This paper describes an empirical method to correct TIMS (Thermal Infrared Multispectral Scanner) data for atmospheric effects by transferring calibration from a laboratory thermal emission spectrometer to the TIMS multispectral image. The method does so by comparing the laboratory spectra of samples gathered in the field with TIMS 6-point spectra for pixels at the location of field sampling sites. The transference of calibration also makes it possible to use spectra from the laboratory as endmembers in unmixing studies of TIMS data

Anderson, Donald L.

Edgett, Kenneth S.

English

NASA Technical Reports Server

N95- 33792
FOR GEOLOGIC INVESTIGATIONS WITH AIRBORNE THERMAL
INFRARED MULTISPECTRAL IMAGES: TRANSFER OF CALIBRATION
FROM LABORATORY SPECTROMETER TO TIMS AS ALTERNATIVE FOR
REMOVING ATMOSPHERIC EFFECTS
Kenneth S. Edgett and Donald L. Anderson
Department of Geology, Arizona State University
Box 871404, Tempe, Arizona, 85287-1404, U.S.A.
I. INTRODUCTION
This paper describes an empirical method to correct TIMS data for atmospheric
of feeLs by transferring calibration from a laboratory thermal emission spectrometer to the
TIMS multispcctral image. The method does so by comparing the laboratory spectra of
samples gathered in the field with TIMS 6-point spectra for pixels at the location of field
sampling sites. The transference of calibration also m_es it possible to use spectra from
the laboratory as endmembers in unmixing studies of TIMS data.
2. THE PROBLEM
The emissivity and spectral unmixing study described in this volume by Edgctt
et al. (1995) was designed to use a TIMS image (obtained September 1991) to address
the geology of a dune field in Christmas Lake Valley, Oregon. In that study, accurate
determination of surface temperature was not required. However, Edgett et al. (1995)
desired to obtain accurate estimates of surface emissivity, a property directly related to
the comIx_sition of surface materials. To obtain surface emissivity, the effects of
atmospheric contributions (including H20 vapor in TIMS band 1 and 03 in band 4) to
d_eTIMS spectrum had to be removed.
Atmospheric correction for TIMS images usually involves the use of a radiative
transfer code such as the Lowtran 6 or Lowtran 7 atmospheric models developed for the
U.S. Air Force (e.g., see Hook et al., 1994). The Lowtmn code uses an input or modeled
atmospheric profile. An input profile might include pressure, temperature, and humidity
as a function of altitude measured at the time of the TIMS overflight. In the case of most
TIMS flights (including that of September 1991 over Christmas Lake Valley), these
parameters are usually not measured and a model atmosphere must be assumed. The
best model atmosphere that can be assumed by Lowtran for a mid-September overllight
in Oregon is for "North American mid-latitude Summer." However, it was found in the
Edgctt et al. (1995) study that the model atmosphere over-predicted the humidity in arid
Christmas Lake Valley, Oregon, resulting in radiances for TIMS band 1 that were
inordinately higher than in the other five bands. The artificially high radiance in band 1
off-set d_e emissivities calculated (using the method of Realmuto (1990)) for all six
bands, Ix-cause the surface kinetic temperature is assumed to be the same as that detected
by the band with the highest radiance. The main problem that resulted was that band 1
emissivities were exceptionally high relative to the other TIMS bands and to what was
observed in laboratory spectra of samples collected from sand dunes located in the TIMS
scene (e.g.. Figure 1). The problem was the same for spectra of water and vegetation,
which should have had relatively flat spectra.
9
https://ntrs.nasa.gov/search.jsp?R=19950027371 2020-06-16T06:58:53+00:00Z
Emissivity
_ 1.0
0.90
0.80 / 1.0
0.90,
0.80 / 1.0
0.90
0.80
B
3 4
C
' ' ' ' I ' " ' ' I ' ' ' ' i " ' ' '
8 9 10 11
Wavelength (micrometers'
2
Figure 1. Comparison of six-
point emissivity spectra of sand
near the crest of one of the
dunes in the Shifting Sand
Dunes field of Christmas Lake
Valley, Oregon (see Edgett et
al., 1995). Band 1 through 6 of
a TIMS image obtained in
September 1991 are indicated.
(A) Spectrum from TIMS image
using Lowtran atmospheric
correction. (B) Spectrum from
TIMS image after empirical
correction as described here.
(C) Laboratory specuum
convolved to TIMS spectral
resolution. Note the
inordinately high emissivity in
band 1 for the Lowtran-
corrected spectrum.
3. PROPOSED SOLUTION
When the relative humidity is not known, the atmosphere correction for water
vapor can be solved by assuming a different humidity, as suggested by Hook et al.
(1994). Alternatively, atmospheric effects can be corrected empirically by calibrating
the TIMS image to a known surface spectrum, such as a body of water, vegetation, or the
spectrum of material collected from the field area. For example, Ondrusek et al. (1993)
used snow in a TIMS scene to calibrate an image. This approach precludes the need for
a radiative transfer code, and therefore might not be easily applied to all thermal infrared
images of remote or inaccessible areas, particularly if seen from very high altitudes.
However, this alternative approach was used because of (1) the emphasis on surface
geology and (2) the difficulty in application of the Lowtran atmospheric model to the
Christmas Lake Valley TIMS image of Edgett et al. (1995).
Conceptually, the approach followed here is a model for transfer of calibration
from the laboratory spectrometer to the TIMS. More sophisticated examples of
laboratory spectrometer calibration transfer models are found in papers by Adhihetty et
al. (1991 ) and Wang and Kowalski (1992). The philosophical j usti fication for th is
approach is that the atmospheric column during acquisition of laboratory spectra has a
very short path length (< 1 m) relative to that of the TIMS in an aircraft (> 3,000 m), and
water vapor (humidity) in the laboratory setting is minimal.
Transference of calibration from the laboratory spectrometer data to the T1MS
emissivity image requires two main steps. The first is to obtain laboratory spectra of
materials that occur within the scene and convolve to TIMS six-band resolution. The
second step is to calibrate the TIMS normalized emittance image using the average of
several (in this case, 10) of the laboratory spectra and then check the calibration transfer
using several other laboratory spectra (here, another 10 were used). The steps followed
are described in more detail below.
10
4. SPECIFIC STEPS
4.1 Laboratory Spectra of Christmas Lake Valley Samples
Laboratory thermal emission spectra of sand samples from the Shifting Sand
Dunes of Christmas Lake Valley, Oregon, were obtained, convolved to TIMS six-band
resolution, and used in the empirical correction for atmospheric effects (see section 4.2,
below). The configuration and calibration of the laboratory thermal emission
spectrometer at Arizona State University was described by Christensen and Harrison
(1993) and in a manuscript presently being prepared by S.W. Ruff, P.W. Barbera,
P.R. Christensen, and D.L. Anderson. Laboratory thermal emission spectra were
acquired for this study using a commercial fast Fourier transform infrared spectrometer
(/vlattson Cygnus, modified for emission measurements) with a spectral resolution of
4 cm -1. Atmospheric contributions to spectra from water and CO2 are minimized by
using a dry nitrogen purge and cold finger to trap excess water. Emission spectra were
obtained according to "method 1" of Christensen and Harrison (1993).
Laboratory spectra were collected for 25 bulk sediment samples from the
Shifting Sand Dunes. The samples were subsequently impregnated with epoxy and cut
for thin section analysis. Twenty of the spectra obtained from the laboratory were used
in the procedure for transferring calibration from laboratory data to the TIMS image of
the dunes obtained in September 1991. The sand samples contain mostly plagioclase
feldspar, volcanic glass, ash aggregates, basalt lithic fragments, pyroxenes, and
magnetite/opaque minerals (Edgett, 1994).
4.2 Empirical Transfer of Calibration to TIMS
The empirical corrections for the 1991 TIMS image of the Shifting Sand Dunes
of Christmas Lake Valley (see Edgett et al., 1995) involved the following steps:
(1) Normalized emittance (Realmuto, 1990) was derived from the radiance
image calibrated to the two onboard blackbodies.
(2) The emittance image was corrected for atmospheric attenuation effects on
the sides of the TIMS scan. These effects are caused by the side-scanning nature of
TIMS, thus there is more atmosphere between the instrument and surface as a function
of angle away from nadir. A low pass filter (1 line by 211 samples) was used to remove
high-frequency features. The resulting image was a map of low frequency brightness (in
this case, cmittance) variations across the image. The image was darker in the middle
and brighter on the sides, consistent with increased atmospheric attenuation on the sides
of the scan. The average of this brightness variation was then subtracted from the
original emittance image, and the result of this step was re-boosted to have the same
slope and offset (DN stretch) as the original cmittance image.
(3) The average of ten laboratory spectra (convolved to TIMS wavelengths) of
samples collected from the field area were compared to the average six-point spectra of
the image pixel corresponding to the location where each of the field samples was
collected. The average difference between the two data sets were used to correct the
emissivity image for atmospheric effects by essentially forcing a transfer of calibration
from laboratory to TIMS image. In this case, the average difference in emissivity
between the laboratory spectra and TIMS emittance image was 0.007 (bands 1,2, 5) to
0.027 (band 6).
(4) The newly calibrated and corrected image was checked by comparing ten
different (than used in step 3) laboratory spectra to the TIMS image. Again, laboratory
spectra arc convolved to TIMS six-point resolution and compared with the image
emissivity for the pixel corresponding to the site where a .sample was collected. The
11
difference between the two should be small. In this case, differences < 0.01 emissivity
were considered to be successful, and the average differences were only 0.001 to 0.003.
5. DISCUSSION AND CONCLUSIONS
In an ideal situation, the spectra used in steps 3 and 4 above should be very
different from those of the material of interest in the study. They should also be
different from each other (for example, if spectra used in step 3 were for quartzite
outcrops and spectra in step 4 were for basalt flows). However, most spectra of samples
collected in the vicinity of the Shifting Sand Dunes of Christmas Lake Valley are very
similar. There are no outcrops of material composed of something completely different
(e.g., quartz-rich rock) relative to the dune sand. All of the geologic materials are glassy
volcaniclastics (Edgett, 1994). The spectra that are different from the dune sand are
those of materials that typically have relatively fiat spectra, like vegetation and fine-
grained playa surfaces. The results for step 4 above were compared with the laboratory
spectrum of playa salts and indeed found to be flat.
The approach used here to remove atmosphere from a TIMS image appears to
be a very promising method for work with airborne thermal infrared images used for
geological purposes. The advantage of this approach is that now laboratory spectra can
be applied to unmixing analysis of the image. Use of laboratory spectra for mixing
analysis means that spectral libraries might be applied with relative ease. Use of
laboratory spectra to unmix the September 1991 Shifting Sand Dunes image is a goal of
future work (Edgett et al., 1995). Before this is done, however, it will be necessary to
further test the empirical atmospheric correction approach by using a different TIMS
scene that has surfaces of widely contrasting composition, such as the Kelso - Cima,
California image described by Barbera (1989).
6. REFERENCES
Adhihetty, I.S., J.A. McGuire, B. Wangmaneerat, T.M. Niemczyk, and D.M. Haaland,
1991, "Achieving transferable multivariate spectral calibration models:
Demonstration with infrared spectra of thin-film dielectrics on silicon,"
Analytical Chemistry, vol. 63, pp. 2329-2338.
Barbera, P.W., 1989, The Geology of the Kelso-Baker region, Mohave Desert,
California, using Thermal Infrared Multispectral Scanner Data, M.S. Thesis,
Arizona State University, Tempe, Arizona, 198 pp.
Christensen, P.R., and S.T. Harrison, 1993, "Thermal infrared emission spectroscopy of
natural surfaces: Application to desert varnish coatings on rocks," J. Geophys.
Res., vol. 98, pp. 19,819-19,834.
Edgett, K.S., 1994, "The volcaniclastic Shifting Sand Dunes of Christmas Lake Valley,
Oregon," in The Sand Component of the Modern Martian Aeolian Sedimentary
System, Ph.D. Dissertation, Arizona State University, Tempe, AZ, pp. 145-201.
Edgett, K.S., M.S. Ramsey, and P.R. Christensen, 1995, "Aeolian erosion, transport, and
deposition of volcaniclastic sands among the Shifting Sand Dunes, Christmas
Lake Valley, Oregon: TIMS image analysis," submitted for this volume.
Hook, S.J., K.E. Karlstrom, C.F. Miller, and K.J.W. McCaffrey, 1994, "Mapping the
Piute Mountains, California, with thermal infrared multispectral scanner
(TIMS) images," J. Geophys. Res., vol. 99, pp. 15,605-15,622.
Ondrusek, J., P.R. Christensen, and J.H. Fink, 1993, "Mapping the distribution of
vesicular textures on silicic iavas using the thermal infrared multispectral
scanner," J. Geophys. Res., vol. 98, pp. 15,903-15,908.
Realmuto, V.J., 1990, "Separating the effects of temperature and emissivity: Emissivity
spectrum normalization," in Proceedings of the Second TIMS Workshop, JPL
Publication 90-55, pp. 31-35.
Wang, Y., and B.R. Kowalski, 1992, "Calibration transfer and measurement stability of
near-infrared spectrometers," Applied Spectroscopy, vol. 46, pp. 764-771.
12


For geological investigations with airborne thermal infrared multispectral images: Transfer of calibration from laboratory spectrometer to TIMS as alternative for removing atmospheric effects

For geological investigations with airborne thermal infrared multispectral images: Transfer of calibration from laboratory spectrometer to TIMS as alternative for removing atmospheric effects

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server