Geophysical and Geological Assessment of Groundwater Occurrences of Northern Kharga, Western Desert, Egypt
Abdellatif Mohammed Abdellatif Younis;
Abstract
The development of the Egyptian desert is one of the most important factors for assuring the availability of food for population and for the export of certain agricultural products, employment possibilities increasing, mining and tourism activity. It is also known that the Egyptian populations increasing devour the agricultural production. When this is coupled with increasing and expansions of urban agricultural land, which annexed to the urban space, the problem is increased considerable. The solution is to reclaim more desert land using all available means of technological progress.
El Kharga Oases depends on groundwater as the sole source for water and are suffering from continuous decrease in groundwater levels due the excessive groundwater withdrawal, so it is necessary to study the aquifer of El Kharga Oasis to delineate the extension of the ground water layers.
The study area is located between longitudes 30°33'0.00" & 30°43'12.00" E and latitudes 25°37'48.00" & 25°25'12.00" N. The area under study covers about 410 km2.
The Nubian Sandstone aquifer in El Kharga Oases belongs to Lower Cretaceous and mainly consist of coarse to medium sand with high permeability. This permeability varied from one area to another depending on the deposition conditions and also by intercalations of silts. The Nubian Sandstone thickness reaches a minimum value in the southern part of the study area and is gradually increasing to the north. This aquifer system overlies the Pre-Cambrian basement Complex, thus, ranging in age from Cambrian to the Pre-Upper Cenomanian.
The present study aims to evaluate the conditions of groundwater occurrences by making an integration between geophysical methods. Whereas these aims can be simplify as following:
• Reviewing the subsurface litho- stratigraphy and hydrogeology in the study area through geological data.
• Identifying the basement relief and thickness of sedimentary cover.
• Delineating the subsurface structuralelements.
• Delineating the groundwater aquifer bearing layers and it’s types of Kharga oasis basin.
To achieve these goals a Conducting geophysical measurements using land magnetic survey, land gravity survey, electrical resistivity survey , time domain electromagnetic, Analyses and processing the available geophysical data sets, Integrating the results from geophysical and well logging data to delineate the geophysical and hydrological parameters of the groundwater aquifer in the study area.
Hydrogeologically The Nubia facies constitutes the main groundwater aquifer in the study area; it consists of alternating beds of clay, shale, sand and sandstone, in El-Kharga Oasis, this succession rest unconformably on the basement complex and passes conformably upward into the Cretaceous. Hydrogeological cross section taking from lithological logs of wells obtained from RIGW 2006moving through N-S the study area show that Nubian sandstone aquifer is divided into four zones (A, B, C & D).
The total intensity of the earth’s magnetic field was measured along 12 profiles covering the study area these profiles taking 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 apply the reduction to the pole technique. This data are analysis and Interpreted qualitative and quantitative to detect the basement relief map or the sedimentary cover.
In Quantitative interpretation of magnetic data the same direction and location of 10 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.
The basement depth value is ranging from 250 m to 1500 m in the study area. and the general dip of basement in the study area is toward South East.
The depth of basment in the study area indicate on the thickness of sedimentery cover, consequentely the depth of Nubian Sandstone is known because it is found directely on basement rocks, which is the main aquifer in study area. So the importance of Magnetic method for determine the upper surface of basement rocks or the lower surface of Nubian Sandston aquifer in the study area.
An accurate gravity survey was conducted at Northern Kharga area. Data were collected using a grid with 500 m spacing between the gravity stations along 12 profiles covering the study area these profiles taking W-E direction to form more or less grid pattern. The station positions and elevations were determined using dual frequency Global Positioning System (GPS). The survey was designed to map the subsurface structure of study area. The spacing between the gravity stations was chosen in order to detect the local and regional changes in the gravity values reflecting density contrasts of the underground. That allows one to precisely determine changes in the subsurface geological structure and faulting zones.
The measured Gravity data are corrected to construct the Bouguer Gravity contour map. After that Frequency analysis technique is applied to the gravity data to separate anomalies related to the shallow structure (residual component) and anomalies related to deep and wide geologic structures (regional component).
From interpretatation of the Gravity separation,The regional map shows major faulttrending NE-SW occupying the central part of the study area and the high gravity anomalies occupying Southern eastern Part of the area about -155 mGal. The northwestern and the central of southern part reflect low gravity anomalies of about -160 mGal. Also theresidualanomalymapshowsmore smallscalefeaturesrelatedtothelocalstructures.By inspectioninthe residualmapmaybedissectedbydifferentfaultstrendingE-W,NW-SE andNE-SWwhichassociatedwithhighfrequencyforgravity anomalyinarea of study. Inaddition,there isanarrowelongatedplainextendingalongthe westernborder of Kharga inNW-SEdirectionandischaracterizedby the presenceofaseries ofnormalfaultsofvaryinglengthsanddisplacements whichreveals inresidualmap.
The Euler deconvolution process is applied at each solution. The best solutions which make the data are concentrated at some places in the study area. The structural index 0 gives better solution, which indicate on many structures as Sill, Dyke, Ribbon, and Step structure in the study area. Also the trend of faults at SI=0 is nearly similar of fault element which detect from high pass (residual) map.so residual gravity anomaly map was used to detect the location and trends of the faults dissecting the study area. This map indicates that the study area is affected by major and minor fault structures dissected the Nubian sandstone aquifer which have different trends NE-SW, N-S and E-W.
After detection the depth of lower surface of Nubian Sandston aquifer by magnetic method, the gravity method was applied to determine the structure element which affected in sedimentary cover especially Nubian sandstone aquifer which controlling in groundwater aquifer in the study area especially fault structures.
Time Domain Electromagnetic (TEM) survey was conducted in the study area using SIROTEM MK-3 conductivity meter. The field measurements have been recorded in 167 stations each station measured three to four Times. These stations are arranged more or less in a grid like pattern, where the distance between stations ranges from 500 m to 1000 m depending on the suitable space for spreading the Loop of transmits and receivers along W-E trending lines and distance between lines in N-S about 2 km made 11 lines through the study area to provide reasonable coverage of the study area.Beside the electromagnetic measurements, the field survey comprises measuring the apparent resistivity values through 28 vertical electrical soundings (VESes). These VESes are arranged along six profiles four of them are N-S and the other is W-E in the available places perpendicular to the TEM Stations to get the primary model and to control the inversion of TEM Data analysis, most of these profiles are conducted along Kharga – Assuit Road because it’s difficult to do them in grid according to large distribution of Sand dunes and highly resistivity surface layer along the area. Electrical resistivity data have been acquired in this area using Syscal R2 resistivity meter. The field survey has been carried out by application of the standard Schlumberger electrode configuration.
El Kharga Oases depends on groundwater as the sole source for water and are suffering from continuous decrease in groundwater levels due the excessive groundwater withdrawal, so it is necessary to study the aquifer of El Kharga Oasis to delineate the extension of the ground water layers.
The study area is located between longitudes 30°33'0.00" & 30°43'12.00" E and latitudes 25°37'48.00" & 25°25'12.00" N. The area under study covers about 410 km2.
The Nubian Sandstone aquifer in El Kharga Oases belongs to Lower Cretaceous and mainly consist of coarse to medium sand with high permeability. This permeability varied from one area to another depending on the deposition conditions and also by intercalations of silts. The Nubian Sandstone thickness reaches a minimum value in the southern part of the study area and is gradually increasing to the north. This aquifer system overlies the Pre-Cambrian basement Complex, thus, ranging in age from Cambrian to the Pre-Upper Cenomanian.
The present study aims to evaluate the conditions of groundwater occurrences by making an integration between geophysical methods. Whereas these aims can be simplify as following:
• Reviewing the subsurface litho- stratigraphy and hydrogeology in the study area through geological data.
• Identifying the basement relief and thickness of sedimentary cover.
• Delineating the subsurface structuralelements.
• Delineating the groundwater aquifer bearing layers and it’s types of Kharga oasis basin.
To achieve these goals a Conducting geophysical measurements using land magnetic survey, land gravity survey, electrical resistivity survey , time domain electromagnetic, Analyses and processing the available geophysical data sets, Integrating the results from geophysical and well logging data to delineate the geophysical and hydrological parameters of the groundwater aquifer in the study area.
Hydrogeologically The Nubia facies constitutes the main groundwater aquifer in the study area; it consists of alternating beds of clay, shale, sand and sandstone, in El-Kharga Oasis, this succession rest unconformably on the basement complex and passes conformably upward into the Cretaceous. Hydrogeological cross section taking from lithological logs of wells obtained from RIGW 2006moving through N-S the study area show that Nubian sandstone aquifer is divided into four zones (A, B, C & D).
The total intensity of the earth’s magnetic field was measured along 12 profiles covering the study area these profiles taking 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 apply the reduction to the pole technique. This data are analysis and Interpreted qualitative and quantitative to detect the basement relief map or the sedimentary cover.
In Quantitative interpretation of magnetic data the same direction and location of 10 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.
The basement depth value is ranging from 250 m to 1500 m in the study area. and the general dip of basement in the study area is toward South East.
The depth of basment in the study area indicate on the thickness of sedimentery cover, consequentely the depth of Nubian Sandstone is known because it is found directely on basement rocks, which is the main aquifer in study area. So the importance of Magnetic method for determine the upper surface of basement rocks or the lower surface of Nubian Sandston aquifer in the study area.
An accurate gravity survey was conducted at Northern Kharga area. Data were collected using a grid with 500 m spacing between the gravity stations along 12 profiles covering the study area these profiles taking W-E direction to form more or less grid pattern. The station positions and elevations were determined using dual frequency Global Positioning System (GPS). The survey was designed to map the subsurface structure of study area. The spacing between the gravity stations was chosen in order to detect the local and regional changes in the gravity values reflecting density contrasts of the underground. That allows one to precisely determine changes in the subsurface geological structure and faulting zones.
The measured Gravity data are corrected to construct the Bouguer Gravity contour map. After that Frequency analysis technique is applied to the gravity data to separate anomalies related to the shallow structure (residual component) and anomalies related to deep and wide geologic structures (regional component).
From interpretatation of the Gravity separation,The regional map shows major faulttrending NE-SW occupying the central part of the study area and the high gravity anomalies occupying Southern eastern Part of the area about -155 mGal. The northwestern and the central of southern part reflect low gravity anomalies of about -160 mGal. Also theresidualanomalymapshowsmore smallscalefeaturesrelatedtothelocalstructures.By inspectioninthe residualmapmaybedissectedbydifferentfaultstrendingE-W,NW-SE andNE-SWwhichassociatedwithhighfrequencyforgravity anomalyinarea of study. Inaddition,there isanarrowelongatedplainextendingalongthe westernborder of Kharga inNW-SEdirectionandischaracterizedby the presenceofaseries ofnormalfaultsofvaryinglengthsanddisplacements whichreveals inresidualmap.
The Euler deconvolution process is applied at each solution. The best solutions which make the data are concentrated at some places in the study area. The structural index 0 gives better solution, which indicate on many structures as Sill, Dyke, Ribbon, and Step structure in the study area. Also the trend of faults at SI=0 is nearly similar of fault element which detect from high pass (residual) map.so residual gravity anomaly map was used to detect the location and trends of the faults dissecting the study area. This map indicates that the study area is affected by major and minor fault structures dissected the Nubian sandstone aquifer which have different trends NE-SW, N-S and E-W.
After detection the depth of lower surface of Nubian Sandston aquifer by magnetic method, the gravity method was applied to determine the structure element which affected in sedimentary cover especially Nubian sandstone aquifer which controlling in groundwater aquifer in the study area especially fault structures.
Time Domain Electromagnetic (TEM) survey was conducted in the study area using SIROTEM MK-3 conductivity meter. The field measurements have been recorded in 167 stations each station measured three to four Times. These stations are arranged more or less in a grid like pattern, where the distance between stations ranges from 500 m to 1000 m depending on the suitable space for spreading the Loop of transmits and receivers along W-E trending lines and distance between lines in N-S about 2 km made 11 lines through the study area to provide reasonable coverage of the study area.Beside the electromagnetic measurements, the field survey comprises measuring the apparent resistivity values through 28 vertical electrical soundings (VESes). These VESes are arranged along six profiles four of them are N-S and the other is W-E in the available places perpendicular to the TEM Stations to get the primary model and to control the inversion of TEM Data analysis, most of these profiles are conducted along Kharga – Assuit Road because it’s difficult to do them in grid according to large distribution of Sand dunes and highly resistivity surface layer along the area. Electrical resistivity data have been acquired in this area using Syscal R2 resistivity meter. The field survey has been carried out by application of the standard Schlumberger electrode configuration.
Other data
| Title | Geophysical and Geological Assessment of Groundwater Occurrences of Northern Kharga, Western Desert, Egypt | Other Titles | "التقييم الجيوفيزيقي والجيولوجي لإمكانيات المياه الجوفية شمال الخارجة، الصحراء الغربية، مصر” | Authors | Abdellatif Mohammed Abdellatif Younis | Issue Date | 2015 |
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