Thermal infrared imagery is obtained by airborne scanning devices that detect thermal radiation from the earth's surface and record it as an image in which bright tones represent relatively warm temperatures. Scanners sensitive to wavelengths between 8 and 14 microns span the radiant power peak of the earth at 9.7 microns and coincide with an atmospheric “window.”
An example of 8 to 14 micron nighttime infrared imagery from the Imperial Valley, California, is interpreted and compared with aerial photographs of the same area. In this monotonous-appearing desert terrain, the imagery exhibits greater contrast and geologic detail than the photography. On the imagery, deformed Tertiary sedimentary bedrock (relatively cool) is distinguished from Holocene windblown sand cover (relatively warm).
Of especial geologic interest is a faulted plunging anticline in flat terrain. It is obscure both on aerial photographs and to a ground observer. On nighttime infrared imagery, however, the fold is clearly shown by the outcrop configuration of the individual siltstone and sandstone strata comprising the structure. Apparently the radiometric temperature differences between strata are sufficient to outline the fold on the imagery. The obscure expression of the fold on aerial photographs may be due to insufficient contrast in light reflectance between the different strata.