Variations in surface roughness within Death Valley, California: Geologic evaluation of 25-cm-wavelength radar images

摘要

Images — processed from 25-cm wavelength, side-looking airborne radar — of the salt flats and gravel fans on the floor of Death Valley, California, show distinctive variations in radar backscatter (that is, variations in image gray tones) that can be correlated with systematic changes in the surface roughness of different geologic units. Well-developed desert pavements on the oldest boulder gravel units of late Pleistocene age are clearly delineated as weak backscatterers on the images. A gradation in the size of gravel constituents near the base of the giant gravel fans is associated with an abrupt change in the backscatter energy. The change takes place at gravel radii between 0.08 and 0.14 (2.0 and 3.5 cm). A breakpoint observed in the Rayleigh scattering region of the total radar cross section is virtually independent of the antenna depression angle as long as the resolution area does not lie in the first pulse width of the echo (the 90° depression angle). With the longer wavelength radar systems, antenna depression angles of 45° to 90° appear to be well suited for investigations of surface roughness because of the suppression of radar shadows and the increased radar return from the near range. Key Words: environmental geology • remote sensing • radar • surface roughness • backscatter