ASTER Spectral Processing
GEOIMAGE has developed a standard methodology to process ASTER imagery for mineral exploration and this procedure is explained and the example graphics are from an ASTER scene over the Sar Cheshmeh Deposit in Iran. The procedure involves:
- The selection of the best cloud free data over the area of interest,
- The purchase of the AST-05 (Emissivity) and AST_07xt (cross-talk corrected surface reflectance VNIR and SWIR) imagery,
- The orthorectification of the ASTER imagery,
- Preparation of Lithological and spectral images in ERMapper format,
- ECW compression of the imagery for display in MapInfo or ArcView,
- Vectorisation of the highest 1% predicted clay mineral groups into TAB and/or Shape files,
- Preparation of a report describing the processing, and
- Supply of all data on DVD.
The VNIR and SWIR files are orthorectified using the Geocover 2000 Landsat panchromatic image for X and Y control and the SRTM DEM for height control. The final file is a 9 band co registered VNIR+SWIR integer ERMapper file at 15 m resolution. The file is usually in WGS84 and the relevant UTM zone (unless the client requires the data in a different projection) and the locational accuracy of the file is probably of the order of 25 m.
The TIR emissivity file is rectified using system parameters to 45 m cell size and its location is fine tuned to the VNIR-SWIR file. The final file is a 5 band integer ERMapper file at 45m resolution. The SRTM data used in the orthorectification process is supplied as well as a coloured ECW file.
Colour composites using bands from the ASTER VNIR, SWIR and TIR subsystems can be used to give complementary information on ground cover and lithological information. The images commonly prepared (bands in RGB order) are:
Bands 3,2,1 Near infrared colour scheme with vegetation in red. Uses the bands with the best spatial resolution (15 m)
Bands 7,3,1 This is the band equivalent of a Landsat 7,4,2 colour composite and gives more spectral
information than the bands 3,2,1 although at the loss of spatial resolution in the Red channel.
Iron ratio image. Ferric iron in red, band 3 (vegetation) in green and ferrous iron in blue.
DS765 Decorrelation stretched image of SWIR bands 7,6,5.
DS876 Decorrelation stretched image of SWIR bands 8,7,6.
DS987 Decorrelation stretched image of SWIR bands 9,8,7.
RBD Larry Rowan’s Relative Band Depth image highlights spectral details in the SWIR subsystem.
Red Kaolinite/alunite group (B4 + B7)/(B5+B6)
Green Illite Group (B5 + B7)/B6
Blue Chlorite Group (B7 + B9)/B8
TIR_indices Several TIR emissivity ratios are used separately or as a colour composite.
Silica Index b11*b11/b10/b12
Carbonate Index b13/b14
Mafic index b12/b13
Quartz Index b14/b12
Spectral Images and Vectors
Minerals of exploration interest that produce recognizable spectral patterns in ASTER SWIR imagery are the epithermal clay minerals groups - kaolinite, alunite-pyrophyllite and illite, and the propylitic minerals. These mineral groups can be predicted using the Relative Band Depth estimator and the output of the estimator is presented as both imagery and as vectors. The estimator is first presented as a greyscale image where the brighter the pixel the more likely the probability that the pixel is the relevant mineral group and then the highest 1% predicted distribution is output as a MapInfo TAB and/or ArcView Shape file.
A report describing the processing with example graphics of each of the spectral images is produced and supplied as a DOC and PDF file.