RESOLVING LITHOLOGIC AND TEMPORAL CONTRASTS OF LAVA FLOWS USING OPTICAL-RADAR FUSION: ERTE' ALE VOLCANO, ETHIOPIA

Fusion of radar and optical remote sensing data is an excellent technique for deriving both compositional (from multispectral optical data), and surface roughness contrasts (from radar data). When this technique is used on active volcanic regions, it allows for the spatial and relative temporal distribution of individual lava flows to be resolved. The fused imagery can be merged with digital topographic data, providing both a means of visualizing the geomorphological differences between individual flows as well as the potential to make a quantitative link between composition, time and slope morphology. In addition, when coupled with historical eruption data, spatio-temporal or volumetric statistics of individual eruptions could be computed. The Erte' Ale volcano in the Afar Depression of Ethiopia provides an outstanding testing ground for the application of radar-optical fusion, because it has an active eruption history and compositionally diverse lava flows. It is a ~50X100 km Quaternary shield volcano made up of basalt, trachyte and rhyolite eruptions. The remoteness of Erte' Ale makes field work difficult, but this approach can be used to guide future investigations and mapping efforts, and the techniques can be used as a template for other volcanics. The optical imagery used in the fusion process was a combination of Advanced Spaceborne Thermal Emission Radiometer (ASTER) and Enhanced Thematic Mapper Plus (ETM+) data. This data was fused with Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) data using Red-Green-Blue (RGB)- Hue-Saturation-Intensity (HSI)-RGB transformation to embed, as an intensity layer, back-scattering information from the radar imagery into the multi-spectral information of the optical imagery. This fused image is then draped on a Digital Terrain Elevation Data (DTED) data set that provides topographic information at approximately 1 km spatial resolution. The fused imagery is used for the delineation of individual lava flows, and 3D visualization of the volcano. The coarseness of the topographic data makes it difficult to extract geomorphological statistics of flows, but it is hoped that future availability of higher-resolution data sets, such as the Shuttle Radar Topography Mission (SRTM), will provide a means for extracting these statistics.