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Abstract SUMMARY AND CONCLUSIONS Western desert acts as one of the most vital regions in Egypt for its size and natural resources. This desert lands owned the greatest aquifer in Egypt (Nubian Sandstone Aquifer System) which can use to increase the national income by increasing the amount of agricultural lands and then amount of crops. Therefore, intensive studies evaluate the condition of groundwater aquifer in the study area via geophysical exploration tools. The investigated area is located in south Eastern portion of Western desert of Egypt. It lies between latitudes 22° 15`& 22° 57` N and longitudes 31° 12` & 31° 54` E. It is bounded by Wadi Halfa road to the west and Tushka Canal to the east (Figure 1). The area under study covers about 2500 km2 . This area is accessible through Cairo – Aswan – and Abu-Simbel asphaltic road. The present study deals with using the application of remote sensing and different geophysical tools such as, magnetic and geoelectrical resistivity to achieve the main object of groundwater aquifer condition evaluation in the investigated area. The geomorphologic features in the study area can be classified into three morphological units that are the Aswan High Dam Lake: occupies the southern part of the study area, Wadi Kurkur Pediplain: covers the area between Aswan High Dam Lake and Tushka depression, and Tushka Depression: topographic depression is considered as a remarkable geomorphologic unit of Tushka area and which occupies the northern part of the study area. Stratigraphically, the area under study is covered by sediments representing three ages (Upper Jurassic – Lower Cretaceous, Lower Cretaceous, and Quaternary deposits) in addition to Pre Cambrian basement rocks. The Stratigraphic formations of the investigated land are subdivided into Nubian SUMMARY AND CONCLUSIONS 144 144 Sandstone presented by Abu Simbel formation, Lake Nasser formation, and Sabaya Formation and Quaternary deposits. Structurally, the study region is affected by fault system. The fault system is represented by normal faults having four directions E-W, N-S, NE-SW, and NW-SE. Two landsat scene images and one scene of digital elevation model image was used as application of remote sensing technique to identify the lands affected by hydrothermal solutions and construct a 3-D view to the surface. The results of this classification are; 1- the surface of the study area shows seven types of classes. These is surface water in lake Nasser, plant cover, dry channels, Nubian sandstone on the floor or covered by sand sheets, Nubian sandstone as hilly lands or mountains, basalt and Nubian sandstone affected by hydrothermal solutions. 2- The effect of hydrothermal solution was defined at the high lands so it appears as isolated points not in polygons. 3- The areas covered by sand sheets must contain some locations affected by hydrothermal solution but it’s not viewed. The total intensity of the earth’s magnetic field was measured along 24 profiles traversing the study area; 10 profiles have N-S direction intersected by 14 profiles with W-E direction to form more or less grid pattern. The measured total magnetic values are corrected diurnally and plotted on profiles and contour maps. The resulting picture represents a total intensity magnetic map. This map is reduced to the pole by applying the reduction to the pole technique. In Quantitative Interpretation of Magnetic Data; the same direction and location of 24 profiles were extracting from the total magnetic intensity map reduced to the north magnetic pole to be modeled to delineate the depth of the basement surface and the basement tectonic framework of the concerned area. With the helpful of all available geologic and topographic information and the obtained results from interpretation of SUMMARY AND CONCLUSIONS 145 145 magnetic and geoelectrical data; the basement cross-sections were assumed. from interpretation of the created magnetic modeled profiles and maps, it was clear that: the maximum depths to the basement rocks in the study area reached to 694 m. the area under study affected by 17 normal faults that represent the two directions NE-SW, and NW-SE. These faults create many grabens and basement uplifts. This structural framework affects the groundwater conditions. A reasonable coverage for the study area was reached by a total of 52 Vertical Electrical Soundings (VES). The sounding stations were distributed, more or less, in the form of a grid. The Schlumberger 4-electrode configuration was applied in the present investigation. The maximum current electrode separation (AB) ranges from 1000m to 3000m. This electrode separation proved to be sufficient to reach the required depth that fulfils the aim of the study in view of evaluating the geologic and hydrogeologic conditions. The resistivity sounding data has been interpreted qualitatively and quantitatively. The qualitative interpretation of the sounding curves revealed that, the apparent resistivity values on the first, second and third logarithmic cycles show unsystematic common trend that is characterized by different apparent resistivity values due to differences in the surface and near surface layers (irrigation lands, sand sheets, and/or hard bed sandstone). In the last terminal, it can be observed that almost all the field curves terminate with uncompleted or completed H-type, which starts at different distances (AB/2) in many curves. This indicates that the main layer (expected water bearing formation) have, most probably, different depths due to the effected of structural elements. Three VES’es stations only show H-A-type that means reaching to the basement rocks. SUMMARY AND CONCLUSIONS 146 146 The detailed results from the quantitative interpretation of the geoelectrical resistivity sounding data at the western portion of the study area are discussed in terms of the geoelectrical parameters (resistivity and thickness) of the resulting geoelectrical layers. The interpretation of the resistivity soundings led to the detection of five main geoelectrical layers (A, B, C, D and E). Some of these layers have not been detected at some sounding stations. The first geoelectrical layer (A) shows average transverse resistivity ranges from 182 Ohm-m at VES 36 as minimum resistivity to 114858 Ohm-m at VES 22 as maximum resistivity. These big differences between values are due to lithological changes of surface layer. The thickness of this layer is ranging from 1.2m at VES 36 to 6.6m at VES 31. The second geoelectrical layer (B) has minimum thickness of 47m at VES 32 and maximum thickness 127m at VES 1. This layer is corresponding to dry layers of Nubian sandstone. On the other hand, it has a wide rang of average transverse resistivity from 38.5Ohm-m at VES 39 to 5382.9 Ohm-m at VES 27. The third geoelectrical layer (C) is corresponding to shall stone and its minimum resistivity value is 8 Ohm-m at VES No. 17, whereas its maximum resistivity value is 33 Ohm-m at VES No. 24. This geoelectrical layer comprises the cape rock of the main aquifer in the study area. The thickness of this geoelectrical layer ranges from 8m at VES No. 29 to 19m at VES No. 1. This layer was not detected through the sounding stations No. 2 and 3 due to the rising of basement rocks in this area. The fourth geoelectrical layer (D) is the water bearing Nubian sandstone in western area (formed from successive layers of sand and shale or clay but the amount of clay is decreasing with depth) and attains an electrical resistivity range from 56.2 Ohm-m at VES No. 39 to 367 Ohm-m at VES No. 5. The minimum thickness was detected for this geoelectrical layer is 56m at VES No. 32, whereas its base was not reached in many VES’es SUMMARY AND CONCLUSIONS 147 147 but we use the result of magnetic data to determine the maximum expected thickness of this layer 442m. The fifth geoelectrical layer (E) which has higher resistivity values reached to 11823 Ohm-m at VES 2. So, it’s interpreted as basement rocks and detected in three VES’es only, which are VES’es No. 2, 3 and 32. The detailed results from the quantitative interpretation of the geoelectrical resistivity sounding data at the north portion of khor Tushka represented by three main geoelectrical units (A, B, and C) as a result of geologic correlation. The first geoelectrical unit “A” mostly is formed from number of thin layers of different resistance, so, the resistivity of this unit is result of average transverse resistivity (ρt) and ranges from 22 Ohm-m at VES 51 as minimum resistivity to 14850 Ohm-m at VES 43 as maximum resistivity. These big differences between the values are due to heterogeneity of surface layer, which is change from wade deposits, soil, sand sheets, to compact sand. The thickness of this layer is ranging from 1.6m at VES 41 to 8.2m at VES 49. The second geoelectrical unit “B” corresponding to dry layers of Nubian sandstone with thickness range between 36.5m at VES 49 and 78.1m at VES 46, the thickness of these layers is increasing and decreasing as factor of land relief . These dry layers of Nubian sandstone have a wide range of average transverse resistivity from 92.9 Ohm-m at VES 42 to 1167 Ohm-m at VES 50. The low resistivity values reflect high amount of silt and shale intercalated with sandstone and high resistivity values becomes from gravelly sand and well sorted sandstone. The third geoelectrical layer “C” this layer was interpreted as water bearing sandstone. It’s formed from different types of sand like gravelly sand fine to medium or medium to coarse sand. The individual sounding interpretations have been used to generate 12 geoelectrical cross sections traversing the investigated area in different directions. Correlation of the SUMMARY AND CONCLUSIONS 148 148 geoelectrical parameters (resistivity and thickness) helps inferring the structural elements that affect the succession. Two sites of 2-D imaging profile with distance between the unit electrode separation 20m was measured at selected two small khors in the study area The first profiles have length 600m and it is located at the south of khor gond. It shows; there is no water invasion from khor Tushka to the dry layers of the study area. Second the water bearing layers are in contact with khor Tushka and it’s appearing at depth 90.5m. The second profiles have length 540m and it is located at the north of the first one and it shows; first, was their water invasion from khor Tushka to the dry layers of the study area so, it’s become wet layer and formed shallow aquifer in this portion with thickness about 20m. Second the water bearing layers are in contact with khor Tushka and it’s appearing at depth 49.8m. One site of 3-D resistivity imaging measurements was carried out at the eastern portion with 81 electrodes as the Pole-Pole array. The electrodes are arranged in a 9 by 9 square grid with a unit spacing of 7 meter between adjacent electrodes to identify the relationship between khor Tushka water and aquifer. According to the result of interpretating this 3-D resistivity imaging; the unconfined aquifer has charged from khor Tushka. from all kinds of the previous applications we can conclude the following result points: 1. There are some locations in the area under study have impervious Nubian sandstone due to the effect of hydrothermal solution. 2. The depth to basement rocks in the study area reached to 694m so, the sedimentary thicknesses is in a positive condition for ground water accumulation. 3. The study area is formed from many grabens and horsts. SUMMARY AND CONCLUSIONS 149 149 4. The saturated Nubian sandstone layer which is represented by the fourth geoelectric unit at the south of khor Tushka area has thickness from 56m to 442m. Moreover, the aquifer in this area is defined as confined aquifer type. 5. The saturated Nubian sandstone layer which is represented by the third geoelectric unit at the north of khor Tushka area has thickness range from 160m to 337m. Moreover, the aquifer in this area is defined as free aquifer type. 6. The study area is affected by different normal fault systems which are affected by hydrothermal solutions that make the sediment on its plain impervious rocks. This classifies the aquifer to isolate and/or semi isolate aquifers system. 7. There are direct connections between khor Tushka and the layers above the confined aquifer in some places formed the shallow aquifer and absent at others. 8. There are similarity between Isoresistivity contour map and water Salinity map. 9. Some of the drilled water wells in the study. |