Hydrogeophysical Study of Sub-Basaltic Alluvial Aquifer in the Southern Part of Al-Madinah Al-Munawarah, Saudi Arabia

Abstract

Groundwater is extremely important in a water-scarce country such as Saudi Arabia, where permanent surface water resources are absent. Sustainable and future developments plans are essentially relying on the clear understanding of water resources. To evaluate the water resources in arid countries, the groundwater should be quantified through either traditional or scientifically advanced techniques. Aquifer characteristics, particularly the hydraulic conductivity and transmissivity, are essential for the evaluation the aquifer properties as well as the management and development of groundwater modelling for specific aquifers. The present study aims to evaluate the sub-basaltic alluvial aquifer in the northern part of Harrat Rahat, south of Al-Madinah city, and then estimates the principal aquifer’s hydraulic parameters based on the interpreted 1D resistivity-depth models along the study area. For that, 49 Vertical Electrical Soundings (VES’s) utilizing a Schlumberger electrode array were performed along the southern part of Al-Madinah city. The resistivity of the water-bearing formation, thickness, porosity, hydraulic conductivity, and transmissivity parameters were calculated along the measured longitudinal profile from the interpreted VES data. The estimated porosity, hydraulic conductivity, and transmissivity were achieved along the whole profile with average values of 0.2, 3.5 m/day, and 369.6 m²/day, respectively. The resulting transmissivity values from the VES models were compared with those of previous pumping test measurements carried out in the area and a reasonable correlation between the two data sets was observed. These results indicate that surface geoelectrical resistivity techniques may provide an alternative, rapid, and cost-effective method of estimating the aquifer hydraulic parameters where pumping data is rare or unavailable.

1. Introduction

Water is one of the essential natural resources for life on earth. However, with increasing the earth populations in arid areas and limited water resources, it is subjected to threat for pollution and over exploitation. Therefore, many research projects are running everywhere to sustain the water quantity and quality. Saudi Arabia is considered one of the dry countries in the Middle East area. It has precipitation of around 111 mm/year in the Makkah area, while the average precipitation reaches 500 mm/year in the southern parts of the country [1]. Al-Madinah Al-Munwarah is an example of the megacities in Saudi Arabia, where the surface water is scarce, while it is subjected to a water stress consumption due to the rapidly increasing population, either from local migrations or as seasonal visitors (Figure 1) [2]. This forced the governmental authorities to pump out/the fossil groundwater extensively to cover the increasing water demands for agricultural, industrial, and human activities [3]. Al-Madinah Al-Munwarah area is located along the central-western part of the country, which prevalence of arid to semi-arid weathering conditions. Consequently, the evaluation of the groundwater resources in such an arid urban area is considered one of the most essential factors for planning its sustainable developments.

Surface geophysical techniques, particularly electrical resistivity measurements can be very useful and widely applied for identifying the sub-surface aquifer parameters, such as rock type, saturation degree, groundwater depth, extension, and other sub-surface properties relating to the aquifer properties [4]. The method is rapid, inexpensive, and popular to use for different shallow applications [5,6]. The Vertical Electrical Sounding (VES) technique is one of the traditional 1D electrical resistivity techniques that is useful and efficient in groundwater exploration [7]. In recent decades, several studies have been carried out successfully to determine the essential hydraulic parameters based on electrical resistivity measurements for specific aquifers [8,9,10,11]. The advantage of this method is based on the acceptable spatial resolution either vertically and laterally in comparison with the hydrogeological parameters derived from hydrogeological measurements, such as pump tests and observations of hydraulic heads. Furthermore, the resistivity measurements are less disturbing the study area compared with the traditional hydrogeological methods, and they are comparatively cheap and fast in progress [12]. The success of applying the resistivity method in hydraulic studies is depending on how the researchers accomplish a scientific relation between the subsurface resistivity data and hydrogeological properties in an appropriate way. Electrical resistivity and hydraulic conductivity of the subsurface aquifer are both related to the pore space structure and heterogeneity [8,13,14]. Therefore, among surface geophysical techniques, the electrical resistivity is the most accurate technique that can be used to estimate an aquifer’s hydraulic parameters, which can be widely utilized to enhance the aquifer’s hydraulic model. So far, there is no successful application of more advanced 2D and 3D geophysical techniques in hydrogeophysical applications; this is probably due to the limitation of Darcy’s law to explain the water flow in more than one dimension [15].

The Al-Madinah area is characterized by the occurrence of two subsurface aquifers which are the sedimentary and alluvial aquifer that is located along the ancient basins, and a volcanic aquifer that extends along the southern part of the city. Most of the alluvial aquifer in the southern part is covered by a basaltic flow forming the sub-basaltic aquifer [16]. Few research projects have explored the sub-basaltic alluvial aquifer due to the presence of basaltic flow along the surface that make the application of any geophysical method is not an easy task. Nevertheless, most of the studies have performed evaluations of the hydrogeological and hydrogeochemical aquifer settings through water samples obtained from private and governmental boreholes [3,16,17,18,19]. These studies were not enough to picture out the general characteristics of the aquifer because they represent only the borehole location. Therefore, the current study is intending to evaluate the sub-basaltic aquifer properties and the depth to the bedrock along the sub-basaltic aquifer in the southern part of the Al-Madinah Al-Munawarah area, as well as the properties of the saturated zone. The true resistivity, porosity, thicknesses of the water-bearing formations, formation factor, and the hydraulic parameters (e.g., hydraulic conductivity and transmissivity) will be estimated. Moreover, the interpreted VES data will be used to establish an empirical relationship between the aquifer parameters and surface resistivity values. Afterward, the aquifer transmissivity values will be estimated from the VES models and will be compared with those values obtained from the previous pumping test performed in the study area. This step is essential for validating such direct estimates based on low cost and effective surface measurements of the sub-basaltic aquifer in the study area.

2. Study Area

The Al-Madinah Al-Munawarah area is located in the western part of the Arabian Peninsula, ~150 km east of the Red Sea coast. Many topographic divisions of plains, hills, and valleys with different elevations can be traced in the area; however, the urban area of Al-Madinah City is located in the semi-flat region (Figure 2). The ground elevation in the plains ranges from 600 m to 620 m above the mean sea level [20]. The area slopes from the east to the west until the Al-Aqiq Valley is reached and from south to north thereafter. Hills surrounding the Holy City from the north, the south, and the west vary in elevation from 800 to 1500 m (asl). Al-Madinah city is characterized by arid to semi-arid weathering conditions, with rainfall of ~62 mm/year, and the relative humidity varies considerably during the year, with average values of ~30% and 60% in the summer and winter, respectively. The average annual temperatures vary from 25 °C to 42 °C in summer and from 10 °C to 24 °C in winter. Most of the rainfall happens during November, December, and January, with occasional rainstorms occurring in April [21]. In the last three decades, the water supply of the city has shifted from full dependence on groundwater to partially reliance on the desalinated water from the Yanbu desalination plant along the Red Sea coast. However, due to the absence of surface water, the groundwater serves as the key source for agricultural, industrial, and domestic purposes.

3. Geological Setting

The study area is located in the northern part of the vast Harrat Rahat basalt traps on the edge of Al-Madinah city that is considered as a part of the Arabian Shield (Figure 3). Al-Madinah City is located in a geological depression surrounded by mountains of volcanic and magmatic plutonic rocks within the western side of the Shield (Figure 4). This region is mainly composed of igneous and metamorphic rocks. Along these basement rocks, various types of plutonic rock intrusions occurred during two orogenic cycles and are all affected by slight regional metamorphism [22,23,24]. The study area (south of Al-Madinah) is located along the northern border of Harrat Rahat that developed in the past 10 million years [18,24]. Based on the previous geological records of the boreholes, two main basaltic layers lie along the succession overlaying the basement rocks. These have been identified as ~150- and 200-m-thick layers separated by soft sediments of clay, sand, and calcareous deposits [25]. The surficial deposits, which cover the area of study, are composed of alluvial deposits, sabkhah, and wadi deposits. The lava plateaus of Harrat Rahat surrounds Al-Madinah particularly from the east, south, and western side of the Tertiary and Quaternary age [19]. Furthermore, the newest lava flow that occurred along the area is dated to 1256 A.D. [26]. The basaltic flow and the lava field around Al-Madinah area, which are considered examples of alkaline volcanic lava flow [18], resulted from continental intra-plate volcanism accompanying the opening of the Red Sea [27].

4. Hydrogeology

The previous hydrogeological studies in the Al-Madinah Al-Munwarah areas indicated that most of the groundwater has saturated the alluvial deposits and the highly permeable volcanic rocks that extend along the southern part of the area. This system can be categorized into two hydrogeologic units based on the lithologic characteristics and hydraulic behavior [3]. These units are hydrologically connected on a regional scale and described as unconfined to semi-confined aquifers (Figure 5). These two units are the topmost weathered basalt flows of Harrat Rahat that are composed of the Tertiary and Quaternary lava flows and the old sub-basaltic alluvial deposits that are extend along the old valleys around the highly mountainous features. The weathered basaltic unit represents the secondary unit of the aquifer, whereas most of the groundwater in the Al-Madinah area is located in the old sub-basaltic alluvial deposits. These deposits are composed of gravel, sand, and clay resulting from the erosion process of the surrounding rocks [20,29,30]. According to previous hydrological studies performed along the Al-Madinah area, the average depth to groundwater ranges from 30 m to 90 m below the ground surface [3]. Two buried wadis lie underneath the Rahat volcanic area with alluvial deposit thickness exceeding 50 m. The average thickness of the alluvial unit associated with the aquifer buried underneath this area varies from 15 m to ~40 m (maximum thickness in the center of the wadi Alhamd: ~50 m) [30]. The average annual input and output from the sub-basaltic alluvium aquifers are about 2.95 × 10⁷ m³ and 4.05 × 10⁷ m³, respectively, reflecting a shortage of 1.1 × 10⁷ m³, causing a decline in the groundwater levels up to 0.1 to 0.15 m/month [31].

The upper layer is an aquitard (thickness: 1–20 m) that extends over Al-Madinah City and is formed mainly from impervious clay and silty clay. This top impervious layer can generally protect the groundwater from the possible surface infiltration and leakage from sewage, agriculture, and industrial activities especially in the central area where the layer is ~20 m thick [32]. The main recharge components of the main sub-basaltic aquifer are the seepage from the irrigation return flow, the seasonal rainfall, and the temporary floods through fractures, which allows for a rapid infiltration of rainfall as well as the connate water that accumulated during the early time. The main discharge components are the groundwater pumping used for irrigation, industries, and domestic purposes. The Digital Elevation Model (DEM) for the Al-Madinah area and its surroundings revealed that most of the drainage pattern area ranges from the high lands toward the center of Al-Madinah city (Figure 4). Such topographic features are appeared clearly from the three topographic cross-sections passing through the most important area around Al-Madinah city (Figure 4b–d). The wide flat area of the city contributes to charge the sub-basaltic alluvial aquifer from the collected seasonal rainfall. The final destination of the collected surface water is Wadi Al Aqiq, which neighbors Al-Madinah from the western side [22]. The present shape of the groundwater level indicates that the regional groundwater flows more from western parts along Wadi Al Aqiq toward the central area and the southeastern parts than toward other areas [32].

The depth of the water table ranges from 60 to 100 m below ground surface based on the surface topography and the sub-surface layer distributions (Figure 5). The uncontrolled drilling wells and over pumping, accompanied by low rainfall rates during the last decade, have directly affect to dewatering of the aquifer. This is evidenced by a severe drop in the water level reaching to over 100 m below the ground surface. The schematic section for displaying the subsurface successions of the sub-basaltic layers and the saturated zone is shown in Figure 5. The saturated zone is dominant in the fractured basaltic layer that is located underneath the surface basaltic flow. Then, the old alluvial deposits are located directly over the Precambrian basement. The sub-basaltic aquifer system includes the alluvial deposits and the above fracture basaltic layer.

5. Methodology

Groundwater quality in many sites of Al-Madinah suffers from the high content of salinity, nitrate, and trace metals. The high salinity is observed for groundwater in both aquifers. The high salinity percentage may be attributed to either dissolved salts from evaporation ponds or anthropogenic activities associated with irrigation return flow and bottled water treatment factories that discharge saline water into the aquifer [20,31,33]. Consequently, an integrated approach for reconstructing and characterizing the sub-surface hydrostratigraphy at the scales of the main aquifer systems and identifying the aquifer hydraulic parameters are essential for planning future sustainable developments. At these scales, classical DC resistivity methods, represented in 1D VESs [34], can be effectively used to reveal and map the characteristics (boundaries, external geometry, vertical–lateral evaluative trends, and hydrofacies). Different beds exhibit contrasts of texture, porosity, water content, and silt–clay fraction. Hence, the application of DC resistivity methods requires the integration of other information (e.g., phreatic head, electrical conductivity of groundwater, and the lithological properties of the geological units) to achieve hydrofacies’ unit properties. Consequently, in this work, the geoelectrical characteristics of the sub-surface saturated zones are obtained by considering the 1D electrical resistivity distribution computed from near-surface VES’s as a “proxy” for the hydrofacy litho-textural properties. The near-surface electrostratigraphy is interpreted, constrained with independent data (electrical conductivity of groundwater), in accordance with the decreasing resolving power of DC soundings at depth relying on the well-established suppression and equivalence principles [35].

The geophysical measurements were performed in the areas with alluvial deposits that represent the basaltic and sub-basaltic aquifers, located along the southern region of Al-Madinah city (northern part of Harrat Rahat (Figure 2)). Investigating the southern part of the city before the expected extension of urbanization activities was essential to achieve the aquifer properties along the study area. The VES data were acquired with a SYSCAL R2 system (IRIS) using a Schlumberger electrode array. The current electrode separation (AB) started with 3 m and extended in successive logarithmic steps distances ranging from 600 m to 1000 m, based on the surface conditions. The field measurements employed a maximum of five stacks, depending on the acquired data quality. At each measured value, the electrode coupling with the ground was checked for each of the four electrodes to be sure of a good contact with the ground. However, along some locations we have to add some saline water around the stainless-steel electrodes to achieve an acceptable contact degree with the ground (Figure 6). The IX1D [36] software was used for data processing and obtaining the sub-surface resistivity model versus depth.

However, there are suite of other cost-effective and promising surface geophysical methods that can be applied in aquifer evaluation and obtaining some essential hydraulic parameters. For example, the electromagnetic [37,38], either in frequency and/or time domain, radio-magnetotelluric (RMT) methods [39], very-low-frequency (VLF) methods [40], as well as the 2D and 3D geoelectric methods [41]. All these are applied to characterize the aquifer geometry, water saturation, and contaminated plumes. The Vertical Electrical Sounding has an advantage that can be applied in obtaining the required parameters to calculate the hydraulic conductivity of the aquifer. Therefore, it will be considered in this study and more result verification will be achieved throughout the direct comparison of results with the hydraulic parameters measured in the available borehole.

Direct Current (DC) resistivity method is still considered a time- and cost-effective geophysical tool for delineating the vertical variation of the subsurface layers, and with increasing the number of measured stations, the lateral properties can be also identified with an acceptable degree [42,43,44,45,46,47,48]. All the VES curves are interpreted as one dimension (1D) based on theoretical considerations that aimed to develop a resistivity-depth model for sub-surface variations (Figure 7). Then the models will be updated to achieve an acceptable fitting between the measured data and the theoretical models. Hence, at specific locations, the resultant resistivity-depth model will be correlated with the subsurface geological units and the available aquifer information to validate the resistivity ranges for the various lithological units. For validating and correlating the different resistivity-depth layers with the subsurface information, a direct comparison with the well (ME-T1) information has been achieved (Figure 8). Three VES’s (No. 16, 17, and 18) that are acquired close to the borehole (ME-T1, Figure 2) have been processed, and the output models layers have been compared with the lithological succession.

Transmissivity, hydraulic conductivity, and storativity are important hydraulic parameters for developing local and regional water plans and predicting the availability of the aquifer water resource [49]. The transmissivity and hydraulic conductivity describe the general ability of an aquifer to transmit water, while the storativity describes the change in volume of water per unit change in the water level per unit area of the aquifer. Evaluation of the groundwater resources for the purposes of sustainable development requires a clear understanding of aquifer parameters such as the porosity, hydraulic conductivity, transmissivity, aquifer thickness, and aquifer type [50,51]. The traditional methods rely on hydrological measurements that are being carried out through the pumping test experiments in the boreholes. However, these costly and time-consuming methods provide sub-surface information at the measured point only [5]. Surface geophysical techniques provide alternative acceptable ways of estimating the essential hydrological parameters with acceptable success and good aquifer coverage. The hydraulic parameters of the aquifer depend on the porosity and the geometry of the saturated pore spaces. These factors also control the electrical resistivity characteristics of the saturated sub-surface layers [50,52]. In addition, the electrical current paths in the sub-surface layers are similar to the hydraulic paths in the pore spaces. Such relationships between these two parameters are controlled by Darcy’s law, which stipulates that the discharge is proportional to the hydraulic gradient between two points. This is similar to Ohm’s law that describes the current flow in a specific layer.

References [53,54] formulated a relationship between the formation factor (F) and the resistivity of sediments and its brine filled media in clay free strata. The relationship is calculated as follows:

$$F=\raisebox{1ex}{${\rho }_a$}\!\left/ \!\raisebox{-1ex}{${\rho }_w$}\right.$$

(1)

where (ρ_a) is the bulk resistivity and (ρ_w) is the fluid (pore water) resistivity.

However, the relation between the formation factor (F) and the porosity of the sedimentary unit can be calculated as follows:

$$F=\raisebox{1ex}{$a$}\!\left/ \!\raisebox{-1ex}{${\varnothing }^m$}\right.$$

(2)

where (∅) is the porosity of the medium, and (a) and (m) are the cementation factors. These factors vary widely for different lithological types. If the pore network is modeled as a set of parallel capillary tubes, a cross-sectional area average of the rock resistivity would yield a porosity dependence described by Equation (1). However, the tortuosity of the rock increases the porosity dependence. This relates the cementation exponent to the permeability of the rock, where the exponent decreases with increasing permeability. A value close to (1) has been observed for the exponent (m) of unconsolidated sands and may increase with cementation to (4.1) for more consolidated rock [12].

Archie’s second law describes the relation among the formation factor, porosity, bulk resistivity, and fluid resistivity (including two variables (a and m)) and is given as follows:

$$F=\frac{{\rho }_a}{{\rho }_w}=a{\varnothing }^{-m}{S}_w^{-n}$$

(3)

where ρ_a is the formation resistivity, ρ_w is the pore water resistivity, ∅ is the porosity, and S_w is the water saturation. The formation factor at the measured VES stations can be estimated from the formation resistivity (determined from the surface electrical resistivity measurements) and the standard fluid resistivity values of the water samples from the wells in the study area (water conductivity at 25°: 8.25 mmhos/cm). The corresponding calculated values of the hydraulic parameters associated with the sub-basaltic aquifer are shown in

Table 1. The calculated aquifer parameters based on the measured electrical resistivity values, compared with those from the previous pumping tests by [55] for the sub-basaltic aquifer in the southern part of Al-Madinah area.

Station No.	Water-Bearing Formation Resistivity (Ohm.m)	Pore Water Resistivity (Ohm.m)	Aquifer Thickness (m)	True Resistivity TR(Ohm.m)	Formation Factor	Porosity	Hydraulic Conductivity (m/day)	Transmissivity (m2/day)
								VESs Model	Pumping Tests
3	179.3	0.002	182	32,636	133.33	0.1737	2.417607	440.00
4	197		172	33,884	266.67	0.1712	2.299740	395.56
5	85		120	10,214	152.38	0.1950	3.606902	432.83
6	105		85	8897	228.57	0.1887	3.215129	271.68	276.48
7	147		151	22,197	76.19	0.1791	2.687904	405.87	216.00
8	127		131	16,580	495.24	0.1834	2.912262	381.51	181.44
9	44		47	2046	666.67	0.2161	5.175094	240.64	475.20
10	23		56	1288		0.2391	7.433548	416.28
11	88		140	12,320		0.1940	3.542405	495.94	604.80
12	69		37	2553		0.2015	4.042918	149.59	97.63
13	136		70	9520		0.1813	2.802125	196.15	155.52
14	157.35		153	24,075		0.1773	2.591947	396.57
15	49.128		72	3537		0.2124	4.869830	350.63
16	36.948		105	3880		0.2221	5.700881	598.59
17	99.931		140	13,990		0.1902	3.307079	462.99	259.2
18	35.3		67	2365		0.2237	5.847329	391.77
19	74		30.5	2257		0.1993	3.891768	118.70
20	90		149.19	13,427		0.1934	3.499566	522.10
21	66.968		150	10,045		0.2025	4.109370	616.41
22	128.2		144	18,461		0.1830	2.892248	416.48
23	73.84375		144	10,634		0.1994	3.896247	561.06
24	191.6		175	33,530		0.1719	2.333938	408.44
25	184.62		101	18,646		0.1729	2.380460	240.43
26	55		66	3630		0.2087	4.576798	302.07
27	193		147	28,371		0.1717	2.324929	341.76
28	323.36		169	54,649		0.1585	1.770227	299.17
29	105		78	8190		0.1888	3.219980	251.16
30	67		87	5829		0.2024	4.108299	357.42
31	66		150	9900		0.2029	4.142155	621.32
32	115		112.72	12,964		0.1861	3.066058	345.63
33	214.76		174	37,369		0.1689	2.196800	382.24
34	303		98	29,694		0.1601	1.831808	179.52
35	55		116	6380		0.2087	4.576798	530.91
36	207.27		189	39,176		0.1698	2.238512	423.08
37	144.33		216	31,176		0.1797	2.714344	586.30
38	134		100	13,400		0.1817	2.824451	282.45
39	131.14		207	27,147		0.1824	2.857264	591.45
40	103		22	2266		0.1893	3.253543	71.58
41	231.9		215	49,859		0.1669	2.109309	453.50
42	15		24	360		0.2555	9.479689	227.51
43	60		52	3120		0.2059	4.363670	226.91
44	249		74	18,426		0.1650	2.031459	150.33
45	147.78		108	15,961		0.1790	2.680256	289.47
46	88		160	14,080		0.1940	3.542405	566.78
47	77.25		137	10,583		0.1980	3.801859	520.85
Min			22.0			0.2	1.8	71.6	97.632
Max			216.0			0.3	9.5	621.3	604.8
Avrg.			118.3			0.2	3.5	369.6	283.28

.

The average hydraulic conductivity (K) can be calculated from the Kozeny–Carman–Bear Equation (4) [56] for the VES stations. The sub-surface geoelectrical parameters indicate the locations and properties of the saturated units (Figure 9 and Figure 10):

$$K=\left(\frac{{\delta }_{w}g}{\mu }\right).\left(\frac{d^2}{180}\right).\left[\frac{{\varnothing }^3}{{\left(1-\varnothing \right)}^2}\right]$$

(4)

where δ_w: fluid density (taken as 1000 kg/m³); μ: dynamic viscosity (0.0014 kg/ms, [57]); and d: grain size (0.01 mm) of the sub-basaltic porous layer.

The transmissivity (T m²/day) values associated with the sub-basaltic saturated layer of thickness (h) for the cross-sections interpreted from the electrical resistivity measurements are calculated from:

T = K × h

(5)

6. Results

To confirm the consistency of the sub-surface model and minimize the uncertainties associated with inverting the 1D model of the VES data, the general trends of the acquired VES curves are examined (Figure 7). Most of the VES curves started with a low or high resistivity character based on the surface and shallow-layer conductivities. Furthermore, most of the acquired curves have a K-type shape, which started with low resistivity values, and then the resistivity values increase due to the dryness and the consolidation properties of the layer. The VES curves decline again due to the water saturation of the sub-basaltic layer (Figure 7b,c). At the far offsets, the resistivity values increase again due to the effect of the massive basement rocks. The other curves exhibit different characteristics, owing to the local changes in the properties of the sub-basaltic zone along the study area. That is, the resistivity is high at small electrodes offsets, then decreases with increasing AB/2, and then shows a high resistivity character with the far electrodes offsets (H-type) (Figure 7a).

More verification for the obtained VES models has been carried out through the direct comparison between the resistivity-depth layers and the lithological units in the borehole (ME-T1) (Figure 2 and Figure 8). Generally, of the subsurface lithology in the well are porous basaltic layer with different degrees of water saturation. The basaltic layer is intersected with thin alluvial deposits at the surface and at depths of 50 m and 65 m. The water table is recorded in the well at a depth of 48 m and has a considerable effect on the resulting model in the nearest VES (16), thereby leading to a sharp decrease in the resistivity values. The resulting VES model (blue line, Figure 8b) exhibits different characteristics ranging from low to high resistivity at shallow depths (<40 m) based on the degree of groundwater saturation in the sub-basaltic porous layer. However, the sharp reduction in the resistivity is comparable to that of the water table at a depth of 48 m. The resistivity values increase again at depths of >120 m due to the dominance of non-porous basement rocks. The other two near VES’s (17 and 18) show more or less the same response characters with variant layer thicknesses due to the strong heterogeneity in the saturated layer thicknesses. Despite such variations in the sub-basaltic saturated thicknesses, they show the same character in the resistivity models. Therefore, they can be utilized in the estimation of aquifer parameters.

Due to the sub-surface heterogeneities having a considerable effect on the inverted resistivity-depth models (Figure 8), three VES’s are acquired at the same location with changing azimuths of the electrode array directions to check the effect of subsurface heterogeneities on the processed VES models (Figure 9). In Figure 8, the shallow sub-surface heterogeneity has a considerable effect on the inverted sub-surface resistivity-depth models. This effect is observed only at the shallow depth and is associated with the variations in the near-surface lithological properties due to the variable water saturations, dry conditions, and the dominant of the basaltic flow. With increasing investigated depth, the homogeneity is dominant, and the effect of the saturated zone is dominant in the measured resistivity data. Therefore, the inverted models for the target-saturated zone exhibit good consistency with increasing investigated depth (Figure 9).

The processed VES data sets are provided in the form of depth-resistivity models that represent the sub-surface lithological variations laterally and vertically to the maximum depth of investigation. Figure 10 shows the data in the form of 2D profiles constructed using the lateral interpolation between the resulted 1D VES models. The surface topography is considered, and the maximum depths have been limited to 250 m from the ground surface for all the models. The measured profile is divided into two parts associated with the gap between the acquired VES stations (Figure 2). The variations in the subsurface resistivity values are indicated by a color scale where the blue color indicates the low resistivity zones corresponding to the sub-basaltic saturated layers. Similarly, the bright red color shows the high resistivity values that represent the massive and dry basaltic layers. Inspecting the resistivity distribution along the profile shows that the cross-section is divided vertically into three main resistivity layers. The first thin, shallow, and high resistivity layer that is dominant along the ground surface and extends to a depth of 10 m. This layer can be attributed to the surface basaltic flow and dry sediments along the surface of the area. For the layer underneath the basaltic surface flow, the resistive values decrease due to the presence of saturated porous sub-basaltic and alluvial sediments saturated with groundwater. The saturated layer extends laterally along the study profile with variant thickness and degree of saturation. Therefore, the optimum regions that can provide a considerable amount of water to the southern part of Al-Madinah city are those with the high thickness and low resistivity values (under VES No. 4, 18–25, 41–36, and 47, Figure 10). The saturated layer is bound from the bottom along most of the measured profile with massive basaltic layer that is characterized by high resistivity values. At some specific locations, this massive basaltic layer extends vertically, thereby minimizing the thickness of the saturated zone (e.g., VES No. 2, 5, 6, and 44, Figure 10).

Based on the resistivity-depth models that are produced from the VES analysis, the 49 VES stations are used in the calculations of hydraulic conductivity and transmissivity of the aquifer. The estimated hydraulic aquifer parameters are compared with the available transmissivity values deduced from the previous pumping test experiments (Table 1 and Figure 11a). The first three VES stations are excluded due to the small aquifer thickness (Figure 10). The two trend lines showing a correlation coefficient of 0.2277 (Figure 11a) reveal reasonable matching for the transmissivity between the estimated (from the VES models) and calculated values (from the pumping test) along eight VES stations. The rest of the estimated transmissivity values cannot be presented in this figure because they do not have equivalent values from the pumping test measurements. However, the transmissivity values along the measured profile show a specified range (71–621 m²/day) and vary with the position of the VES stations based on the specifications of porous rocks and the aquifer thickness (Table 1 and Figure 11b). The consistency of the estimated hydraulic parameters can also be determined from the relation between the transverse resistivity and the transmissivity (Figure 11b). The transverse resistivity is the product of the formation resistivity and the layer thickness (ρ_a × T) and can be calculated from the measured VES stations and plotted against the transmissivity values. Moreover, the correlation coefficient (R² = 0.0313) indicates that the transmissivity values are related to the resistivity values and the aquifer thickness. Therefore, surface geoelectrical measurements can contribute significantly to determining the essential hydraulic parameters of the sub-basaltic aquifer in this strategic place; however, special considerations should be considered for determining the thickness and resistivity values of the saturated zone. Obtaining full coverage for the aquifer hydraulic parameters, whereas the pumping test results are limited or aquifer parameters may be missing, is essential in enhancing the groundwater models and assisting in the sustainable development planning.

7. Discussion

Satisfying the current and future water demands of strategic cities such as Al-Madinah Al-Munawarah is considered a significant task, especially as the number of inhabitants and visitors increases annually. The sustainable developments in such an arid area rely on an intensive understanding of the subsurface aquifer’s properties to maintain the groundwater demand. Therefore, surface geoelectrical resistivity measurements were performed in order to demonstrate the sub-basaltic aquifer characteristics along the southern part of Al Madinah city before urbanization activities extend to this area. The resulting 1D VES models were verified by comparing them with the available well information to reduce the uncertainty dominant in the inverted data sets. Obtaining the geoelectrical resistivity response from the subsurface lithological units in the vicinity of the available borehole enabled the careful analysis of the acquired VES data sets to obtain the subsurface property changes that can be attributed to the aquifer properties such as depth to basement, aquifer thickness, homogeny, and extension. Afterward, the VES model data are used to estimate the aquifer hydraulic parameters that are essential for producing a correct and updated groundwater model. The estimated aquifer parameters include the water-bearing formation resistivity, thickness, transmissivity, and hydraulic conductivity. Based on the Arche’s law that control the electrical resistivity of the subsurface aquifer layers with the water resistivity and the other version that related these two parameters with the porosity and saturation degree, the layer porosity and the hydraulic conductivity can be estimated. Then, the transmissivity can be estimated based on the thickness of the saturated zone of the aquifer along the acquired VES stations. The estimated porosity and hydraulic conductivity range from 0.2 to 0.3 and 1.8 to 9.5 m/day, respectively (Table 1). In addition, the aquifer transmissivity values range from 71.6 m²/day to 621.3 m²/day, while the measured values from the previous pumping tests range from 97.63 m²/day to 604.8 m²/day (Figure 11). The transmissivity values from the interpreted VES models coincide with those obtained from pump test measurements. Such direct comparisons between the estimated and measured hydraulic parameters demonstrate that the surface geoelectrical methods (VES) are a feasible alternative way for estimating the aquifer hydraulic parameters that can be used for enhancing the aquifer model parameters, whereas there are not boreholes available (Figure 11 and Table 1).

8. Conclusions

Al-Madinah City is one of the important strategic and religious cities in the western part of Saudi Arabia. Recently, there have been different kinds of mega development projects running along this area to accommodate the increasing number of population either due to internal migration or seasonal visitors. Therefore, the water consumption rate is increasing, whereas the amount of annually precipitation is not enough to cover the increasing water demands. Sustainable and future developments plans are essentially relying on the clear understanding of water resources in such arid area particularly the groundwater. The groundwater resources should be quantified either throughout the traditional and/or advanced methods to understand the aquifer properties and keep it as a sustainable water source. As the area is almost covered with basaltic flow from an early volcanic eruption, it is not easy to carry out pumping test measurements to cover the whole area because it is expensive, time consuming, and only shows the information along the borehole location. Therefore, a total of 49 VES data sets have been acquired along the southern part of Al-Madinah City, where the urbanization activities are expected to cover this area during the coming years. The careful processing the 1D VES data has been accomplished using a comparison with the available borehole information to reduce the equivalence and uncertainty problem. Then, the aquifer properties have been achieved along the measured data sets that almost covered the southern part of the city. Applying the Arche’s law that relates the resistivity of the saturated zone with the porosity and degree of water saturation, the hydraulic conductivity, and transmissivity parameters were calculated along the measured longitudinal profile from the interpreted VES data. The essential hydraulic parameters were compared with those obtained from pumping test measurements. The reasonable correlation between the estimated and measured hydraulic parameters indicated that surface geoelectrical resistivity techniques may provide an alternative, rapid, and cost-effective method of estimating the main aquifer hydraulic parameters where pumping data is unavailable. Such hydraulic information will be utilized to enhance the groundwater modelling that essentially used for sustainable developments.