Exploration geologists have made extensive use of aerial photography and orbital Landsat imagery, primarily for purposes of structural mapping. The Landsat 4 spacecraft launched in July 1982 is carrying a new imaging instrument called the Thematic Mapper which represents a significant advance over earlier Landsat sensors. The Thematic Mapper possesses improved

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spectral resolution (seven spectral channels distributed throughout the visible and infrared portions of the electromagnetic spectrum), improved spatial resolution (30 × 30 m [98 × 98 ft] ground resolution), and improved sensitivity (256 gray levels in each spectral channel). Experimental studies with airborne Thematic Mapper simulators tentatively indicate that these measurement capabilities will have a major payoff in terms of our ability to detect variations in clay mineralogy and abundance, to map bleaching effects in surficial rocks and soils that may be produced by hydrocarbon seepage, and to detect variations in the distribution and vigor of natural vegetation that are also related to seepage phenomena. The improved spatial resolution of the Thematic Mapper will enabl photogeologists to identify smaller scale landforms and drainage features which will also contribute to improved structural mapping capabilities. Research is currently underway to determine the utility of Thematic Mapper measurements for geologic mapping in complex areas characterized by large relief and extensive vegetation. Recent experimental results have underscored the need for improved spectral resolution in the next generation of visible and infrared sensors.

Radar imaging techniques also represent an important source of information concerning geological conditions at the earth's surface. Imaging radars artificially illuminate the earth at an angle of incidence and azimuthal direction that can be controlled by the radar operator. Variations in radar backscatter intensity observed at different angles of incidence are largely governed by areal variations in surface relief and roughness. Exploration geologists have made extensive use of airborne radar surveys for terrain analysis and structural mapping, particularly in tropical environments. However, it is difficult to obtain quantitative information on surface roughness conditions from airborne surveys because of the large variation in incidence angle that occurs across the radar's ground sw th. Spaceborne radar systems offer the unique advantage of being able to illuminate large areas at a nearly constant angle of incidence due to their much greater elevation above the earth's surface. A Shuttle Imaging Radar Experiment (SIR-B), which will be conducted in August 1984, is designed to acquire multiple images of selected regions at different radar incidence angles. In principle, the coregistration of these images will enable researchers to differentiate surficial materials on the basis of their roughness characteristics in much the same way that multispectral measurements at visible wave-lengths are used to detect variations in surface pigmentation. Experimental studies tentatively indicate that variations in surface roughness detected in orbital radar surveys can be related t subtle differences in the lithology of sedimentary rock units. Orbital radar techniques may provide an important new tool for mapping facies variations within sedimentary basins. Additional orbital experiments are being planned during the latter 1980s to evaluate the geological utility of radar imagery obtained at multiple frequencies and multiple polarizations.

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