Hyperspectral Mapping of Cuprite, Nevada with CASI and SFSI-2 Fused Data

José Lim, and Gary Borstad
(February 2000)

Overview

Recent case studies (Table 1) have evaluated and demonstrated the geological and environmental applications of CASI and SFSI-2.  The studies used either CASI or SFSI-2 in successfully identifying minerals, mapping alteration zones and delineating vegetation cover.

Airborne hyperspectral missions made during the 1998 and 1999 flight seasons over Idaho, Utah, Nevada and Arizona used both CASI and SFSI-2 sensors.  Both platforms were mounted in tandem and covered the same target area.  One recent improvement Borstad Associates Ltd. has developed is the fusion of CASI images with that of SFSI-2 data.  One data set now covers both the VNIR (.47 um to .90um - 7 bands) and SWIR (1.2 um to 2.4 um - 240 bands) regions.

The main objective of this note is to document a comparison of mineral maps produced by the newly fused CASI/SFSI-2 image using a standard hyperspectral image processing program (ENVI).  The classified map will then be compared with an AVIRIS produced mineral map of the area.

Study Area

The study area covers the eastern region of the Cuprite Mining District, Nevada (Figure 1).  For comparison, a side by side partial coverage of a fused CASI/SFSI-2 image and an AVIRIS image is shown in Figure 2.

This area was chosen because it has been extensively studied and well mapped.  Ground truth data of the study area in terms of a classified mineral map done by AVIRIS (1998) was also conveniently available on line (http://speclab.cr.usgs.gov/).

Methodology

The fused CASI/SFSI-2 data (1999) was transformed to apparent reflectance via a flat field correction using the nearby Stonewall playa.  The data set was then processed using ENVI's standard hyperspectral mapping tools in obtaining pure pixel endmembers.  For simplicity, only three spectral endmembers - Alunite, Buddingtonite and Kaolinite, were extracted from this process.

Using the three spectra collected from 'pure pixel targets' within the image as endmembers, Spectral Angle Mapping (SAM) in 2.0 um to 2.4 um range was then applied to the fused CASI/SFSI-2 image.  The rule classes from the SAM classification were then thresholded subjectively to produce the final classification map (Figure 3).  The final classified image was then compared to the AVIRIS 17-meter resolution mineral map of the area (Figure 4).  Collected endmembers were also compared with corresponding library spectra collected from JPL.

Results

A comparison of the crudely classified map produced from the fused CASI/SFSI-2 image with that of the AVIRIS mineral map shows very good spatial correlation for Alunite and Buddingtonite (Figures 5A & 5B).  An oblique aerial photo of the mapped Buddingtonite area is shown in Figure 6.

The Kaolinite regions delineated in the fused CASI/SFSI-2 image are observed to be larger than that of the AVIRIS map (Figure 5C).  This minor misclassification is probably due to atmospheric effects and residual artifacts brought about in using the playa to normalize the image

The overall spectral shape and form of the 'flat field reflectance' spectral endmembers collected from the CASI/SFSI-2 fused image display good correlation with JPL's spectral library (Figures 3A to 3C).  As a means to compare the absorption features of spectra originating from different sensors to a common baseline, a Continuum Removal was done to each of the collected endmember spectra.  The Continuum Removed spectra again show good agreement between the fused CASi/SFSI-2 image and JPL's spectral library (Figures 4A to 4C).

Conclusion

This quick attempt to produce a mineral map from a CASI/SFSI-2 fused image using a standard packaged hyperspectral program (ENVI) was deemed successful.   Mineral maps classified from the newly fused CASI/SFSI-2 image display good overall correlation spatially with AVIRIS maps.  Collected endmember spectra of the fused image also exhibit good agreement with existing spectral libraries.

TABLE 1. Recent Case Studies on the Use of Borstad's CASI and SFSI-2 Data

Figure 1. A single CASI/SFSI-2 fused image (Red: 1.32 um, Green: 1.75 um, Blue: 2.17 um) of the Cuprite Mining District acquired in September 1999.  Yellow inset is study area.  Click on image for full resolution (400kb).

Figure 2. Side by side comparison of CASI/SFSI-2 fused and AVIRIS image (Red: 2.10 um, Green: 2.20 um, Blue: 2.35 um).  AVIRIS image taken from ENVI tutorial CD.

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Figure 3. SAM classification map derived from CASI/SFSI-2 fused image.
Collected Endmember spectra with corresponding JPL library spectra are shown on the right.

Figure 4. Comparison of Continuum Removed spectra of collected endmembers derived from CASI/SFSI-2 fused image with JPL spectral library.

Figure 5. Comparison of mineral map derived from CASI/SFSI-2 fused image with AVIRIS mineral map from the USGS website.

Figure 6. Oblique view of study area showing delineated Buddingtonite area.  A) Oblique air photo (1998) overview of Cuprite looking southeast.  B) Drape image of classified CASI/SFSI-2 fused image over Digital Terrain Model (3X vertical exaggeration).  C) Close-up oblique air photo of exposed Buddingtonite area (green).

Bibliography

Borstad Associates Ltd. Internal Document. 1999.  "Airborne Hyperspectral Mapping the Dragon Mine, Tintic District, Utah with SFSI-2/CASI Systems."

Staenz, K., R.A. Neville, J. Levesque, T. Szeredi, V. Singhroy, G.A. Borstad, P. Hauff. 1999. "Evaluation of CASI and SFSI Hyperspectral Data for Environmental and Geological Applications - Two Case Studies," Canadian Journal of Remote Sensing, Vol. 25, No.3, pp. 311-322.

Coeur d'Alene River Basin Project. 1998. "Test Study of the Application of Airborne Hyperspectral Data to Chemical Characterization of Mine Wastes in the Coeur d'Alene River Basin," Under contract with URS Greiner Woodward Clyde, Seattle, WA.

Utah Hyperspectral Project. 1998 - on going. "Application of Airborne Hyperspectral Data to Characterization of Mined Lands and Analysis of Associated Watersheds and Impacts for Environmental Management," Under contract with NASA EOCAP.

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