COMPARISON AND DATA FUSION OF ASTER, ETM+ AND SIR-C/X-SAR IMAGERY FOR GEOLOGICAL MAPPING IN ARID REGIONS: THE AFAR DEPRESSION, ETHIOPIA

The Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) and Enhanced Thematic Mapper Plus (ETM+) are recent satellite optical systems that started collecting remote sensing data in 2000. The 1994 Shuttle Imaging Radar (SIR)-C/X-band Synthetic Aperture Radar (SAR) is a shuttle-borne system, which collected radar remote sensing data in 1994. ASTER has 14 bands in different regions of the electromagnetic spectrum; 3 in the visible and near infrared (VNIR) with 15m spatial resolution, 6 in the short wavelength infrared (SWIR) with 30m spatial resolution, and 5 in the thermal infrared (TIR) with 90m spatial resolution. The ETM+ has 7 bands; 6 in the VNIR with 30m spatial resolution and1 in the TIR with 60m spatial resolution. In addition ETM+ has 1 panchromatic band with 15m spatial resolution. The multi-frequency and multi-polarization SIR-C/X-SAR data has L (l=24cm), C (l=6cm) and X (l=3cm) bands in the microwave region of the electromagnetic spectrum with like- and cross-polarizations. We used the Afar Depression of Ethiopia as a natural laboratory to examine the usefulness of those remote sensing data for geological mapping in arid regions. The Afar Depression is where the Red Sea, Gulf of Aden and the Ethiopian Main Rift meet in a rift-rift-rift triple junction. It is dominated by volcanic rocks and normal faults where major structures are NE trending parallel to the Ethiopian Main Rift, NW tending parallel to the Red Seas and E-W trending parallel to the Gulf Aden. Lack of vegetation and excellent exposure makes the Afar Depression a suburb site for testing new orbital optical and radar remote sensing systems. Our results show that: (1) Fusion of 3 - 2 - 1 ETM+ and C-band SIR-C/X-SAR images using Red - Green - Blue (RGB) to Intensity - Hue - Saturation (IHS) transformation is useful in outlining lithologies as well as highlighting morphologically-defined structures; (2) RGB color display of ETM+ band 1 (blue) - ASTER band 1 (green) and ASTER band 2 (red) produces normal color images with 15m spatial resolution which is 4 times better than if ETM+ data is to be used alone; and (3) ASTER is advanced compared to ETM+ in that it has a better spectral range especially in the SWIR and TIR. This makes ASTER more useful in identifying geological materials that has spectral features within these ranges such as felsic volcanics.