Aquifer Characterization and Groundwater Potential Using Integrated Geoelectric Sounding and Geoinformatics in West Maghagha Area, Minia Governorate, Egypt

The study area is located in the western extension of the Nile Valley near the boundary with the Western Desert, where the groundwater represents the potential water resource for future land development for both industrial purposes and agricultural reclamation. Historically, geoelectric methods proved prospective and practical in exploring for groundwater resources. In this study, 17 Vertical Electric Sounding (VES) were acquired and processed to reveal the subsurface distribution of the water bearing layers and identify the groundwater potential in West Maghagha area. After routine data analysis and calibration, the preliminary results are interpreted in light of the available geological data and indicated the presence of at least four geoelectric layers with model resistivity values up to 2000 Ω·m. The potential aquifer was encountered down to ~120 m depth with average thickness of 100 m and is made of argillaceous fractured carbonates. Despite the overall poor quality of this aquifer, the integrated geoelectric and hydrogeologic information indicated a possible potential occurrence of potable groundwater at the southern and northeastern parts of the study area. To improve understanding of the groundwater systems in the study area, detailed aquifer characterization is discussed through integration of the available geologic data, maps, and the geoelectric sections constructed from the VES.


Introduction
Due to the enormous overpopulation in Egypt over the past century (11 million in 1907 to ~80 million in 2010) the available resources in the traditionally inhibited lands of the Nile Valley and Delta has undergone sever deterioration and deficiency. Therefore, the government considered agricultural expansion and urban growth on the desert especially that located at the periphery of traditionally cultivated lands in the Nil Delta and Valley, as an obligation for accommodating the growing population [1]. Typically, water resources represent the fundamental parameter for initiation and sustainability of desert land development for both agricultural such industrial purposes. Therefore, exploring the quantity and quality of such valuable resources has the prime importance for these projects [2] [3] [4]. Over decades, Vertical Electrical Sounding (VES) has not only proved practical in exploring groundwater resources but also useful in investigating hydrological, engineering, and environmental problems [5] [6] [7] [8]. It provides a quick and cost-effective technique to explore subsurface patterns but the success of the method is always dependent to the contrast in electric properties between the targets and hosting environment [9].
After modern improvement in hardware technology and data processing software, resistivity survey represents the most common exploration technique in exploring the subsurface for potential groundwater resources and revealing the layer parameters, true resistivity, and thickness of the near subsurface units (~200 m). VES involves acquiring apparent resistivity measurements through injecting a direct current (I) into the ground from two current electrodes conventionally called A-B and measuring the resulting potential difference (ΔV) at two potential electrodes known as M-N ( Figure 1). The resulting potential difference is subdivided by the injected current and multiplied by the geometrical factor (K), a function of the spacing between current and potential electrodes.
For a geologic medium the apparent resistivity is typically a function of lithologic composition and fluid content [10] and therefore, geoelectric units usually define parastratigraphic units with boundaries that commonly don't coincide with the recorded stratigraphic boundaries [11]. Various resistivity configurations of current and potential electrodes were introduced and discussed in literatures (e.g. [12] [13] [14]). The symmetrical Schlumberger array represents the most widely used configuration in DC resistivity survey [12] that is not only easy to conduct in the field but also provides a reasonable resolution associating reliable depth sensitivity. Traditionally, in a resistivity survey the depth of penetration is usually accepted to be about one-third of the separating distance between the current electrodes [15].
For a geologic medium, the apparent resistivity is calculated as a function of the resistance value and a geometric factor using the modified Ohm's formula (Equation (1)): where ρ is the apparent resistivity in Ω•m, K is a geometrical factor that accounts for the separating distance of both current and potential electrodes, ν is the potential difference in mV, and δ is the electric current in mA. Mathematically, the geometric factor of the layers can be calculated using the survey parameters, current electrode spacing, and potential electrode spacing presented in Figure 1, as inputs for Equation (2).
During the VES data acquisition, the mid-point of the survey remains permanent as the separating distance between the electrodes is gradually increased to enable investigating deeper layers while achieving continuous vertical data record ( Figure 1). The interpretation of apparent resistivity data is known to be relatively ambiguous, and occasionally obtaining a unique interpretation becomes relatively impossible [10]. However, a good control and wise constraints using borehole data and/or stratigraphic information greatly diminish ambiguity and facilitate interpretation.
The present study aims at characterizing the near-surface geological units to reveal the groundwater potential in West Maghagha using VES integrated to the available geologic information. Such integration helps identifying in details the vertical and horizontal distribution of the subsurface sedimentary units and delineating the important water-bearing horizons. Accordingly, the hydrostratigraphic framework influencing groundwater occurrence in West Maghagha area can be delineated and hence groundwater potential is determined.

Location
The study area constitutes the western boundary of the Nile Valley between Maghagha and Al-Fashn and covers an area of ~41,000 Km 2 between the latitudes 28˚39'00" and 28˚45'12"N and the longitudes 30˚30'30" and 30˚39'00"E  and infiltration from surface water courses, with a relatively hot summer (avg. Temp. 31˚C) and warm winter (avg. Temp. 16˚C). Topographically, the dominant slope of the study area follows West-East towards the Nile course, with ground level between 60 m amsl at the Northeast to 210 m amsl to the West and Northwest. The slope of ground surface is typically shallow at the proximity of the Nile but changes to a relatively steeper slope (~150 m/10 km) away towards the West. This particularly may explain the present course of the Nile River as being located in the far eastern boundary of the Nile Valley.

The Geological Setting
The Nile Valley is surrounded on both sides by plateaux topped by rocks ranging in age from Precambrian to Eocene. The Nile Valley itself is filled with sediments of Lower Eocene to Recent. Holocene silt and clay occupy young alluvial plains whereas Pleistocene sand and gravels dominate the old alluvial plains  The Eocene limestone includes the limestone exposed on both sides of the Nile at the latitude of Minia province that was subdivided into Minia at the base and Samalut at the top and was previously lumped together by Said et al. in [19] and involves 80 -160 m of reefal white limestone that locally changes into chalky limestone and stratigraphically separated from the conformably underlying Minia Formation by clay beds [24]. The post Eocene sediments form a series of various heights above the flood plain, covering both sides of the Nile banks at the foot of the scarps. Said in [16] classified these sediments from base to top as; i) Paleonile sediments, ii) Paleo-Protonile sediments (Armant and Issawia Formation), iii) Prenile sediments (Qena Formation), iv) Neonile sediments and v) Recent to subrecent alluvial deposits. Each of these classes is assumed to be unique in lithological composition and fluvial cycle [16] but practically categorization is hard to define in subsurface using conventional techniques [24].

Groundwater System
Previous investigation to the groundwater system of the study area and its environ indicated the presence of four different aquifer systems including; Quaternary, Plio-Pleistocene, Lower Eocene and Nubian Sandstone aquifers. Being accessible and possess economic potential, the Quaternary and Plio-Pleistocene aquifers are typically targeted where they exist. The Quaternary aquifer extends north-south along the central part of the study area following the Nile Valley. The geometry and aquifer characteristics are well determined by the Research Institute for Groundwater using the available well data [27] and geoelectric survey. As shown in the hydrogeological cross section (

Methods
The dominant subsurface hydrostratigraphic architecture of the study area is investigated using 17 VES of 600 to 800 m long within Schlumberger configuration. Despite the high-signal-to noise ratio, Schlumberger array provides a reasonable resolution in horizontal layers with adequate depth sensitivity [32]. In the present study, VESs are distributed to cover the different accessible parts in the study area and the acquired data are processed to reveal the subsurface architecture ( Figure 2). The locations of the VESs are accurately surveyed using ~410 ground control points (GCPs), divided equally among the resistivity lines, using a handheld Garmin eTrex GPS unit with an expected horizontal accuracy less than 1 m. Similarly, the coordinates of the midpoint at each VES-line are acquired while the ground level is determined using the available topographic map and the obtained value was subsequently checked for accuracy and consistency with Google Earth altitudes. A successive increase in electrode separation with a series of measurements was applied to identify datum levels [33]. The measurements normally start with a unit spacing (typically 2, 3, 5, or 10 m) in the first traverse and then increasing the electrode separation by adding a unit spacing to acquire a subsequent measurement. A detailed description of this field method is described by Abdulaziz in [34]. For each measurement, the potential difference (ΔV) between potential electrodes and the electric current intensity between the current electrodes are measured using a highly sensitive Voltmeter and micro-Ammeter respectively (Figure 1). To minimize the soil effect and maintain the contact resistance below or within 2 kΩ, a saline solution is dispensed around the electrode. The power supply that normally secures 300 to 500 volt that sometimes approaches 800 volt in long profiles and insulated cables of armored copper are utilized during field work and data acquisition. The field measurements were carried out using Res-Master 8I6lP Terrameter in a Schlumberger configuration that enabled depth of penetration of approximately 130 -160 m.
Interpretation of the obtained apparent resistivity data is accomplished using forward modeling and automatic inversion by IPI-2 win (2005) software [34]. Data processing of the VES typically starts with investigating all data set for consistency using log-log plot of the apparent resistivity (ρ) versus AB/2 ( Figure 6). This enables data smoothing through filtering false data points that unfit to the overall profile and attenuate resistivity variations at greater depths. A great caution is taken during this step to avoid replacing real field reading by false data points [35]. The resulting curve of each VES is compared to Orellana and Money master curves for layered structures [36]. This identifies geoelectric unit of known thickness (m) with physical properties differs from the other units located above and below and interpretation results are constrained as possible using available geological data ( Figure 6).

Results and Discussion
Normally, the resulting geoelectric sections don't coincide with the acknowledged stratigraphic column of the study area. Consequently, careful visual interpretation of the obtained geoelectric section and stratigraphic column of the study area is maintained in order to define the optimum match that acknowledges both datasets within the available ground truth. A hydrostratigraphic framework for modeled resistivity interpretation (Figure 7) is developed using  available lithostratigraphic data of the study area and published literatures (e.g. [24] [31]). The final output of such interpretation involves 5 composite hydrostratigraphic/geoelectric sections that not only provides complete hydrostratigraphic units but also indicates their lateral extension within the recognized geologic structures (Figures 8-12). The model resistivity of each individual hydrostratigraphic units is presented in Table 1 and is used to identify the possible water-bearing horizons. In addition, a fault map ( Figure 13) and aquifer thick-     Figure 2 and symbols as shown in Figure 8.   Figure 2 and symbols as shown in Figure 8. Five VESs (VES1 to VES5) situated along the E-W direction ( Figure 2   Ohm•m but shale impurities may significantly lower these value ( Table 1). Each of BB − and CC − geoelectric section ( Figure 9 and Figure 10) is constructed using three VESs (VES6, VES7, and VES8 for BB − and VES12, VES13, and VES14 for CC − ) and extend parallel to the AA − geoelectric section but located southward (Figure 2). The subsurface stratigraphic successions developed in BB − and CC − (Figure 9 and Figure 10) show geoelectric units closely similar to that imaged in AA − section ( Figure 8) but with different unit thicknesses and the complete absence of the fifth resistive carbonate unit. The upper geoelectric unit seems match-able in thickness but the model resistivity measured in BB − and CC − is notably lower. In contrast the second geoelectric unit seems to increase to the South with much lower model resistivity in BB − and CC − sections compared to that recorded in AA − section. The average thickness of the second unit reports 20 and 40 m in BB − and CC − sections respectively (Figure 9 and Figure 10) while the same layer did not approach 10 m thick in AA − section ( Figure 8). The second geoelectric unit overlies argillaceous fractured carbonate aquifer (Samalut Formation) with a dominant model resistivity less than 10 Ohm•m. The thickness of this aquifer increases northward from 90 m in CC − section ( Figure 10) to over 110 m in AA − section ( Figure 8) with a relatively consistent model resistivity throughout this aquifer (~10 Ohm•m). The displacement of the normal fault was significantly perceptible in BB − section ( Figure 9) but was barely seen in the lower part of CC − section ( Figure 10) displacing the slightly fractured carbonate unit based underneath the argillaceous carbonate aquifer. This lower geoelectric unit showed various resistivity measurements that dominantly fall between 120 and 200 Ohm•m but may reach 15 Ohm•m due to enhanced local water saturation and/or higher clay content (Figure 9 and Figure 10).
Both DD − and EE − geoelectric sections ( Figure 11 and Figure 12) follow N-S direction with the VES5, VES7, VES10, VES15, and VES16 utilized to construct DD − and VES5, VES5, VES5, and VES5 used to prepare EE − section (Figure 2).  (Figure 14, Right) shows a moderate aquifer thickness towards the southeastern corner of the study area but local increase towards the northeast is also reported. Thus, the southern part of the investigated area has relatively a slight potential relative to the northern part, given that the water quality remains unchanged.

Conclusion
The integration of the present geoelectric study in West Maghagha area with the available geological and hydrogeological information allowed evaluating subsur-