remote sensing

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

Eman Ghoneim

Department of Earth and Sciences, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5944, USA; [email protected]; Tel.: +1-910-962-2795

 Received: 24 April 2018; Accepted: 4 June 2018; Published: 5 June 2018 

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 . Residing in the Nubian 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.

Keywords: ALOS PALSAR; Sentinel-1; SRTM; data fusion; minimum noise fraction

1. Introduction Bombardment by meteorites was a major geological process during the early history of Earth [1]. Unlike the Earth, the lack of the geological and atmospheric process on the Moon and other planets have prevented erosion of their impact craters. However, scars of these geologic forces and the past cratered Earth’s surfaces have been deeply erased by erosion and plate tectonic processes [2]. The discovery of new impact structures is not only of high importance to gain an understanding of the Earth’s early history, but also has economic significance as possible hosts of valuable ore deposits (e.g., gold and uranium) and hydrocarbon resources (e.g., oil and natural gas) [3,4]. For example, in North America alone, approximately nine out of 19 confirmed impact craters were exploited for oil and gas with a daily production of 2–30 million barrels of oil and more than 42 million m3 of gas of gas [5]. Consequently, it is worthwhile to search for new unknown terrestrial impact structures.

Remote Sens. 2018, 10, 880; doi:10.3390/rs10060880 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 880 2 of 17

Remote Sens. 2018, 10, x FOR PEER REVIEW 2 of 17 Terrestrial craters are composed largely of two basic types: small, simple craters (<4 km in diameter) and large,Terrestrial complex craterscraters (>4are kmcomposed in diameter) largely [6of]. two A simple basic cratertypes: issmall, a small simp bowl-shapedle craters (<4 depression,km in withdiameter) a structurally and large, uplifted complex rim, craters which (>4 includeskm in diameter) an overturned [6]. A simple flap crater and is ejecta. a smallA bowl-shaped complex crater, on thedepression, other hand with, exhibits a structurally a structurally uplifted rim, complex which outerincludes rim an with overturned a terraced flap and wall, ejecta. a topographically A complex lowercrater, annular on trough, the other and hand, a central exhibits topographic a structurally peak [7 ].complex Today, thereouter arerim about with 190a terraced confirmed wall, terrestrial a topographically lower annular trough, and a central topographic peak [7]. Today, there are about 190 impact craters (simple and complex) listed in the Earth Impact Database (http://www.unb.ca/passc/ confirmed terrestrial impact craters (simple and complex) listed in the Earth Impact Database ImpactDatabase). The distribution of these impact structures, as mapped by [4], illustrates a significant (http://www.unb.ca/passc/ImpactDatabase). The distribution of these impact structures, as mapped by crater[4], clustering illustrates in Northa significant America, crater Eurasia, clustering and in Australia, North America, whereas Eurasia, the remaining and Australia, continents, whereas particularly the Africa,remaining lack their continents, share ofimpact particularly craters Africa, recorded lack their at the share same of impact level. craters recorded at the same level. To date,To onlydate, 20only structures 20 structures were were confirmed confirmed to be to of be an of impact an impact origin origin in the in entire the entire African African continent. In addition,continent. 49 In structures addition, 49 are structures currently are cited currently as being cited of as potentialbeing of potential impact impact origin origin and still and awaiting still field verification.awaiting field Approximately,verification. Approxim 55%ately, of the 55% confirmed of the confirmed structures structures in Africa in Africa are foundare found in in the the Great SaharaGreat alone Sahara [4] (Figure alone [4]1). (Figure In fact, 1). the In fact, Sahara the hasSahara long has been long knownbeen known to host to host a large a large number number of of these structuresthese whichstructures were which found were either found during either oilduring field oil explorations field explorations (e.g., the(e.g., B.P. the confirmed B.P. confirmed structure structure by [8]) or during the mapping of ancient river courses and lake basins for groundwater by [8]) or during the mapping of ancient river courses and lake basins for groundwater exploration exploration (e.g., the Ibn-Batutah structure by [9]). There are seven structures that still remain to be (e.g., the Ibn-Batutah structure by [9]). There are seven structures that still remain to be discovered, discovered, buried beneath the dry sand of the desert. Presently, the harsh conditions and difficult buriedaccessibility beneath the are dry impeding sand of the the exploration desert. Presently, of the vast the harsharea of conditions the Great andSahara difficult by ground-based accessibility are impedingtechniques, the exploration making satellite of the data vast the area only offeasible the Great means Sahara to hunt byfor ground-basedsuspected impact techniques, structures (e.g., making satellite[9,10]). data the only feasible means to hunt for suspected impact structures (e.g., [9,10]).

(a)

(b)

Figure 1. Confirmed and potential impact structures in the Great Sahara of North Africa. (a) Figure 1. Confirmed and potential impact structures in the Great Sahara of North Africa. (a) Distribution of Distribution of the structures plotted on an EarthSat mosaic. Confirmed structures are mapped in the structures plotted on an EarthSat mosaic. Confirmed structures are mapped in green, whereas proposed green, whereas proposed structures (including Rimaal), which are awaiting field confirmation, are structures (including Rimaal), which are awaiting field confirmation, are plotted in red. (b) Confirmed and plotted in red. (b) Confirmed and proposed craters in the Great Sahara with their reported diameters. proposed craters in the Great Sahara with their reported diameters.

Remote Sens. 2018, 10, 880 3 of 17

Over the last four decades, the growing number and quality of space imagers has strongly contributed to the identification of possible impact structures (e.g., [9,11]) which usually appear as distinct ring anomalies marking the original Earth surface. Unlike optical sensors, which can only Remote Sens. 2018, 10, x FOR PEER REVIEW 3 of 17 image the surface of the desert, long-wave microwave sensors can unveil sand-hidden subsurface features [12].Over In particular,the last four radardecades, images the growing at C and numb Ler bands and quality have successfully of space imagers revealed has strongly sand-buried paleorivercontributed courses to and the lake identification basins in of several possible parts impact of structures the Great (e.g., Sahara [9,11]) [13 which–17]. Anusually important appear as property of imagingdistinct radar ring isanomalies its ability marking to penetrate the original dry Earth desert surface. sand Unlike and optical produce sensors, images which ofcan the only shallow subsurfaceimage terrain the surface [15]. Forof the radar desert, wave long-wave penetration, microw theave sand sensors cover can needsunveil tosand-hidden be fine-grained, subsurface physically features [12]. In particular, radar images at C and L bands have successfully revealed sand-buried homogeneous, and extremely dry with a moisture content of less than 1%. The low dielectric constant paleoriver courses and lake basins in several parts of the Great Sahara [13–17]. An important property of the Saharaof imaging sand radar results is its in ability a low to attenuation penetrate dry and desert maximum sand and penetration produce images for theof the radar shallow waves [18]. These criteriasubsurface are allterrain satisfied [15]. inFor the radar sandy wave desert penetrat of Northion, the Africa. sand cover needs to be fine-grained, Remotephysically sensing homogeneous, has contributed and extremely to the dry discovery with a moisture of several content suspected of less impactthan 1%. structures The low in the Sahara region.dielectric Outconstant of theof the 13 Sahara confirmed sand results structures in a low cited attenuation in North and Africa,maximum about penetration five impact for the craters were foundradarin waves the eastern[18]. These sector criteria of are the all Great satisfied Sahara in the clusteredsandy desert in of Southeastern North Africa. , Southwestern Remote sensing has contributed to the discovery of several suspected impact structures in the , andSahara Northern region. Out Chad. of the Yet, 13 no confirmed circular structures structure cited has in been North cited Africa, in the about neighboring five impact Sahara craters area in Northwesternwere found . in the With eastern vast sector areas of the of desert,Great Saha Sudanra clustered still holds in Southeastern high potential Libya, forSouthwestern the discovery of impact craters.Egypt, and This Northern work, therefore,Chad. Yet, no is aimingcircular struct at exploringure has been a new cited large in the crater-like neighboring structure Sahara area of possible impact originin Northwestern in the Northwestern Sudan. With vast Sudan areas of desert desert using, Sudan optical still holds and high radar potential satellite for the data. discovery A thorough examinationof impact of the craters. morphology This work, and therefore, the regional is aiming influence at exploring exerted a new by large the conceivablecrater-like structure impact of event is possible impact origin in the Northwestern Sudan desert using optical and radar satellite data. A considered in this work. thorough examination of the morphology and the regional ◦influence0 exerted by◦ the0 conceivable Theimpact discovered event is Rimaal considered circular in this structure work. is centered at 21 34 10” N and 20 50 15” E, about 280 km south of UweinatThe discovered Mountain. Rimaal It iscircular located structure in the is prefecture centered at of21°34 the′10 State″ N and of Northern20°50′15″ E, Darfur about 280 in Sudan about 654km kmsouth north of Uweinat of Al-Fashir Mountain. City. It is located As there in the are pr noefecture prominent of the State landmarks of Northern or Darfur geographic in Sudan features in the immediateabout 654 km vicinity north of of Al-Fashir the structure, City. As Rimaal,there are meaningno prominent “sand” landmarks in Arabic, or geographic is given features as the name for thisin structure the immediate since vicinity it is largely of the concealedstructure, Rimaal, beneath meaning Aeolian “sand” sand in Arabic, cover. is The given subsurface as the name geology for this structure since it is largely concealed beneath Aeolian sand cover. The subsurface geology of of the regionthe region isnot is not very very wellwell known; known; nevertheless, nevertheless, the region, the region, in general, in is general, mapped as is Mesozoic mapped Nubian as Mesozoic NubianSandstone Sandstone [19] [19 (Figure] (Figure 2). 2The). The thickness thickness of the of Nu thebian Nubian Sandston Sandston in the eastern in the easternSahara region Sahara is region is unknown,unknown, but itbut can it can generally generally reach reach aa maximum thic thicknesskness of 1700 of 1700 m in msome in someparts [20]. parts [20].

Figure 2.FigureA simplified 2. A simplified geological geological map map of of the the regionregion (map (map modified modified after after [21] [showing21] showing the Rimaal the Rimaal structure (marked as a circle) residing in the Cretaceous Nubian Sandstone formation. structure (marked as a circle) residing in the Cretaceous Nubian Sandstone formation.

Remote Sens. 2018, 10, 880 4 of 17

2. Materials and Methods Remote sensing is an important tool for the initial recognition of suspected impact structures, particularly in regions that are made up of the same rock as the surroundings. Optical imagery (Multispectral Sentinel-2 and Landsat-8 (LS8), microwave data (Sentinel-1 and ALOS PALSAR), and digital elevation data (Shuttle Radar Topography Mission—SRTM) were used in this study.

2.1. Optical Data It is vital to use multispectral data and compare it with microwave data in desert regions in order to determine if the portrayed features are on the surface or in the near-surface. Both Sentinel-2 and Landsat-8 (LS-8) data were employed in this study. Sentinel-2, acquired in 25 April 2018, provides thirteen spectral bands with a spatial resolution that ranged from 10 m to 60 m for the visible, NIR (near infrared), and SWIR (shortwave infrared) wavelengths. All spectral bands, except channels 9 and 10 (the water vapor and cirrus detection bands, respectively), were used in this research. LS8, acquired in 22 October 2017, provides eleven spectral channels with a spatial resolution of 30 m for the visible, NIR and SWIR wavelengths, 100 m for the TIR (thermal infrared), and 15 m for the panchromatic band. All spectral channels, except bands 8 and 9 (the panchromatic and cirrus detection bands, respectively), were used in the present study. The Sentinel-2 and LS-8 raw images show little contrast due to the relative homogeneity of the surface cover type in the study area. The high inter-spectral-band correlation, which commonly encountered in multispectral data, results in a reduction of the interpretability of the satellite image.

2.2. Microwave Data Radar data from both Sentinel-1 and ALOS Phased Array L-band Synthetic Aperture Radar (PALSAR) sensors were used. The Sentinel-1 system is a radar satellite constellation consisting of two C-band synthetic aperture radar sensors, Sentinel-1A (launched in April 2014) and Sentinel-1B (launched in April 2016), operating at 5.405 GHz, and dual-polarization support (HH+HV, VV+VH). Both satellites fly in the same near-polar, sun-synchronous orbit [22]. The Sentinel-1 image used in this study was acquired on 7 January 2015 in a descending orbit with an interferometric wide swath mode (IW), a large swath width of 250 km at ground resolutions of 5 m × 20 m, and dual polarizations of VV+VH. An ascending ALOS PALSAR, Level 1.5 (geo-referenced amplitude image) data product, along with a radiometrically terrain-corrected (RTC) data product, was utilized in this study. The PALSAR scene with a multi-look complex (MLC) and geo-referenced to UTM coordinate data, was acquired in 13 July 2008 in a fine beam mode (FBD). It operates at 1.27 GHz (L-Band), and dual-polarization support of HH and HV at a spatial resolution of 12.5 m. The process began by applying a precise orbit to the radar data and conducting image radiometric calibration to obtain backscattering values. The radar images were then speckle filtered using a Lee Sigma algorithm with a 5 × 5 pixel window. Terrain geocoding was then implemented using the SRTM 30 m DEM, along with the ALOS orbit data. Ortho images were projected to UTM coordinates, zone 35. A multi-looking function was then applied to generate accurate pixel dimensions (to produce square pixels out of rectangular SAR pixels). These processing sequences generated a geo-coded, orthorectified, terrain-corrected, radiometrically-calibrated and normalized Sentinel-1 and PALSAR scenes with a pixel size of 12.5 m. A LS8 and PALSAR image fusion, using the Gram Schmid algorithm [23], along with image transformation, using the minimum noise fraction (MNF) approach [24] were, therefore, applied to enhance the visual interpretability of the structure morphology. LS8 was fused together with the PALSAR L-HH imagery to produce a hybrid image that combines information of spectral properties (from the LS8) and surface roughness (from the PALSAR image) to improve the visual interpretation of the structural features and drainage systems in the surrounding area. The minimum noise fraction Remote Sens. 2018, 10, 880 5 of 17

(MNF), a principal component transformation algorithm [24], was applied on a stacked multispectral LS8 and PALSAR microwave image to eliminate data redundancy and enhance the surficial mapping of the Rimaal structure. In the MNF procedure, the first phase results in transformed data in which the noise has unit variance and no band-to-band correlations, whereas the second phase is a standard PC transformation of the noise-whitened data, which gives rise to final outputs that are not correlated and are arranged in terms of decreasing information content [24]. The first three MNF components were used in the image analysis as they explain most of the variance in the scene (96.2%). The remaining components were discarded from the analysis since they contain proportion of noise contents. This procedure generated a hybrid MNF image that contains spectral and texture information and, thus, compensates for the limitation of using single remote sensing data products alone.

2.3. Digital Elevation Data For digital topographic data, the 30 m SRTM and the 12.5 m ALOS PALSAR terrain dataset were used. Drainages were extracted based on a procedure by [25], with drainage pathways and flow accumulation delineated with the widely used 8D flow direction algorithm [17,26]. Drainage networks were generated using a threshold of 2000 cells and were then compared against the actual subsurface channels revealed in Sentinel-1 and PALSAR radar scenes. A good agreement was found between the derived drainage networks and those revealed by radar images, validating the hydrologic delineation process. All the above data were projected to the Universal Transverse Mercator (UTM) and WGS84 datum. Satellite images and terrain model processing were carried out using ENVI 5.4 software and SARscape “image processing workbench” (Exelis Visual Information Solutions, Boulder, CO, USA) as well as ArcGIS 10.5.1 software (Environmental Systems Research Institute, Redlands, CA, USA). The above data were grouped in a geographic information system (GIS) to allow overlaying and correlation of the surface and near-surface features. Each data source added significant information which facilitated a clear examination of the Rimaal structure. In fact, the adopted procedure which utilizes optical and radar data was proven by the author to be effective in revealing crater structures is sandy desert regions, such as the Ibn-Batutah structure in Libya [9].

3. Results

3.1. Multispectral Data Both Sentinel-2 and LS8 optical satellite images faintly show the Rimaal structure consists of a single circular wrinkle ridge surrounding a basin that is completely infilled by smooth sandy material (Figure3). The area appears as a featureless desert surface, except for segments of ancient rivers that are most probably of Tertiary age [13]. The images barely reveal the structure as a light tonal circular feature coated by Aeolian sand. The topography of the area in Sentinel-2 and LS8 is relatively subdued with no obvious expression of a topographic rim to the Rimaal structure. In the visible part of the spectrum, the albedo of the filling sand is close to the albedo of the surrounding country rocks, leading to the subdued expression of the surface features [9]. In commonly used moderate to high spatial resolution multispectral data, such as LS, Sentinel-2, and ASTER (30 m and 10 m in the VNIR), sandy surfaces in areas that are made up of relatively low contrasting bedrocks show similar spectral responses that hinder detailed mapping or classification of the area. This issue could likely be improved when using very high spatial resolution multispectral data with spectral channels in the mid-infrared region of the spectrum (e.g., WorldView-3) [27]. Remote Sens. 2018, 10, 880 6 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 6 of 17

Figure 3. Multispectral and radar images of the Rimaal structure. (a) Sentinel-2 using bands 432 as Figure 3. MultispectralRGB, (b) Landsat-8 and using radar bandsimages 432 as RGB of and the (c Rimaal) ALOS PALSAR structure. image( ofa) the Sentinel-2 structure. While using the bands 432 as RGB, (bstructure) Landsat-8 is barely using noticeable bands as a 432bright as circular RGB feature and ( cin) ALOSa homogeneous PALSAR low-contrasted image of sandy the structure. While the structureterrain in bothis barely Sentinel-2 noticeable an Landsat-8 as aimages, bright it is circular clearly visible feature in the in amicrowave homogeneous imagery low-contrasteddue to sandy terrainthe penetration in both Sentinel-2 capability anof the Landsat-8 radar signals images, to desert it issand. clearly visible in the microwave imagery due

to the penetration3.2. Radar Data capability of the radar signals to desert sand. The aptitude of radar signals to differentiate among distinct surface roughnesses is particularly 3.2. Radar Dataadvantageous in low contrasting sandy desert regions that show analogous spectral behaviors on multispectral data, as is the case of the present study area. Radar data clearly show an isolated crater- The aptitude of radar signals to differentiate among distinct surface roughnesses is particularly like structure of nearly spherical shape with a dark radar backscatter delimited by bright radar signals advantageous(Figure in 3b). low This contrasting can be interpreted sandy as deserta smooth-sand regions floored that basin show (crater’s analogous interior)spectral delimited behaviorsby a on multispectralrough data, bedrock as surface is the (crater’s case of outer the rim). present At thestudy center of area. the structure, Radar a data near-circular clearly ring show feature an isolated crater-likeof structure a bright radar of nearly backscatter, spherical surrounded shape by with a dark a annular dark radar trough, backscatter is clearly visible. delimited by bright radar In the Sentinel-1 image (Figure 4), the overall circularity of the Rimaal structure, however, was signals (Figure3b). This can be interpreted as a smooth-sand floored basin (crater’s interior) delimited concealed, to some extent, along its eastern flank. This could be due to the presence of a coarse-grained by a roughmaterial bedrock that surfacecovers a vast (crater’s part of the outer structure rim). an Atd, thus, the acts center as a ofrough the surface structure, to the shorter a near-circular radar ring feature of awavelength bright radar of the backscatter,Sentinel-1 C-band surrounded, causing a specular by a darkbackscatter annular of bright trough, radar signals. is clearly The Sentinel- visible. In the1 C-band Sentinel-1 VV like-polarized image (Figure image is,4 ),however, the overall substantially circularity superior in of portraying the Rimaal the gullies structure, that incised however, the interior wall of the structure’s outer rim. A close-up look at the structure divulges traces of concentric was concealed, to some extent, along its eastern flank. This could be due to the presence of a faults on the inner wall of the northern flank of the structure (Figure 4). Considering a penetration depth coarse-grained material that covers a vast part of the structure and, thus, acts as a rough surface to the shorter radar wavelength of the Sentinel-1 C-band, causing a specular backscatter of bright radar signals. The Sentinel-1 C-band VV like-polarized image is, however, substantially superior in portraying the gullies that incised the interior wall of the structure’s outer rim. A close-up look at the structure divulges traces of concentric faults on the inner wall of the northern flank of the structure (Figure4). Considering a penetration depth of 0.5 m for the Sentinel-1 C-band signals in a dry sand surface [18] leads to the proposition that the western and northern flanks of the Rimaal structure are covered by, at most, 50 cm of dry desert sand. Remote Sens. 2018, 10, x FOR PEER REVIEW 7 of 17

Remote Sens. 2018, 10, 880 7 of 17 of 0.5 m for the Sentinel-1 C-band signals in a dry sand surface [18] leads to the proposition that the western and northern flanks of the Rimaal structure are covered by, at most, 50 cm of dry desert sand.

Figure 4. Radar images of the structure with different wavelengths and polarizations. A close-up look Figure 4. Radar images of the structure with different wavelengths and polarizations. A close-up at the structure in (a) Sentinel-1 C-HV, (b) Sentinel-1 C-VV, (c) PALSAR L-HV, and (d) PALSAR L-HH look at thepolarizations. structure in Bright (a) Sentinel-1areas in the C-HV,radar images (b) Sentinel-1 indicate rocky C-VV, surfaces (c) and PALSAR coarse-grained L-HV, materials, and (d) PALSAR L-HH polarizations.whereas dark areas Bright are fine-grained areas in thematerials, radar such images as Aeolian indicate and fluvial rocky deposits. surfaces Note the and superiority coarse-grained materials,of whereas the PALSAR dark L-bands areas ( arec,d) in fine-grained revealing the complete materials, circularit suchy asof the Aeolian structure and when fluvial compared deposits. to Note the superioritySentinel-1 of C-bands. the PALSAR This is due L-bands to the longer (c,d) wavele in revealingngth of the theL-band complete which is capable circularity of penetrating of the structure sand deeper than the shorter wavelength of the C-band. (e) A close-up look of the Sentinel-1 C-band. when comparedNote the to superiority Sentinel-1 of the C-bands. shorter wavelength This is due in revealing to the longerthe radial wavelength pattern drainage of thenetwork L-band and which is capable oftraces penetrating of concentric sand faults deeper on the thaninner wa thell shorterof the northern wavelength flank of the of structure. the C-band. (e) A close-up look of the Sentinel-1 C-band. Note the superiority of the shorter wavelength in revealing the radial pattern drainage networkThe longer and wavelength traces of concentricof the PALSAR faults L-band on the (Figure inner 4) wall was of capable the northern of penetrating flank ofdeeper the structure. (~3 m in dry fine sand) and reveal larger parts of the hidden eastern flank of the inferred crater when compared to that of Sentinel-1 C-band (with max penetration of 0.5 m in dry fine sand) [15,27]. Coarse The longergrained wavelengthmaterial are smooth of the to the PALSAR radar L-band L-band and, (Figurethus, radar4) waves was capable are capable of of penetrating penetrating deeper (~3 m in drydeeper fine and sand) reflect andless energy reveal back larger to the parts radar’s of receiving the hidden antenna. eastern This allows flank key ofmorphometric the inferred crater elements, like the structure’s annual trough, to appear dark in the radar L-band imagery. As a when comparedconsequence, to that it provides of Sentinel-1 greater contrast C-band between (with the max structure’s penetration troughof and 0.5 its mbright in dry outer fine rim sand)and [15,27]. Coarse grainedcentral topographic material arering smoothand peak, toemphasizing the radar the L-band overall circularity and, thus, of the radar Rimaal waves structure. are The capable of penetratingco-polarized deeperand L-HH reflect imagery less shows energy a significant back toimprovement the radar’s in exposing receiving the antenna.structure rim This when allows key morphometric elements, like the structure’s annual trough, to appear dark in the radar L-band imagery. As a consequence, it provides greater contrast between the structure’s trough and its bright outer rim and central topographic ring and peak, emphasizing the overall circularity of the Rimaal structure. The co-polarized L-HH imagery shows a significant improvement in exposing the structure rim when compared to the cross-polarized imager (L-HV). In the L-HH imagery, the rim shows a stronger backscatter than the surroundings, therefore becoming easier to detect.

3.3. Fusion and Transformation of Multispectral and Radar Data LS8 and PALSAR image fusion (Figure5), using the Gram Schmid algorithm, along with multispectral and radar image transformation, using the MNF approach, have significantly boosted Remote Sens. 2018, 10, x FOR PEER REVIEW 8 of 17

compared to the cross-polarized imager (L-HV). In the L-HH imagery, the rim shows a stronger Remotebackscatter Sens. 2018, 10 than, 880 the surroundings, therefore becoming easier to detect. 8 of 17

3.3. Fusion and Transformation of Multispectral and Radar Data the visual interpretability of the structure’s morphology. Fusion and transformation of LS8 and radar LS8 and PALSAR image fusion (Figure 5), using the Gram Schmid algorithm, along with data enabled the combining of information of spectral properties (from multispectral data) and surface multispectral and radar image transformation, using the MNF approach, have significantly boosted roughness (from microwave data) to enhance the visibility of subtle surface and near-surface features the visual interpretability of the structure’s morphology. Fusion and transformation of LS8 and radar of thedata Rimaal enabled structure. the combining The fused of image information reveals of clearly spectral the properties circularity (from of the multispectral structures anddata) the and radial drainagesurface patterns roughness buried (from beneath microwave the Aeolian data) to deposits.enhance the The visibility fused imageof subtle highlights surface and extensive near-surface systems of NE–SWfeatures trending of the Rimaal elongated structure. sand The dunes fused that image run reveals across clearly the Rimaal the circularity structure of andthe structures obscure muchand of the detailthe radial of the drainage inner ring patterns and the buried outer rim.beneath The th dunes’e Aeolian orientation deposits. indicates The fused a strong image influence highlights of the easterlyextensive trade windsystems in of the NE–SW area. trending elongated sand dunes that run across the Rimaal structure and Figureobscure5 displaysmuch of anthe RGBdetail color of the composite inner ring image and the of aouter stacked rim. Sentinel-1The dunes’ C-VV orientation and PALSAR indicates L-HH a and L-HVstrong channels, influence of which the easter clearlyly trade reveals windthe in the morphology area. of a large part of the Rimaal structure. Figure 5 displays an RGB color composite image of a stacked Sentinel-1 C-VV and PALSAR L- The structure is a 10.5 km-diameter near-circular bowl-shaped depression. Impact craters larger than HH and L-HV channels, which clearly reveals the morphology of a large part of the Rimaal structure. 10 km in diameter are most probably created by a meteorite with diameter larger than 1 km [28]. The structure is a 10.5 km-diameter near-circular bowl-shaped depression. Impact craters larger than Assuming10 km a in spherical diameter projectile, are most probably the volume created of the by impactor a meteorite of with the Rimaal diameter structure larger than is estimated 1 km [28]. to be 3 aboutAssuming 606 km . a It spherical comprises projectile, a small, the irregularly-shaped volume of the impactor central of the peak Rimaal surrounded structure byis estimated a near-circular to 1 to 1.5be km aboutwide 606 ringkm3. featureIt comprises of a stronga small, radar irregularly-shaped backscatter. Thiscentral is peak followed surrounded by a slightly by a near-circular flat radardark annular1 to trough, 1.5 km ofwide a fairly ring uniformfeature of width a strong of ~2.5radar km backscatter. surrounded This by is afollowed radar bright by a slightly outer rim. flat Theradar inner wall ofdark the annular rim exhibits trough, tracesof a fairly of concentricuniform width faults. of ~2.5 PALSAR km surrounded surface by roughness a radar bright profile outer indicates rim. a muchThe coarser inner textured wall of the material rim exhibits of the traces crater of interior concentric than faults. the surroundings. PALSAR surface Here, roughness the inner profile ring of the structureindicates is amade much upcoarser of a significantlytextured material coarse of the textured crater materialinterior than (e.g., the bedrocks surroundings. and boulders) Here, the than inner ring of the structure is made up of a significantly coarse textured material (e.g., bedrocks and that of the annual trough of fine textured grains (e.g., fine Aeolian and fluvial deposits) (Figure6). boulders) than that of the annual trough of fine textured grains (e.g., fine Aeolian and fluvial deposits) Typically, in complex impact craters, the central uplift structures are often more resistant to weathering (Figure 6). Typically, in complex impact craters, the central uplift structures are often more resistant than theto weathering surrounding than crater the surrounding fill of impact crater breccias fill of im and,pact thus, breccias could and, be thus, the onlycould relicbe the of only deeply relic eroded of impactdeeply craters eroded [4]. impact craters [4].

FigureFigure 5. Fusion 5. Fusion and transformation and transformation of multispectral of multispectral andradar and radar data ofdata the of Rimaal the Rimaal structure. structure. (a) Landsat-8 (a) true colorLandsat-8 (bands true 4, color 3, 2 as(bands R, G, 4, B); 3, (2b as) fusionR, G, B); of ( LS8b) fusion with of Palsar LS8 with HH Palsar polarization HH polarization images using images Gram Schmid algorithm; (c) MNF image transformation of a stacked LS8 multispectral and PALSAR microwave image. The figure shows that combining of spectral and roughness data dramatically enhances the visibility of the subtle surface and near-surface features of the structure. (d) Sentinel-1 and PALSAR stacked image (C-VV, L-HH, and L-HV as RGB) clearly reveals large parts of the morphology of the Rimaal structure. Remote Sens. 2018, 10, x FOR PEER REVIEW 9 of 17

using Gram Schmid algorithm; (c) MNF image transformation of a stacked LS8 multispectral and PALSAR microwave image. The figure shows that combining of spectral and roughness data dramatically enhances the visibility of the subtle surface and near-surface features of the structure. (d) Sentinel-1 and PALSAR stacked image (C-VV, L-HH, and L-HV as RGB) clearly reveals large parts Remote Sens. 2018, 10, 880 9 of 17 of the morphology of the Rimaal structure.

Figure 6. The topography of the Rimaal structure. ( a) A terrain oblique 3D perspective view of the structure. (b) An oblique, 3D perspectiv perspectivee view produced by superimposing the PALSAR L-HH image over the SRTM data.

3.4. Digital Elevation Data In the the terrain terrain data, data, the the Rimaal Rimaal structure structure is gent is gentlyly undulating undulating with with low lowtopographic topographic relief. relief. The Theradar radar defined defined crater crater periphery periphery is coincident is coincident with with a semicircular-like a semicircular-like basi basinn in the in DEM the DEM (Figure (Figure 6). 6). A topographic profileprofile derived from these data shows the structure as a bowl-like shallow depression that is nearly 45 m deepdeep withwith aa generalgeneral tilttilt fromfrom westwest toto easteast (Figure(Figure7 7).). TheThe formationformation does not display display a a clear clear raised raised rim, rim, particularly particularly at atits itseastern eastern side, side, and and the thewall wall is not is as not steep. as steep. The Theperpendicular perpendicular alignmentalignment of the of eastern the eastern side of the side rim of to the the rim direction to the of direction the prevailing of the northeastern prevailing northeasterntrade wind could trade be wind the reason could be for the the reason rapid forerosion the rapid of the erosion eastern of side the in eastern comparison side in comparisonto the western to theside western of the structure. side of the Although structure. the Although terrain theprofile terrain shows profile the showsinterior the of interiorthe structure of the structureto be flat towith be flatbarely with any barely sign any of topographic sign of topographic expressions, expressions, PALSAR PALSAR reveals reveals a distinct a distinct surface surface roughness roughness to the to thestructure’s structure’s interior interior (Figure (Figure 7).7 In). InPALSAR, PALSAR, the the inner inner ring ring and and central central peak peak appears appears to be mademade upup ofof significantlysignificantly coarser textured material than that of the structure annular basin which is characterized by fine-texturedfine-textured material.material. The shallowshallow depth depth of of the the Rimaal Rimaal structure structure could could be the be resultthe result of filling of filling by deposits, by deposits, including including impact breccia,impact asbreccia, well as as Aeolian well as and Aeolian fluvial sediments.and fluvial According sediments. to [According7] for a complex to [7] impact for a structure,complex theimpact true depthstructure, (the depththe true to thedepth crater (the true depth floor to from the thecrater top true of the floor outer from rim), the and top the of apparent the outer depth rim), (the and depth the toapparent the top ofdepth the breccia(the depth lens fromto the the top top of ofthe the breccia outer rim),lens from can follow the top the of following the outer empirical rim), can relationship: follow the following empirical relationship: 1.06 1.02 da = 0.13 D. and dt = 0.28 d . (1) = 0.13 =0.28 (1) where D is is the the final final rimrim diameterdiameter in km, km, and and da andand dt are are the the apparent apparent and andtrue true depthsdepths in meters, respectively. Applying the formula in [7] [7] to the Rimaal structure of a 10.5 km in diameter, yields an apparent depth of 243 m and an actualactual depthdepth ofof 412412 m.m. Based on the topographictopographic data, the current depth of the structure structure is is only only 45 45 m, m, which which means means that that about about 80% 80% of ofthe the crater’s crater’s depression depression is filled is filled by bydebris debris and and sediments. sediments. The The deep deep erosion erosion to tothe the outer outer rim rim would would have have also also contributed contributed to the shallowness of the RimaalRimaal structure.structure. As As appare apparentnt from the DEM topographic data, the structure betrays considerable modificationsmodifications by extensiveextensive long-termlong-term fluvialfluvial erosion.erosion. The SRTM-delineatedSRTM-delineated drainage network in the area illustrates the presence of river channels of southern flowflow directions (Figure(Figure8 8),), which which drain drain toward toward the the ancient ancient mega-lake mega-lake of of Northern Northern Darfur Darfur [ 29[29].].

Remote Sens. 2018, 10, 880 10 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 10 of 17

FigureFigure 7. NW–SE7. NW–SE topographic topographic and and roughness roughness profilesprofiles of the Rimal Structure. Structure. (a ()a )Roughness Roughness profile profile generatedgenerated from from PALSAR PALSAR L-band L-band radar radar image. image. ((bb)) Topographic profile profile extracted extracted from from SRTM SRTM elevation elevation data.data. It showsIt shows the the structure structure as as a a bowl-like bowl-like depressiondepression with a a depth depth of of approximately approximately 45 45 mm deep deep and and a generala general tilt tilt from from west west to to east. east. Although Although thethe terrainterrain profile profile shows shows the the interior interior of of the the structure structure to tobebe relativelyrelatively flat, flat, PALSAR PALSAR reveals reveals a distincta distinct surface surface roughness roughness expression expression toto thethe structure’s interior with with a cleara clear central central peak peak which which appears appears to beto be made made up up of of significantly significantly coarse coarse textured textured material materialthan than thatthat of theof structure’s the structure’s annular annular trough. trough.

Figure 8. Cont.

Remote Sens. 2018, 10, 880 11 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 11 of 17

FigureFigure 8. Hydrology8. Hydrology and and morphology morphology of the of Rimaal the Rimaal structure structure (a) SRTM-delineated (a) SRTM-delineated paleorivers paleorivers draining intodraining the ancient into mega-lakethe ancient ofmega-lake Northern of Darfur.Northern The Darfur. white The box white indicates box indicates the structure the structure area shown area in (b).shown (b) Morphometric in (b). (b) Morphometric elements mapped elements from mapped hybrid from spectral hybrid and spectral roughness and remote roughness sensing remote data. Thesensing interior data.wall The of theinterior structure wall of is the incised structure with is radialincised pattern with radial gullies pattern that gullies originate that at originate the crater’s at northernthe crater’s rim, northern implying rim, a much implying steeper a much rim steeper wall in rim the wall past. in Thethe past. inner The wall inner of thewall northern of the northern flankof theflank structure of the divulges structure traces divulges of concentric traces of concentric faults. Whereas, faults. Whereas, to the west to the of thewest structure, of the structure, the presence the of apresence curved paleoriverof a curved course paleoriver (marked course as (marked R2) indicates as R2) plausible indicates presence plausible of presence a concentric of a concentric ring graben relatedring tograben the inferred related to Rimaal the inferred structure. Rimaal structure.

3.5.3.5. Hybrid Hybrid Remote Remote Sensing Sensing Data Data BasedBased on theon hybridthe hybrid remote remote sensing sensing images images (from passive,(from passive, active, and active, elevation and elevation data), the morphologydata), the morphology of the Rimaal structure is sketched (Figure 8). The spatial configurations of the of the Rimaal structure is sketched (Figure8). The spatial configurations of the paleorivers in the vicinity of paleorivers in the vicinity of the Rimaal structure show interesting patterns. The arrangements of the Rimaal structure show interesting patterns. The arrangements of these rivers can be used as indicators these rivers can be used as indicators of the presence of the inferred crater at this locality. For example, of the presence of the inferred crater at this locality. For example, the interior wall of the structure is the interior wall of the structure is incised with gullies that originate at or near the crater northern incised with gullies that originate at or near the crater northern rim, implying a much steeper rim wall rim, implying a much steeper rim wall and erosion by precipitation-derived surface runoff in the andpast. erosion The bygullies precipitation-derived clearly developed surface in a radial runoff patte inrn the and past. cut The into gullies radar-bright clearly developedbedrock, providing in a radial patternevidence and cutof the into circularity radar-bright of the bedrock, structure. providing A few of evidence these gullies of the flow circularity southeast of and the joined structure. together A few of theseto form gullies one water flow southeast course. This and course joined flows together easterly to form to merge one water with course.a major Thispaleoriver course course flows (R1 easterly in to mergeFigure with8). This a major eastern paleoriver river, which course is (R1now in dry, Figure breaches8). This the eastern structure’s river, northeastern which is now rim dry, and breaches runs thethrough structure’s its eastern northeastern annular rim trough. and runs Although through it has its easterna general annular south flow trough. trend Although in the plateau, it has a the general R1 southriver flow course trend shows in the an plateau, abrupt thechange R1 riverto the course west wh showsen it anreaches abrupt the change inferred to crater the west basin. when The it course reaches theof inferred the R1 river crater has basin. an obvious The course narrowness of the R1 as riverit enters has and an obviousexits the narrownessputative crater as periphery, it enters and inferring exits the putativea more crater resistant periphery, rim material, inferring whereas a more the resistant river rimcourse material, becomes whereas wider thewithin river the course Rimaal becomes formation wider

Remote Sens. 2018, 10, 880 12 of 17 within the Rimaal formation itself. Moreover, to the west of the Rimaal structure, a circumferential of a second paleoriver (R2 in Figure8) is clearly visible in the microwave data. The shape of the river course indicates the plausible presence of a concentric ring graben related to the inferred structure. In the DEM, this waterway appears to flow along a curved path parallel to the western periphery of the structure, suggesting a causal relationship. In view of that, it appears that there is an intriguing spatial association of the paleorivers’ geomorphology with the inferred location of the structure. The geomorphological properties of the paleorivers provide additional evidence for the inferred crater and their presence suggests fluvial processes were among the key factors in the crater degradation.

4. Discussion The four multispectral and radar and microwave satellite datasets of LS8, Sentinel-1, PALSAR, and SRTM were used to perform multisource data fusion and roughness analysis, in order to map the Rimaal circular structure. The results indicate that fusing satellite data collected from both the optical and microwave regions of the spectrum adds value over using a single data source. However, despite the benefits of combining optical and microwave data, image fusion techniques are not commonly used in studying and mapping possible impact craters. The actual Rimaal structure could also be a smaller relic of a formerly larger structure that has afterwards been eroded. Noticeable traces of a corroded central peak in the hybrid imagery might imply the old geological age of the Rimaal structure. Prominent central uplifts at impact structures are typically quite susceptible to erosion. At the present time, the interior of the Rimaal structure is mostly filled with quaternary sediments. The shallowness of the structure is believed to be attributed to the amount of sediment layers that have deposited within it by wind and surface runoff during wet pluvial. The fluvial activities of the incoming eastern paleoriver were most likely responsible for a large fraction of these sediments. If the impact origin hypothesis is confirmed, it is expected that the central peak of the structure is buried under layers of Aeolian and fluvial deposits. It is plausible that in its early stage the Rimaal basin held a considerable amount of water that was supplied from the incoming eastern drainage. Accordingly, lacustrine conditions may have prevailed when water was copious in the past. The ponding water may have formed a local lake occupying at least its deepest part. This lake was probably broadly similar to the modern-day Bosumtwi Lake in Ghana. The latter lake is situated within an ancient complex impact crater that is about 10.5 km in diameter [30], equal to that of the Rimaal structure. The principal drainage cutting through the southern wall of the structure would have served as a spillway for the proposed lake. Lakes in rainy warm regions, such as the proposed Rimaal Lake, are known to accumulate organic-rich lacustrine sediments and, thus, are commonly viewed as analogues for other impact structures that contain economic reserves of oil and gas [31,32]. In fact, meteorite impact craters of comparable size and geologic setting to the Rimaal structure were found to host significant amounts of oil and gas reservoirs worldwide. In the US, for instance, the total annual commercial hydrocarbon production from nine exploited impact structures was estimated to be between USD$5 and USD$16 billion [5]. Among the impact structures that host rich oil and gas fields in the US are the Ames (16 km), Red Wing (9 km), Sierra Madera (~13 km), and the Marquez (~12.5 km). For example, the Red Wing Creek impact structure in North Dakota, one of the most productive oil basins in the US, has an estimated production of 12.7 million barrels of oil, whereas the Ames impact structure in Oklahoma produces about 7200 barrels of oil per day. For gas production, the Sierra Madera in Texas boasts about 122 million m3 daily. These structures are all formed in sedimentary rocks. In fact, the Sierra Madera and Marquez craters, in particular, resemble that of the Rimaal structure as they both have a comparable size and are formed in Cretaceous Sedimentary rocks with ages of less than 100 million years. Typically, fracturing and brecciation resulting from meteorite impacts can cause significant porosity and permeability of the target rocks [3,33], therefore, most the likely produce a good quality oil reservoir. Here, the shock deformation and the generation of central uplift caused by the impact Remote Sens. 2018, 10, 880 13 of 17 would make a substantially productive reservoir if filled with hydrocarbons [34]. Large impact events commonly fracture the ground surface to several kilometers deep and create pathways in a sedimentary basin for fluid migration from the hydrocarbon reservoirs at depth. The exact age of the proposed structure is yet to be estimated; however, a maximum age of the Rimaal structure is probably ≤65 Ma, as it most likely formed in of the Cretaceous age. In the Eastern Sahara, just west of the region where the Rimaal structure is found, lies the Al-Kufrah Basin, with a high potential hydrocarbon reservoir [35]. Since this reservoir is estimated as Late Triassic to Early Cretaceous in age [36], it can be assumed that the Rimaal structure postdates the hydrocarbon reservoir in this area. This suggests that the Rimaal structure might represent preferential conduits for fluid migration from such a potential reservoir. The Rimaal structure is situated entirely in a vast area of monotonous Mesozoic sediment and is far from the large volcanic province of Awaynat massif and any known volcanic features in the region. Therefore, the volcanic origin for the structure formation is unlikely. Likewise, Maar formation and sand volcanism processes, produced by gas bubbles erupting through moist sands, can be discarded by the large size of the Rimaal structure (~86.6 km2). No salt plug or gypsum- features are known in the study area. The Rimaal structure is entirely carved in sandstone; accordingly, a dissolution karst origin for the structure can be ruled out. Lastly, a glacial origin cannot be inferred for Rimaal since the structure is located in the Great Sahara and circular glacial features, like kettles, could only be formed in loose sediments and not in the firm Nubian Sandstone where the structure exists. Based on the above discussion, the endogenic processes for the formation of the Rimaal structure are improbable; therefore, an impact origin is proposed. To date, nine circular structures have been discovered in the Eastern Sahara, and all are excavated within the Nubian Sandstone Formation. Five of these structures have been confirmed as impact craters, which are Kamil in Egypt [37]; BP and Oasis in Libya [38,39]; and Aorounga and Gweni-Fada in Chad [40]. None was discovered yet in Sudan. The morphology of the Rimaal structure bears a distinct morphological similarity to the Upheaval Dome impact complex crater in Utah, USA [41] (Figure9). Both craters have central peaks that exhibit inner morphological depressions (appear as inner ring structures), and their outer rims are partially breached by river courses. The defined central inner ring of the Rimaal structure was also described for the Aorounga impact crater in the neighboring country of Chad [40] (Figure9). In fact, other resemblances were found between the Rimaal and the Aorounga Crater, where both structures are situated in sandstone and are shielded by the Sahara sand and, thus, hardly visible by optical satellite images. The subsurface radar imaging capabilities facilitated the exposure of both structures. With a diameter of 10.5 km, if the Rimaal structure is confirmed to be of an impact origin, it would be among the very few large complex impact craters found in Africa and the fourth largest structure in North Africa after Oasis, Gweni-Fada, and Aorounga impact craters. Rimaal might be a smaller relic of a formally larger structure that has been deeply eroded. Remote Sens. 2018, 10, 880 14 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 14 of 17

FigureFigure 9.9. Oblique satellite image view of the (a) Upheaval Dome impact complex cratercrater inin Utah, USAUSA andand ((bb)) thethe AoroungaAorounga cratercrater inin ChadChad (images(images acquiredacquired fromfrom GoogleGoogle Earth).Earth). TheThe morphologymorphology ofof thethe RimaalRimaal structurestructure bearsbears aa distinct morphological resemblanceresemblance toto thesethese impact craters, particularly with thethe presencepresence ofof anan innerinner ringring structure.structure.

5.5. Conclusions TheThe presence of the the vast vast ocean ocean of of sand sand sheets sheets and and dunes dunes in inthe the Great Great Sahara Sahara of Africa of Africa limits limits the theability ability to map, to map, in indetail, detail, the the region region and and understand understand its its past past landscape. landscape. The The use use of of optical andand microwavemicrowave satellitesatellite datadata hashas ledled toto thethe identificationidentification ofof aa sandysandy buriedburied complexcomplex crater-likecrater-like structurestructure inin NorthwesternNorthwestern Sudan.Sudan. TheThe long wavelengthwavelength ofof radar data, in particular, enabledenabled thethe penetrationpenetration ofof desertdesert sandssands andand revealedrevealed thethe hiddenhidden RimaalRimaal structure.structure. TheThe structurestructure hashas aa diameterdiameter ofof ~10.5~10.5 kmkm with aa lowlow topographictopographic expression.expression. ItIt exhibits a clear outerouter rim with traces of concentric faults, an annular basin,basin, andand anan innerinner ringring surroundingsurrounding remnantsremnants ofof anan extensively-erodedextensively-eroded centralcentral peak.peak. TheThe structurestructure isis mostmost likely of an old old origin origin ( (≤≤6565 Ma) Ma) considering considering the the situation situation in inNubian Nubian Sandstone Sandstone formation formation of ofCretaceous Cretaceous age age and and the the strongly strongly eroded eroded nature nature of its of morphological its morphological elements. elements. The Thediscovery discovery of this of thisburied buried structure structure in the in desert the desert of Sudan of Sudan demonstrates demonstrates that thatthe database the database of possible of possible impact impact craters craters can canbe greatly be greatly improved improved in sandy in sandy desert desert regions regions worldwide worldwide by combining by combining optical optical and radar and radar satellite satellite data through image fusion and transformation. This work shows that fusing multispectral data with

Remote Sens. 2018, 10, 880 15 of 17 data through image fusion and transformation. This work shows that fusing multispectral data with microwave data of different frequencies (C and L-bands) and polarizations (HH, HV, and VV), can be of fundamental help for further in situ studies of the impact structures. Field visits and rock samples are still required from in and around the structure for confirming or discarding Rimaal as an impact crater. Yet, visiting the region is currently quite challenging due to the environmental inaccessibility of the area and for security reasons. If proven to be of an impact origin, the Rimaal structure could be the first complex crater ever found in Sudan and one of the few structures with a developed inner ring known so far in Africa. More importantly, if confirmed, this structure could hold promise for hosting economically valuable ore deposits, such as gold and uranium, as well as hydrocarbon resources, including oil and natural gas, for the benefit of the nation of Sudan.

Author Contributions: The author conducted all the research work solely, including the detection of the previously-unknown buried circular structure, image processing, analyzing the data, obtaining the results, and writing the manuscript. Acknowledgments: The author would like to express her appreciation for the anonymous reviewers and editors, whose comments have helped to improve the overall quality of this paper. Conflicts of Interest: The authors declare no conflict of interest.

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