EVALUATION OF AN EMPIRICAL RADAR BACKSCATTER MODEL FOR PREDICTING BACKSCATTER CHARACTERISTICS OF GEOLOGIC UNITS AT PISGAH VOLCANIC FIELD, CALIFORNIA

Comparison of radar backscatter coefficients (sigma-degrees, in dB), c alculated by using the empirical model of Oh et al, [1992], to sigma-d egrees extracted from AIRSAR data of four geologic units at Pisgah sho ws that the model predicts measured sigma-degrees(vv) and sigma-degree s(hv) to within +/- 3 dB. The model predicts higher sigma-degrees(hh) than those observed. For smooth surfaces (rms height = s, s < 8 cm), m odel results depend strongly on the accuracy of the surface measuremen ts (s and dielectric constant, epsilon(r)). For rougher surfaces, the model is less dependent on the accuracy of surface characterizations. The model may be inverted to estimate s from measured sigma-degrees fo r surfaces with ks < 3 (k = wavenumber, or 2pi/lambda, where lambda = radar wavelength). Model inversion for a pahoehoe unit at 30-degrees t o 50-degrees incidence angles (theta) resulted in an estimate of s to within < 1 cm of the measured 3 cm. The inability of the model to esti mate accurately sigma-degrees(hh) and the anomalously high nadir Fresn el reflection coefficients (GAMMA(o)) and epsilon(r) required in the m odel inversion may both be due to approximately equal co-polarized rat ios (sigma-degrees(hh)/sigma-degrees(vv) = P approximately 1) of the s oils used to derive the model. For effective application to many geolo gic surfaces, for which p < 1 is often observed at theta > 30-degrees, the model will require modification to include surfaces with non-unit y sigma-degrees(hh)/sigma-degrees(vv).