SHALLOW SEISMIC REFRACTION INVISTIGATION AT THE NORTHERN EXTENSION OF SAHARY CITY, ASWAN
by : Mohamed H. A. Ahmed Bachelor of Geophysics faculty of science University of Aswan
Supervised by : Prof. Dr. Raafat E. Fat-helbary Director of Aswan Regional Earthquake Research Center June2016 1
Content I. Introduction……………………………………………………………. 3 II. Geology of the study area and its surroundings……………………..5 III. Theoretical Basis ……………………………………………………..10 IV. Basic Requirements and Field Procedure…………………………. 14 V. Seismic refraction survey at the study area………………………….16 VI. Data processing……………………………………………………….17 VII.RESULTS…………………………………………………………………………..19
Conclusion…………………………………………………………………22 Refrences…………………………………………………………………..23
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I. Introduction The shallow seismic refraction technique is considered as one of the most effective geophysical method, which can be applied in the field of engineering seismology i.e. tunnels, dam site, landslides, roads, and Cavities. The maذin targets of the shallow seismic refraction method are determining the depth to the bedrock, the depth to the ground water, lithology type, the lateral and vertical change in lithology. Seismic exploration involves generation of seismic waves and recording the arrival times of these waves from the source to the series of geophones . Seismic refraction is used to evaluate the necessary parameters for constructions, or to solve the problems related to the geological nature of sub-surfaces, mining works, and the environmental conditions overcame in the site (Stumpel et al., 1984). In the shallow seismic refraction method, the seismic waves, created by artificial sources such as an auto-hammer viberator, propagate through the medium and are refracted at interfaces, where the seismic velocity or density changes. Geophones laid on a single line record the waves returning to the surface after travelling different distances through the ground. By measuring the travel time between the break and the recording of a seismic signal, the seismic velocity in the subsurface and the depth of the interfaces may be inferred. Many researchers have used seismic refraction technique to determine the characteristics of the sites, and the necessary parameters for constructions (e.g. (Dutta, 1984), (Marzouk, 1995), (Mohamed, 1993), (El- Behiry, 1994), (Hatherly, 1986), (Sjogren & Sandberg, 1979) and (Abdelmotaal, 2010). In this study, shallow seismic refraction method was used to determine the lateral variation in the lithological changes of the subsurface earth materials in the surveyed area. Six seismic refraction P-wave profiles were conducted at the studied area. The formations obtained from refraction method include velocity of P waves which are used in turn to calculate the depth to bedrock and determine the dynamic characteristics of subsurface layers. The study area is located at the northern extension of Sahary city, about 4 Km west of Aswan High Dam as shown in Fig. 1.
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II. Geology of the study area and its surroundings II.1. Geomorphic Features Four major geomorphic features are present in the Aswan Area and adjacent portions, westerly; these are the Aswan Hills, the Nile River valley, the Nubia Plain, and Sinn-el Kaddab Plateau. The latter two are within the area referred to as the Western Desert. The general distribution of rock units and the basic structural and topographical characteristics of these features are shown in figure (2(.
Fig. 1. Location of the Study Area II.1. 1 Aswan Hills The Aswan Hills are located along the east side of the Nile valley and extend southwards from nearness of Aswan. As defined for this study, they include portions of the Dabud Hills, Umm Naga uplands, and Ambukol plain "described by Butzer and Hansen (1968)". Physiographically, the Aswan Hills are characterized by rugged topography produced by a dense, complicated system of steep-walled wades. The hills typically reach elevations of 200 to 400m. Account hill tops in the Aswan Hills represent remnants of ancient erosion surfaces possibly formed at middle Tertiary period. II.1.2. Nile Valley and the Aswan High Dam Reservoir. The Nile Valley is located along the western edge of the Aswan hills. The former Nile Valley is a narrow, steep-sided bed rock cut into the Nubia formation and pre-Cambrian basement complex . it trends northward and sub parallel to the trends of several faults , that cross the Nubia plain to the west. The high Dam lake through its cycles of filling extends 4
mostly to cover lowlands to the west of the Nile Valley. Embayments are formed due west. The greatest embayments cover the former Kalabsha depression. II.1.3. Nubia plain The Nubia Plain is 30 to 50 Km wide and located on the west side of the Nile River valley approximately 100 to 200 m above the Nile valley flood plain. It represents an ancient erosion surface (by Butzer and Hansen, 1968) that was probably formed in the early to middle Miocene. Local relief on the surface is very low, generally less than 10 m; the average elevation hills. The low areas are primarily closed precessions formed by aeolian deflation. Near the eastern margin of the Nubia Plain west of the High Dam Lake, several east- flowing wades have incised the plain. The largest one is Wadi Kurkur and Wadi Kalabsha. The lower reaches of these two wades have been inundated by the High Dam Lack. II.1.4. Sinn El-Kaddab Plateau The Sinn el-Kaddab Plateau (called the "Limestone Plateau" by Issawi, 1968) extends westward from a high, steep escarpment that forms the western boundary of Nubia Plain. The relief on the plateau is variable and ranges from extensive flat areas to rugged with resistant rock hills and benches. The average elevation of the plateau is approximately 350m. The Sinn el-Kaddab escarpment is retreating westward in an irregular pattern that may be structurally affected by E-W and N-S fault sets in the region. North of Gebel Kalabsha, the scarp has a northerly trend; to the north, the scarp bends sharply to as east-west trend. The lower portion of the Sinn el-Kaddab escarpment is predominately composed of easily erodible shale beds of the Dakhla Formation; the upper portion is composed of hard, resistant limestone
beds of Kurkur Formation. The steepness of the escarpment is a result of the differential erosion of these lithologic units.
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Figure (2): Geological map of Aswan area Modified from EGSMA (1981) and Main faults (after WCC, 1985).
II.2. Stratigraphy The geology of the area is essentially covered by foreland sediments ranging in age from Cretaceous to Quaternary, with some exposures of igneous and metamorphic rocks which belong to the basement complex of late pre-Cambrian to Early Paleozoic age (see figure 3). The exposed rocks of the whole area surrounding the High Dam Lake from the top to the base are as follows:
1-The basement rocks. 2-The Nubia Formation. 3-The Dakhla Formation. 4-The Kurkur Formation. 5-The Garra Formation. 6-The Dungul Formation. 7-The Quaternary deposits
II.2.1. Basement rocks Precambrian basement rocks are exposed along the crest of the uplift of the Aswan Hill, along portions of the Nile River canyon, and in isolated outcrops on the Nubia Plain. Detailed studies of the Precambrian crystalline complex have been carried out at the northern 6
end of the Aswan Hills by Attia (1955) and El Shazly (1954). Their work indicates that both metamorphic and platonic rocks are abundant within the complex. The metamorphic rocks include biotic and hornblende schist, amphibole, and gneiss, and form a low topography that contrasts with the more rugged topography developed on the platonic rocks of the Aswan Hills. The platonic rocks are primarily granite and granodiorite that were intruded into the metamorphic rocks. Intrusion of the granite probably occurred in several stages, resulting in different granitic rocktypes. In the Nubia Plain and along the Nile River canyon, the Precambrian rocks are primarily composed of coarse-to-fine grained Aswan granite. This granite is characterized by its pink color, which is produced by abundant prophyroblasts of pink feldspar. Precambrian metamorphic and granitic rocks were subsequently intruded by swarms of dikes. Three different compositions of dikes have been identified: 1) Diabase dikes and sheets; 2) Volcanic breccias dikes; and 3) andesite-comptonite-bostonite group dikes. These dikes were probably emplaced during the early Paleozoic. Faults and shear zones are common within the Precambrian rocks.
II.2.2. Nubia Formation The principal rock stratigraphic unit that underlies the Nubia Plain is Nubia Formation. The unit also lanks the basement complex along the Aswan Hills uplift, and has been incised by the Nile River. The dominant lithology of the formation is well-sorted sandstone composed almost entirely of quartz grains that are poorly cemented by rather silica or calcite. The unit unconformable overlies the granitic basement rock, which has an irregular upper surface. The thickness of the Nubia Formation is variable as a result of this irregular base. A maximum thickness of 592 m was measured in a drill hole near Kurkur oasis; in general, there is a decrease in unit thickness toward the south and southeast. Sediments of this formation in southeastern Egypt were deposited during the early to middle part of the Late Cretaceous in three essentially conformable lithofacies : a lower regressive non-marine sandy faces deposited in a fluvial environment ; a middle paralic ( swampy ) to coastalmarine unit of fine-grained sandstone, mudstone, and ferruginous oolitic ironstone; and an upper non-marine to paralic progradational sandstone characterized by large scale crossbedding (Van Houten and others,1984).
II.2.3. Dakhala Formation The Dakhala Formation, which crops out in the lower portion of the Sinn el-Kaddab escarpment and on the western edge of the Nubia Plain, conformably overlies the Nubia Formation. The Dakhla Formation is principally composed of shale, with sandstone ntercalation's and conglomeratic beds near the base, and carbonate interbeds near the top. The shale is fissile and of varied colors, most commonly gray to green. The formation ranges in thickness from approximately 39m to 155 m, and thins to the west. The sediments were deposited during the latest part of the Late Cretaceous in marine environment that ranged from deep water (shale units) through shallow neritic and littoral environments (sandstone units and reefal limestone's).
II.2.4. Kurkur Formation The Kurkur Formation conformably overlies the Dakhla shale, and is found in the upper portion of the Sinn el Kaddab escarpment. Gebel Marwa, the only limestone hill within the sandstone of the Nubia Plain, is a syncline of the Kurkur Formation . In the type section at Kurkur Oasis, the formation contains two siliceous limestone beds with clastic intercalations. To the north and south of the type area, a thick sandstone unit is present at the base and is overlain by one limestone bed. The color of the formation is typically brown, and locally 7
contains abundant marine invertebrate fossils. The Kurkur Formation has a total thickness of 11 m at the cross section; the maximum reported thickness is 57.2 m sediments of the formation were deposited during the early Paleocene in shallow marine environments.
II.2.5. Garra Formation A portion of the Sinn el-Kaddab Plateau surface, and locally the top part of the escarpment, are composed of Garra Formation. This unit unconformable overlies the Kurkur Formation. The Garra Formation is composed of thick limestone beds with chalk, marl, and shale intercalations. It has a distinctive white to grayish-white color that contrasts with the dark-brown color of the underlying Kurkur Formation. Marine invertebrate fossils are locally abundant. The formation attains a thickness of 110 m and thins westward to a thickness of about 60 m. The age of the unit is late Paleocene to early Eocene. The sediments were deposited in shallow, protected, marine shelf through deep marine environments.
II.2.6. Dungul Formation The Dungul Formation is the principal rock formation exposed on the Sinn el-Kaddab Plateau. Conformably overlying the Garra Formation, the unit consists of alteration shale and limestone beds at the base, and limestone with flint bands and abundant concretions at the top. The dungul beds are gray to pale green and yellow, and are highly fossiliferous. The formation ranges in thickness from approximately 60 to 127 m. the sediments were deposited on a shallow marine shelf during the early Eocene.
II.2.7. Quaternary deposits Quaternary deposits of Pleistocene age are represented by two units of calcite and tufa deposits in the form of irregular patches of calcite deposits formed near the face of Sinn ElKaddab escarpment and Gebel Kalabsha. At the southern part of SinnEl-Kaddabescarpment irregular conglomerate sheets are recorded overlying Nubia sandstone. Pleistocene – Holocene sediments have been distinguished into Darb ElGallaba Conglomerate sheets. Playa and silty clay lake deposits are recorded in some localities of the area. Holocene sediments are mainly represented by the alluvium terraces deposits sand dunes surficial and alluvial deposits which are recorded in some localities of the area.
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Figure (3):stratigraphic composite section of the Kurkur area (after Issawi 1968;Tantawy 1994; Berggren et al. 2003)
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III. Theoretical Basis The seismic refraction technique is based on the refraction of seismic energy at the interfaces between subsurface geological layers of different velocity. The seismic refraction method uses very similar equipment to seismic reflection, typically utilizing geophones in an array, and a seismic source (shot).
Fig. 4 Fig.4.illustrates the path of seismic waves propagating from a source at the surface. Some of the seismic energy travels along the surface in the form of a direct wave. However, when a seismic wave encounters an interface between two different soil or rock layers a portion of the energy is reflected and the remainder will propagate through the layer boundary at a refracted angle.Fig.4. At a critical angle of incidence, the wave is critically refracted and will travel parallel to the interface at the speed of the underlying layer. Energy from this critically refracted wave returns to the surface in the form of a head wave, which may arrive at the more distant
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geophones before the direct wave. By picking the time of the first arrival of seismic energy at each geophone, a plot of traveltime against distance along the survey line can be generated. This type of graph is shown in fig.4. The gradients of the lines in this type of plot are related to the seismic velocity of the subsurface layers. The final output is a velocity/depth profile for the refractors as fig.5 above.(http://www.environmental-geophysics.co.uk/)
Head Waves we discovered that if a low-velocity layer overlies a higher velocity halfspace that in addition to the direct and reflected arrivals, we also observe what is called a head wave. In refraction seismic surveying, we measure the earliest times of arrival of the seismic waves at various distances from the source. For the layer over a halfspace model, this earliest arriving energy could be associated with either the direct wave or the head wave. Computing the time of arrival of the direct wave is relatively simple. It is nothing more than the horizontal distance between the source and the receiver divided by the speed at which the wave propagates in the layer. Raypaths must be perpendicular to wavefronts. As shown in the figure 6. below, we can sketch three raypaths from the boundary between the layer and the halfspace (red) and the wavefront describing the head wave. The angle between each of these raypaths and a perpendicular to the boundary is given by ic.
Fig.6. Substituting ic for i1 into Snell's law and solving for i2, we find that i2 equals 90 degrees. That is, the ray describing the head wave does not penetrate into the halfspace, but rather propagates along the interface separating the layer and the halfspace. ic is called the critical angle, and it describes the angle that the incident raypath, i1, must assume for i2 to be equal to 90 degrees. Although the head wave must travel along a longer path than the direct arrival before it can be recorded at the Earth's surface, it travels along the bottom of the layer at a faster speed than the direct arrival. Therefore, as is apparent in the movie showing the head wave, it can be recorded prior to the time of arrival of the direct wave at certain instances.
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Records of Ground Motion When seismic waves interact with a boundary in the subsurface, some of the energy is transmitted through the boundary, some is reflected from the boundary, and if the velocities of the media separated by the boundary represent a velocity increase to the propagating wave, some of the energy is transmitted along the boundary in the form of head waves. Instruments that are capable of recording ground motion are referred to as seismometers or geophones. These instruments will be described later. Suffice it to say now, that they are capable of recording the ground motion produced by the seismic waves we are interested in studying.
First Arrivals When performing an exploration refraction experiment, this is the only information extracted from the recorded seismograms that is used. Plotting the arrival times of the first arrival versus offset produces the travel-time curve shown below.
Fig.7.
From fig.7 above. Notice that the travel-time curve associated with the first arrivals is given by two, straight-line segments. At small offsets (green), the travel-time curve corresponds to the direct arrival. At larger offsets (red), the travel-time curve corresponds to that of the refracted arrival. The two segments are clearly distinguished from each other by a change in slope at some critical offset commonly called the crossover distance. This distance represents the offset beyond which the direct arrival is no longer the first arrival recorded.
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In going from the recorded seismograms to our first arrival travel-time curves, we must determine the time instant at which ground motion was initiated on each seismogram fig.8.This time corresponds to the time indicated by the red line. On this record, choosing the first arrival time is not difficult, because the seismogram shows no signal before this time.
Fig.8.
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IV. Basic Requirements and Field Procedure. The basic requirements for seismic field procedure are:-
IV. 1. Seismic Source The aim of using any seismic source is to produce a large enough signal into the ground to ensure sufficient depth penetration and high enough resolution to image the subsurface. Generally, there are three type of seismic source: impact, impulsive and vibrator In the present study, the Geometric seismic source PWD-80 was used as shown in figure (9.A). This source is a hydraulic source mounted on a car and works by an electric generator. The source uses proven elastomers technology, shortened to accelerate a steel hammer to high velocities, guaranteeing high frequencies and superior energy level. This can be used to get P-wave by hitting the ground vertically; furthermore it is even available with an optional pivot to cant the mast to 45 degrees to get shear waves as shown in figure (5.1b). The PWD-80 is suitable for study from a few meters to several ten meters in depth. IV. 2. Detectors The common detector is known as a geophone and similar to microphone but the remains stationary while the magnet moves with the ground. On marine work, the coil detector is called hydrophone. However the seismic signals can be recorded by using a group of detectors which are distributed in a profile. Every geophone should be constructed with specific natural frequencies that can be apple to record the dominant frequencies of seismic pulse. It has strong effect on the data quality. In the present study the geophone which used are of vertical type (14 HZ natural frequency) see figure (9.C). For shallow seismic refraction surveys, seismic signals tend to have a much higher frequency and a geophone with a natural frequency around 14 HZ is a good compromise. IV. 3. The System used in present study. The Geometric Strata Visor-NZ 48 channels, is the system which used in present study, this system was manufactured in USA (figure 9.D). The system contains 48 detectors, each detector consist of a coil and a magnet, one rigidly attached to the frame and the other suspended from a fixed support by a spring. When the ground is displaced, it causes a relative motion between the coil and the magnet so that an oscillatory voltage is generated in the coil. This is proportional to the velocity of the motion. The detectors that have been used in the present study have a natural frequency of earth. The signal is amplified and undesirable frequencies can be filtered out. The Strata Visor-NZ can be configured either as a field- rugged personal computer with no internal channels or include up to 64 high resolution seismic channels within the same chassis. Additional chassis can be added with more channels if required. The strata Visor- NZ has a daylight visible color screen and is the preferred
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control device for Geodes in harsh environments. The Strata Visor-NZ operates from a 12 V powers supply. The Strata Visor-NZ can also connect with several external Geode seismic modules via a high speed network interface. The NZ comes with software that is already configured for multiple Geode operation (MGOS). The Strata Visor-NZ is unlike any earlier seismograph, it is easy to use and it gives you answering in the field. A menu bar makes it easy to set up the instrument to take and display your data. The menus include an option for standard setting, which will be adequate for any initial refraction survey. The Strata Visor-NZ employs a new concept in portable exploration seismograph. It combines the ruggedness and high signal quality of a distributed system with the convenience and cost effectiveness of personal computer-based control devices.
A. Seismic source PWD-80 in vertical position
C. Geophone used in recording P-wave.
B. Geophones Profile
D. Geometric Strata Visor-NZ48 channels.
Fig. (9) Field instruments 15
V. Seismic refraction survey at the study area Shallow seismic refraction survey was carried out at 6 sites in the study area in order to estimate seismic velocities (Vp ), determine the ground model and subsurface structures. The distribution of the measured sites is shown in Figure (10).
Fig.(10) Location of measured seismic profile in the Study area
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For each measured site, one seismic profiles were conducted to record Compressionalwaves (P-waves) . The profiles carried out with 24 geophones spaced at 5 m with total spread length of 115 m. Along the profile, three shots were done, at distance of 5 m far from both ends (normal and reverse shooting), and the mid-point (between geophones12 and
13) see figure (11). Figure (11): geophone spread for a refraction survey with shot locations indicated.
VI. Data processing In the current study, the recorded seismic signals were processed using SeisImager software, which is a complete seismic data processing and modeling software. It is based on the time delay and ray tracing methods. The wave forms for compressional obtained for each shot were analyzed by picking the first arrivals (see fig.11) and the corresponding travel timedistance (T-D) curves are illustrated in Figure (12). Arrivals from the second layer were always recognizable on all shot records as first layer arrivals. On the other hand, arrivals from the third layer were not recognizable on any shot record as the arrivals from the first and second layers, probably due to (a) the gradual increase of seismic velocities with depth. In other words, the occurrence of slight interface between the second and third layers; (b) the limited extent of the profile relative to the depth of the third layer. The picked times together with the distances between the shooting points and geophones were used to calculate the travel time distance (T-D) curves. The resulted travel time-distance (T-D) curves were analyzed and the 2-D ground models were constructed based on the refracted waves from subsurface interfaces. The P -wave velocities for each layer were calculated.
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Fig.11. picking
Fig.12. T-D diagram for the three shots over the profile
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VII.RESULTS Interpretations of Geoseismic cross sections The corresponding 2- D ground model for each site is described as following. Profile No.1:
This profile is located in the southern part of the study area. Into NE-SW direction. Though out analyzing P-waves of the produced pulse, we are able to detect the subsurface strata below this profile. The first layer is composed of loose sand which appears on the surface and below it a layer of a Muddy Sand stone as the velocity of P-waves in this layer lies between 900-1400 m/sec and its thickness is about 4 to 11 m. The second layer or the base layer is composed of a hard compacted Sandstone as the Pwaves velocity in this layer lies between 1400-2600 m/sec. Profile No. 2:
This profile is located north to profile 1. Into NE-SW direction. Though out analyzing P-waves of the produced pulse, we are able to detect the subsurface strata below this profile. The first layer is composed of loose sand which appears on the surface and below it a layer of a Muddy Sand stone as the velocity of P-waves in this layer lies between 1350-1400 m/sec and its thickness is about 0 to 9 m. The second layer or the base layer is composed of a hard compacted Sandstone as the P-
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waves velocity in this layer lies between 1400-2400 m/sec. part of this layer is appears on the surface. Profile No. 3 :
This profile is located north to profile 2. Into NS-SE direction. Though out analyzing P-waves of the produced pulse, we are able to detect the subsurface strata below this profile. The first layer is composed of loose sand which appears on the surface and below it a layer of a Muddy Sand stone as the velocity of P-waves in this layer lies between 1050-1400 m/sec and its thickness is about 1 to 8 m. The second layer or the base layer is composed of a hard compacted Sandstone as the Pwaves velocity in this layer lies between 1400-1850 m/sec. Profile No. 4:
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Profile No. 5:
This profile is located north to profile 4. Into NW-SE direction. Though out analyzing P-waves of the produced pulse, we are able to detect the subsurface strata below this profile. The first layer is composed of loose sand which appears on the surface and below it a layer of a Muddy Sand stone as the velocity of P-waves in this layer lies between 1100-1400 m/sec and its thickness is about 6 to 8 m. The second layer or the base layer is composed of a hard compacted Sandstone as the Pwaves velocity in this layer lies between 1400-3000 m/sec.
Profile No.6:
This profile is located in the northern part of the study area. Into NE-SE direction. Though out analyzing P-waves of the produced pulse, we are able to detect the subsurface strata below this profile. The first layer is composed of loose sand which appears on the surface and below it a layer of a Muddy Sand stone as the velocity of P-waves in this layer lies between 700-1400 m/sec and its thickness is about 0 to 9 m. The second layer or the base layer is composed of a hard compacted Sandstone as the Pwaves velocity in this layer lies between 1400-2800 m/sec. part of this layer appears from the distance of 105m to the end of the profile.
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Conclusion The studied profiles of the area. Though out analyzing P-waves of the produced pulse on the six profiles, we are able to detect the subsurface strata below these profiles. The first layer is composed of loose sand which appears on the surface and below it a layer of a Muddy Sand stone as the velocity of P-waves in this layer lies between 700-1400 m/sec in most profiles and its thickness is about 0 to 11 m. The second layer or the base layer is composed of a hard compacted Sandstone as the Pwaves velocity in this layer lies between 1400-3000 m/sec. part of this layer appears on the surface in some distinct areas in 2,4 and 6 profiles.
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Tantawy AA (1994): Paleontological and Sedimentological Studies on the Late Cretaceous– Early Tertiary Succession at Wadi Abu Ghurra–Gabal El-Kaddab Stretch, South Western Desert, Egypt. M.Sc. Thesis, Aswan Faculty of Science, Assiut Univ. Van Houten, F. B., D. P. Bhattacharyya, S. E. I. Mansoun (1984): Cretacous Nubia Formation and Correlative deposits, eastern Egypt; major regression-tangression complex, Grological Society of America Bulletin, V. 95, p. 397-405. WCC (Woodward-Clyde Consultants) (1985): Identification of earthquake sources and estimation of magnitudes and recurrence intervals, Internal Report, High and Aswan Dams Authority, Egypt http://www.environmental-geophysics.co.uk/ (http://www.environmental-geophysics.co.uk/documentation/info/seismics.pdf)
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