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MODULAR OPTOELECTRONIC MULTISPECTRAL STEREO SCANNER IMAGING: ETHIOPIA AND ERITREA (DECEMBER 1996) Michael Beyth I, Johann Bodechtel 2 and Lei Qingxia 2 I. Geological Survey of Israel. 30 Malkhei Israel St., Jerusalem 95501, Israel

2. Working Group Remote Sensing. University of Munich, Luisenstr. 37, Munich, Germany

INTRODUCTION The high precision German MOMS- 2P has been used since May 1996 to observe oceans and land surfaces within MIR's orbital path with (DARA, 1997). Three orbits were made over Ethiopia and Eritrea (Fig. I) by the Modular Optoelectronic Multispectral Stereo Scanner on Priroda - MOMS-2P - docked on the MIR Space Station and operating on Mode B containing four multispectral bands (DARA, 1997). The central and most significant orbit was measured on 18/12/96 from Ras Dashan in northern Ethiopia, through the Danakil Depression, to the Dahlak Archipelage in NW Eritrea (Fig. 1). The southern orbit was measured on 211I2/96 from Addis Ababa in SW Ethiopia, through the East African Rift - Red Sea - Gulf of Aden triple junction, to the Bay of Beylul in Eritrea. The most northerly orbit made over a partial cloud cover was measured on 23/12/96 from the Gash River in the southwest through the Baraka River, and to the Red Sea coast. The goal of the present study is to evaluate the geology of this area based on the new high-resolution multispectral data and to assess its feasibility as a new remote sensing tool.

rocks and soils. Band 2 (S30-S7Snm)- this band can be useful to discriminate between rocks and soils containing ferric and ferrous iron. Band 3 (64S-680nm)- this band includes slight crystal field absorptions of Fe and expresses laterites well. Band 4 (770-81 Onm)- this band is placed in the high renectance plateau for vegetation. Data Reduction Data reduction was carried out in three main stages: (1) Radiometric, including atmospheric and geometric calibrations of the relevant data set. (2) For the present study, bands 4-3-1 as RGB were selected for colour composites. (3) IHS transform was applied to the selected data set for colour enhancement of the spectral information.

OPTICAL CONCEPT In order to fulfill different user requirements, a modular concept was defined with four datatake modes (A to D), each having a specific channel combination, such as Mode B which comprises four multispectral bands. The system consists of five lenses, three of which were designated for stereoscopic images, and the other two for the multispectral images. The central lens, with a focal length of 660 mm, forms the core of the camera system. It allows high-reSOlution imagery with a ground pixel size of 6x6 m2 (flight altitude 400 km). Two lenses each with a focal length of 220 mm enable the multispectral imaging with a total of four channels. A ground pixel size of 18x 18 m2 for the spectral channels was selected. MOMS-2P was designed for spectral data acquisition in the VIS and NIR range (four channel with Nadir orientation) and for along-track stereo rccording: Band I (440-S0Snm)- this band is most valuable for hydrologic-water applications due to shortwave transmission and is diagnostic for Fe-bearing minerals,

Figure 1. LocatioD map.

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GEOLOGICAL BACKGROUND The main physiographic units, which comprise the EthiopialEritrea area, are (from west to east): - The plateau, up to 4200 m at Ras Dashan, with lowgrade metasediments and metavolcanic rocks overlain by a Paleo-Mesozoic sedimentary sequence and Tertiary trap volcanics (Beyth, 1972; Merla et al., 1973; Kazmin, 1973). - The eastern escarpment, bordering the western margins of the Danakil and Afar depressions and the Ethiopian East African Rift. - The Danakil and the Afar depressions, to 128 m below sea level, with an attenuated continental crust and embryonic ruptwing and spreading expressed by young volcanic activity and sedimentary fill of the red-bed type. - The Danakil Alps; these are an internal block forming the eastern margins of the depressions and are composed of metamorphic basement overlain by Mesozoic Antalo limestones. - The Red Sea coastal lowlands with young sediments and volcanic rocks.

EVALUATION OF SELECTED AREAS In order to apply this new data set and to assess its feasibility as a tool for geological interpretation, we selected one study area on the plateau, one area in the escarpment and at the northern edge of the Danakil Depression. Plateau-low grade metamorphic sequence Mai-Kenetal and Ras-Dashan Most of this area, including the Mai-Kenetal and Negash synclinoriums and the Plio-Mesozoic sedimentary rocks· of the Mekele Outlier, were previously described by Beyth (1972), Arkin et al. (1971) and Garland (1980). The large, oval-shaped MaiKenetal synclinorium (Fig. 2) is located west of the town of Mai-Kenetal. It trends SWS-NEN. This structure is 50 km long and 8 km wide at the center where it is intruded by a round stock of the Mareb Granite. To the south, it terminates against another Mareb Granite intrusion and to the north, partly against a north-south striking fault where it is partly covered by Paleozoic sandstones and trap volcanics. The synclinorium consists of the Tembian Group metasediments (type section) which overlie the Tzaliet Metavolcanic Group, both of Neoproterozoic age. The core of the synclinorium is built of black algal (Cyaophicia) limestone which is part of the Mai-Kenetal Limestone (upper unit of the Tembian Group).


Escarpment high grade metamorphic sequence - Boya Wadi The high-grade metamorphic rocks of the eastern escarpment were described by Beytb et al. (1997) and Ghebread (1999). They are composed of kyanitestaurolite parascbist and granitic orthogneiss. From the MOMS-2P image this sequence is structurally connected to the Negash SynClinorium which is built of the Tambian Group overlying the Tzeliat Metavolcanics (Beyth, 1972). The Negash Synclinorium axis strikes NEN-SWS. Its continuation is visible on the MOMS2P image from the Ethiopian plateau through the escarpment up to the Boya River on the margins of the Alid Graben, where the high grade paraschist was mapped. Here, the light colored rocks are probably the orthogneiss while the dark bluish ones are the paraschist (Fig. 3). Alid volcanic range The Alid volcanic axial volcanic range or graben (Fig. 3) was described by Marinelli et al. (1980), Beyth (1996) and Clynne et al. (1996) and is located at the northern edge of the DanakiU depression. This depression is the most northern part of the Afar triangle located south of the Gulf of Zula A preliminary geological map of this volcanic range was prepared by Marinelli et aI. (1980) who divided the geological history into four stages. During the first stage, stratoid basalts erupted. The second stage corresponds to the formation and updoming of the alkaline or peralkaline felsic volcanic rocks of the Alid, and Gelti or Jallula. The age of this felsic intrusion, which was the cause for the final desiccation of the Danakil Depression from the Gulf of Zula, is less than 32,000 years (Bonatti et al., 1971). The third stage was an extensional phase that caused marine invasion into the Alid Graben; marine sediments were deposited and submarine volcanoes erupted. Olivine basaltic fissure eruptions north and south of the Alid volcano dwing recent historic times is the fourth and final stage. Fumarolic and hot spring activity is presently associated with the Alid volcano and the fissure basalt. The Alid caldera was formed on the top of the Alid stratovolcano, 904 m above sea level and about 700 m above the plains containing Quaternary sediments and bordering it from west. This caldera is elliptical, elongated in an E-W direction. It is some 3 km long and 500 m in width. The caldera walls are up to 100 m high and are composed mainly of pumice as pyroclastic fragments, syenite as accidental fragments, and intermediate to rhyolitic lava flows. The floor of most of the caldera is flat and filled with clay-type sediments.

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Figure 2. Mai Kenetal Sync linorium image.

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Figure 3. Alid Volcanic Range and Eastern Escarpment image.


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There are a few indications of caldera collapse: ring intrusions of olivine basalt inside the caldera; hot springs and fumarola along the internal margins of the caldera; a collapsed block of bedded pyroclastics at the center of the caldera Much of the caldera pyroclastics and flows are strongly altered by fumarolic activity into multi-colored clays (mainly illite according to XRD analysis of one sample). Salts deposited at the hot springs of Ertakara and Ilegedi were found to consist mainly of ammonium sulfates: tschermigite, mascagnite, koktaite, plus gypsum. Blocks of high grade Neoproterozoic paraschist were mapped within the caldera and east of the Alid volcano (Clynne et al., 1996).

SUMMARY This note presents a new remote-sensing tool that may support the sustainable use of natural resources in arid countries like Eritrea and Ethiopia. Different lithologies and structures are well expressed with excellent ground resolution in Mode B.

Acknowledgments We thank our Eritrean cOllegues for supporting our field work. References Arkin. Y.• Beyth, M., Dow, D.B .• Levitte, D., Temegen Haile and Tsegaye Hailu, 1971. Geological Map of Mekele Sheet Area ND 37-11 Tigre Province: Imperial Ethiopian Government, Ministry of Mines, Geol. Surv. Ethiopia.

Beyth M., 1972. The geology of central-western Tigre. Ph.D. Thesis, Univ. Bonn, 159 p. Beyth, M., 1996. Preliminary assessment of the Alid geothermal field, Eritrea. Isr. Geol. Surv., Current Research, 10: 124-127. Beyth, M., Stern, R.J. and Matthews, A., 1997. Significance of high-grade metasediments from the Neoproterozoic basement of Eritrea. Precam. Res., 86: 45-58. Bonatti, E., Emiliani, C., Ostlund, G. and Rydell, H., 1971. Final desiccation of the Afar rift, Ethiopia. Science, 172: 468-469. Clynne, M.A., Duffield, W.A., Fournier, R.O., Giorgis, W., Janik, C.J., Kashai, G., Lowenstern, J., Mariam, K., Smith, J.G. and Tesfai., T., 1996. Geothermal potential of the Alid Volcanic Center, Danakil Depression, Eritrea. U.S.Aid Fmal Rep. PASA# AOT-0002-P-OO5033-00, 46 p. DARA, 1997. Modular Optoelectronic Multispectral Stereo Scanner MOMS-2P, MOMS on PRIRODA: DLR, DFD, Germany, 64 p. Garland, C.R., 1980. Geology of the Adigrat area, 1:250,000 scale - including explanatory notes. The Provisional Military Government of Socialist Ethiopia, Ministry of Mines, Energy and Water Resources, Geol. Surv. Ethiopia. Ghebread, W., 1999. Tecto-metamorphic history of Neoproterozoic rocks in Eastern Eritrea. Precam. Res., 98: 83-104. Kazmin, V., 1973. Geological Map of Ethiopia (compiled) 1:2,000,000 scale - including explanatory notes. Imperial Ethiopian Governent, Ministry of Mines, Geological Survey of Ethiopia. Marinelli, G., Quaia, R and Santacroce, R., 1980. Volcanism and spreading in the northernmost segment of the Afar rift (Gulf of Zula). Geodynamic Evolution of the Afro-Arabian Rift System, Accademia Nazionale dei Lincei 47: 421-435. Merta. G., Abbate, E., Canuti. P., Sagri, M. and Tacconi, P., 1973. Geological Map of Ethiopia and Somalia, 1:2,000,000 scale. Consiglio Naz. Della RicercheItaly.

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