Rimaal: A Sand Buried Structure of Possible Impact Origin in the Sahara: Optical and Radar Remote Sensing Investigation

Abstract

This work communicates the discovery of a sandy buried 10.5 km diameter near-circular structure in the eastern part of the Great Sahara in North Africa. Rimaal, meaning "sand" in Arabic, is given as the name for this structure since it is largely concealed beneath the Sahara Aeolian sand. Remote sensing image fusion and transformation of multispectral data (from Landsat-8) and synthetic aperture radar (from Sentinel-1 and ALOS PALSAR), of dual wavelengths (C and L-bands) and multi-polarization (HV, VV, HH, and HV), were adopted in this work. The optical and microwave hybrid imagery enabled the combining of surface spectral properties and subsurface roughness information for better understanding of the Rimaal structure. The long wavelength of the radar, in particular, enabled the penetration of desert sands and the revealing of the proposed structure. The structure exhibits a clear outer rim with traces of concentric faults, an annular flat basin and an inner ring surrounding remnants of a highly eroded central peak. Radar imagery clearly shows the interior wall of the structure is incised with radial pattern gullies that originate at or near the crater periphery, implying a much steeper rim wall in the past. In addition, data reveals a circumferential of a paleoriver course that flows along a curved path parallel to the crater's western margin indicating the plausible presence of a concentric ring graben related to the inferred structure. The defined crater boundary is coincident with a shallow semi-circular-like basin in the SRTM elevation data. The structure portrays considerable modifications by extensive long-term Aeolian and fluvial erosion. Residing in the Cretaceous Nubian Sandstone formation suggests an old age of ≤65 Ma for the structure. If proven to be of an impact origin, the Rimaal structure could help in understanding the early evolution of the landscape of the Eastern Sahara and holds promise for hosting economically valuable ore deposits and hydrocarbon resources in the region.

Flash Flood Hazard Using Optical, Radar, and Stereo-Pair Derived DEM: Eastern Desert, Egypt

“…The soil texture properties along the main wadi channel bed were mapped using PALSAR radar imagery ( Figure 3f). Unlike multispectral satellite imagery, radar data provides surface roughness (grain size relative to the radar wavelength) that can be used to distinguish fine-grained from coarse-grained material [13,14,25]. Radar backscatter coefficient values extracted from radar data were correlated and checked in the field, confirming that radar wavelengths produced similar textural classes that correspond to specific grain or fragment sizes of alluvial deposits in dry lands [25].…”

“…Important surface topology variables such as stream network, catchment area, and surface gradient can be derived from a digital elevation model (DEM). Land use and land cover characteristics can be derived from the classification of multispectral satellite imagery, whereas surface lithology and soil texture properties can be extracted from a fusion of multispectral and spaceborne radar images [13,14]. The ultimate goal of this work is to integrate remote sensing and GIS techniques, along with hydrological and hydraulic models, to explore the flood response of Wadi El-Ambagi to intense, short-lived precipitation events.…”

The Egyptian pyramid chain was built along the now abandoned Ahramat Nile Branch

“…In this context, radar waves strip away the surface sand layer and expose previously unidentified buried channels. The penetration capability of radar waves in the hyper-arid regions of North Africa is well documented 4,[34][35][36][37] . The penetration depth varies according to the radar wavelength used at the time of imaging.…”

“…Topographic data represents a primary tool in investigating surface landforms and geomorphological change both spatially and temporally. This data is vital in mapping past river systems due to its ability to show subtle variations in landform morphology 37 . In low lying areas, such as the Nile floodplain, detailed elevation data can detect abandoned channels, fossilized natural levees, river meander scars and former islands, which are all crucial elements for reconstructing the ancient Nile hydrological network.…”

Integration of Well Logging and Remote Sensing Data for Detecting Potential Geothermal Sites along the Gulf of Suez, Egypt

“…A high drainage density, particularly of those structurally controlled channels, is favorable for geothermal energy availability. The drainage network was delineated based on a procedure by [53], using the widely used 8D flow direction algorithm [54][55][56]. The stream network was generated using a threshold of 200 cells and matched with those visible in Google Earth Pro imagery for verification.…”