Inversions of SIR-C and AIRSAR data for the roughness of geological surfaces

The surface roughness of alluvial fans in Death Valley, California, changes as the fans age. Because radar backscatter is sensitive to surface roughness, it is possible to map relative surface age using Synthetic Aperture Radar (SAR) data. We have examined surface roughness estimates in Death Valley from Shuttle Imaging Radar-C SAR data with the use of a technique that we call foreground/background analysis (FBA), with the goal of establishing a robust inversion method that is extendible to a range of surfaces and conditions. In this method, a foreground entity (in this case roughness) is distinguished from complicating background factors (including the distribution of intermediate-scale slopes, vegetation, and dielectric constant). The inversion for roughness is non-unique even when constrained by the use of field measurements. When the range of possible solutions is examined, it is observed that they fall into a small number of domains, each with distinct characteristics that are probably associated with physical factors such as the scale of roughness. Solutions were compared with those determined from the semiempirical (SEM) and integral equation (IEM) models, and all solutions were evaluated with respect to field knowledge. The SEM and IEM solutions, and those obtained using FBA together with the field measurements, fall into a common domain of solutions that are susceptible to contamination by background effects and hence may not be extendible to other geographic locations. A domain of stable solutions that are more extendible does exist; however, this extendibility is achieved at the expense of reduced resolution of roughness levels. For Death Valley, we estimate that it is possible to resolve only four levels of roughness, which is far fewer than can be theoretically resolved with existing inversion algorithms.