Farouk El-Baz

Document Type

Article

Publication Date

1998

Abstract

Nearly all sand dune fields in the eastern Sahara are located within topographic depressions. The sand, mostly composed of quartz grains, occurs south of lime. stone plateaus that border the Mediterranean seacoast, over which the wind blows southward. The source ofthe sand is the "Nubian Sandstone," which is exposed throughout the southern part of the eastern Sahara. Satellite images, particularly radar data, reveal that sand-covered, northward-trending courses of dry rivers end at the depressions. The sand appears to have been deposited, most likely in lake beds, during wet climates. Alternating dry climatic episodes resulted in sculpturing these deposits intfr sand dunes and sheets by southward flowing wind. The depressions must have hosted great volumes of surface water during the wet climates. Much S that water would have seeped into the underlying rock through primary andlor secondary porosity. It foL- lows that areas of large accumulations of sand may host vast groundwater resources.Mapper (TM as well as radar images from the Spaceborne Imaging Radar (SIR) of thc American Space Shuttle and the Radarsat spacc- craft of the Canadian Space Agency. These data provide unique per- spectives that allow the recognition of regional influences on groundwater concentration, as well as the necessary information for detailed evaluation of the groundwater potential in a given area. In addition to the primary porosity of the sandstone in the east- ern Sahara, structural stresses causing the formation of faults induce porosity along fracture zones. The latter appear to have significant control on the trends of exposed and sand-buried drainage lines, thus, on the enhancement of ground water recharge into the substrate. Therefore, it would be prudent to apply the concept of fracture zone aquifers (Bisson and El-Baz, 1991) to the exploration for groundwa- ter in this descrt. It is hoped that the ideas presented in this paper will influence the strategy of groundwater exploration in the eastern Sahara and similar desert regions worldwide.

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