Lava-flow characterization at Pisgah Volcanic Field, California, with multiparameter imaging radar

Multi-incidence-angle (in the 25° to 55° range) radar data acquired by the NASA/JPL Airborne Synthetic Aperture Radar (AIRSAR) at three wavelengths simultaneously and displayed at three polarizations are examined for their utility in characterizing lava flows at Pisgah volcanic field, California. Pisgah lava flows were erupted in three phases; flow textures consist of hummocky pahoehoe, smooth pahoehoe, and aa (with and without thin sedimentary cover). Of the eight AIRSAR images used here, four were calibrated to within an accuracy of ±2 dB with trihedral corner reflectors, and data from these calibrations were used to process the additional images to a conservatively estimated ±5 dB level of accuracy. Calibrated radar backscatter data (σ°, in dB) were plotted as a function of incidence angle at three wavelengths (P-band, 68 cm; L-band, 24 cm; and C-band, 5.6 cm) and three polarizations (HH, horizontal transmit/horizontal receive; HV, horizontal transmit/vertical receive; and VV, vertical transmit/vertical receive) for eight major units at Pisgah for which multi-incidence-angle AIRSAR data were available. The eight units consist of near-vent and distal aa flows; near-vent and distal, hummocky pahoehoe flows; a mantled, hummocky pahoehoe flow; a platform pahoehoe flow; an alluvial fan; and a playa. Analyses of these backscatter data show that major unmodified volcanic units at Pisgah are readily distinguishable from each other and that they exhibit diffuse (HH, VV) and/or multiple (HV) scattering behavior typical of rough surfaces at these wavelengths. These analyses show that discrimination of smooth lavas (platform pahoehoe) from mantled units with greater primary roughness (hummocky pahoehoe) is difficult and must rely on supporting observations (such as evidence of localized weathering and/or sediment deposition, contrast with surrounding units, and superposition of flow units). L-band backscatter and image data at HV polarization show the best discrimination of Pisgah lava flows, with optimal unit separation observed between ∼40° and 50° incidence angles. Backscatter data shown as a function of relative age of Pisgah flows indicate that dating of lava flows on the basis of average radar backscatter may yield ambiguous results if primary flow textures and modification processes are not well understood.