GEOLOGIC MAPPING AT EMI KOUSSI VOLCANO IN THE CENTRAL SAHARA DESERT USING LANDSAT ETM+ DATA

Emi Koussi is a stratovolcano located on an intraplate continental hotspot in the Tibesti Volcanic Province (TVP), northern Chad. Active from 2.4 Ma to late Quaternary (Gourgaud and Vincent, 2004), Emi Koussi is the highest peak (3445 m) in the Sahara and an important volcano within the province which covers approximately 30,000 km². Fieldwork required for geologic mapping of this volcano has been limited given its remote location and arid climate (Gèze et al., 1959; Gourgaud and Vincent, 2004). The purpose of this research was to produce a geologic map of the Emi Koussi volcano using satellite-based Landsat Enhanced Thematic Mapper Plus (ETM+) data. Volcanoes within the TVP are well suited for remote mapping technology due to the climate, very limited vegetation cover, and late Cenozoic age.

Volcanic stratigraphic units and their relative ages were determined using calibrated Landsat ETM+ spectra combined with descriptions of units from on-site field work by previous researchers (Gèze et al., 1959; Gourgaud and Vincent, 2004). Given the arid climate an Internal Average Relative Reflectance (IARR) algorithm was used to minimize spectral atmospheric artifacts and reduce data to apparent surface reflectance. The algorithm normalizes each spectrum in the scene by using the scene average. We validated calibrated spectra using those from oases within deeply carved valleys with known similar targets from spectral libraries. A Minimum Noise Fraction (MNF) transformation was applied to segregate coherent spectra from noise. The transformation produces the bands consistent with the number of input bands with the first band being the most coherent surface reflectance.

The final (geologic) map is a detailed false color image with RGB= MNF bands 1, 2, and 3. Mapping results include seven volcanic units, two pre-volcanism units, three caldera boundaries, and one late Quaternary eolian sand unit. All are distinctive given the 30 m resolution of Landsat ETM+ satellite data and the roughly 70 km diameter of Emi Koussi.