الفهرس 
CHAPTER I: INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 393 |  |  |
| | | from | 393 | | |
Abstract
Egypt is facing an annual water deficit of around seven billion cubic meters. In fact, United Nations is already warning that Egypt could run out of water by the year 2025. Accordingly, groundwater prospecting considers part of the solution to the mentioned problem. The area under investigation is located in the western part of the Nile Delta, northwestern of Cairo city. It spreads along the both sides of the Cairo-Alexandria desert road, Beheira Governorate.
The main purpose of this study to evaluate the area under study in terms of ground water prospecting and to outline the subsurface geological conditions and hydro-geological conditions based on the most suitable and pronounced geophysical methods; gravity, electrical resistivity method in addition to shallow seismic refraction techniques.
The area lies west El-Nubariya is covered mainly by sedimentary rocks, which belong to the Quaternary and Neogene age. The Quaternary sediments are exposed in nearly all over the studied area which is mainly clastic with essential sand facies and occasional gravel and clay intercalations. Sand sheet and sand dunes are detected towards north. Neogene sediments are generally exposed western and southern parts at Wadi El Natrun. They composed of sand and sandstone with clay and limestone intercalations.
In the subsurface, the sedimentary succession rocks are ranges in age starts from base by Triassic rocks resting on the Basement Complex and ends at top with Pliocene sediments and have a thickness of about 4000 m.
The western Nile Delta is generally characterized by low relief and mild topography with elevation varying from zero to +100 m. The land features have been developed through the interaction of the geologic structures, the geomorphologic processes and the climatic conditions, which affects the scenery in the different province.
The western Nile Delta has a thick sedimentary succession that ranges in age from the late Cretaceous to Quaternary. In Wadi El Natrun, the sedimentary section is greater than 4 km in thickness and ranges in age from Triassic to Pliocene. The water-bearing formations belong to Oligocene, Miocene, Pliocene and Quaternary.
The area under study and its vicinities, is mainly affected by faults, folds and unconformities. These structural elements are the most important factors influencing the groundwater conditions. The Northern Egypt was affected by three tectonic events. The oldest resulted in NE-SW trending structure, which followed by (Syrian Arc) major trending structures. The minor tectonic event resulted in the NW-SE (Gulf) trending structures.
The gravity data of the study area is extracted from the Bouguer gravity map executed by the General Petroleum Company, (1977) with scale 1:100,000 and contour interval of 1 mgal; The resistivity data is represented in the 44 Vertical Electrical Soundings (VES) obtained from Research Institute for Groundwater while
shallow seismic refraction survey includes 32 profiles near selected VES are executed by the author himself. The survey is carried out using a 12-channel signal enhancement seismograph model (GEOMETRICS) version ES-1225 along 130 m length of profiles and 10 m geophone interval.
The treatment and processing of the Bouguer gravity map of El-Nubariya area played an important role in the determination of the structures and tectonic elements of the study area. Qualitative and quantitative interpretation of the available gravity data were conducted to indicated the main subsurface geological implication with special emphasis to the structural-tectonic inferences. Gravity interpretations were done first in the form of separation the effect of shallow causative bodies of residual anomalies from the effect of the deep-seated causatives regional anomalies by one of the traditional gridding methods. Then, wavelength filtering of the Bouguer gravity anomalies was operated. Such a technique proved helpful in separating the residual anomalies from the regional anomalies or the anomalies reflecting a certain interval of depth. It is also beneficial in attenuating the extraneous factors affecting a certain depth range and enhancing the wanted effects of the other ranges.
Bouguer gravity anomaly map is characterized in general by the presence of three main gravity belts of varying polarities; north western gravity high belt, north eastern gravity low belt and large central gravity high belt. Every of these gravity belts are formed of a number of gravity anomalies of variable areal extensions, vertical relief, directions and shapes. Each of these anomalies reflects a certain causable mass distribution of certain shape, depth, density and configuration.
In shallow levels, the main tectonic trends have the NE-SW, NNE-SSW and E-W directions, where are in deep levels, major tectonic trends have the NE-SW in addition to the presence of NW-SE and E-W directions which suggest that the area under investigation is subjected to vertical uplift more than lateral compression force.
Three shallow drilled holes (Wadi El Natrun, Khatatba-1 and Abu Roash-1) reached to the basement surface to determine the depth to the basement. The resulted depth to the basement values in the area under study revealed that, the depths to the basement in the area under investigation are ranged between 1900 and 4000 m. Finally, the regional and local structures were established. The fault elements are varied in depth from less than 2.0 km. to more than 5 km. and their trends are varied from NE-SW to NNE-SSW to NW-SE and E-W but the more predominant trends are the NE-SW and NNW-SSE.
The basement configuration map of the area under study shows that, the average depth to the basement surface exceeds 4000 m and lies within a depth to the basement highs (swells) and lies within a depth range of 1900 m and 4000 m. This map reflects a number of basement highs (swells) and lows (troughs) having varying sizes and trends.
Generally, it could be safely stated that, the major basement troughs are associated with large low gravity anomalies, while the major basement swells are accompained with high gravity anomalies. These interpretations
are convenient, since the basement troughs are normally associated with thick sedimentary sections and low gravity effects and vice versa.
Forty-four vertical electrical soundings in the form of Schlumberger configuration in the El-Nubariya area with maximum spread AB/2 = 200 m. The interpretation procedure of the obtained field data executed both qualitatively and quantitatively. The qualitatively interpretation of the geoelectrical resistivity data were executed by preparation of the iso-apparent resistivity maps, iso-apparent resistivity profiles and pseudo sections. On the other hand, the quantitative interpretations include the preparation of VES curves, the true resistivity maps, true thickness maps and geo-electrical cross sections.
from the discussion of the iso-apparent electric resistivity contour maps, the following manifestations can be exhibited; there is a great similarity in the shape of the major anomalous features and contours with their directions through the studied maps. This reflects that, the subsurface strata inferences are nearly similar; the conspicuous lateral variations of the iso-apparent resistivities indicate comparable lateral changes in the encountered types of rocks. The comparison among the three iso-apparent resistivity contour maps at AB/2=1.5, 4, and 10 m (for the surface layer). reveals that, the resistivity values for the smaller depth of penetration generally begin with relatively high-very high values in the northern and northeastern zone and medium-high values in the central zones, then increase gradually in the western zone and decrease in southern zones. This gives indications to the expected surface layer in the most locations in the northern zone is sands, sandy gravels or gravels and their derivatives, while
in the central and southern zones the expected surface layer is sands and silty sand.
The comparison among the iso-apparent resistivity contour maps, from AB/2 = 15 to 50 m, for the aquifer layers indicates that, the resistivity values decrease gradually all over the area for all these maps. This gives indications to the resistivity of the aquifer layers that begins with relatively medium resistivity of sands.
- The comparison among the other iso-apparent resistivity contour maps, from AB/2= 100 to 200 m., indicates the decrease in the resistivity in all over the area for all these maps that indicate the presence of the sandy clay and clay layer.
The visual inspection of the apparent resistivity profiles and the apparent resistivity pseudo-sections shows the followings:
The apparent resistivity behavior is more or less the same along all branches of the profiles and the pseudo-sections. This appears to be in a good concordance with the results obtained from the iso-apparent resistivity maps.
The surface cover in the northern profiles, generally, is of relatively high-very high apparent resistivity values (medium sand with gravel). while the surface cover in the central and southern profiles is of a relatively high apparent resistivity values (medium to fine sands).
Generally, by increase the depth of propping, (after passing the surface layer) the apparent resistivity behavior begin with relatively low values (clayey sand), increase (coarse-medium sand) and decrease again (sandy clay) and further decrease (clays or clays with sand).
The interpretation of the electrical resistivity sounding data aims firstly to determine quantitatively the true resistivities and thicknesses of the successive strata below the different stations utilizing the measured field curves. It is achieved purely on theoretical considerations and is popularly known as quantitative interpretation. Secondly, the geological interpretation translates the quantitative values into a realistic picture within the known geological framework. For the present work, the interpretation of the geoelectrical resistivity data involves the following items:
Interpretation of the vertical electrical sounding curves is the method used to convert the values of AB/2 and a into a multi-layer model, is that of (Zohdy, 1989). The multi-layer models are reduced manually to less number of layers and then, subjected to successive iterations till best fit between the calculated curves and the observed curves using (WinSev, 1997) program.
Construction and analysis of the geoelectrical cross-sections, when they correlated with the available geologic informations can provide an interpretation for the hydrogeologic conditions predominant within the investigated area.
Construction and interpretation of the true resistivity maps, depth to the aquifer map thickness maps for the encountered layers. The maximum and minimum thicknesses of the aquifer are determined.
- The quantitative interpretation of these sounding curves plays an important role in reflecting a clear picture about the depth, thickness, resistivity and the extension of the layers constituting the shallow section of the study area.
The subsurface sequence in the area under investigation is considered essentially of five geoelectrical units (1<2>3<4<5) with average resistivity values of 39, 12, 23, 14 and 7.69 m. and average thicknesses of 3.78, 10.2, 34.40, 20.89 and 4 m.
The first geoelectric unit is characterized by relatively high-very high electric resistivity value ranged between 23.10 m to 51.27 m and The thickness of this unit is ranged between 1.03 m and 7.29 m which corresponds to the surface coarse to medium sand with gravel layer.
The second geoelectric unit is characterized by relatively low-very low electric resistivity value ranged between 8.75 m to 16.77 m. The thickness of this unit ranged between 3.76 m. and 20.75 m., which partly or totally correspond to clayey sand or sandy clay and on some time with lenses of clay.
The third geoelectric unit is characterized by relatively moderate electric resistivity value ranged between 19 m. to 27.39 m. and the thickness of this unit is ranged between 10.35 m. to 42 m., which partly or totally corresponds to sandy layer (Quaternary aquifer unit).
The fourth geoelectric unit is characterized by relatively low electric resistivity value ranged between 11.74 m and 16.43 m., and the thickness of this unit is ranged between 15.83 m. to 21 m., which partly or totally corresponds to the sandy clay or clayey sand.
The fifth geoelectric unit is characterized by relatively low-very low electric resistivity value ranged between 3.66 m and 11 m and the thickness of this unit is ranged approximately 3.78 m. which partly or totally corresponds to the clay or clay with sand.
Seismic refraction data interpretation has been carried out in El-Nubariya study area using modern computer software that is based on advanced interpretation methods. It can be concluded that the shallow geological section in the area under study is composed of three layers from seismic point of view.
Seismic refraction measurements were carried out at thirty-two profiles in El-Nubariya area. The survey was carried out using a 12-channel signal enhancement seismograph model GEOMETRICS version ES-1225 along 130 m length of profiles and 10 m geophone interval. The spread used in recording the P-waves was linear with 14 Hz vertical
geophones placed on the ground along a straight line through the shot point to detect the direct and refracted P-waves. A sledgehammer (about 10 kg) used as a source of seismic energy.
Three shots (normal at 0 m; middle at 65 m and reverse at 130 m) were carried out at each profile and each shot was stacked to enhance the arrivals at distant geophones, improve data quality, and to increase the signal to noise ratio at all the geophones. The normal and reverse shooting were carried out 10 m far from the two ends of the profile.
The resulted shot records were printed out and converted to SEG-Y format valid for using any computer software data files. The data might be not clear due to the noises that were overlapped with seismic signals. Many modern software were used for filtering the data such as Geogiga Seismic Pro 8.0, after filtering the data picking the first arrival (P-wave) is the next step to get time-distance curve. Finally, using computer program software called ZONDST2D to construct the time distance curves and velocity depth model of profile.
from all of the seismic refraction data interpretation, it can be concluded that:
The shallow geological section in the area under study is composed of three layers from seismic point of view.
The first layer is a surface layer composed mainly of medium sand with gravel with compressional wave velocity ranges between 300 m/s and 710 m/s and thickness ranges from 2.6 m to 8.8 m. This thickness was
geologically correlated and matched with the drilled boreholes close to each seismic profile.
The second layer is composed sometimes under some profiles of sand and other times under other profiles of clayey sand with P-wave velocity ranges between 460 m/s and 750 m/s and thickness ranges from 7.7 m to 25.7 m.
The third layer is composed mainly of medium to coarse sand with a relatively higher P-wave velocity in the range of 910-1440 m/s.
Finally, According to the formational study, it is show that, the gravely methods is focused on the detection of sedimentary basin which is characterized by negative anomalies which conformed with electrical resistivity method and shallow seismic refraction method in recognition the lithological characteristics and the thicknesses of the encountered geological units respectively.