Delineation of Nubian sandstone aquifer using geophysical data around Nuweiba area, Sinai, Egypt

The Nubian sandstone aquifer (NSA) is defined by using 19 vertical electrical sounding (VES) stations, 201 gravity stations, which define the structures that control the configuration of the NSA, and 183 land magnetic stations, which define the lower surface of the NSA by determining the depth of the basement surface. In order to assess the top of the NSA, we collected and analyzed 19 deep VESes. The upper surface depth of the NSA spans from 707 to 1154 m, according to the interpretation results for various geophysical data. Additionally, the aquifer’s resistivities ranged from 30.2 to 477 Ω m, which indicates good groundwater quality. According to the interpretation of the gravity result, the study region is influenced by many structural characteristics of different trends, including northwest–southeast, northeast–southwest, and east–west trends. The upper surface depth of crystalline rocks (also known as basement rocks) is determined by three-dimensional magnetic modeling to range between 967 and 4122 m.


Introduction
The Sinai Peninsula is a significant part of Egypt, having an area of ~ 62,000 km 2 . It lies toward the east of Northern Egypt and is bounded by the Suez Gulf, Aqaba Gulf, Mediterranean Sea, and the Red Sea (Fig. 1). In Sinai, most areas experience water scarcity, particularly in central Sinai. Nubian sandstone aquifer (NSA) is the principal aquifer in the region. It is part of the Nubian sandstone aquifer system (NSAS), which is considered one of the most significant and potable groundwater basins in the world. It extends over a vast area in Egypt, Libya, Sudan, and Chad in NE Africa and bears its largest fossil water reservoir. The Nubian sandstone formation ranges from Cambrian to Upper Cretaceous and overlies a Precambrian basement composed of sandstones intercalated with some clays. The water sources that recharge the Nubian sandstone aquifer (NSA) in Sinai are rainfalls and flash floods at high altitudes in the mountains in the south of Sinai, with regions with high topography and characterized by densely populated areas rain throughout most days of the year. The rainfall distribution varies spatially across the area, hence affecting the groundwater recharge within the area (Shebl et al 2022;Araffa et al 2022;El-Badrawy et al. 2021). For exploring groundwater potentiality, geophysical methods such as geoelectric, magnetic, and gravity are important (Telford et al. 1995;Zohdy et al. 1974;Nabighian and Macnae 1991). The area under investigation is bounded by 28° 46′ and 29° 41′ N and 33° 51′ and 34° 41′ E and represents an area of 3900 km 2 . This study assesses the NSA in the investigated area. It aims to delineate the tectonic features and structures, which are essential in the distribution and configuration of aquifers in the investigated area.
Moreover, in addition to the thickness of NSA, it defines the depth of the top of crystalline rocks. The area under investigation has been the subject of numerous geophysical applications. (Sultan et al. 2009) integrated geophysical investigation in northwestern Sinai to specify the subsurface structures and assess water occurrences. Araffa et al.  (Irving and Singha 2010;Hinnell et al. 2010;Pollock and Cirpka 2012;Van Dam, 2014;Gian, 2003). Furthermore, geoelectric tools have been employed for geotechnical studies (Araffa 2010;Sultan and Santos 2008;Mohamed et al 2012;Al Deep et al. 2021;Abdel Zaher et al. 2018).

Geologic setting
The Sinai Peninsula is characterized by different topographic features where high mountains occupy the southern part; these high mountains have the height of the highest peak that reaches 2750 meters. The Nubian sandstone aquifer (NSA) of the Lower Cretaceous in the Sinai Peninsula extends the sandstone formation of the African shield. In the central and southern Sinai, the aquifer is composed of alternating beds of sandstone and shale, and north of the folded zone of the El-Maghara-El-Halal Mountains, the facies changes to carbonate and shale. Low lands characterize the northern Sinai; most Wadis are in the southerly to a northerly direction. The investigation area is covered by various rocks that belong to different formations of different ages. The geological setting of the examined region is represented by the surface geological map, which is developed by UNESCO Cairo Office (2005) (Fig. 1); this map shows that most geological units belong to different geological ages. The subsurface lithostratigraphic succession is obtained from existing boreholes drilled in the examined area by Japan International Cooperation Agency JICA (1992) as in JICA-2, JICA-4, and JICA-5 (Fig. 1). The oldest rocks, which comprise thick-bedded sandstone, belong to the Cambrian age in the examined area. In contrast, the Jurassic rock units of Raqabah FM are composed of sand and sandstone with gravel bands intercalated with shale and outcrops at Wadi Qudayrah. Lower Cretaceous rock units in the investigated area include different formations such as Malhah, Jalalah, Wata, Matallah, Duwwi, and Sudr. These formations comprise sandstones intercalated with shale and represent the target under the groundwater aquifer in NSA (Hassanin 1997). The NSA, which overlooks various rocks of different formations, belongs to the Paleocene-Early Eocene. These rocks are reported in the Esna Shale Formation. Egma Formation belongs to the Pliocene, which comprises gravels of a thickness of ~ 15 m. Quaternary deposits occupy most low land and Wadis. The investigated area is the plurality of intensively faulted regions in Sinai (Said 1962). Faults dissect certain pieces of faulted zones in these trends: E-W, N-S, NE-SW, and NW-SE.   . 3 a First-order regional gravity anomaly map. b First-order residual gravity anomaly map c Second-order regional gravity anomaly map. d Second-order residual gravity anomaly map. e Third-order regional gravity anomaly map. f Third-order residual gravity anomaly map. g Fourth-order regional gravity anomaly map. h Fourth-order residual gravity anomaly map. i Fifth-order regional gravity anomaly map. j Fifth-order residual gravity anomaly

Geophysical methods
We used three geophysical tools: a geoelectrical tool for delineating the upper surface of NSA; a gravity tool to detect the structural features that have a direct effect on the distribution of NSA; a magnetic tool to estimate the depth of crystalline rocks and delineating the lower surface of the aquifer; and thickness of NSA.

Gravity method
This study uses gravity data for delineating structures that directly impact the groundwater distribution and aquifer geometry. Gravity measurements are obtained using CG-3 Autograv with a sensitivity of 0.01 mGal via 201 gravity stations of space between stations 2 and 3 km apart, distributed based on topography and tracks in the lowlands of the investigated area. The gravity data measurements are reduced for various corrections such as drift, latitude, elevation, tide, and topographic corrections (Oasis Montaj 2015). The final corrected gravity data can be used for developing the gravity anomaly map (Fig. 2), which reflects various anomalies of high gravity (20 mGal) in the northwestern, eastern, southwestern, and central parts. However, the southeastern part and other regions in the southern, northeastern, eastern, and central parts show low gravity anomalies ( − 71 mGal).

Gravity data filtration
The regional and residual components are separated to filter anomalies from deep-seated and shallow sources. We used the least-squares technique and the high-and low-pass technique for gravity filtration. The least-squares technique was used until the fifth order. Table 1 shows the correlation factor of every two successive orders (r), with the fourth order, referred to as the best (Fig. 3).

Results of gravity interpretation
The interpretation of gravity data started with separating the two components of low-and high-pass components. They are separated through wavenumbers' high-and low-pass technique (0.018 1/km) (Fig. 4). The average depth of deep sources is 5.91 km, whereas the depth of the shallow source is 2.71 km. The residual gravity map for fourth-order and low-pass filter maps ( Fig. 5a and b) is used to delineate the study area's structural elements. Also, the Euler technique is used to estimate the different Euler solutions, which are superimposed gravity maps with structural index (SI) = 0 where various sources' depth locations are reported by different color circles, which vary from ~1 to 5 km (Fig. 5e). Moreover, the Euler deconvolution delineates faults or contact at different depth levels dissecting the examined area. The important trends of structural elements are determined by the fourth-order residual gravity anomaly map, high-pass gravity map, and Euler deconvolution of structural index equal 0. The trends of these fault elements are represented by rose diagrams (Fig. 5a-f). The major trend is northeast-southwest (Aqaba Gulf trend).

Magnetic method
Magnetic data were obtained via 183 magnetic land stations at the same sites of gravity stations using two ENVI-MAG magnetometers of a sensitivity of 1 nT. One of them is used in the field survey, whereas the other is used for base station recording to calculate the diurnal variation (diurnal correction). As per (Oasis Montaj 2015), the magnetic data were corrected for IGRF correction; moreover, after correlations, they are represented by the total intensity magnetic map (Fig. 6a). The total intensity of the magnetic map for corrected magnetic data can be reduced to the pole before applying any interpretation. Thus, magnetic data interpretation begins with RTP (Fig. 6b); the IGRF parameters applied to generate the RTP map were inclination of 43.157°, declination of 3.34°, and field strength of 43,076.2 nT. Previously, Baranov 1957;Baranov 1975;Baranov, and Naudy 1964;Bhattacharyya 1965 explained the RTP procedure. The RTP map was used for magnetic processing and interpretation to determine the depth of the upper surface.

3D magnetic modeling
The three-dimensional (3D) magnetic modeling is applied to RTP data using GMSYS-3D, which estimates the depth of the crystalline basement rocks and susceptibility of 7850 µCGS. There is a correlation between observed and calculated maps (Fig. 7).

Basement relief map
The basement depth map is drawn based on the results of 3D magnetic modeling (Fig. 8), which shows the upper surface of crystalline rocks (basement) and has a value between 967 and 4122 m.

Geoelectric method
Geoelectrical methods are primary tools for groundwater exploration, particularly vertical electrical soundings (VES). The subsurface and water-saturated layers can be precisely defined based on electrical resistivity. The penetration depth for electrical current is directly proportional to the distance between current electrodes (AB); the electrical resistivity data are acquired and represented by 19 VESes (Fig. 9a). The data acquired through the Schlumberger array of AB/2 vary from 5 to 3000 m using certain devices produced by IRIS (Syscal-R2 instrument). The Syscal-R2 instrument is combined with a 1200 W AC/DC converter. The collected geoelectrical data are arranged to cover the investigated area along southwest-northeast and west-east trending lines as per topographic features. Certain VES stations, such as VES4, VES8, and VES19, are acquired besides drilled boreholes JICA-4, JICA-5, and JICA-2, respectively, for correlation between the results of geoelectrical and borehole data. The quantitative interpretation of resistivity data was used via a manual method that used two-layer master curves (Koefoed 1960). The manual interpretation of results was used as a preliminary model for IPI2WIN-1D 2000 (Bobachev et al. 2001) to estimate true resistivities and depths for the curve of each VES (Fig. 9b). The interpretation consequences for geoelectrical data show that most results of VES stations comprise six to nine geoelectric units of different thicknesses.

Results of geoelectrical data
Geoelectrical cross sections Five geoelectric cross sections are obtained from the final quantitative interpretation of VES stations along with five profiles: A-A′, B-B′, C-C′, D-D′, and E-E′ (Fig. 10). These geoelectrical cross sections show the six geoelectrical units of different lithological compositions. The first geoelectric unit comprises multiple rock fragments representing alluvial deposits with varying thicknesses, ranging from 0.7 to 202.6 m, and resistivity values ranging from 7 to 980 Ω m of the Quaternary age. The second geoelectrical unit belongs to the Upper Cretaceous age and comprises limestone of various thicknesses between 13.8 and 259 m; the differentiating resistivity is between 20.2 and 1196 Ω m. The third geoelectrical unit includes a sandstone unit that belongs to the rocks of the Upper Cretaceous with thicknesses ranging from 34.1 to 372 m and resistivities between 8.6 and 848 Ω m. The fourth geoelectrical unit comprises limestone and dolomitic limestone, reflecting the number of resistivity values ranging from 28.5 to 1971 Ω m, as well as thickness values between 116 and 459 m. The fifth geoelectrical unit comprises limestone, dolomite limestone, marls, and shales of the Upper Cretaceous with various resistivity values ranging from 1.98 to 223 Ω m, and the thickness values are between 142 and 397 m. The sixth geological unit represents NSA in the examined area. The depth at the top of this unit is between 707 and 1154 m, which comprises thick-bedded grained sandstones. The age of this unit (aquifer) is the Lower Cretaceous and has resistivities ranging from 30.2 to 477 Ω m, which exhibits low salinity water in the investigated area.

Depth map to the top of NSA (sixth geological unit)
The interpretation of VES data for developing the depth to the top of NSA reflected the low values of depths at the southeastern part of the investigated area (654-700 m); however, the depth was ~ 1300 m in the western part (Fig. 11a).

Isoresistivity Map of NSA (sixth geological unit)
The true resistivity values of quantitative VES data interpretation are obtained, and northern parts of the investigated area have high resistivity values of ~ 350-477 Ω m. In contrast, the Fig. 7 Outputs of GMSYS-3D modeling: a observed magnetic anomalies; b calculated magnetic anomalies; c error percentage ranged from 0.1 to 1 northwestern and northeastern parts have low resistivity values ranging from 30 to 210 Ω m. This layer comprises sand and sandstone belonging to the Lower Cretaceous age representing the NSA (Fig. 11).

Porosity map (sixth geological unit)
The porosity percentage was explained by the relation between porosity and electrical conductivity. Archie formulated the following equation (Archie 1942): where σ b and σ f are the bulk and fluid, electrical conductivities, respectively; and m are constants for a rock of definite kind, and is the porosity. The aquifer in the investigated area comprises pure sandstone, as estimated from the results of the drilled borehole (JICA-4). Both and m are defined as per Senet al. 1988;Das et al. 2017, where = 1 and m = 2. The electric conductivity of the fluid, which is 0.1636 Siemens/m, is determined from the drilled borehole (JICA-4) in the central part of the investigated area. The results suggested that porosity had values of ~ 11-36%. The northeastern, southwestern, and eastern parts are determined b = a f m by low porosity ranging from 11 to 12%, whereas the northwestern and eastern parts exhibit high porosity ranging from 24 to 36%. However, the central part is occupied by moderate porosity ranging from 12 to 24%; high porosity values are recorded at VES-17, whereas low porosity values are recorded at VES-7 (Fig. 11c).
Thickness map of NSA The depth of crystalline rocks can estimate NSA thickness from the magnetic interpretation and the depth of the top of NSA from the interpretation of geoelectrical data. The depth of the upper surface of the basement rocks represents the lower surface of NSA, where all rocks overlay the basement rocks represent the NSA. The Priority map The priority map for the drilling plan is based on the interpretation of geophysical data. Figure 12b is divided into A, B, and C zones. A is the priority for the drilling plan, indicating low depth for the surface of the top of NSA, high thickness, and high resistivity values. These high resistivity values indicate good water quality (low salinity). The second and third priorities are B and C, respectively.

Discussion
The integrated geological boreholes and geophysical data confirm the results of the examined area where the results obtained from the interpretation of VES-19, VES-8, and VES-4 are compared with those of boreholes JICA-2, JICA-5, and JICA-4 ( Fig. 1), indicating that results obtained from both VES and boreholes are compatible for the lithological successions, depth, and thickness of waterbearing zone (NSA) and water quality. Both borehole data and VES data interpretation indicated that the subsurface succession consists of six layers; the first layer comprises multiple rock fragments representing alluvial deposits with varying thicknesses of the Quaternary age. The second layer belongs to the Upper Cretaceous age and contains limestone of various thicknesses. The third geoelectrical unit includes a sandstone unit that belongs to the rocks of the Upper Cretaceous. The fourth layer comprises limestone and dolomitic limestone. The fifth layer comprises limestone, dolomite limestone, marls, and shales of the Upper Cretaceous. The sixth layer represents NSA in the examined area. The depth at the top of this unit is between 707 and 1154 m, which comprises thick-bedded grained sandstones, where the depth of groundwater aquifer (NSA) increases from the southeastern part to southwestern and western parts, then the flow of groundwater from southeast to the southwestern and west direction (Fig. 11a). Also, the recharge coming from the southeastern part of the area the age of this unit (aquifer) is the Lower Cretaceous which exhibits low salinity water in the investigated area. The resistivity values for groundwater aquifer are compatible with the salinity of borehole JICA-4 where both results show that the water quality is the freshwater of resistivity values for NSA ranging from 30.2 to 477 Ω m, as well as TDS recorded from the borehole of ~1047 ppm (Fig. 11b). The coupling of electrical resistivity, gravity, and magnetic techniques managed the structural conditions of the subsurface and assessment of the groundwater aquifer in the investigated area. Furthermore, the delineating fault elements derived from, the residual gravity anomaly map for the fourth-order, a high-pass filter map, and Euler deconvolution are all compatible (Fig. 5), where the main trend for most structural elements is NE-SW parallel to the Gulf of Aqaba. The investigated area is dissected by several fault elements, and these faults have been a direct effect on the distribution and hydraulic parameters of NSA. The large variation of hydraulic conductivity and transmissivity is due to structural features in the area.

Conclusion
From the results of drilled boreholes and geophysical data, the results obtained are as follows. The investigated area is determined using multiple structural features of northwest-southeast, northeast-southwest, and east-west trends; however, the important trend is [northeast-southwest (parallel to the Gulf of Aqaba direction)]. The subsurface sequence of the investigated area comprises six geoelectrical units composed of different lithological units belonging to different geological ages from the Lower Cretaceous to the Quaternary. The upper surface of the sixth geoelectrical unit (NSA) is delineated at depths varying from 707 to 1154 m; this unit is the primary aquifer in the investigated area characterized by freshwater quality as per the hydrochemistry of boreholes and resistivity values. The basement depth extends from 967 to 4122 m; the results suggest that the investigated area's southeastern part (zone A) is the best location for drilling, as reported in the priority map.
Acknowledgements The authors thank the teamwork of the National Research Institute of Astronomy and Geophysics (NRIAG) for helping in data collection.
Author contributions All authors shared in data collection, data interpretation, and preparation of the manuscript.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). This work was not funded.

Data availability
The data that were collected for this work are available.
Code availability Not applicable.

Conflict of interest
The authors declare that they have no competing interests.

Ethical approval
The authors approve for all states of this work.

Consent to participate All authors consent to participate.
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