampled to SEBASS wavelengths for 7.5-10.5 m. .............................................. 24
Figure 17 Transmittance of H2O, CO2 and O3 at 1cm-1 resolution (MODTRAN) and
resampled to SEBASS wavelengths for 10.5-13.5 m. ............................................ 25
Figure 18 Transmittance of H2O, CO2 and O3 at 1cm-1 resolution (MODTRAN) and
resamapled to SEBASS bands for 8.0-13.0m wavelengths. The presence of water
vapour shown in the raw SEBASS data is still apparent within the emissivity data at
11.6 m and 12.6 m, although most water vapour spectral line features have been
removed by the atmosphere correction. .................................................................... 26
iii
Figure 19 Comparison of atmospheric transmittances, and mean and standard deviations
of raw, atmosphere corrected and emissivity converted SEBASS Run3. ................. 27
Figure 20 SEBASS quartz, alunite and kaolinite endmember spectra ............................. 28
Figure 21 SEBASS carbonate endmember spectra .......................................................... 28
Figure 22 SEBASS phyllosilicate endmember spectra ..................................................... 29
Figure 23 SEBASS quartz, and quartz-like endmember spectra ..................................... 29
Figure 24 Comparison of SEBASS and AVIRIS imagery for calcite endmembers, Run 3
................................................................................................................................... 30
Figure 25 Comparison of SEBASS and AVIRIS imagery for calcite endmembers, Run 1
................................................................................................................................... 31
Figure 26 Comparison of SEBASS and AVIRIS imagery for alunite, kaolinite and
muscovite .................................................................................................................. 32
Figure 27 RGB colour composite of SEBASS quartz, kaolinite and alunite endmembers
................................................................................................................................... 33
Figure 28 RGB colour composite of SEBASS “quartz A”, quartz and kaolinite
endmembers .............................................................................................................. 34
Figure 29 SEBASS fine quartz, calcite endmember imagery and the comparison with
field emissivity spectral measurements of the Oatman Mine tailings....................... 35
iv
Summary
This report summarises the results of work carried out on the SEBASS data collected
over the Oatman mining district, Arizona in June 1998 and presented at the Thirteenth
ERIM Applied Geological Remote Sensing Conference, Vancouver, 1-3 March, 1999
(attached Abstract). This research follows a preliminary study of SEBASS data from
Cuprite (Huntington, et al., pers.com.). The current study at Oatman found processed
SEBASS spectral signatures that could be directly related to the laboratory TIR spectra of
known mineralogy. In addition, AVIRIS data over the same SEBASS survey areas were
used to study the complementary or supplementary potential of hyperspectral thermalinfrared data with more commonly used hyperspectral visible-shortwave infrared systems.
The processing of the Oatman SEBASS data encountered “stripping” noise effects similar
to those observed within the Cuprite data (Huntington, et al., pers. comm.). This appears
to be a result of a slight misalignment of the SEBASS detector array. This report
summarises the processing procedures applied to the SEBASS data, the effects of
instrumental noise and uncorrected atmospheric features, the results of spectral unmixing,
and a comparison with mineral endmembers derived from AVIRIS data over areas of
hydrothermal alteration at Oatman.
The objectives of this study were to :

Evaluate SEBASS for the mapping of alteration minerals at thermal-infrared
wavelengths; and

Investigate the complementary nature of hyperspectral thermal-infrared remote
sensing to traditional shortwave-infrared remote sensing techniques.
1
* Supplementary mineral mapping of an epithermal alteration system,
Oatman Arizona, using hyperspectral thermal-infrared data.
Robert Hewson1, Peter Hausknecht2, Thomas Cudahy1, Jon Huntington3,
Peter Mason3, John Hackwell4, John Nikitas4, Kazuya Okada5
1 CSIRO Exploration and Mining, Floreat Park, Western Australia
2 World Geoscience Corporation, Floreat Park, Western Australia
3 CSIRO Exploration and Mining, North Ryde, NSW, Australia
4 Aerospace Corporation, Los Angeles, USA
5 Sumitomo Metal Mining. Tokyo, Japan
ABSTRACT
Hyperspectral thermal-infrared (TIR) SEBASS (Spectrally Enhanced Broadband Array Spectrograph
System) image data collected from an area of epithermal alteration over the Oatman Mining District,
Arizona were evaluated for their ability to identify and map surface mineralogy.
From laboratory studies it is well known that the TIR has the potential to provide complementary
mineralogical information to that available at shorter wavelengths, especially for a range of Si-O bearing
minerals. In this environment, opaline silica is a critical epithermal alteration mineral.
This study examines the ability of SEBASS image and spectral data to map alteration and host rock
mineralogies. A number of these minerals can also be detected efficiently using remote sensing systems that
operate at shorter wavelength, such as AVIRIS. This study, therefore, also investigates the similarity and
complementary nature of the results from the TIR and the shortwave infrared wavelength region.
The airborne SEBASS system, developed by Aerospace Corporation comprises a two-dimensional
pushbroom spectrometer imaging 128 detectors across track and 128 contiguous spectral bands between 7.6
and 13.5 m. The pixel size of the Oatman SEBASS data was approximately 2.5 metres. This data
acquisition was unfortunately flown under non-ideal conditions. These conditions would theoretically make
detection of the diagnostic mineral emissivity features difficult, as significant down welling atmospheric
radiance was present. Despite the less than ideal conditions, we found a range of mineral-specific spectral
signatures that were used to generate a series of mineral abundance maps consistent with the known
geology. Areas of epithermal alteration characterised by silicification, kaolinite and alunite were
delineated. In addition, two types of carbonates were identified from the SEBASS data, based on the
narrow reststrahlen bands centred at 11.3 and 11.4 um. Other SEBASS spectral signatures appear to be
related to chlorite/biotite and muscovite/clay. These preliminary results, from data collected under nonideal conditions, show the exciting potential the TIR offers for enhanced mineralogical mapping.
* As presented at the Thirteenth ERIM Applied Geological Remote Sensing Conference,
Vancouver, 1-3 March, 1999.
2
1.0 Introduction
A series of four areas were surveyed for the CSIRO and World Geoscience Corporation
by the SEBASS (Spectrally Enhanced Broadband Array Spectrograph System) thermalinfrared push broom scanner developed by Aerospace Corporation. The SEBASS
scanner consists of an array of 128 x 128 detectors recording thermal data from 7.4 to
13.4 m for a 128 pixel-wide swath at an instantaneous field of view (IFOV) of 1
milliradian. The Oatman survey was recorded at an average altitude of 2500 metres,
resulting in an approximate across-swath pixel dimension of 2.5 m. Helium cooled Si:As
detectors are credited with an impressive +/- 0.05 degree Ko temperature precision.
The comparative resolution of SEBASS versus TIMS (Kahle and Goetz, 1983) and
MIRACO2LAS (Whitbourn, et al., 1994) are shown in Figures 1 and 2, indicating the
superior spectral resolution and wavelength range of the SEBASS instrument. Note that
the spectral resolution of the MIRACO2LAS refers to the band intervals between
consecutive channels.
The area studied at Oatman, Arizona encompasses an epithermal-altered Tertiary volcanic
complex containing areas of pervasive argillic, propylitic and silicic alteration (Clifton et
al., 1980). Gold mining is actively carried out within the SEBASS survey area at Oatman
and at the Gold Road Mine. The orebodies are typically low sulphur, epithermal quartzcalcite (and possibly adularia) lode deposits (Clifton et al., 1980). They occupy dilatant
fault zones containing fissure fillings to complex stockworks with quartz calcite veins
ranging from several centimetres to 5 metres wide.
Figure 3 shows a SPOT panchromatic image with the flight passes of SEBASS Runs 1
and 3 overlayed. Figure 3 also shows the results of partial unmixing AVIRIS data into a
compositional mineral map of alunite, kaolinite and muscovite, highlighting areas of
epithermal alteration within the Oatman mining district (Boardman and Huntington,
1997). Note that the 2.5 m pixel resolution of the SEBASS data produces an image with
a swath-width of only 300 metres, preventing significant overlap with the AVIRIS data
for any of the flightlines. Comparison between the AVIRIS and SEBASS data must
allow for the 20 m pixel resolution of the AVIRIS data, nearly an order of magnitude
coarser in spatial resolution than the SEBASS data.
2.0 Processing procedures
2.1 SEBASS data
Aerospace Corporation provided SEBASS data for each survey run as raw radiance-atsensor and atmospherically corrected radiance-at-surface datasets. In addition, emissivity
values were provided for Run 3. Figure 4 shows an example of the SEBASS responses at
3
image coordinate pixel 21, line 708 of Run 3 for the raw, atmosphere corrected and
emissivity datasets.
Atmospheric gas and water vapour line features (Section 3.3) are apparent as narrow
spectral features in the raw SEBASS data in Figure 4. The atmosphere correction
algorithm described by Johnson (1998) and Young (1998) was used by Aerospace to
effectively remove the atmospheric line features in this example (Figure 4). The overall
shape of the atmosphere corrected data, mimics approximately a Planck blackbody curve
as shown in Figure 4, assuming a temperature of 297o Kelvin. In order to separate the
emissivity information from the temperature of each pixel Aerospace fitted a smooth
Planck-like function to the atmosphere corrected data. The ratio of the atmosphere
corrected data to the fitted, smooth Planck curve produced apparent emissivity spectra as
shown in Figures 4 and 5. Although the emissivity information from is contaminated by
instrumental and atmospheric noise (Section 3.0), spectral signatures of quartz, clay
mineral and carbonate minerals were observed using a simple wavelength profile search
within the data. SEBASS clearly distinguished the diagnostic 8.625 m quartz
reststrahlen feature (Salisbury et al., 1992), indicating that the wavelength calibration of
the SEBASS data was of a high accuracy (Figure 5).
2.2 Endmember extraction.
Data reduction of the SEBASS data was deemed necessary to remove redundancy
produced by the sometimes highly correlated 128 SEBASS channels. The process was
performed by the Maximum Noise Fraction ( MNF) transformation that ordered the
multi-band thermal data in terms of signal to noise quality (Green et al., 1988). A shift
difference operation was used to estimate noise (ENVI vers. 3.2 software) within the
SEBASS data for the MNF transformation. Initial MNF processing of the entire
SEBASS emissivity data in the 7.4-13.4 m wavelength range encountered significant
problems shown as noisy striping features within the image data (Section 3.1). Improved
signal to noise results were obtained by limiting the MNF processing to the 103 of the
original 128 SEBASS bands that measured the 8-13 m wavelength region. Further
reduction and separation of the effects of noise was enabled by identifying noisy spectra
and excluding those endmembers from the analysis of the Pixel Purity Index (PPI)
(Boardman and Kruse, 1994). The PPI process effectively performed a spatial
compression, limiting the number of pixels required to determine the “spectrally pure” (or
extreme) pixels within the image data. These spectrally pure pixels represent the various
surface components or “endmembers”of the survey area. Mineral endmembers were
highlighted using n-D visualisation where the extreme pixels are displayed as a multidimensional scatterplot. The mineral endmembers were extracted by collapsing the
obvious “noisy endmembers”, indicated by atmospheric line spectral features and
uncorrected blackbody behaviour/temperature effects.
Processing of the SEBASS data was further split and processed into three wavelength
regions: 8.0-9.5 m; 9.0-10.5 m and 10.5-12.0 m. MNFs were calculated for each
4
region and those with apparent geological meaningful information were selected for PPI
processing. The first MNF bands(s) were found to typically contain noisy “nongeological” anomalies. Examination of the MNF bands for each wavelength region
revealed that there were 13 of the 26 MNF bands useful in the 8-9.5 m region, 11 from
the 28 MNF bands in the 9.0-10.5 m region and 6 from the 23 MNF bands in the 10.512.0 m region. The resulting mineral endmembers from each wavelength region
showed some redundancy, particularly for clay minerals within the first two wavelength
regions. However, minerals with narrow spectral features, such as alunite at
approximately 8.8 m and calcite at approximately 11.3 m, were more reliably
identified by examining the MNF and PPI processed data from the relevant wavelength
regions. (Section 4).
SEBASS Run1 atmosphere corrected data were also processed into endmembers using
MNFs and PPI despite no emissivity extraction. A hull-removal process (Green and
Craig, 1985) was used to approximate “emissivity” type spectra and proved successful in
identifying quartz, alunite, calcite and kaolinite spectral features (but not absolute values)
similar to those observed in the emissivity-corrected Run 3 data. The results indicate that
the main endmembers from both processing approaches show reasonable correspondence,
despite the variability of the temperature and background blackbody effects on the
radiance data over the survey area.
Partial unmixing was achieved by matched filtering unmixing and also to a lesser degree
by mixture tuned matched filtering (ENVI version 3.2 User’s Guide, 1999). Confidence
in the anomalies within the resulting unmixed endmember images was obtained by
comparing their spectral features with mineral libaray spectra using the wavelength
profile. Added confidence in the interpretation of these anomalies was obtained by
examining the mixture tuned matched filtering infeasibility score which identified those
pixels with incorrect matches to the endmember spectra. However, the choice of
threshold/cutoffs and the stretch applied to the grey scale or RGB colour composites of
endmember images remains a problematic issue. The images presented here are at best,
semi-quantitative and highlight the “highest estimated” abundances only of the mineral
endmembers (Section 4.2).
Imagery of SEBASS endmembers were exceedingly difficult to geometrically register
using traditional techniques because of the limited approximate 300 metre swath view,
even with 1:24,000 topographic maps. However, reliable locating of the SEBASS
imagery was achieved using mapped flight-line coordinate information and several
ground features identified along the flightline.
5
3.0 Analysis of spatial noise within SEBASS data.
3.1 MNF transformations
An examination of the spatial distribution of “noise” within the SEBASS data was
undertaken by examining the MNF bands of Run 3 as raw, atmosphere corrected and
derived emissivity datasets, processed for its entire wavelength range, and also for those
bands within the 8-13 m wavelength region (Figures 6-11). This spatial noise took the
form of striping or vertical banding parrallel to the flightline without any obvious
geological origin. SEBASS bands 1-3 (7.422 – 7.560 m) and bands 126-128 (13.5 –
13.8 m) were omitted from the above analysis due to their poor signal to noise
characteristics. Bands1-3 and 125 -128, occur at the edges of the atmospheric window
for the propagation of thermal infrared energy, increasing the influence of systematic
instrumental and non-systematic atmospheric noise.
Figures 6 and 7 show the images of the first MNF bands processed from the raw SEBASS
data for the 7.628-13.438 m and 8.0-13.0 m wavelength ranges respectively. An
apparent distortion along the eastern boundary of the imagery can be observed in several
of the MNF bands of the original data. A dark area shown at the upper end of MNF band
2 (Figure 6) also appears within other MNFs although it is not aparent within the original
band data. Detailed examination of spectra within this feature at Pixel, Line coordinates:
44, 2003 (Figure 18, Section 3.3) suggest this anomaly is a cloud. Note that the raw data
limited between 8.0 and 13.0 m (Figure 7) show the first mineralogical features (mine
tailings drainage containing quartz and calcite) by MNF 5 and clear geological boundaries
by MNF 8. MNFs of the unrestricted raw data (Figure 6), however, do not show such
features until MNF 8, and no clear boundaries of these features until MNF 9.
MNFs of the atmospheric corrected data in Figures 8 and 9, reveal an eight-pixel striping
artefact introduced by the atmospheric correction procedure. Restricting the wavelength
range to 8-13 m again improved the definition of the geological boundaries. The
improvement in geological information over residual atmospheric effects by restricting
the MNF processing to the 8 to 13 m region can be explained by the reduced water
vapour and CO2 gas transmittance for wavelengths less than 8 m and greater than 13 m
respectively (Figures 16 and 17, Section 3.3). Outside the 8-13 m region a higher
proportion of ground emitted radiation is absorbed (and potentially re-emitted as
upwelling radiation) by the intervening water and CO2 in the atmosphere. Also, in
accordance with Wien’s Displacement Law, thermal radiation measured by SEBASS will
tend to detect an overall lower radiation from the emitting ground outside 8-13 m for
most land surface temperatures relative to the upwelling atmospheric radiation. For
example, assuming Wien’s Displacement Law the highest emitted radiation will occur at
9.9 m for a ground surface at 20o C.
6
The atmospheric correction used by Aerospace involves the calculation of column
averages of eight pixels wide and is used to assist the removal of the broader
“instrumental” column stripping effect, revealed by the raw SEBASS data. The presence
of clouds and artefacts introduced by the atmosphere correction process are still present in
the MNF bands from derived emissivity data (Figures 10 and 11), however, clear
geological boundaries can be observed, particularly in the restricted 8-13 m processed
SEBASS data (Figure 11).
3.2 Statistics
The mean for the raw and atmosphere-corrected SEBASS Run 3 are shown in Figure 12.
The atmosphere gas and water vapour features appear to be removed by the atmospheric
correction, leaving a blackbody function-like curve. A Planck blackbody function,
approximately matching the mean atmospheric corrected data, was determined assuming
Wien’s Displacement Law to calculate an average temperature of 297 K from a peak
radiance at 9.75 m. The ratios of the mean SEBASS data and calculated Planck
function to their maximums are shown in Figure 13. The fit of the Planck function to the
mean SEBASS data is closest within the 8-11 m range, however, divergance occurs at
longer wavelengths caused possibly by uncorrected H2O continuum absorption which is
not accommodated in the Aerospace data reduction. Figure 13 also reveals that the effect
of geological emissivity spectra within the entire length of the SEBASS data is subtle and
the overall signal is dominated by blackbody behaviour.
Figure 14 shows the standard deviation of the SEBASS Run 3 data and indicates that the
atmospheric correction did not alter the gross variation within the SEBASS data and
removed the atmospheric line features within the 8-13 m region. The relationship of
this variation to the mean is shown in Figure 15 indicating that the signal variation due to
noise and also geological signatures decreases with wavelength. A possible explanation
for this decrease in Stdev/Mean (Figure 15), and for the increasing discrepancy between
the SEBASS mean and the fitted Blackbody curve (Figure 13) at increasing wavelengths,
is possibly the reduced quartz and clay thermal infrared spectral reflectances at longer
wavelengths. A decrease in the reflectances of these major occuring minerals would
result in a decrease in sky/downwelling radiance reflected from the surface. Therefore
reflected downwelling radiance from quartz and clay minerals would decrease at longer
thermal infrared wavelengths relative to the emiited ground radiance. Alternatively the
decrease in Stdev/Mean may be caused by a change in the detector response sensitivity
with longer wavelengths. The discrepancy between the SEBASS mean and the fitted
Blackbody curve could also be the result of the effects of atmosheric continuum
absorption (i.e. water vapour and CO2) due to the height of the survey (approximately
2500 metres). Further study would be useful to understand the results of these standard
statistical outputs as an aid for quality control measures of the SEBASS data and
observing the effects of the atmosphere.
7
3.3 Atmospheric spectral line features
The source of sharp narrow spectral features in the raw SEBASS data are the gas and
water vapour emission lines still present within the 8-13 m atmospheric window.
MODTRAN output at 1 cm-1 resolution (Davies, pers. comm.) of gas transmittance
reveals the abundance of such narrow but significant spectral features for water vapour,
and ozone and carbon dioxide (Figures 16 and 17). Resampling these emission lines to
SEBASS bands degrades or removes the narrowest spectral features. However,
significant spectral features remain when two or more emission lines are convolved into
broader features, particularly for water vapour (Figures 16 and 17).
Figure 18 compares the SEBASS resampled gas emission lines with the raw SEBASS
and emissivity data at an area within the cloud highlighted by the MNFs (i.e. MNF 1,
Figure 7; MNF 1, Figure 11). Several water vapour emission line features present within
the raw data are still apparent within the emissivity data (eg. 11.6 m, 12.6 m). A
comparison of the standard deviations of the raw SEBASS data and the derived
emissivity with the water vapour transmittance data, reveals a similar correlation of
spectral features, particularly for wavelengths greater than 10.5 m (Figure 19). However
the variation displayed by the SEBASS emissivity at shorter wavelengths than 10.5 m
suggests that geological causes have a greater effect. The standard deviation of the Run 3
emissivity shown in Figure 19 confirms this geological influence, by the appearance of
significant changes in the emissivity variation at the diagnostic 8.625 m quartz feature,
and the 9.0 m and 9.9 m kaolinite spectral features.
4.0 Results
4.1 Endmember spectra extracted
A total of twelve endmember spectra were extracted using n-D visualisation of PPI results
(Boardman and Kruse, 1994) from the Run 3 SEBASS emissivity data using the three
different wavelength regions. Quartz, alunite and kaolinite spectra were extracted from
both the 8-9.5 m and 9-10.5 m wavelength regions (Figure 20). The John Hopkins
University spectral library for thermal-infrared wavelengths (Salisbury et al., 1992, ENVI
version 3.2) was used to interpret these and other endmembers.
Three different carbonates were distinguished from the processing of the 10.5-12 m
wavelength region (Figure 21). Previous processing of the entire SEBASS wavelength
region between 8-13.0 m failed to enable the extraction of “carbonate 2”. However,
restricting the processing to the the 10.5-12 m wavelength region, highlighted the
carbonate spectral features between 11.0-11.5 m. The emissivity feature at 11.4 m
suggests that carbonate 2 is possibly aragonite, a variation of calcite commonly associated
with carbonate shells and vugs/vesicles of volcanic igneous units. However, no field
observations/measurements have been conducted and no interpretation of the AVIRIS
8
data has confirmed the presence of aragonite. The presence of fine grained calcite and
quartz was indicated by the pronounced volume scattering quartz spectral feature between
10-12.5 m and the inverted emissivity calcite feature at 11.4 m (Hunt and Vincent,
1968; Salisbury and Wald, 1992). Laboratory measurements by Hunt and Vincent (1968)
indicated that the 11.3 m calcite feature inverts from a reflectance peak (emissivity
trough) to a “type 2” reflectance low (emissivity peak), slightly shifted to longer
wavelengths for grain sizes finer than 74 m. Research by Salisbury and Wald (1992)
observed the presence of volume scattering effects for quartz and calcite textures finer
than 75 m by the broad and low emissivity spectral features between 10-12.5 m, and
also the presence of very fine calcite by the inverted carbonate absorption band near 11.3
m. Emissivity measurements with a portable FTIR spectrometer (Design and Prototypes
micro FTIR Model 101) of a very fine calcite and quartz rich sample from the Oatman
Mine tailings showed similar spectral volume scattering features observed within the
SEBASS data (Figure 29).
Four additional endmembers, broadly classified as phyllosillicates, were also extracted
from the SEBASS data using the 8-9.5 m and 9-10.5 m regions (Figure 22). The
emissivity peak at 9.4-9.5 m is suggestive of muscovite although it appears likely that
these endmembers are a mixture of various silicate minerals, producing a broad spectral
feature between 8.5-10.5 m.
Two distinct variations of the quartz extracted from the 8-9.5 m region (Figure 23) have
clearly defined geological/geomorphological boundaries within the SEBASS images
(Figure 28). Previous research by Salisbury and Wald (1992) noted that the presence of
clay particles clinging to quartz grains appears to modify the quartz reststrahlen feature,
reducing the shorter wavelength 8-8.6 m lobe. An intimate mixture of clay and quartz
may explain these variants on the quartz endmember spectra although further laboratory
and field work is required to confirm this interpretation.
4.2 Imagery of SEBASS endmembers
AVIRIS endmember abundance anomalies draped over SPOT panchromatic imagery
were compared with the corresponding SEBASS endmember imagery. The calcite
endmember derived from SEBASS data compare favourably with the AVIRIS-derived
calcite anomalies in several locations (Figures 24 and 25). The SEBASS discrimination
of veining is also superior to the AVIRIS because of its higher spatial resolution (2.5 m vs
20 m). The SEBASS data also have the added ability to map quartz veining in detail
either as separate outcropping veins (Figure 24-blue) or mixed intimately with the calcite
(Figure 25-magenta). Figure 24 also highlighted the intimate association of carbonate 2
with calcite as the yellow areas within the central core of the calcite veining.
SEBASS endmember images of kaolinite and, to a lesser extent, muscovite
(“phyllosillicate 4”) compare reasonably with the AVIRIS equivalent endmembers for
some areas of alteration at Oatman (Figure 26). Comparisons for the alunite endmember
9
abundances between the two techniques show there are problems possibly with the
interpretation of the SEBASS data although the limited swath of the SEBASS and
uncertainty of its location make direct comparisons difficult. SEBASS mineral map
results for quartz and kaolinite show some intimate mixing of areas of silicification and
possible alteration kaolinite (Run 3-yellow areas, Figure 27) although weathering and
sediment derived clays are also possible sources. RGB composite image combinations of
kaolinite, quartz, and the interpreted quartz variation, “quartz A”, show interesting results
with some intimate mixing between quartz and “quartz A” and other areas where there
are distinct and separate occurrences (Figure 28). “Quartz A” appears to be either
associated with alluvial channel deposits or as isolated occurrences within kaolinite-rich
areas (possible alteration) shown within the lower half of Run 1 (Figure 28). This
observation suggests that “quartz A” is a distinctly separate endmember, identified by a
significant variation of the primary quartz reststrahlen feature. Laboratory observations
by Salisbury and Wald (1992) suggest that this endmember may consist of quartz grains
coated in kaolinite, producing a quartz reststrahlen spectral signature with a reduced
shorter wavelength feature.
The SEBASS endmember interpreted as fine quartz/calcite is only mapped in the area of
tailings outflowing from Goldroad Mine along Silver Creek (Figure 29). The location of
this material is clearly identified in the SPOT panchromatic imagery downstream from
the Goldroad Mine (Figure 29). Figure 29 also shows a close comparison between the
SEBASS endmember spectra and the field spectrometer “microFTIR” measurement for a
similar tailings sample from the Oatman Mine. XRD analysis confirmed the quartz and
calcite composition of the measured Oatman tailings sample. Grain size analysis of the
tailings sample indicated the whole sample was finer than 75 m. Such a fine grain
texture is consistent with the laboratory results of Hunt and Vincent (1968), and the
above-mentioned type 2 calcite spectral features and volume scattering behaviour from
quartz.
5.0 Discussion
This initial evaluation of SEBASS hyperspectral thermal-infrared imagery shows that it
can successively map areas of silicification, quartz-calcite veining, kaolinite-rich areas of
alteration and/or weathering, and possibly alunite alteration. In the Oatman survey area
other phyllosillicates, including muscovite are delineated although specific identification
appeared ambiguous. SEBASS also highlighted the ability of hyperspectral thermalinfrared data to discriminate different carbonates and grain size effects. Mapping of
degraded quartz reststrahlen signatures with SEBASS also appears to provide new
information related to quartz-clay mixing and encourages further investigation.
These results indicate there is a useful synergy between hyperspectral shortwave and
thermal-infrared systems such as AVIRIS and SEBASS. Kaolinite and calcite mineral
maps developed from both systems correlated well with each other in the study area at
Oatman. SEBASS also provided additional quartz and carbonate information, highly
10
relevant in the mapping of quartz-calcite veining and its possible association with
epithermal lode deposits at Oatman. Further studies examining the
complementary/supplementary relationship between these two systems over different
geological terrains and alteration types, would be of great benefit.
The results from SEBASS to date are impressive, however, some instrumentation
misalignment/“noise”, processing artefacts and some residual water vapour line features
are still apparent in the data. Further work with SEBASS data would benefit from an
increased examination of these instrumentation and atmospheric effects and the
algorithms used for their removal, and the separation of temperature-emissivity
information.
A study of the effects of downwelling radiance as a residual component to the atmosphere
line-corrected radiance data would help to reduce possible errors in the separation of
temperature and emissivity information in SEBASS datasets.
6.0 References
Boardman, J. W. and Kruse, F.A., (1994). Automated spectral analysis: a geological
example using AVIRIS data, North Grapevine Mountains, Nevada. Proc. Tenth
Thematic Conference Geologic Remote Sensing, San Antonio, Texas, 9-12th May,
vol. I, pp. 407-418.
Boardman, J.W. and Huntington, J.F., (1997). Data analysis and product simulation
workshop manual. CSIRO Exploration and Mining Report 369R, AMIRA Project P465
Clifton, C.G., Buchanan, L.J. and Durning, W.P., (1980). Exploration procedure and
controls of mineralisation in the Oatman Mining District, Oatman, Arizona. New York,
Soc of Mining Engineers of AIME, Reprint, 80-143.
ENVI version 3.2 User’s Guide, (1999). Research Systems Inc., Boulder, Colorado.
Green, A.A, Berman, M., Switzer, P. and Craig, M.D., (1988). A transformation for
ordering multispectral data in terms of image quality with implications for noise
removal. IEEE Trans. Geoscience Remote Sens., v. 26, n. 1, pp. 65-74.
Green, A.A and Craig, M.D., (1985). Analysis of aircraft spectrometer data with
logarithmic residuals. JPL Publ. 85-41, pp. 111-119.
Hunt, G.R. and Vincent, R.K., (1968), The behaviour of spectral features in the
infrared emission from particulate surfaces of various grain sizes. J. Geophysical
Research, v 73, No 18, pp. 6039-6046.
11
Johnson, B.R., (1998). Inscene atmospheric Compensation-Apllication to SEBASS
data collected at the ARM Site. Part I. Aerospace Report No. ATR-99 (8407)-1
Kahle A.B. and Goetz, A.F.H., (1983). Mineralogic information from a new airborne
thermal infrared multispectral scanner. Science, v. 222, pp. 24-27.
Salisbury, J.W. and Wald, A., (1992). The role of volume scattering in reducing
spectral contrast of Reststrahlen Bands in spectra of powdered minerals. Icarus, v.
96, pp. 121-128.
Salisbury, J.W., Walter, L.S., Vergo, N. and D’Aria, D.M., (1992). Mid-infrared (2.125 um) spectra of minerals. John Hopkins University Press, Baltimore.
Whitbourn L.B., Hausknecht P., Huntington J.F., Connor P.M., Cudahy T.J. and
Phillips R.N., (1994). Airborne CO2 laser remote sensing system. In Proceedings of
the 1st International Airborne Remote Sensing Conference and Exhibition:
Applications, Technology and Science, Strasbourg, France, pp. II-94 - II-103, 12-15
September 1994.
Young, S.J., (1998). Inscene atmospheric Compensation-Apllication to SEBASS data
collected at the ARM Site. Part II. Aerospace Report No. ATR-99 (8407)-1.
12