Assessment of groundwater aquifer using geophysical and remote sensing data on the area of Central Sinai, Egypt

The study aims to assess groundwater resources in Sinai's central area using remote sensing, geoelectric, and well-logging data, utilising techniques for modelling hydrogeological frameworks and evaluating desert regions' groundwater potential. Its utilized satellite image sources, soil maps, and geological maps to map the effects of various factors on groundwater potentiality recharge, dividing it into five zones. Eighteen deep VES stations were used to examine the upper part of the groundwater aquifer in Central Sinai, Egypt, comparing it with available borehole information (Well-1, and JICA-1) to establish subsurface geology and hydro-geology positioning. Borehole data, VES interpretation results, hydro-geophysical maps, and four geoelectrical cross-sections were used to visualize the rearward expansion of eight lithological units, groundwater-bearing sections, and aquifer-filled thicknesses. From interpretation data output reveal three zones with significant recharge and storage potential, including two groundwater aquifers. The shallow aquifer has a saturation thickness of the fractured limestone of 35–250 m, while the deep aquifer Nubian sandstone is detected at depths ranging from 660–1030 m. NW–SE and NE–SW faults likely recharge conduits connecting shallow and deep aquifers, providing sites with acceptable groundwater potential for living, agriculture, and development in Sinai.


Geologic setting
The Sinai geology has a fascinating history that began at the end of the first quarter of the twentieth century.Works 35 provided us with some excellent background material.Many authors, including [36][37][38][39][40] , have conducted various geomorphologic investigations in the area.Sinai's middle zone is separated into a number of geomorphic units, including the tableland area (El Tih-El Egma plateau), the dissected hilly area, inland depressions, coastal plains, and alluvial deposits (deltas) of the hydrographic basin.The age distribution of the majority of outcropping rock units in the research area ranges from upper Mesozoic to Cenozoic (Fig. 2).Sinai's geology map, produced by 41 shows local surface geology.Quaternary deposits, including sand dunes, wadi deposits, and Hamadah deposits, dominate the study surface.These deposits have been identified throughout the majority of the lowlands in the research region, including Wadi El Arish.The surface and subsurface lithostratigraphic successions were characterized by geologic maps and wells acquired during the inquiry 42,43 .The full depth of the JICA-2 well is 1260 m, and the Nakhl well-1 has a total depth of 1700 m (Fig. 3a,b).Tertiary deposits are classified into stratigraphic units ranging in age from younger to older: Fanglomerates, Gravels and boulders, Wasiyt Formation (limestone with Nummulites desert and conglomerate bands at base), Al-Kuntilla Formation (claystone and limestone, followed upwards by gritty sandstone intercalated with calcareous claystone), Extrusive basaltic rocks, Minya Formation (white chalky in the upper part, greyish and yellowish white in the lower part), Rudays Formation (Marl and sandstone with fossiliferous carbonate beds), Egma Formation: Chalky limestone with flint and chert bands.Thebes Formation (Nummulitic limestone, dolomitic in the upper part), Esna Formation (Greenish marly shale and marly limestone rich in fossils), Southern Jalalah Formation (argillaceous limestone, intercalated with marl and sandy marl lenses).Cretaceous deposits were split into a number of stratigraphic units that ranged in age from younger to older: Sudr Formation (white to pale grey chalk, marly and shale bed), Duwi Formation (alternating clastic and carbonate layering with phosphatic intercalations and a chert band cap), Matullah Formation (Argillaceous limestone, marl and shale), Wata Formation (limestone www.nature.com/scientificreports/and dolomitic, intercalated with sandstone and shale), Buttum Formation (Vari-colored shales alternating with crystalline gypsum and fine-grained sandstone), Abu Qa' da Formation (Carbonate rocks, shale, and marlylimestone), Halal Formation(dolomitic limestone, marl, and claystone), Jalalah Formation (marl, claystone, and limestone), Rizan Unayzah Formation (Fossiliferous limestone, sandstone, claystone beds).The area under study contains a substantial groundwater aquifer known as Nubian Sandstone, which includes fresh water at depths of (663 m) and (1030 m) at well JICA-2 42,43 , Malhah Formation (kaolinized sandstone embedded in mudstone and conglomerate) is a good example of this.

Remote sensing data
Recharge zones and groundwater potential were identified using Landsat- www.nature.com/scientificreports/using SRTM-30m data.LULC was created using the LANDSAT-8 image.Several theme layers were created using the RS and GIS tools, as discussed in the following parts.

Land use land cover (LULC)
LULC analyses infiltration rates, soil moisture, groundwater, and recharge, highlighting human activity's impact on ecological systems 44,45 .The majority of the study locations are bare land or scrubland, according to the LU/ LC map in Fig. 4a, with some developed areas.

Lineament
It differs from satellite imagery in that it usually has straight alignments.Lineaments are faults and cracks that cause secondary porosity and permeability to increase.DEM satellite data is used to extract the lines of this area.The lineament densities map, illustrated in Fig. 4b, was constructed using GIS software.Finding potential groundwater occurrence places can be done well by correlating structural elements such as fractures, joints, faults, and bedding planes for lineaments 46 .Based on a thorough analysis of the results, the data were divided into three groups: low (orange), moderate (green), high (blue), and extremely high (on the line with red colour).
The density of lineaments is ranked based on their proximity and correlation with a geological map.It is found that as one moves away from the lineaments, the intensity of the groundwater potential decreases.Classes with a high density are given a heavy weight, whereas classes with a low density are given a lighter weight.

Drainage density
The density of drainage has a considerable impact on groundwater availability and contamination 47 .Lithology impacts the drainage system, indicating infiltration rate; permeability and drainage density are inverse.Determining potential groundwater zones relies on drainage density, calculated by dividing stream lengths by basin

Slope
Slope significantly influences the groundwater potential zone, affecting infiltration and runoff rates.Moderate to steep slopes promote surface runoff, while low or near-level slopes promote robust infiltration and good www.nature.com/scientificreports/groundwater recharge.A slope map was produced using SRTM elevation data 48,49 and ArcGIS software.The slope map has been split into five major categories (Fig. 4d).Slopes that are thick and gentle are given larger weights.Low weight is offered for slopes that are steep and extremely steep.

Soil
A key factor in identifying the potential groundwater occurrence zone is the soil.Calculating infiltration rate considers hydraulic properties and soil texture, with four main types: clay loam, clay, sandy loam, and sandy clay loam (Fig. 4e).One of the key elements influencing surface runoff and precipitation infiltration in the area is the soil texture.Sandy soil has a low runoff ratio and a high groundwater potential, in contrast to clay soil, which has a high runoff ratio and a very low groundwater potential.Sandy soil has a higher infiltration rate than clayey soil, which has the lowest ability for infiltration.

Precipitation (rainfall distribution)
The hydrological cycle's principal source of water is rainfall.The rainfall pattern related to the outermost gradient affects runoff and penetration rate, revealing probable groundwater zones.IDW interpolation creates rainfall patterns based on highest and minimum values, which range from 40.18 to 72.59; the rainfall has been split into various groups.Infiltration is influenced by rainfall quantity and duration.High-intensity and short-duration rain impact infiltration less, while low-intensity and long-duration rain have a greater impact.High rainfall produces high weights, and oppositely.Figure 4f displays stations and rainfall interpolation map.

Groundwater potential map
Many studies, including that of [14][15][16]18,48,50,51 have used the Multi-Criteria Evaluation technique developed by 16 to predict groundwater. The weighte overlay approach is used to weigh thematic layers based on expert knowledge.Several rasters are multiplied by their weight and combined 15,16,50,51 .Utilized ArcGIS weighted sum overlay strategy to create groundwater potential map using various layers mentioned above.Each raster layer was ranked according to exactly how much it was believed to contribute to groundwater infiltration and occurrences.Rainfall rate, slope, drainage density, lineaments, geologic map, soil type map, and LULC, for example, were assigned weights of 38%, 25%, 13%, 9%, 6%, 5%, and 3%, respectively, based on their importance in controlling groundwater recharging processes.Normalized weights for theme layers were determined by multiplying weights by total thematic layers 15,52 (Table 1).Thematic layers are weighted based on infiltration qualities, and their degree is interpreted into GIS weights.Table 1 displays the significance of thematic layers in enhancing groundwater potential. Lower weigh indicates reduced recharging capacity, calculated element weight as a proportion to potential recharge weight.The final GWPZ map is generated by multiplying the weights of thematic layers 50,52,53 .
where, GWPZ = Groundwater Potential zones, Wi = normalized thematic layer weight, CVi = capability value (normalized feature/subclass weight).Seven classifications were made for the GWPZ map that was created through the integration of all of the thematic maps, which vary from extremely low to exceptional potentiality (Fig. 5).Whereas the dark green areas have an excellent chance for groundwater potential, the red areas have a low potential for groundwater; in shallow aquifers (fractured limestone; second layer).

Gravity data acquisition and interpretation
The Egyptian Geological Survey has acquired the One thousand two hundred and two (1202) gravity stations (Fig. 6) 54 .To make the Bouguer anomaly map (Fig. 7a) for the area, gridded data and contoured by 55 using the minimum curvature option.There is a maximum gravity anomaly field in this investigation area (− 15 mGal) at the NW, SW, and certain portions of the SE, and a minimum amount (− 51 mGal) at the SE, E, and central part of the research area, according to an examination for the map Bouguer anomaly.Uplift of denser basement rock is primarily responsible for the high gravity anomaly, whereas sedimentary basins are indicated by lower gravity readings.

Gravity separations and structural
Separating anomalies of various wavelengths from one another is a primary objective of the filtering techniques.Native anomalies caused by shallow sources correspond to short wavelength or high-frequency anomalies, whereas regional variations caused by deep sources correspond to lengthy wavelength or lower frequency anomalies.Using wavenumber (0.018 cycles/K-unit) from the power spectrum curve, Bouguer anomaly mapping 55 , which was previously gridded and contoured, is dynamically divided into regional and residual components (Fig. 7b).A low-pass (regional) gravity filter accepts long wavelengths while rejecting any shorter wavelengths than the cut-off wave number.After the regional influence is affected, the gravity field's distribution at shallower depths is shown on the high-pass gravity anomaly map (Fig. 7c,d).Mapping the research area's residual gravity anomaly field shows the greatest values (9 mGal) in the NW, S, and SE and some areas of the research area, as well as minimum values (− 16 mGal) in the SE, NW, and SW and some study area portions.Lower gravity values reflect sedimentary basins, while the positive gravity anomaly is mostly caused by the uplift of high-density basement rock.High pass map interprets the research area's structural formations (Fig. 7d) to achieve the best resolution of the angle and lengths of the derived lineaments to aid in the identification of the main structural patterns in the area.www.nature.com/scientificreports/

VES data analysis and interpretation
VES study determines geoelectrical layers' number, resistivity, and thickness beneath VES.Eighteen VES stations were used in the study (Fig. 6).It depicts a subset of the computed field curve, and Four profiles were inspected in Fig. 6.Two VES stations (1 and 9) were measured beside water wells to correlate with borehole data and provide them with geological and hydrogeological significance.In the VES measurements, the Schlumberger electrode design was employed with the greatest possible current electrode separation of 3000 m using a Syscal R2 resistivity meter.The tool measures resistance immediately and accurately.The exact locations and ground elevations were determined using a land topography survey and a GPS device.The VES-9, 1D model was calibrated using the lithology log from Well JICA-1 (Figs. 8 and 9).The calibration revealed that the VES-9 site had eight geoelectrical layers.VES field curve analysis uses calibration output as an indicator.Thus, the stratigraphic sequence of the geoelectrical succession was separated into eight main layers, which correspond to an age range from the Cretaceous to the Quaternary (Table 2).The distribution of horizontal and vertical subsurface resistivity can be seen in cross-sections of geoelectrical resistivity, which are established as vertical slices along the subsurface 56,57 .The obtainable multilayer models were used to build four 1-D geoelectrical cross-sections along (North West-South East) trending profiles (A-A′, B-B′, C-C′, and D-D′).The point of these cross-sections is a representation of the subsurface geoelectrical layers, the aquifer geometry, and the extension zone.According to resistivity, these cross-sections show both horizontal and vertical lithology variations.Figure 6 depicts the locations and directions of these profiles, which reveal the actual resistivities, thickness, and predicted lithology of the subsurface layers.The geoelectrical cross-sections show eight geoelectrical units (Figs. 10, 11, 12, and 13), which include all VESes and borehole data.The final unit of the section is the eighth geoelectrical unit.The top part of this layer consists of thick-bedded grained Nubian sandstones, which form the main freshwater aquifer in this research area and the aquifer from which the majority of wells produce, at depths between (640 and 950 m), but the lower surface is inaccessible due to the geometry of the electrode arrangement used.Also, the sections are intervened by a number of faults between some VESes and boreholes, which correspond with the residual gravity anomaly map.Considerable variations in the thickness and facies of some of the encountered units are observed.
Table 1.The weights assigned to different thematic layers and classifications.

Iso-pach map
The Isopach map is constructed to indicate the variation of the thicknesses of eighth geoelectrical units.The thickness of the eighth geoelectrical unit in the study area is varied from 1301 to 3127 m (Fig. 14c).The lower surface of the eighth geoelectrical unit detects from the upper surface of the 2-D basement relief map according to 58 .The thickness increases mainly to the northern part, southern parts and a small part of the southeast in the study area.The lithology of this layer consists of sandstone and Sandy Shale for the Lower Cretaceous age.

Priority map
Figure 15 displays the priority map of drilling initiatives planned in the study area; Zone (A) offers favorable drilling opportunities in the freshwater aquifer (low depth, high resistivity values and large thickness).Zone (B) reflects medium depth, high resistivity values, and moderate thickness, and Zone (C) reflects large depth, low resistivity values, and Moderated thickness of the weighted overlay from Arc GIS.

Results of integration and discussion
Geophysical field measurements such as VES surveys, land gravity data, borehole availability, and remote sensing were employed in the area under study to detect groundwater potential and locate drilling locations.The geological zone is separated into eight geoelectric layers that reflect Wadi deposits, limestone, clay, limestone intercalated with clay, dolomitic limestone, clay, and Nubian sandstone, following that basement complex, and can be utilized to describe the subsurface in the research area.Identification of groundwater potential requires a multi-factor problem, the use of GIS, both primary and secondary information, and four essential processes.First, identify the primary elements influencing groundwater potential.Rainfall as the primary source of water, slope, drainage density, lineaments, geology map, soil type map, and LULC are all elements that influence groundwater storage.Second, weights are assigned to these factors.Third, Utilize GIS tools to create raster maps with theme layers for relevant criteria.The GIS model generates significant groundwater potential maps in NW, NE, and N zones.Land gravity investigations have identified subsurface faults that have an impact on aquifer connections.The geoelectric method is one of the best approaches for exploring groundwater resources since it provides valuable data on the

Figure 1 .
Figure 1.Landsat image and research area location map.

Figure 5 .
Figure 5. Map representing the potential groundwater zones.

Figure 6 .
Figure 6.Locations of station points for measurements of gravity, locations of wells, VESes, and cross sections of electrical resistivity.

Figure 2. Geological map of the study area (modified after 41,43 ).
8, DEM, soil, lithological, and rainfall data.Data from boreholes were used to validate the results.Spatial analysis plots factors affecting groundwater potential (LULC, soil, lithology,

Table 2 .
Describes the lithology of the geoelectric layers.