Magnetic Applications to Subsurface and Groundwater Investigations: A Case Study from Wadi El Assiuti, Egypt ()
1. Introduction
Wadi El Assiuti is one of the most important wadis in middle Egypt, with a dry drainage basin, whose main stream reaches ~186 km in length [1]. Its watershed basin (Figure 1) is located directly east of the Nile River between longitudes 31˚12'E and 32˚30'E, and latitudes 27˚00'N and 27˚48'N. It is a natural extension of Assiut Governorate in the Eastern Desert due to its groundwater potentialities and uncultivated soils. The catchment basin of this wadi is located entirely in the Mazza limestone plateau. Wadi El Assiuti (Figure 1) is a desert area, except for some parts of urbanization, and very small agricultural parts, which are located close to the entrance of the wadi. The increase of land reclamations for different agricultural activities and vast growth of new settlements (e.g. new Assiut city) in Assiut governorate has led to a search for new groundwater resources, necessary for sustainable development of the agricultural expansion.
One of the major plans of Egypt 2030 is to cultivate new 1.5 million Feddans in the desert to save foods and jobs for people and to build new communities far
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Figure 1. Showing the location of Wadi El Assiuti watershed. Stream networks extracted over the watershed of Wadi El Assiuti. It shows also the available aeromagnetic data; and the locations of isotopically (O, H) analyzed groundwater samples [2].
from the overpopulated areas in the Nile valley. Wadi El Assiuti represents one of the most promising desert areas for sustainable development. Shallow groundwater resources in the wadi have been used for agricultural purposes for more than 20 years; however, the heavy exploitation is leading to a decline in the groundwater table. As a consequence, extensive efforts and resources are required to explore the occurrence of groundwater in the different aquifers to sustain the development activities in the investigated areas. Moreover, suitable soils for agricultural expansion are common. The magnetic data are used to delineate the structural trends of the subsurface and the depth to the basement rocks. It is widely used in many areas [3] [4] [5]. These structural trends and the depth to the basement rocks may affect the groundwater-bearing reservoirs.
Many geological studies have been carried out by many researchers and companies on different regions of the Eastern Desert, the Nile Valley, and the entrance of Wadi El Assiuti [6] - [12]. However, No detailed geophysical studies have been conducted on the uncultivated area of the wadi.
The aim of the current work is to evaluate the occurrence of groundwater resources and to delineate the structural trends and the depth to the basement rocks affecting the groundwater in the studied area of Wadi El Assiuti, based on airborne magnetic data. The results of the geophysical data were integrated with the available borehole data to get better results and interpretations. Furthermore, this study was aimed to determine the more preferable place/places to drill new production wells in the study area.
2. Litho-Stratigraphy
The lithostratigraphic units [6] [7] (Figure 2) in the area of the wadi are discussed in descending order as below:
1) Recent to sub-recent alluvial sediments, represented by sandy deposits and wadi filling.
2) Pleistocene sediments are traced and mapped into the following sediments:
a) Recent to sub recent alluvial cover;
b) Neonile silts and clays;
c) Prenile sands;
d) Paleonile/protonile sediments, represented by the Armant and Issawia Formations; and
e) Pliocene (Paleonile) sediments of interbedded red-brown clays and thin fine-grained sand and silt laminae.
3) Lower Eocene carbonate rocks.
3. Material and Methods
3.1. Satellite-Based Data
Shuttle Radar Topography Mission (SRTM) images with a 90 m resolution were utilized to construct a Digital Elevation Model (DEM) for the Wadi El Assiuti watershed. The DEM was used for delineation of the basin and the stream networks
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Figure 2. Composite stratigraphic cross section of the study area [6] [7].
in the area (Figure 1). Multi-temporal satellite images of Landsat 7 and 8 datasets are used in the current study to detect the changes in the land use/land cover (Figure 3). Landsat 7 Enhanced Thematic Mapper Plus images, acquired in 2001 and 2011 for Path 176 and Row 41, covering the study area, have been radiometrically and geometrically corrected, with spatial resolutions of 30 * 30 and 15 * 15 m for six multispectral and one panchromatic band, respectively. Landsat 8 Operational Land Imager images, acquired in 2019 for Path 176 and Row 41, covering the Wadi El Assiuti, have been radiometrically and geometrically corrected, with spatial resolutions of 30 * 30 and 15 * 15 m for eight multispectral and one panchromatic band, respectively. A 742 (Red Green Blue) image (Figure 3) is used each time to detect the changes in the wadi and to define the uncultivated areas.
3.2. Aeromagnetic Data
The aeromagnetic data, collected by the Western Geophysical Company of America (1983), are used in the current study. The total magnetic intensity (TMI) data were subjected to different processing techniques. These techniques include the reduction to the pole (RTP) that is carried out to remove the anomaly asymmetry caused by inclination. The average magnetic inclination of 38.75˚ and declination of 1.90˚ were applied. It relocates the magnetic anomalies above the causative bodies assuming that the remnant magnetism is small compared to the induced magnetism. Upward continuation, horizontal, and tilt derivative techniques are used to delineate the surface and subsurface structural trends affecting the study area and the groundwater. A 2D magnetic modeling was carried out to estimate the depth to the basement rocks. The processing and analysis of the aeromagnetic data were done using the Geosoft program [13]. The TMI map is shown in Figure 4 with values varying between 41,593.4 and 41,812.1 nT.
3.3. Borehole Data
The available borehole data (Figure 1) were used to study the groundwater aquifers in the area.
4. Data Interpretation
4.1. Land Use/Land Cover
Remote sensing data have many different applications in the change detection
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Figure 3. Landsat 742 (RGB) images showing the Land use changes in the years 2001 (A), 2011 (B) and 2019 (C).
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Figure 4. The TMI map of the study area.
analysis. It allows decision-makers to monitor changes in the area of interest without in situ field observations. The analysis of land cover and land use is an important remote sensing application in change detection studies. It is used to detect the changes that are happening in the cultivated lands by reclamation in the Wadi El Assiuti. Inspection of Figure 3 shows the land use in 2001 (A), 2011 (B), and 2019 (C). It has experienced dramatic changes since 2001. This is evidenced by the increase in the agricultural activities in the wadi, particularly at the entrance, close to the Nile River. The large increase in land reclamations, agricultural activities, and the heavy exploitation of the groundwater during recent years is making stress on the groundwater of the shallow aquifers. Therefore, a rapid decline in the water table is happening. However, the central and northeastern regions of the wadi show no development.
4.2. The Aeromagnetic Data
4.2.1. Qualitative Interpretation
Inspection of the reduced to the magnetic pole map (Figure 5) shows that the values of the measured magnetic intensity are varying from 41,574.8 to 41,852.1 nT with the average of 41,664.7 nT. The variations in the magnetic intensity may be attributed to either lithology changes, basement topography, or faulting and folding. Low magnetic anomalies are located at the eastern parts of the study area, their values may indicate that there is a relatively deep magnetized source at those anomalies. Higher magnetic anomalies are shown at the
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Figure 5. The RTP map of the study area and the inferred faults.
western and northern parts of the area, reflecting the shallower depths of the magnetic sources.
4.2.2. Quantitative Interpretation
The magnetic anomaly data were quantitatively interpreted to delineate the structural trends affecting the area and to estimate the depth to the magnetic sources.
1) Delineation of the structural trends from the RTP magnetic anomaly map
Assiut region may be regarded as a good example that reflects the general structures of the stable paleogeographic and structural domain of the Arabo-Nubian shelf, bounded to the northwest by the mobile zone of the unstable shelf [14]. It was subjected to various tectonic events along the geologic time [9] [15] [16] [17] [18]. These tectonic events have resulted in changes in the structural setting of the area. These changes are appeared on the RTP magnetic map (Figure 5). The density and shape of the magnetic contour lines reflect these changes [19]. Therefore, the subsurface structural trends of the basement structures can be visually traced on the RTP map. Three structural trends are delineated in NW, NNW, and NE directions affecting the study area.
2) Delineation of the structural trends using the Total Horizontal Derivative (THDR) technique
The THDR technique was extensively used to detect the edges of the magnetized structures [20] [21]. It calculates the rate of change of the potential field in horizontal directions. The faulted boundaries might have high gradient areas and can be delineated from the horizontal derivative map.
Inspection of the THDR map (Figure 6) shows that the structural trends in the NW and NNW directions can be traced from it.
3) Delineation of the deeper structural trends using the continuation technique
Magnetic measurements are derived or observed at the Earth surface, at the airborne flight path, or at satellite altitude. In many instances, however, the magnetic information is not required at the measurement location. Thus, an upward or downward continuation process to a different height level has to be performed. Continuation filters are used to transform measured data into some new form; this is carried out by isolating, enhancing, and projecting the observed magnetic anomaly to other surfaces either above (upward continuation) or below (downward continuation) the original observation surface [22] [23].
The upward continuation maps of magnetic data (Z = 500, 1000, 2000, 3000, and 4000 m; Figure 7) made it possible to highlight the great wavelength anomalies. The form and the extension of the anomalies are a function of the continuation distance. Inspection of Figure 7 in comparison to the RTP map (Figure 5) shows the adjacent anomaly sources produced increasingly overlapping anomalies as the height of the continuation level increasing and the effects of the shallow sources and noise are minimized.
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Figure 6. THDR magnetic map showing the faulting system of the magnetic structures.
The regional magnetic anomaly maps (Figure 7) indicate that the anomalies observed on the RTP map (Figure 5) are the manifestations of the deeper magnetic anomalies. Inspection of these maps suggests deep-seated faults alignment in the NNW, NW, NE, E-W directions at the deeper part.
4) Delineation of the structural trends using the Tilt Derivative (TDR) technique
The TDR technique was used to sharpen the potential field anomalies. It was defined by [24]. The tilt angle is positive when over the source, near the edge as the vertical derivative is zero and the horizontal derivative is maximum and is negative outside the source region. From the zero value of the TDR, the outline of the edges can be identified [24]. The tilt amplitudes have values ranging between −π/2 to +π/2. Its zero contour line is located over or close to the edges of the faults. Inspection of the TDR of the RTP and the upward-continuation maps (Figure 8) and the zero contour line shows the magnetic anomalies are almost aligned in definite directions forming structural trends in NNW, NW, and NE directions.
4.2.3. Depth to the Basement Rocks
The 2D modeling of the magnetic data was conducted to investigate the subsurface geology and to calculate the depth to the basement rocks. The 2D modeling of the magnetic data was performed based on the techniques, given by [25] [26]. The 2D models were generated along east-west profiles (Figure 9) on the reduced to pole magnetic map using GM-SYS Geosoft [13]. The GM-SYS inversion option
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Figure 9. Locations of the 2D magnetic profiles on the RTP map.
allows us to optimize the model. Several earth models can produce the same magnetic and/or gravity response. The interpreter can evaluate the geologic reasonableness of any model [27]. These 2D models assume the earth is two dimensional and changes are in Z and X directions of the profile. The magnetic susceptibility values of 0.00050 - 0.00085 CGS units were adopted from the work of [28] [29]. The final models of the profiles are shown in Figure 10. Inspection of these models shows that the depth to the basement rocks is generally increasing southeastward from ~2.24 km at the western parts of the plateau to ~4.84 km at the southeastern parts of the study area, close to the Nile Valley.
5. Analysis of Structural Trends
The analysis of the magnetic data shows that the positive anomalies indicate higher relief of crystalline rocks, while the negative anomalies indicate lower relief or down-thrown blocks of crystalline rocks. No subsurface igneous activity was observed from the analysis of the data. Moreover, the negative anomaly values indicate the presence of deep sedimentary basins. A group of NNW, NW, NE, and E-W striking deep-seated faults is delineated from the reduced to pole map, total horizontal and tilt derivatives, and the upward continuation maps.
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Figure 10. The 2D magnetic models for profiles P1 - P9.
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Figure 11. Main fault systems of Wadi El Assiuti [9].
These faults are responsible for the formation of basinal areas. [9] has delineated three main surface fault systems in Wadi El Assiuti using thematic mapper satellite data. The three trends are arranged in decreasing order as NW (Clysmic), NE, E-W (Tethyian), and N-S (Figure 11), with the active NW fault system. This is obvious from the higher extension and continuity of the NW fault trend [9]. These major surface features are a reflection of the basement structures [16]. The deep-seated sub-vertical NW-trending Neoproterozoic faults have been reactivated during the Neogene rifting events resulted from the Red Sea opening. These faults are part of the Najd Shear System [30], the largest pre-Mesozoic faulting system on the Earth [31], alignment in an NW-SW direction (Figure 12). The system extends into the Eastern Desert [31] [32]. The integration of the surface major fault features with the structural trends delineated from the magnetic
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Figure 12. A geological cross-section (AA', Figure 1) extending from west Assiut area (west) to Red Sea Hills (east) [44].
data indicates the continuity of the faults within the sedimentary succession and the basement rocks. This leads to a hydraulic connection between the deep and shallow aquifers.
6. Groundwater Aquifers
Two shallow aquifers represent the groundwater resources in the wadi. The shallow Quaternary aquifer is the recent water-bearing source in the Nile valley and the main wadis in Egypt. It is represented by Quaternary alluvial deposits, which are considered as a promising aquifer [10]. It is consisted of course to medium sands with a thickness of about 55 m in the eastern part, and coarse to fine sands with a thickness of ~75 m in the western part. The Quaternary aquifer is considered as a semi-confined aquifer in the eastern part and a confined in the western part. The lower Plio-Pleistocene aquifer underlines the Quaternary sediments and overlies the Pliocene clays in Wadi El Assiuti area. It is composed of successions of sandy layers and/or lenses separated by clayey layers [33]. This aquifer is represented by two water-bearing zones, the upper one with ~40 m thick, while the lower zone has more thickness. Sandy clays are intercalated between these two zones. Two deep aquifers (Nubian Sandstone and Fractured Limestone aquifers; Figure 12 & Figure 13) generally represent the main groundwater resources in the Eastern and Western deserts.
The Fractured Limestone aquifer overlies the Nubian Sandstone aquifer and underlies the Quaternary aquifer. The fissured carbonate rocks are represented by a three-horizon system: Upper Cretaceous, Paleogene, and Neogene horizons, separated by other facies layers. Based on a three-horizon concept, it is possible to study the surface and subsurface occurrences, the depth and thickness of each horizon, and the presence and quality of groundwater.
These large groundwater aquifers show good behaviors in response to natural and/or anthropogenic activities. The Earth’s gravity field anomalies from the Gravity Recovery and Climate Experiment satellite mission have been widely
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Figure 13. Borehole data used in the current study. The RWEDEA well was drilled by Apache Petroleum Company.
applied for monitoring the groundwater storage variations and estimating the aquifer depletion and/or recharge worldwide [34] - [41].
7. Suitable Sites for Well Drilling
Based on the integration of the results and interpretations of the magnetic data with the borehole data of the available wells, the first zone of groundwater resources is represented by the Quaternary aquifer that is consisted of sands, silt, and gravels. The second zone is represented by about 400 m thick of Eocene carbonate rocks forming the Fractured Limestone aquifer. The transitional zone is represented by the Upper Cretaceous Sandstone/Limestone aquifer (Figure 13). It is a transitional aquifer between the overlying Fractured Limestone aquifer and the underlying Nubian Sandstone aquifer (NSA). The NSA extends over 2 × 106 km2 in NE Africa. It is represented mainly by water-bearing Paleozoic-Mesozoic Nubian Sandstone with Tertiary marine shale and clay intercalations [42]. The NSA represents the most important deeper aquifers, provided that the higher thickness of the sandstone in the sedimentary basin of the study area of good hydrological properties.
The depth and thickness of the NSA in the Eastern desert show a progressive increase from the Red Sea Hills toward the Nile River based on geophysical and borehole information (Figure 12 & Figure 13). Surface and subsurface water are feeding the Nubian sandstone outcrops, close to the Red Sea Hills, and flowing toward the Nile Valley following the gentle slope of the layers. As a consequence, the groundwater level is lower in the study area in comparison to the source areas at the Red Sea Hills. From another side, the NSA is confined north of latitude 25˚N in Egypt. The confining layer is composed of marine shale [42]. All these conditions help the sub-vertical faults act as conduits for ascending
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Figure 14. Showing the suggested locations for water well drilling in the study area.
groundwater from the deeper NSA [2] [43] into the Quaternary aquifer proximal to Nile River (Figure 12). This is evidenced by the depleted isotopic compositions of groundwater samples, collected from the entrance of the wadi (Figure 1) [2]. This indicates that these paleowaters are ascending from the Nubian Aquifer along the deep-seated faults at depths exceeding 1 km. As a result of the upward leakage along the sub-vertical faults, drilling shallow wells within the zones of these faults can get water from the deeper aquifer and minimize the cost of deep drilling. Thus, based on the integration of the sedimentary thickness and the locations of the faults from the magnetic data, the best sites (Figure 14) could be identified for drilling newly deep wells tapping the deeper NSA in the deep sedimentary basin and/or shallow wells, close to the deep-seated faults. Zones of these faults, enhance porosity, and facilitate the upward flow of the groundwater.
8. Conclusion
The magnetic data were used to delineate the structural trends affecting the groundwater resource of Wadi El Assiuti, and to calculate the depth of the basement rocks. The thickness of the sediments is increasing from ~2.24 km at the western parts of the plateau to ~4.84 km toward the eastern parts of the wadi, where the wadi forming deep sedimentary basins. Several structural trends are delineated in the NNW, NW, NE, and E-W directions affecting the area, with no igneous activity. These deep-seated faults are the most active trends cutting the basement and sedimentary cover, which is providing conduits for groundwater flow from the deeper aquifers into the shallow aquifers. The deltas of Wadi El Rewishad and Wadi Habib represent the most promising areas for drilling new wells in the uncultivated area, given the higher thickness of the sediments and the occurrence of the deep-seated NW, NE and E-W trending sub-vertical faults.
Acknowledgements
Funding was provided by Science & Technology Development Fund (Project ID: 25274) awarded to Assiut University. We thank the editor and reviewers of the IJG for their instructive comments and suggestions.