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Interpretation of aerial images, produced on photographic emulsions by radiation in the visible portion of the electromagnetic spectrum, is a well known and important tool of the exploration geologist. Visible light, however, comprises an extremely small portion of the electromagnetic spectrum. Airborne systems have been developed that are capable of recording images produced by radiation in the infrared, radar, and other spectral regions. Most infrared systems, as well as those employed in conventional aerial photography, are passive. In this case, radiation emitted by the material itself and reflected or reradiated energy, originating in some natural source such as the sun, are recorded. Radar, on the other hand, is an active system, i.e., energy of known characteristic is artificially propagated and the reflected or reradiated energy recorded.
Photographic emulsions are only sensitive to radiation having wavelengths of less than 1.35 microns (near infrared). The majority of infrared and radar systems, therefore, utilize various types of scanning devices which translate fluctuations in received energy into fluctuations in electrical currents. These electrical currents may be recorded directly or converted to light energy to produce images on photographic emulsions. Such images thus present the infrared or radar energy intensities of the viewed materials and differ from conventional visible-light aerial photographs. As in photogeology, identification of specific geologic conditions from infrared and radar images should be possible by analysis of such factors as image tone, texture, and pattern. Interpretation of infrared and adar strip maps holds great promise for complementing photogeologic investigations and increasing the amount of geologic data obtainable by airborne methods.
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