الفهرس 
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Abstract
Horizontal expansion in new lands in the Egyptian desert has long been a key strategic target pursued by successive governments. However, there are still many questions about the possibility of providing water for the reclamation of new land without resorting to the Nile water, at a time when Egypt suffers from severe shortage of surface water resources. One of the targeted promising new lands for reclamation is El Gallaba Plain (also known as El Kubbaniya basin) which is a desert area at the western fringes of the Nile Valley in Upper Egypt and lies to the Northwest of Aswan City and west of Kom Ombo town. It is a vast plain with an area of about 5500 km2 (≈550,000 Hectare).
The hydrological setting and the groundwater potential of El Gallaba Plain is still not well understood. Such information deficiency necessitates more investigations to provide the needed subsurface geological and hydrogeological settings for safe groundwater exploitation. Therefore, the area of El Gallaba Plain has been selected for investigation in the present work.
The investigated area is about 2574 km2 and lies between Latitudes 24.4o and 25.0o N and Longitudes 32.4o and 32.9o E with an average E-W width of about 39 km and N-S length of about 66 km.
The present work depends on using the Transient Electromagnetic (TEM) technique for groundwater exploration over El Gallaba Plain.
The exploration work has been carried out with the following main objectives:
- Delineation of the subsurface geologic setting including the horizontal and vertical lithologic variations and the structural elements that affect the succession.
- Throw light on the groundwater occurrence in El Gallaba Plain, its level, source and flow direction.
- Study the impact of the lithological and structural elements on the hydrogeological setting.
- Proposing priorities of localities in the area for groundwater exploitation.
To reach results consistent with the above mentioned objectives, the following field and office activities have been conducted:
- Review of the previous geological and hydrogeological works about the concerned study area.
- Collecting the hydrogeological data from the existing wells in the area, the available lithological description and well logging records of some of the existing wells in the area.
- Carrying out transient electromagnetic (TEM) measurements at 66 sites distributed along EW and NS lines forming a grid within the plain.
- Critical analysis, qualitative and quantitative interpretation of the acquired TEM data putting into consideration the subsurface information derived from the available lithological description and well logging records of the existing wells.
- Presentation of the interpretation results in the form of maps and cross- sections.
- Presenting spatially distributed zones according to their priorities for drilling wells and groundwater exploitation.
Generally, the area belongs to the arid belt of North African which is characterized by arid climate conditions where rainfall is not significant throughout the year except for some rare and irregular storms which take place over different localities during the winter season.
Two main geomorphic landforms are recognized in the study area, which are the watershed highlands and collector lowlands. The watershed highland comprises two units, the plateau (Sin El Kaddab Plateau) and the isolated hills (e.g. Gebel El Barqa). The collector lowlands include four units which are the nubian and sandy silty plains, the drainage lines, the sand dunes and the alluvial fans.
Geologically, the stratigraphic sequence of the area, from Aswan to El Galaba plain, ranges in age from Pre-Cambrian to Quaternary. The Pre-Cambrian rocks consist mainly of igneous and metamorphic rocks. A short distance to the north of Aswan the igneous rocks do not show above ground. The sedimentary section overlying the basement complex ranges in age from Paleozoic to Recent. The exposed rock units in the study area from base to top include the following:
Upper Cretaceous: It is formed of the following geologic formations:
- Abu Aggag Formation (Nubian sandstone), that consists of coarse sandstone with mudstone intercalations.
- Timsah Formation (Nubian sandstone), which is made up of siltstone, sandstone, and shales with iron ore bed.
- Um Barmil Formation (Nubian sandstone), that is composed of medium sandstone with claystone intercalation.
- Quseir Formation, which consists of varicolored shale, siltstone and sandstone.
- Duwi Formation, which consists of glauconitic sandstone with grey shale.
- Keseiba Formation, which consists of fine grained sandstone with shale and silt intercalations.
Tertiary: It is formed of the following geologic formations:
- Dakhla Formation (Paleocene- Eocene), which is made up of laminated shale.
- Kurkur Formation (Paleocene) that is mainly dolomitic and marly limestone.
- Garra Formation (Paleocene) that is made up of limestone, partly chalky and intercalations of marl and shale.
- Thebes group (Lower Eocene) which consists of earthy brown grading into greyish white limestone with chert.
Quaternary: It represents the youngest deposits in the study area and consists mainly of alluvial sediments (mixture of gravels, sands, silts and mud).
Structurally, the study area is situated within the African Platform with its Pre-Cambrian folded basement, thus its tectonic framework is related to the Last African Orogenic belt. The study area is a part of the Nile Valley in Egypt which is, entirely, controlled by wrench faults that are generally parallel either to the Gulf of Suez or the Gulf of Aqaba directions.
Based on satellite imagery and ground penetrating radar measurements (GPR), there are offsets in the subsurface strata resulted from two sets of buried faults (NW–SE and E–W). These sets of faults are believed to be connected with the Nile River through Wadi El- Kubbaniya and may possibly represent favorable zones of groundwater Accumulation. A set of prominent structures mainly striking in NW-SE and N-S direction have been also detected.
Hydrogeologically, the aquifer system in the investigated area is not homogeneous but consists of several horizontal layers that are affected by deep seated structures causing in some cases uplifting or subsidence of aquifer sections. Some of these structures are also responsible for seepage from Lake Nasser into the aquifer as well as across the aquifer layers. from the collected hydrological field information obtained from drilled wells, the water bearing formation is mainly composed of sand and clayey sand. The depth to water ranges from 4 m at the northeastern part of El Gallaba Plain to 80 m at the center of the plain. The total depth of the wells ranges from 100 to 150 m. Most of these wells tap, only, the Quaternary aquifer.
The Transient Electromagnetic (TEM) technique has been applied for groundwater exploration over El Gallaba Plain. This technique is being increasingly used in geotechnical, environmental, groundwater surveys and geological mapping with very good findings due to its sensitivity to conductive bodies.
The acquisition of the TEM soundings took place using the TEM-FAST 48 (AEMR Company, Netherlands). A total of 66 TEM soundings were acquired in different locations along successive profiles in E-W direction forming a survey grid covering the study area. The location coordinates of the TEM soundings were used to import their corresponding ground elevation with high accuracy from SRTM DEM 30 by means of the GIS software application (Global Mapper, v 11.01, 2009).
The TEM surveys were carried out using a single square loop configuration (Coincident loop) of dimensions 200 X 200 m. The TEM-FAST 48 system was set to transmit current up to 4A with 48 active time gates for most of sounding stations and with 40 active time gates and 44 active time gates for some other soundings. To improve quality of the collected TEM measurements in case of noise existence, the filter was set to 50 Hz and 5 stacks. The data is recorded digitally, reviewed, stored in memory and then downloaded to a PC. Prior to the inversion process, the acquired TEM data were filtered, smoothed and converted to apparent resistivity versus time (ρ and t) by making use of the computer software IX1D V.3.39 (2008).
The TEM sounding curves have been qualitatively analyzed before carrying out the detailed quantitative interpretation (inversion process). Emphasis has been given to the common features that characterize the field curves.
The qualitative analysis showed that most of the TEM sounding curves have, more or less, the same resistivity descending trend. However, some curves show deviation from that general trend specially the last segment which show an increase in resistivity. The latter group of soundings was measured on an older rock unit (Duwi Formation) than the prevailing sediments in the rest of the plain and reflecting different succession from that within the plain. Pseudo-2D sections were produced along 6 profiles covering the study area in W-E direction from the interpolation of a group of 1D processed (apparent resistivity and time) models. The pseudo- sections revealed that there are five main layers with downward decrease in their resistivities. The pseudo- sections confirmed the presence of a layering succession, at the eastern part of the area, different from that dominating the plain. These features and some other features have been considered in the quantitative inversion process.
The quantitative interpretation of the TEM sounding data was made by the computer software IX1D V.3.39 (2008). The initial model needed for TEM data inversion has been generated in view of the general geology of the area and from the lithological description and well logging records of 4 existing wells in the area.
The inversion of the TEM sounding data revealed that two groups of layering succession have been detected in the study area. The first group (I) consists of a sequence of six layers which dominates most of El- Gallaba Plain, whereas, the second group (II) consists of a sequence of four other layers detected in the north-eastern part of the study area.
The determined geoelectrical parameters of the six layers of the first group (I) showed that the topmost layer consists originally of 2 or 3 successive thin layers with different resistivities. The resistivities of these thin layers have been combined into a single equivalent resistivity by making use of the transverse resistivity (ρt). The resistivity of this combined layer ranges from 400 to 1200 Ohm.m and its total thickness varies between 16.5 m to 73 m. The second geoelectrical layer is intermittently detected at some localities with low resistivity values (2 to 4 Ohm.m) and a thickness ranging from 5 m to 20 m. The third geoelectrical layer has been detected at most of the locations where the first layer is found. It attains resistivity values ranging from 100 to 400 Ohm.m. The thickness of this layer ranges from 5 m to 25 m and increases up to 46 m at TEM sounding 41. The fourth geoelectrical layer is subdivided into two zones. The upper one is dry and attains resistivities range from 15 to 80 Ohm.m. The lower zone of this layer attains lower resistivity than the upper one (5- 12 Ohm.m) due to saturation with water. The total thickness of the fourth layer ranges from 6 m to 108 m, while, the thickness of this dry zone varies between 3 m and 75 m. The fifth geoelectrical layer is the lowermost detected layer from the inversion of most TEM soundings. This layer attains a resistivity ranging between 2 and 4 Ohm.m and its base has not been reached. The sixth layer has been detected at the locations of TEM soundings 53 and 55 with resistivities 9.8 Ohm.m and 12.5 Ohm.m, respectively. This layer is, possibly saturated with water and its base has not been reached.
According to the correlation with the geologic information, the upper four layers are related to the Quaternary deposits (Gravel, sand, clay, silty sand) whereas, the fifth layer corresponds to the Dakhla shale and the lower most detected layer corresponds to Keseiba Formation (fine grained sandstone).
The determined geoelectrical parameters of the four layers of the group (II) revealed that the uppermost geoelectrical layer attains resistivity ranging from 90 Ohm.m to 240 Ohm.m and a thickness varies from 12.5 m to 46 m. The second geoelectrical layer exhibits resistivity varies between 10 Ohm.m and 26 Ohm.m with a thickness ranging from 42 m to 76 m. The third geoelectrical layer has low resistivity (4 to 8 Ohm.m) and thickness varies between 53 m and 66 m. The fourth geoelectrical layer attains resistivity ranges from 34 Ohm.m to 52 Ohm.m. The base of this layer has not been reached.
Based on the geological information, the second group of layers is older in age than those of the first group. The four layers are related to Duwi Formation (sandstone), Qusseir Formation which forms the second and third layer and consists of silty sandstone and sandy siltstone, respectively. The lowermost layer consists of sandstone corresponding to Umm Barmil Formation and most probably saturated with groundwater.
The results of the inversion of the TEM soundings have been used to develop 2D geoelectrical cross- sections. The cross- sections are correlated and visualized according to their corresponding geologic rock units. A total of six geoelectrical cross- sections showed the lateral and vertical extension of the different layers in W - E direction and another geoelectrical cross- section traversed the central part of El Gallaba plain in N – S direction. The cross-sections revealed the geological and the hydrogeological features in the area.
The geological and hydrogeological information from the drilled boreholes in the study area indicated that the lower zone of the fourth geoelectrical layer has the same lithological composition as the upper zone (silty sand) and corresponds to water bearing zone. The resistivity distribution of the water bearing formation throughout the investigated area has been illustrated as iso-resistivty contour map. This map showed that the groundwater salinity in the northern and southern parts of the area is less than central part. The electrical resistivity is also affected by the lithologic composition of the formation. The isopach map of the water bearing formation showed that the water bearing formation attains its maximum thickness at the southern and the central parts of the investigated plain. The water table map (generated from TEM data inversion results) indicated that there is a clear groundwater flow from the south (at Wadi El-Kubbaniya) toward the central part of the plain in the north. The low level of the shale layer at that locality along with the southern NNW- SSE striking faults and the groundwater flow direction indicated that the alluvial deposits (Quaternary) aquifer is likely recharged from the Nubian Aquifer in the south. Occurrence of groundwater at the northeastern part of the area is due, mainly, to seepage of irrigation water of the cultivated area at that location.
A practical augmentation of the feasibility of the geophysical results for use by beneficiaries and decision makers is presented by a group of priority maps for groundwater exploitation in El Gallaba Plain. Setting priorities depended on three criteria, i.e. groundwater salinity (from the water bearing formation resistivity distribution), groundwater quantity (from the water bearing formation thickness distribution) or the borehole drilling depth (from the distribution of the depth to water bearing formation). Separate priority map for each criterion has been delivered to be used in case it represents the main priority of the end user. Additionally, the three criteria have been collectively considered in illustrating a single general priority map (decision map) for groundwater exploitation in the investigated area.
Automatic generation of the priorities has been reached by making use of a simple QBASIC program specially designed to determine a number of classes (priorities) from a data set includes up to four parameters.
According to the present geophysical study, the following results have been reached:
- The Transient electromagnetic (TEM) technique proved to be an effective tool for groundwater exploration in terms of resolving power, cost and time.
- The geological and hydrogeological settings in El Gallaba plain have been delineated.
- The structural setting and the relief of the Dakhla shale play the major role in groundwater flow, recharge and occurrence.
- The Nubian sandstone aquifer is likely to recharge the Quaternary aquifer in El Gallaba plain.
- The lithologic composition of the water bearing formation (silty sand) necessitates reasonable location distribution when drilling new wells and discharge rate.
- The generated priority map (decision map) delineated the best sites for drilling water wells in view of the combined characteristics of resistivity, aquifer thickness and depth to water. Accordingly, some localities in the south, middle and north of El Gallaba plain have been selected.