j ourna l homepage: www.e lsev ie r .com/ locate /geomorph
Remote sensing and geophysical investigations of Moghra Lake in theQattara Depression, Western Desert, Egypt
Shuhab D. Khan a,⁎, Mohamed S. Fathy b, Maha Abdelazeem c
a Department of Earth and Atmospheric Sciences, University of Houston, Houston, USAb Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egyptc National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Egypt
TheWestern Desert covers two-thirds of the land area of Egypt and occupies one of the driest regions of theSahara. Seven depressions within the desert – Siwa, Qattara, Fayum, Bahariya, Farafra, Dakhla, and Kharga –may represent parts of old drainage systems with deflation, extensive erosion, and possibly, some tectonicactivity. Oases with freshwater exist in these depressions. Geological and geophysical investigations in theQattara Depression indicate the presence of buried fluvial channels with southeast to northwest flow direc-tions from the highland areas. The origin of these fluvial systems, as well as the origin of the depressionsthemselves, is still unresolved, and many ideas have been suggested. Moghra Lake at the northeastern tipof the Qattara basin may be a remnant of a larger paleolake, including the mouth of a paleo-river.We present here the results of our recent work in this area using ALOS PALSAR radar remote sensing data, whichindicated the presence of buried channels that may have fed the larger Moghra paleolake. Ground penetratingradar (GPR) data along 2D profiles were acquired; the migrated GPR sections identified a major paleochannelwith numerous minor channels at its margins. GPR interpretations are verified by field observations, trenching,and stratigraphic information from outcrop data. Potential field analyses identify possible aquifers that arecontrolled by regional structures. Density contrasts within the sedimentary units, physical boundaries ofuplifted basement blocks and depths to causative sources were also identified. This work contributes tothe reconstruction of paleodrainage of this region and helps in understanding processes involved in theformation of the Qattara Depression.
TheWestern Desert of Egypt, one of the driest regions of the Sahara,covers two-thirds of the land area of Egypt and contains seven majordepressions (Fig. 1). Important oases with fresh and/or brackish wateroccur within the depressions. Dune fields on the downwind flanks ofthe depressions indicate that deflation has played a part in their forma-tion, but they also may be manifestations of an old drainage systeminvolving extensive erosion and, possibly, some tectonic activity.The Qattara Depression has been suggested to be, at least in part, akarst feature related to the Messinian low-stand of the Mediterranean(e.g., Albritton et al., 1990). Qattara is a large depression, with a maxi-mum length of ~300 km and a width of 145 km (Albritton et al.,1990). About 19,605 km2 of the depression is below sea level, withthe lowest elevation being−133 m (Fig. 1; Said, 1990). The depressionis bounded to the north and northwest by a steep escarpment, whichrises to heights of over 200 m in the El Diffa Plateau, and separates thedepression from the Mediterranean Sea (Peel, 1966). In contrast, the
ights reserved.
southern and eastern sides of the depression rise more gradually tothe plateau of the Western Desert, covered in the southwest by theGreat Sand Sea.
Investigations in the Qattara Depression have identified buriedstream channels with flow directions from southeast to northwest.The origin of these fluvial systems, as well the origin of the depressionitself, is still unresolved, and many ideas have been suggested. Forexample, Ball (1927, 1933) and Squyres and Bradley (1964) suggesteddeflation to a base level, with a role for groundwater, as an origin.Albritton et al. (1990) proposed that the depression originated as astream valley that was subsequently enlarged by karstic processesduring the late Miocene and later deepened by deflation, mass wasting,and fluviatile processes. Gindy (1991) suggested structural control asthe origin of the depression. Finally, salt weathering is a mechanismthat was suggested by Aref et al. (2002) for the formation of the depres-sion. They concluded that the depressionwas subjected to at least threelong, alternating wet–dry phases in the Quaternary, which coincidedwith the formation of the evaporites at elevations of 25, 75, and100 m below sea level. In addition to these aridity phases, present-day aridity allows continuation of the salt-weathering mechanismand deflation of the depression. This work presents field observation,
Fig. 1. Landsat image showing key locations. Bahariya, Dakhla, Fayum, Farafra, Kharga, Qattara, and Siwa are seven depressions that may represent old drainage basins (Said, 1990).
Fig. 2. Field photographs of important features in the study area. (A)Moghra Formation. (B) Petrifiedwood exposed nearMoghra Lake. (C) Recent sand dunes, north and south of MoghraLake. (D) Quaternary sabkha and saline deposits around Moghra Lake.
11S.D. Khan et al. / Geomorphology 207 (2014) 10–22
Fig. 3.Optical and radar remote sensing images of the study area. (A) Landsat 8, 15-mpan-sharpened, bands 4, 3, 2 combination showing surface geology. Fractures/faults from geologicalmap of Conoco (1986) are also shown. The inset rose diagram shows the directions of the fractures/faults. (B) PALSAR image showing shallow subsurface fluvial systems. Present-daystreams are also shown, extracted from SRTM 90-m data.
12 S.D. Khan et al. / Geomorphology 207 (2014) 10–22
13S.D. Khan et al. / Geomorphology 207 (2014) 10–22
remote sensing, and geophysical data aimed at helping to resolvethis debate.
In the Qattara Depression, there are six semi-permanent saline lakes,the best known of which is Moghra Lake, located at the extreme north-eastern end of the depression (Fig. 1). The lake is around 4 km2 in areaand contains brackish water; it is surrounded by phragmite swamps(Hughes and Hughes, 1992). We carried out geological, remote sensing,and geophysical investigations around the Moghra oasis to deduce theorigin of the lake and delineate the paleochannels that might havecharged the depression. Gravity and aeromagnetic data are also usedto look for possible subsurface faults and/or contacts and their role forground water flow.
2. Geological setting
Geologically, the lithostratigraphic column in the northern partof the Western Desert of Egypt contains the sedimentary successionfrom Pre-Cambrian basement rocks to recent deposits. The Pre-Cambrian basement complex of the study area is unconformablyoverlain by a thick sedimentary cover of varying thickness. The generalincrease in sedimentary thickness is from south to north, due to a gentledipping of basement rocks toward the north. The Qattara Depression iscut into nearly horizontal beds of Miocene to Eocene age (Said, 1962).Sand and clay-rich units of early Miocene (Moghra Formation) formthe bottomand the surroundings of the northeastern part of the depres-sion, while calcareous sands and clay-rich sediments of middle and lateEocene and Oligocene age (Mokattam, Qasr El Sagha and Gebel Qatraniformations, respectively) form the southern and western boundaries ofthe depression. At its type section, the Moghra Formation is a 230 mthick sequence of sandstone, siltstone, and calcareous shale, withvertebrate remains and petrified tree trunks (Fig. 2A, B). The northernborder of the Qattara Depression is marked by a steep escarpment(250 m a.s.l.) of white limestone of the Middle Miocene MarmaricaFormation. Over large areas of the depression's floor, the bedrock iscovered with surficial deposits, including sand dunes, sabkha deposits,and Quaternary evaporites (Fig. 2C, D). The area in and around MoghraLake ismostly covered by Quaternary depositswith occasional outcropsof Moghra Formation as small mesas and residual hills within theQuaternary deposits. The western part of this northern escarpment iscovered by arenaceous and argillaceous carbonate sediments of theMiddleMioceneMarmarica Formation. The Pliocene El-Hagif Formationis exposed in northeast of the study area and is composed ofwhite shal-low marine limestone with yellow marl intercalations.
The Quaternary deposits are represented by unconsolidated eoliansands, sabkha deposits, and wadi fill unconformably overlying theMiocene rocks. The eolian sands occur as large parallel, longitudinal,lunette, seif, and complex dunes composed of very fine sands withfew detrital carbonates. The large dune belts are distributed in thecentral and southern parts of the studied area. The dune axes trendNW–SE, parallelwith theprevailingwinddirection (Fig. 3A). The sabkhasediments are also exposed around Moghra Lake, at or below 40 mbelow present sea level. Moreover, the western side of Moghra Lake ischaracterized by recent eolian sand dunes with vegetation cover.The eastern side of the study area is covered by pebbly sand sheets,with no marks of surface drainage in the northern part of the lake.The southern margin of the lake is also flanked by dunes.
3. Data and methods
3.1. Remote sensing
In this study, we used Landsat 8 imagery, Shuttle Radar TopographyMission (SRTM) data, and dual polarization remote sensing data—thePhased Array type L-band Synthetic Aperture Radar (PALSAR). Landsat8 is the latest American Earth observation satellite that was launchedon February 11, 2013. It provides data with spatial resolution of 15 to
100 m in eleven spectral bands in the visible, near, short-wave, andthermal infrared spectrums (Jhabvala et al., 2011). Landsat 8 data usedin this work were acquired on July 16, 2013 and were obtained fromthe United States Geological Survey for identification of lithology andgeomorphic features of the study area.
Table 1Description and interpretation of the observed radar facies in the studied area.
Radar facies (RF) Description Interpretation
RF 1 Continuous, high-frequency, moderately parallel configuration with high reflection amplitude Bedrock unit (Moghra Formation)RF 2 Reflection-free pattern with horizontal, continuous, parallel configuration Saline clay sedimentsRF 3 Trough-shaped reflection pattern, with moderately sub-parallel continuous to discontinuous reflectors Channel fill sedimentsRF 4 Oblique clinoform reflection pattern with a sub-parallel continuous to discontinuous configuration Delta foreset sedimentsRF 5 Chaotic high-frequency reflection pattern with microscale hyperbolas Buried petrified wood or bouldersRF 6 Short, discontinuous, irregular or wavy parallel to sub-parallel reflection patterns, with moderate amplitude Overlapping flood sediments of adjacent
small braided channelsRF 7 Horizontal continuous, parallel reflection pattern, with moderate to high amplitude Stacked sequences of horizontal sedimentsRF 8 Complex sigmoid oblique reflection pattern with parallel to sub-parallel reflectors and high amplitude Channel and bar migration
14 S.D. Khan et al. / Geomorphology 207 (2014) 10–22
PALSAR was a polarimetric instrument on the Japanese AdvancedLand Observing Satellite (ALOS) that acquired data from December2006 to April 2011. In fine-beammode, PALSAR acquired data in singlepolarization (HH or VV), dual polarization (HH + HV or VV + VH), orfull polarimetry (HH + HV + VH + VV) (Koch and El-Baz, 2001;Rosenqvist et al., 2004; JAXA, 2008). The center frequency for thissystem was 1270 MHz (23.6 cm), with a 28-MHz bandwidth in fine-beam single polarization mode, and 14 MHz in the dual-, quad-pol,and Scan SAR modes (Rosenqvist et al., 2004). In fine-beam mode, therange resolution varies 7–44 m at 28 MHz and 14–88 m at 14 MHzbandwidth (JAXA, 2008). One of the most useful attributes of PALSARis its ability to penetrate to the subsurface. The depth of penetration isinfluenced by the dryness of the target area, roughness of the subsurfacefeature, and wavelength of the radar system. Longer wavelengthsoptimize the depth of investigation of the radar signal. In general, theapproximate depth of penetration is equal to the radar's nominal wave-length (Henderson and Lewis, 1998). PALSAR datawere used to identify
Fig. 5. Field surveys. (A) GPR survey locations. (B) GPR survey in
surface features, such as channels and channel fill. In addition, thesubsurface signal penetration of PALSAR allowed analysis of the nearsurface geology and the generation of images that helped in delineatingareas of interest for GPR surveys. PALSAR interpretation also identifiedpossible shallow, buried channels.
We also used the remotely sensed Shuttle Radar TopographyMission (SRTM) elevation data to extract watershed boundariesand surface stream networks. The data have a spatial resolutionof 90 m. SRTMand Landsat 8 data helped to identify different topograph-ic features such as ridges, escarpments, sand dunes and lake boundaries.
3.2. Ground penetrating radar (GPR)
Ground-penetrating radar has been used in a wide range of applica-tions in the field of sedimentology and shallow stratigraphy over thelast two decades, as major GPR reflections are generally parallel toprimary depositional structures. Using GPR, it is now possible to image
progress. (C) 2-m-deep trench, dug to confirm GPR findings.
Fig. 6. GPR profile showing two different stages of paleolake. Note the erosional truncation (unconformity) radar surface between the two stages.
15S.D. Khan et al. / Geomorphology 207 (2014) 10–22
two- and three-dimensional sedimentary structures in unconsolidatedsediments and sedimentary rocks, including sets of laminae, beds,bedsets, bounding surfaces, and architectural elements (Neal, 2004).The GPR technique was used in this study to investigate the suspectedshallow subsurface streams east of Moghra Lake.
GPR data were acquired in May 2012 along 12, 2D profiles east ofMoghra Lake, using the GSSI SIR-3000 systemwith a 400-MHz antenna,which provided images to approximately 4 m in depth. Most profileshave an SW–NE direction (across the suspected streams), and each pro-file is roughly 250 m long. The GPR system transmits electromagneticwaves at a user-defined frequency and records the backscatteredwave as a function of time (Olhoeft, 1984). Radar waves can penetratethe subsurface down to several tens of meters, depending on the fre-quency used in the survey and the soil's electromagnetic properties.GPR has an enormously wide range of applications, and it has provento be a perfect tool for shallow subsurface investigations (e.g., Stenibergand McGill, 1995; Albert et al., 1999; Khan et al., 2007; Bonomo et al.,2009; Mukherjee et al., 2012; Khan et al., 2013).
Pre-acquisition parameters were set, including a gain function thathelped counteract the natural earth attenuation of the signal, as wellas a band-pass filter to maintain frequencies within the signal bandvelocity of 0.1–0.2 m ns−1 was used. Post-survey processing wascarried out using RADAN 6.6 software. A conventional processing flowwas used for data processing, as follows. The positional correction toolremoved the airwave, a range-gain balanced the amplitudes, and afinal band-pass filter (250 to 500 kHz) was applied to the data.Deconvolution was also used for highlighting the finer details. In addi-tion, spatial filters were used to attenuate continuous vertical noise.
Fig. 7. GPR profile showing continuous flood st
3.3. Potential field data
A critical prerequisite for understanding subsurface water flow andaquifer system comes from the knowledge of structural and tectonicsetting of the study area. Integrated potential fields and modern nu-merical data analyses succeeded in mapping basement topographyand intrasedimentary structures assuming clear physical boundaries.Gravity and magnetic methods are extensively used for this kind ofstudies (i.e., Al-Garni and Gobashy, 2007; Khamies and El-Tarras,2010; Wilkes et al., 2011; Yang et al., 2011).
For probing deeper subsurface and tomap regional structures of theeastern Qattara Depression, two types of potential field data, aeromag-netic and land gravity data, were used. For our study both shallow anddeep structures were investigated. The reduced-to-pole (RTP)magneticintensity of the study area was calculated from the original total inten-sity aeromagnetic map of Egypt (EGPC, 1989). The RTPmagnetic anom-aly map (digitized to 87 × 100 data points) was filtered in thefrequency domain to calculate the different directional derivatives toenhance the structural features in the area. Moreover, to estimatedepths of magnetic causative source bodies in the study area, assuminga vertical-contact model, we used the tilt angle approach proposed bySalem et al. (2008). The magnetic tilt angle is a normalized derivativebased on the ratio of the vertical and horizontal derivatives of the RTPfield; it provides an intuitive means of understanding variations in thedepth of a magnetic source (Salem andWilliams, 2007). The land grav-ity data, on the other hand (260 × 226 data points), were extractedfrom the Bouguer anomaly maps of Egypt (sheets 71, 72, 55, and 56;Kamel and Nakhla, 1985) at a scale of 1:100,000. For precise edge
Fig. 8. GPR profile displaying different radar facies (RF) in transverse and longitudinal sections of different distributaries.
16 S.D. Khan et al. / Geomorphology 207 (2014) 10–22
tracing anddepth enhancement,we applied the tilt angle directly on thefirst vertical gradient of the gravity field, as proposed by Oruç (2010).
4. Results and discussion
4.1. Remote sensing
A total of 290 surface faults and fractures were identified from theLandsat 8 data (Fig. 3A). The rose diagram showing their orientations(Fig. 3A) indicates two trends: NE–SW (major) and NW–SE (minor).Generally the frequency of these fractures/faults increases from southto north and their length varies from a few to tens of kilometers.
Present-day ephemeral streams from the SRTM data are plotted inFig. 3B. Using the dual polarization PALSAR radar data (HV, HH), severalshallow, subsurface fluvial channels were identified. The PALSAR HVimage also allowed identification of several buried fluvial channels,which cannot be seen in optical remote sensing data such as the Landsator ASTER images for the same area (Fig. 3A). Radar images show chang-es in the orientation of fluvial channels and that of sand dunemigration(Fig. 3B).
Sand dunes are generally oriented NW to SE directions, and ridgesand escarpments tend to run also from NW to SE (Fig. 3). Areas under-lain by some lithological units are also shown in Fig. 3, includingQuater-nary deposits in the upper right, surrounded by the El-Hagif Formationand limestone of the Marmarica Formation.
RF5 RF7
RF2
200
4 m
0
Fig. 9. GPR data manifesting sand dune migration with diffe
4.2. GPR
The full length of all GPR profiles was analyzed and interpretedbased on radar facies and stratigraphy. Radar facies is defined as thesum of all characteristics of a reflection pattern produced by a specificrock formation (van Overmeeren, 1998). Both structural and texturalfeatures in the subsurface influence the radar response and producecharacteristic effects in the radar observations.
Eight macro-scale radar facies are distinguished by reflection pat-terns (Fig. 4; Table 1). These are described in terms of reflection conti-nuity, shape, amplitude, internal reflection configuration, and externalform, using the approach used by van Heteren et al. (1998), Bereset al. (1999), and Neal (2004). The radar facies types of Ekes andHickin (2001) were followed in the present study. These facies include:RF 1 (Radar facies 1) — High-frequency micro-scale reflection pattern;RF 2 — Reflection-free pattern; RF 3 — Trough-shaped reflectionpattern; RF 4 — Oblique clinoform reflection pattern; RF 5 — Chaotichigh-frequency reflection pattern; RF 6 — Discontinuous, hummocky,wavy reflection pattern; RF 7—Horizontally continuous, parallel reflec-tion pattern; and RF 8 — Complex sigmoid oblique reflection pattern(Table 1 and Figs. 5 to 8).
The GPR transverse (SW–NE) and longitudinal (SE–NW) profilesare about 3.6 km east of the present-day lake (Fig. 5A). The migratedGPR sections identified a major paleochannel distributary system withnumerous minor channels at its margins. Field observations and GPR
RF1
300 m
RF4
rent types of radar facies (RF1, RF2, RF5, RF7 and RF8).
Fig. 10. Schematic map showing outline of the paleo-lake with braided discharge distributaries and present-day sand dunes.
17S.D. Khan et al. / Geomorphology 207 (2014) 10–22
data suggest the presence of a complicated fluvial system with mainchannels and several distributary channels, as well as sedimentarystructures and unconformities. The area around present-day MoghraLake is covered by about 2-m thick lacustrine sediments. Several chan-nels can be identified on the southeast side of the lake in the rechargearea of the paleolake. However, the southeast side of the present-daylake is covered by recent aeolian sand dunes and/or sand sheets.
To confirm the presence of paleochannels on the southeast side ofthe lake, one of the main channel distributaries was probed by GPR,along with small channels on the sides. The 2D migrated GPR profileand its interpretation show amain channel with several small subaque-ous channels. The middle part represents the main channel stream(10 m wide), characterized by RF6, which changes laterally on bothsides to RF7 intercalating with RF2. The change in radar facies resultedfrom lateral and vertical change of grain size from pebbles to clays.In general, these sediments were deposited in a lower energy regimeby rapidly flowing water and/or by underflow turbidity currents. Cyclic
Fig. 11.Qualitative analysis of aeromagnetic data of the study area. (A) Total magnetic intensitywith major trends.
change of facies may be related to seasonal variations and/or differentstages of flooding.
Fig. 6 shows part of (35 m) a 2D GPR of a 195-m long profile. Thisprofile shows fascinating radar facies and surfaces that help in deducingthe prevailing depositional environment and extrapolation of thegeological history of the paleolake. The lower part of the radar signalis represented by RF1 for bedrock, which is unconformably (erosionaltruncation radar surface) overlain by RF7. The horizontal, continuousparallel configuration of RF7 resembles the horizontal fine sediments(silt to sand) of a deep lacustrine environment. Dillenburg et al.(2011) observed the same facies in a lower/middle shoreface envi-ronment. Another erosional truncation (unconformity) radar surfaceseparates RF7 from shallow lacustrine sediments of different radarfacies types. The upper part of the SW segment is characterized byhummocky and trough cross-bedding (RF8) sedimentary structures.The undulating, non-continuous, and concave/convex pattern of radarfacies characterizes an upper shoreface environment (Dillenburg et al.,
(TMI) map of the study area reduced to pole (RTP). (B) Shaded relief map of the RTP field
Fig. 12.Magnetic directional derivatives of the study area and regional linear features interpreted. (A) Tzz, (B) Txx, (C) Tyy, (D) h, and (E) a.
18 S.D. Khan et al. / Geomorphology 207 (2014) 10–22
2011). To the NE the pattern gradually changes from horizontal layers(RF7) of fine sediments (lower/middle shoreface) to cross-beddingstructures (RF8) of coarse-grained sediments of a shallow (upper face)lacustrine environment. All sediments are truncated by recent aeoliansand sheet sediments. The well-defined unconformity surface repre-sents periods of non-deposition or erosion and separate beds or groupsof beds with distinct, but laterally variable, sedimentary characteristics.Based on these observations, we deduce that the lake deposits wereformed in at least two stages.
Fig. 7 displays an 81-m-long profile that is part of a 160-m-longprofile. This profile represents the right side of one of the main incisedvalleys that recharged to the paleolake. The lower part represents the
bedrock of the Moghra Formation. The deposits record the regressionhistory of the lake with their upward course and change in the depo-sitional environment from the middle/lower shoreface (RF7) to theupper shoreface (RF8). In the upper part of the GPR profile, we canobserve several flooding stages, truncations, onlaps, downlaps, andtoplap stratal termination of radar surfaces, and we can deduce theflow direction in this 2D line. Transverse and longitudinal sectionsof channels can be identified in Fig. 8A and B, respectively. RF4 isthe main facies identified in the longitudinal profile (Fig. 8B),which might represent the foreset beds of the delta resulting fromthe sediment supply of the recharged channel. The deltaic structurerefers to the lake margin. Cross-bedding shows flow direction of
19S.D. Khan et al. / Geomorphology 207 (2014) 10–22
water, and unconformity radar surfaces can also be identified. Fig. 9shows sand dune (RF4) migration that may have resulted in theblocking of the paleochannel.
Most of the hydrogeological studies on Moghra Lake state that thewater is derived from the intersection of the Moghra aquifer watertable with the ground surface and from rainfall runoff. From the GPR in-vestigation,we deduced thatMoghra Lake is a remnant of amuch largerlake that was fed by numerous channels flowing from the southeast(Fig. 10) during glacial and interglacial stages of post Miocene period.Consequently, the lake basin was eventually filled with sediments,most of which were converted into fluvial plains as they were overrunby fluvial systems. Lake filling is commonly regarded as a regressiveprocess. However, the gradual landward siliciclastic facies changedfrom lake-offshore mud to lake-margin sand and pebbles, which isgood evidence of decreasing lake size during the Quaternary period.
Fig. 13. Quantitative analysis of the aeromagnetic data. (A) Tilt angle map (TAM). (B) Geologic(C) Solution depth map using the tilt angle method. (D) Interpreted faults from the tilt gradien
The final stage of lake filling commonly involves amigration of eoliansand dunes and sand sheets (Figs. 5 and 9).
4.3. Potential field
4.3.1. Magnetic dataReduced-to-pole (RTP) results are shown in Fig. 11A. Two zones
with high magnetic values can be seen, one trending N–S (b and c)and the other trending NW–SE (a). These two zones are surroundedin the east and south by lowmagnetic zones (d, e, and f). Surface linea-ments for all faults and linear contacts are plotted on RTP anomalies inFig. 11B, suggesting N–S, NW–SE, NE–SW, and E–W as the dominantstructural trend. The N–S trend is a characteristic of the Nubian Shield(Said, 1962, 1990). This trend extends northward and mainly underliesthe area between the Nile Valley and the Qattara Depression. The
cross-section across theWD 8-1, RABAT-1, and TIBA-1 wells (modified after Ezzat, 1982).t map (in black) and major faults (in blue, modified after RRI, 1982).
Fig. 14. Gravity data analysis. (A) Solution depths to the vertical contacts in the study area as calculated from tilt gradients. (B) Linear trends as deduced from the tilt gravity gradient.
20 S.D. Khan et al. / Geomorphology 207 (2014) 10–22
21S.D. Khan et al. / Geomorphology 207 (2014) 10–22
northeasterly trend is characteristic of the Tibesti Massif and thePelusium Line, which were probably related genetically. This trend ex-tends below the western part of the Western Desert, and it can befollowed in the study area. The E–W trend has no manifestation at thesurface, but it is strongly indicated by the present RTP and previousaeromagnetic measurements of the Western Desert (Said, 1990).
When themagnetic mineral content of the sedimentary cover is lowor negligible, the sedimentary rocks overlying the basement are consid-eredmagnetically “transparent,” and the RTPmaps reflect the basementtopographydirectly. For the purpose of this study, several interpretationmethods were applied, with a final goal of enhancing the signatures ofhidden faults and/or magnetic contacts, mainly on the basement sur-face. The estimation of the locations of themagnetic contacts associatedwith faults and other structural discontinuities was achieved by theapplication of gradients (Schmidt and Clark, 2006).
All the calculated gradient anomalies reflect evidence of lateralmag-netic susceptibility contrasts in the subsurface rock units of the studyarea, most probably due to undulations on the surface of the basement.Fig. 12A–C shows thesemean trends as follows: for the Txx anomaly, themean vector is 67.3°; for the Tyy anomaly, the mean vector is 296.2°;and for the Tzz anomaly, the average mean direction is 21.8°. The hori-zontal gradient (h) and the total gradient (a) are shown in Fig. 12Dand E. The total gradient attains special importance where it peaksover vertical contacts and/or edges. Directional analysis of the extractedlinear features correlates well with the Txx, Tyy, and Tzz anomalies(mean vector 352.5° for a and 24.2° for h).
To estimate the depths of magnetic source bodies in the study area,we assumed a vertical-contact model, and used the tilt angle approachproposed by Salem et al. (2008). Fig. 13A shows the tilt angle map(TAM) in degrees; the zero contour line identifies the location of possi-ble vertical contacts or faults in the study area,whichprobably identifiesdeep basement boundaries. Two main massive igneous bodies can beoutlined (marked by arrows on the map) — a massive, narrow body tothe west of the study area and a central NW elongated body over shal-low basement regions. Three wells in the study area confirmed thephysical boundaries of the basement and/or faults associated withthem — the WD 8-1, RABAT-1, and TIBA-1 wells (Said, 1962; Ezzat,1982; Said, 1990). The wide physical distance between the ±45° con-tours directly indicates a deep basement contact/fault in this region.Solution depths to source bodies and/or faults are shown in Fig. 13C,using the tilt angle method. The distribution of the calculated depthsagrees with the drilling information from the wells (Said, 1962; Ezzat,1982; Said, 1990). The saline-water-bearing sandstone aquifer systemin the area seems to be well controlled by the deep faults (i.e., thosedirectly affecting the basement), whereas the fresh-water-bearingsandstone aquifer system is affected by more shallow discontinuitiesor surface faults, which is not clear using aeromagnetic analysis(Fig. 13B). A more detailed map, showing the distribution of faultsas deduced from aeromagnetic analysis in the present work, is shownin Fig. 13D. For comparison, we posted the major faults shown by RRI(1982).
4.3.2. Gravity field dataThe Bouguer gravitymap in Fig. 14A is characterized by amajor cen-
tral gravity high (~24 mGal) directly south of Moghra Lake, trendingNE–SW, and a sub-anomaly trending NW–SE directly over the fareastern margin of the Qattara Depression. Both anomalous zonesare surrounded by low gravity zones (approaching ~−22 mGal) tothe east and south. Qualitatively, such high anomalies are probablyproduced by the massive basement uplifting blocks revealed by themagnetic analysis, as shown in the previous section. However, manyminor anomalies in the study area, due to density inhomogeneities(i.e., lateral density variations across massive bodies and/or contact-like structures) within the thick sedimentary section, may be hiddenunder such major gravity fields. Fig. 14A shows the Bouguer anomalymap with calculated depths (colored posted dots) using the tilt angle
technique applied directly to the first vertical gravity field gradient, asproposed by Oruç (2010). The most common range of depths of thesestructures is 250 to 1820 m.A veryminor percentage of these structureshave depths exceeding these limits (brown dots ~8 km), reflectingstrong basement contacts. Fig. 14B shows the trends of these possiblevertical contacts/faults as extracted from the tilt angle map in Fig. 14A.It is evident that most of these contacts represent shallow densitycontrast interfaces located in the sedimentary cover overlying thedeeper basement. The general trend of these density contacts/faultsare N70°W, N35°E, and NE. These faults probably played a majorrole in controlling freshwater flow in the uppermost section of thesedimentary cover.
5. Conclusions
This study integrated different datasets including PALSAR radarremote sensing, Landsat 8, GPR and geophysical potential fields todecipher the paleo-drainage of Moghra Lake in the eastern QattaraDepression. PALSAR radar remote sensing data identified several buriedand exposed fluvial channels and recognized some fractures/faults.Most of the surface fractures/faults of the study area trend NE–SW andNW–SE directions. The migrated GPR sections together with field ob-servations identified a major paleochannel distributary system withnumerous minor channels at its margins. These results support thatMoghra Lake is a remnant of a much larger lake that was fed by numer-ous channels flowing from an SE to NW direction. Analyses of magneticdata show that the saline-water-bearing sandstone aquifer system iscontrolled by deep faults, whereas the fresh-water-bearing sandstoneaquifer system is affected by shallow faults. Gravity data analysis con-firmed the presence of these faults and density interfaces in the sedi-mentary cover overlying the deeper basement (depth ranges from 250to 1820 m). The trend of these shallow structures (faults) agrees withthe flow direction of the paleochannel system identified by remotesensing and GPR data. This study leads to an important conclusionthat tectonics have a possible contribution to the surface and subsurfacewater flow in the eastern Qattara Depression.
Acknowledgments
This work is funded by the National Science Foundation. SK thanksDr. Yasser Metwally for his assistance in organizing field work inEgypt; Jessica Quintinar for her assistance in field work; Dr. KevinBurke and Dr. Ian Evans for their suggestions; and Unal Okyay forhis help with one figure. MA thanks Prof. Mohamed Gobashy for hisuseful comments and suggestions for potential field analysis.
References
Albert, M., Koh, G., Perron, F., 1999. Radar investigations of melt pathways in a naturalsnowpack. Hydrol. Process. 13, 2991–3000.
Albritton, C.C., Brooks, J.E., Issawi, B., Swedan, A., 1990. Origin of the Qattara Depression,Egypt. Geol. Soc. Am. Bull. 102, 952–960.
Al-Garni, M.A., Gobashy, M.M., 2007. High resolution ground magnetic survey (HRGM)for determining the optimum location of subsurface dam in Wadi No'man, Makkahal-Mukarammah, KSA. J. King Abdulaziz Univ., Earth Sci. (JKAU) 19, 57–83.
Aref, M.A.M., El-Khoriby, E., Hamdan, M.A.A., 2002. The role of salt weathering in theorigin of the Qattara Depression. Western Desert, Egypt. Geomorphology 45,181–195.
Ball, J., 1927. Problems of the Libyan Desert. Geogr. J. 70, 21–38.Ball, J., 1933. The Qattara Depression of the Libyan Desert and the possibility of its utiliza-
tion for power production. Geogr. J. 82, 289–314.Beres, M., Huggenberger, P., Green, A.G., Horstmeyer, H., 1999. Using two- and three-
dimensional georadar methods to characterize glacio-fluvial architecture. Sediment.Geol. 129, 1–24.
Bonomo, N., Cedrina, L., Osella, A., Ratto, N., 2009. GPR prospecting in a prehispanicvillage, NW Argentina. J. Appl. Geophys. 67, 80–87.
22 S.D. Khan et al. / Geomorphology 207 (2014) 10–22
EGPC (Egyptian General Petroleum Corporation), 1989. Reduction to pole magneticanomaly map of Egypt. Minerals, Petroleum and Groundwater Assessment ProgramAid Project 263-0105 and Amendment No.1.
Ekes, C., Hickin, E.J., 2001. Ground penetrating radar facies of the paraglacial Cheekye Fan,southwestern British Columbia, Canada. Sediment. Geol. 143, 199–217.
Ezzat, M.A., 1982. Impact of a future Qattara salt-water lake on the Nubian Sandstoneaquifer system in the Western Desert, Egypt. Improvements of Methods of LongTerm Prediction of Variations in Groundwater Resources and Regimes Due toHuman Activity (Proceeding of the Exeter Symposium, July 1982), 136. IAHS Publ.,pp. 297–314.
Henderson, F.M., Lewis, A.J., 1998. Principles and applications of imaging radar, Manual ofRemote Sensing3rd ed. John Wiley and Sons, New York.
Hughes, R.H., Hughes, J.S., 1992. A Directory of African Wetlands. IUCN, Gland,Switzerland.
JAXA (Japan Aerospace Exploration Agency), 2008. ALOS User Handbook. NDX-070015 .Jhabvala, M., Choi, K., Waczynski, A., La, A., Sundaram, M., Costard, E., Jhabvala, C., Kan, E.,
Kahle, D., Foltz, R., Boehm, N., Hickey, M., Sun, J., Adachi, T., Costen, N., Hess, L.,Facoetti, H., Montanaro, M., 2011. Performance of the QWIP focal plane arrays forNASA's Landsat Data Continuity Mission. Proceedings of SPIE, Infrared Technologyand Applications XXXVII. , vol. 8012 (1) (April, 2011).
Kamel, H.A., Nakhla, A.F., 1985. Gravity map of Egypt. A Report Prepared for the EgyptianAcademy of Scientific Research and Technology.
Khamies, A.A., El-Tarras, M.M., 2010. Surface and subsurface structures of Kalabsha area,southern Egypt, from remote sensing, aeromagnetic and gravity data. Egypt.J. Remote. Sens. Space Sci. 13, 43–52.
Khan, S.D., Heggy, E., Fernandez, J., 2007. Mapping exposed and buried lava flows usingsynthetic aperture radar and ground-penetrating radar in Craters of the Moon lavafield. Geophysics 72 (6), 161–174.
Khan, S.D., Stewart, R.R., Otoum, M., Chang, L., 2013. A geophysical investigationof the active Hockley Fault System near Houston, Texas. Geophysics 78 (4),B177–B185.
Koch, M., El-Baz, F., 2001. Application of ALOS in arid land studies: land degradation,natural hazards and water resources. Proceedings of the 1st ALOS PI Workshop,Tokyo, Japan, 28–30 March, 2001, pp. 190–192.
Mukherjee, D., Khan, S.D., Sullivan, C., 2012. Upper Albian rudist buildups of the EdwardsFormation in central Texas: a GPR-assisted reservoir analog study. Sediment. Geol.247/248, 71–81.
Neal, A., 2004. Ground-penetrating radar and its use in sedimentology: principles,problems and progress. Earth Sci. Rev. 66, 261–330.
Olhoeft, G.R., 1984. Applications and limitations of ground penetrating radar: society ofexploration geophysicists. 54thAnnual InternationalMeeting and Exposition, Atlanta,GA, pp. 147–148.
Oruc, B., 2010. Edge detection and depth estimation using a tilt angle map from gravitygradient data of Kozakli-Central Anatolia region, Turkey. Pure Appl. Geophys. 168,1769–1780.
Peel, R.F., 1966. The landscape in aridity. Institute British Geographers Robertson ResearchInternational (RRI) & Associated Research Consultants in Association With ScottPickford&Associates Limited and Ere EnergyResource Consultants Limited1982. Petro-leum Potential Evaluation, Western Desert, ARE. . Report Prepared for EGPC, vol. 8.
Rosenqvist, A., Shimada, M., Watanabe, M., 2004. ALOS PALSAR: technical outline andmission concepts. 4th International Symposium on Retrieval of Bio- and GeophysicalParameters from SAR Data for Land Applications Innsbruck, Austria, pp. 1–7.
RRI (Robertson Research International), Associated Research Consultants in associationwith Scott Pickford, associates limited, ERC Energy Resource Consultants limited,1982. Petroleum potential evaluation, Western Desert, ARE. Report Prepared forEGPC, 8 vol.
Said, R., 1962. Geology of Egypt. Elsevier, Amsterdam (370 pp.).Said, R., 1990. The Geology of Egypt. Balkema, Rotterdam, Brookfield (734 pp.).Salem, A., Williams, S., 2007. Tilt-depth method: a simple depth estimation method using
first-order magnetic derivatives. Lead. Edge 26, 1502–1505.Salem, A., Williams, S., Fairhead, D., Smith, R., Ravat, D., 2008. Interpretation of magnetic
data using tilt-angle derivatives. Geophysics 73 (1), L1–L10.Schmidt, P.W., Clark, D.A., 2006. The magnetic gradient tensor: its properties and uses
insource characterization. Lead. Edge 25, 75–78.Squyres, C.H., Bradley, W., 1964. Notes on the Western Desert of Egypt. Conference
Petroleum Exploration Society, Libya, pp. 99–105.Steniberg, B.K., McGill, J.W., 1995. Archaeology studies in Southern Arizona using ground-
penetrating radar. J. Appl. Geophys. 33, 209–225.Van Heteren, S., Fitzgerald, D.M., McKinlay, P.A., Buynevich, I.V., 1998. Radar facies of
paraglacial barrier systems: coastal New England, USA. Sedimentology 45, 181–200.VanOvermeeren, R.A., 1998. Radar facies of unconsolidated sediments in theNetherlands:
a radar stratigraphy interpretation method for hydro-geology. J. Appl. Geophys. 40,1–8.
Wilkes, P.G., Timms, N., Corbel, S., Horowitz, F.G., 2011. Using gravity and magneticmethods with geomorphology and geology for basement and structural studies toassist geothermal applications in the Perth Basin. Australian Geothermal EnergyConference.
Yang, X., Scuderi, L., Liu, T., Paillou, P., Li, H., Dong, J., Zhu, B., Jiang, W., Jochems, A.,Weissmann, G., 2011. Formation of the highest sand dunes on Earth. Geomorphology135, 108–116.
