K&C Science Report – Phase 1 Mapping Subsurface Geology in Desert Areas Philippe PAILLOU OASU, Université de Bordeaux, France [email protected] Abstract—Using JERS-1 and PALSAR radar While the geographical coverage of the images provided by JAXA, we built regional Shuttle Imaging Radar missions was limited, and continental scale mosaics of Sahara that a more complete L-band radar coverage of the allowed to discover major geological features. eastern Sahara by the Japanese JERS-1 The unique capability of L-band SAR to map satellite was used to realize the first regional- subsurface structures in arid areas revealed scale radar mosaic covering Egypt, northern several impact craters and paleo-rivers in Egypt and Libya. Sudan, eastern Libya and northern Chad. This data set helped discover numerous unknown geological structures, particularly impact Index Terms—ALOS PALSAR, K&C Initiative, craters: a double impact crater was found in Sahara, subsurface geology, impact craters, southern Libya, in a flat and hyper arid area paleo-hydrology. covered by active aeolian deposits. More than 1300 small crater-like structures, distributed over an area of 40,000 km2, were also INTRODUCTION detected in the western Egyptian desert. Continental-scale exploration is now being Low frequency orbital Synthetic Aperture conducted using higher quality data from the Radar (SAR) has the capability to probe the new high-performance PALSAR L-band radar subsurface down to several meters in arid of the Japanese ALOS satellite. A new mosaic areas. Previous studies have shown that L- of the eastern Sahara made from PALSAR band SAR is able to penetrate meters of low scenes shows excellent data quality, allowing electrical loss material such as sand. The first a better detection of subsurface features. Shuttle Imaging Radar (SIR-A) obtained Using this unique data set, we discovered a some of the first subsurface imaging results major paleodrainage river in eastern Libya. for a site located in the Bir Safsaf region, in southern Egypt: SIR-A L-band radar revealed buried and previously unknown paleodrainage SUBSURFACE IMAGING USING JERS-1 channels, which afterwards were confirmed DATA during field expeditions. Subsequently, SIR-C data were used to map subsurface basement JERS-1 was launched in 1992 and acquired structures that control the Nile's course in L-band (1.275 GHz) SAR images of the Earth northeastern Sudan : numerous hidden faults until end of 1998. It provided 18m resolution were detected, thus helping to better images in HH polarization, with an off-nadir understand the Cenozoic uplift of the Nubian angle of 35o. Due to power feed problems, the Swell. More recent studies have shown that data present a high NEσ0 (noise equivalent σ0) combining SRTM – Shuttle Radar of -18dB. It is a crucial parameter for Topography Mission – topographic data with subsurface imaging since buried structures are SAR images better reveals subsurface likely to have a low backscattering return. features which still present a topographic Also, geocoding of JERS-1 SAR data was signature. New paleodrainage flow directions poor, with location errors reaching several have been mapped in the eastern Sahara, hundreds of meters. A complete L-band radar allowing better definition of drainage lines coverage of the eastern Sahara by the Japanese leading to oases and valleys, as well as a JERS-1 satellite exists and was used in 2003 better understanding of the Nubian aquifer in to realize the first regional-scale radar mosaic Libya. covering Egypt, northern Sudan, eastern Libya SW-NE, while the corresponding L-band radar and northern Chad [1]. The production and image extracted from our JERS-1 mosaic scientific analysis of more than 1600 SAR reveals two circular structures partially hidden scenes was used to study the near-surface by Quaternary deposits (fig. 1). The NE crater, geology hidden by the superficial sand layer, 6.8 km in diameter, is composed of concentric and we discovered numerous unknown inner and outer rings separated by a geological structures. depression filled with sediments. Its morphology is very similar to the Aorounga We thus revealed a double impact crater in crater in Chad, corresponding to a typical southern Libya: the structure is located 110 complex crater. Shatter cones and breccia km west of Djebel Arkenu and 250 km south were observed during field work in April of Kufra oasis in Libya, at coordinates 2003. Planar fractures were also found into 22o04’N, 23o45’E. It is a flat and hyperarid rock samples, confirming the impact origin of area covered by active aeolian deposits. The the craters [2]. optical Landsat 7 image of the region shows a sandy region with large sand dunes trending Figure 1. Landsat 7 ETM+ image of the Arkenu double crater (left), and corresponding JERS-1 radar image (right) at a resolution of 50m (JAXA/METI ). Figure 2. Surface view of the outcropping part of GKCF28 crater (40m in diameter, top) and corresponding GPR profile at 270 MHz (bottom) showing its subsurface shape. We also detected more than 1300 small resolution optical sensors (AVNIR-2 and crater-like structures distributed over an area PRISM) and one full polarimetric L-band of 40,000 km2 in the Western Egyptian Desert, SAR, PALSAR. This phased array SAR close to the Gilf Kebir plateau. Sixty-two of provides high resolution (10m) imagery with them were visited in the field during February variable incidence angle, with a much and December 2004 [3, 4]. Morphological improved value of NEσ0 around -25dB. The observations, rock samples and ground- geolocation accuracy is better than 10m and penetrating radar data were obtained [5] (fig. the radiometric accuracy is better than 1dB. 2). Shatter cone-like features, breccia and sub- The PALSAR instrument is operated to planar fractures were observed in the vincinity provide systematic wall-to-wall observations of most of the craters, but the impact origin of of all land areas on the Earth on a repetitive the field still has to be confirmed: basis. First acquisitions over North Africa took hydrothermal vent complexes could also place during ascending cycle 9 in January explain some of our observations [6]. 2007 (Fine Beam mode, HH polarization, Whatever its origin, the Gilf Kebir crater field incidence angle 34.3o). We produced a first is of great scientific interest and is a major small mosaic of 50 PALSAR scenes, element of the geological history of Western extending between 18-23oN and 24-30oE over Egypt. Further field and laboratory studies are southern Egypt and northern Sudan. required in order to better understand its Comparison between the JERS-1 SAR and nature and origin. PALSAR data clearly shows the superior capacity of the PALSAR sensor to map FIRST RESULTS USING ALOS/PALSAR subsurface features (fig. 3). Due to its DATA improved NEσ0 and finer resolution, PALSAR In January 2006, the JAXA successfully allows in particular a better detection of fine launched the Advanced Land Observing paleo-hydrological networks [7]. Satellite (ALOS). It carries two high Figure 3. Landsat (top left), JERS-1 (top right) and PALSAR image (bottom) of a region located around 21o55'N, 26o36'E. PALSAR much better reveals paleodrainage channels in the lower part of the scene (JAXA/METI ). Figure 4. PALSAR mosaic from acquisition cycles 12 and 13, covering Sahara and Arabia (JAXA/METI ). A full coverage of Sahara in HH and HV northern Uweinat and western Gilf Kebir / polarizations was acquired in June and July Abu Ras) that connect in the Kufrah oasis 2007 (ascending cycles 12 and 13). Using our region. The river system then flows north own data processing chain, we produced a through the Jebel Dalmah, and forms a large geocoded mosaic of Sahara and Arabia from alluvial fan in the Sarir Dalmah. The sand more than 400 dual-polarization PALSAR dunes of the Calanscio Sand Sea prevent deep strips (fig. 4). It covers latitude between 17- orbital radar penetration and preclude detailed 37oN and longitude between 17oW and 60oE. reconstruction of any possible connection to This dataset constitutes a unique tool for the the Mediterranean Sea, but a 300 km-long link scientific community to study the paleo- to the Gulf of Sirt through the Wadi Sahabi environment and paleoclimate of North paleochannel is likely. If this connection is Africa. It will also help build more complete confirmed, and its Miocene antiquity is geological maps in support to future water established, then the Kufrah River, prospecting in arid and semi-arid regions [8]. comparable in length to the Egyptian Nile, will have important implications for the We started the analysis of the PALSAR understanding of the past environments and mosaic over eastern Sahara. As a first result, climates of northern Africa from the middle we mapped a major paleodrainage system in Miocene to the Holocene [9]. Future work eastern Libya, that could have linked the concern the analysis of tha PALSAR mosaic Kufrah Basin to the Mediterranean coast to map paleodrainage networks in western through the Sirt Basin, possibly as far back as Sahara (Mauritania, Niger, Mali). We aslo the middle Miocene. Images from the plan to apply the same approach to study PALSAR sensor clearly reveal a 900 km-long subsurface geology in arid regions of northen river system (fig. 5), which starts with three China (Sinkiang and Badain Jaran deserts). main tributaries (north-eastern Tibesti, Figure 5. The Kufrah River (in blue) mapped onto SRTM topography (left) and PALSAR mosaic (right). The red dotted line represents a possible path to the Mediterranean coast (JAXA/METI ). ACKNOWLEDGMENTS crater-shaped structures in the Gilf Kebir region – This work has been undertaken within the Egypt: Observations and hypotheses about their origin”, Journal of African Earth Sciences, vol.