Importance of Spatial and Radiometric Resolution of AVIRIS Data for Recognition of Mineral Endmembers in the Geiger Grade Area , Nevada ,

In September 1998 NASA deployed the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) on a NOAA Twin Otter aircraft and acquired an approximately East-West line at 2000m above ground level over the Steamboat Springs Geiger Grade area, Nevada. Data were acquired at a spatial resolution of 2.2 meters for the low-altitude data. A high altitude AVIRIS mission, flown on NASA ER-2 aircraft at 20,000m, provided data at approximately 20 meter spatial resolution. There are about 100 two meter spatial resolution elements in a 20 meter pixel. These data sets provided an excellent opportunity to evaluate the effects of linear mixtures of surface cover types on classification accuracy and the determination of end members. We found that high altitude data, for the most part, do not require extensive pre-processing and offer majority of the useful spectral content. Even though, high altitude data do not allow the mixture separation of low altitude data, they maintain an integrated data set, without the artifacts and line drops. 1.0 Geography and Geology Geiger Grade (Figure 1) is situated on the southwestern slopes of Virginia Range, 10 miles south of Reno, NV, between the famous mines of Comstock Lode and the well-known thermal area of Steamboat Springs. The area is mountainous with elevation ranging between 4,400 ft and 6,500 ft and characterized by semi-arid climate. The exposed rocks are Tertiary andesite flows and breccias of the Alta and Kate Peak Formations. The Alta formation is most affected by hydrothermal alteration where the groundmass is devitrified and hyperstene replaced by secondary minerals indicative of propylitic and argillic alteration. (Thompson, 1956; Vikre, 1998). Geiger Grade exhibits the most spectacular evidence of alteration with bleaching, iron staining and widespread propylitization (Hudson, 1984, Vikre 1986). Predominant minerals found in/around Geiger grade are jarosite, illite, halloysite, quartz, alunite, minor biotite and iron oxides (Hudson, 1999). Figure 1 Area Location 2.0 Spectral Characteristics of Alteration Minerals The altered zones of Geiger Grade contain iron oxides, clay mineral assemblages and silicates (Hudson, 1999). Weathering of pyrite has produced heavy bleaching of the host rocks and alteration to jarosite while the propylitic and occasionally argillic alteration are characterized by chlorite, kaolinite and illite, respectively. The alteration to the host rock are visible even to the naked eye. Most iron minerals exhibit subtle spectral features in the 400-700 nm spectral range (Spatz, 1988; Taranik, 1989). The hydroxyl (OH) ion characteristic for clay minerals exhibits absorption features in 2200 – 2400 nm range (Figure 2). Figure 2 – Characteristic mineral spectral features in 400-2500nm range 3.0 AVIRIS Data Collection and Processing AVIRIS Instrument is usually flown on NASA ER-2 aircraft at an altitude of 20,000 meters. This imposes certain limitations on data collection: the ER-2 has a narrow operational altitude range, fixing the AVIRIS ground pixel at 20 meters. In 1998, JPL began experimenting with a different platform (Twin Otter aircraft) and lower flight altitudes (3000 – 10,000 meters) to improve spatial resolution (down to <3 meters) (Sarture et. al, 1999). Low altitude data acquistion requires special pre-processing. Because AVIRIS is a whisk-broom type scanner, aircraft preturbations and scanner undersampling introduce geometric distortions in the data (Boardman, 1999). Furthermore, low altitude AVIRIS flight-line require greater computer processing memory than the high altitude data, and could introduce additional computer processing problems (6 Gigabytes per flightline). 4.0 Low-Altitude Issues Even though the spatial resolution of the sensor has improved, the low altitude acquisition creates geometric distortions and undersampling in the down-track direction, particularly at ground elevations less than 6000 ft (Figure 3). In order to make coherent imagery, JPL has requested that the data be geo-rectified and undersampling gaps be “filled” with the nearest neighbor replicates. The data are filled using a 7x7 kernel, which means that up to 12-line gap in sensor measurements can be spanned in especially bad conditions. The locations of actual and “artificial” pixels are then recorded to a file (*.glt file), in which the actual pixels have positive values and artificial pixels have negative values (Boardman, 1999). Figure 3 – Geo-rectification image map of actual pixels (white) and artificial pixels (black). Note jagged edges caused by kerneling.