ANALYSIS OF LANDSAT 7 IMAGERY FOR GEOLOGIC MAPPING IN NEW MEXICO

The advent of readily available, low-cost Landsat 7 imagery has provided new opportunities for the application of remote sensing to geologic investigations. The Development of Geologic Imaging Techniques (DGIT) project within the USGS National Cooperative Geologic Mapping Program has evaluated the use of newly available Landsat 7 imagery for enhancing lithologic information and speeding the production of digital geologic maps in northern New Mexico. Improved resolution from band 8 15-m resolution panchromatic image data allows creation of sharp photographic-quality images at scales as large as 1:100,000 (100K) to 50K. Band combinations using Landsat bands 7-4-2 or 5-4-1 as RGB color images, with color infrared band 4 assigned green for vegetation, generate true-color appearing images. Enhanced lithologic content in the red band results from use of the short-wave infrared bands 7 or 5. We have prepared seasonally matched mosaics of cloud-free Fall 1999 imagery for all of New Mexico that optimize natural shaded-relief illumination for coupled geomorphic and lithologic interpretation. Merged with band 8 data, a selection of images enhanced in Hue-Saturation-Intensity color space emphasize significant geologic and physiographic features in New Mexico at scales from 250K to 50K. Interactive image processing to discriminate spectral signatures in the Landsat bandwidths has been evaluated by geologists for consistency with geologic map units mapped by common ground-control methods. Traditional field and laboratory characterization of representative samples for mineralogical, chemical, and physical rock properties have been compared with short-wave infrared spectra determined on selected samples by imaging spectroscopy. Resolution merging of Landsat 7 RGB color images with widely available 12K scale USGS digital orthophoto quads provides a good geologic map image base that provides more spectral detail at 24K scale than color or color-infrared aerial photography. Determining the geospatial accuracy of satellite imagery in the scale range of most geologic maps (100K to 24K) is critical for more widespread use; we are rigorously evaluating this using ground control, digital photogrammetry, and GIS methods.