The geophysical study was performed east of Rutba town due to vertical electrical sounding in a net of forty points between Dhalaa and Dhabaa valleys. Geophysical electrical model applicated using Winsev6 program to determine the geo-electrical layers. Three geo-electrical layers were derived from geophysical survey. These layers are composed of four sediment types, such as clays, marls, marly carbonates, carbonates (dolomitic limestone), characterized by resistivity less than 20 ohm-m, 20 - 100 ohm-m, 100 - 350 ohm-m and more than 350 ohm-m, respectively. The thickness of the geo-electrical horizons are increased in Dhabaa Fault zone which characterized by multi karst shapes reflected as karst topography on the surface, which represents subsurface structural boundary for Mullusi aquifer, where this aquifer considered as main water supply for Rutba people in drinking water throughout 17 water wells located in Dhabaa site. Two empirical relation between Formation Factors (F) and Hydraulic Conductivity (K) obtained using linear and Polynomial regression techniques. The first equation of linear fit (F = 11.82 + 116.45 K; with a Correlation Coefficient of 0.94) represents the contribution between formation factor and hydraulic conductivity of a 2<sup>nd</sup> layer in Mullusi aquifer. The second equation of 3<sup>rd</sup> degree Polynomial Fit (F = 20.32 - 203.33 K + 1554.99 K<sup>2</sup> -3127.30 K<sup>3</sup>; with a Correlation Coefficient of 0.75) represents the contribution between formation factor and hydraulic conductivity of a 3<sup>rd</sup> layer in Mullusi aquifer.
The initial application of electrical resistivity methods in geophysical prospecting started with the work of Wenner [1] and Schlumberger [2], both of whom proposed fourpoint electrode configuration for field measurements. A third general class of electrode arrays is the dipole-dipole array described by Alpin [3], for deep investigations .The electrical resistivity method is utilized in diverse ways for groundwater ([4-12]).
The purpose of this paper is to use the electrical resistivity data as vertical electrical sounding (VES) to study Mullusi aquifer conditions, such as depth, boundaries, water bearing horizons. Geo-physical survey aims to support the geological and hydrogeological data in determining the aquifer and their vertical and lateral extensions which are influenced by geological, structural and/ or geo-morphological settings. This is done by comparing the values of the electrical resistance of rocks, which represent their effectiveness to electric current passage and its relationship to the type of metal-forming layers and the amount of moisture in the pores as well as salt content, also determine the contribution of hydraulic conductivity (permeability) with formation factor includeing resistivity of groundwater and resistivity of saturated rocks (Mullusi aquifer).
2. Materials and Methods
The study area is located to the east of Rutba City, west of Iraq, between latitudes (32˚59'50" - 33˚03'48") and longitudes (40˚34'42" - 40˚24'44''), Figure 1, covers an area of about (112) km2, that extends between Al Dhalaa and Al Dhabaa valleys, with an elevation ranging between (575 - 616) meters above sea level , including the area where the system of water wells for Dhabaa water project consisting of 17 production wells, which are used for supply to the population of Rutba City, Figure 2.
Depending on the classification program of the United Nations Environment UNEP-1991 the region is climatically classified within the dry arid zone for the duration of the second half of the twentieth century [13].
2.1. Geology
Phisographically, the study area lies in the western part
of the upper valleys province to the east of Rutba city, and possibly classified as part of the transitional zone between the upper valleys and Al-Hammad provinces. Region is characterized by undulating terrain rises gradually from the east to the west. Decline in the Earth’s surface ranges between (0.07 - 13) m/km at a rate of decline of 2.05 m/km towards the north-east. From a geological perspective, the study area is characterized by its proximity to the Quaternary deposits , Rutba Sandstone Formation (Cenomanian-Upper Cretaceous), Maudud-Naher Umer Formations (Lower Cretaceous-Albian), Ubaid clayey dolomitic limestone (lower Jurassic), in addition to Mullusi dolomitic Limestone (Upper Triassic) [14]. The geological section of the study area can be seen in the sections of wells (W-1 and W-9), Figure 3. Structurally, the region is located at the southern limb of Horan anticline, whose fold axis extends along SW-NE direction.
The region is intersected by Dhabaa Fault that belongs to the system of Horan strike slip faults and extends with Dhabaa Valley of SW-NE direction, which is confirmed by Al-Mubarakstudy [15], while Al-Bassam et al. [16] classified Dhabaa Fault as normal fault with horizontal slip.
2.2. Electrical Resistivity Method
The electrical resistivity method depends on AC passage of low-frequency underground by a pair of metal electrodes installed on the Earth’s surface and measure the voltage between two electrodes are installed between them and all these poles are located on a straight line. Current flows emitted from pole and bend toward the other pole and be in vertical position on equipotential lines, which is distributed on a semi-spherical and its center position at the current poles [17]. Values of the measured voltage depend on the deployment location of the center poles and the distance between them and the direction of the path of the survey, in addition to the values of electrical conductivity of minerals and rocks layered solutions in pores [18]. Rock resistance values ranging from one to a few tens (ohm-m) in the mud and marl, and (10 - 1000) ohm-m in sand and sandstones, and more than 100 ohm-m in the limestone. The process of vertical electric sounding takes sequential measurements of the resistance by increasing the virtual distance between the poles of the current deployment, while the center of array and the trend remains constant [19]. This involves the principle of increased access current, by increasing of a deployment distance. The ratio between the depth of current penetration
and the distance between the electrodes is called penetration factor. The depth of current penetration is of about (1/4 to 1/3) the distance between the poles of power [20].
2.3. Vertical Electrical Sounding
Vertical electrical sounding provides information concerning the vertical succession of different conducting zones and their individual thicknesses and resistivities. By installing two electrodes into the ground and inducing an electric current through the ground, a potential field is created. Two additional electrodes are used to measure the potential at some location. Increasingly deeper measurements are achieved by using a larger separation between the current electrodes. Moving the current electrodes and having the potential electrodes fixed is named the Schulumberger array. In the electrical sounding with the Schulumberger array, the mid point of the electrodes array remains fixed but the spacing between the electrodes is generally increased to obtain more information about the deeper sections of the subsurface. For Schulumberger array, apparent resistivity is given by [21].
where, L = Half current electrode separation.
ℓ = Half potential electrode separation.
∆V = potential difference.
I = electrical current.
2.4. Data Acquisition
The sounding locations are shown in Figure 4. Forty Vertical electrical sounding (VES) were acquired using Schulumberger array with a maximum current electrode separation (2L or AB) of 350 meters. The instrument used was the SYSCAL R2 UNIT, IRIS Company, a digital averaging instrument for direct current resistivity work. Actual data acquisition begins with the selection of sounding point. Once the sounding location was determined, the instrument was deployed to the position. The geographic location and elevation of the chosen sounding point was measured by GARMIN SUMMIT-e TREX GPS apparatus. The vertical electrical sounding (VES) field data were processed using Winsev6, a computer iteration resistivity software and layers, which depends on the following geophysical references [22-26]. Detailed quantitative interpretation was done with the Winsev6 software.
3. Results and Discussion
The results of the VES interpreted through Winsev6 software are given in Table 1. A geo-electric layer is described by two fundamental parameters including its resistivity and thickness. The geo-electric sections for seventeen vertical electrical sounding (VES) indicate that there were two geo-electric layers as in VES-1, Figure 5. At the other vertical electrical sounding points, the geoelectric sections indicate that there were three geo-electric layers as shown in VES-5 for example, Figure 6.
3.1. Geo-Electrical Layers
Spatial distribution maps of resistivity , thickness and lithology of the three geo-electrical layers are outputted from the results of geophysical models, using a computer software (Surfer8 program), in addition to the three-dimensional models which explained the lower surface of each layer in the study area.
3.1.1. First Geo-Electrical Layer
In this layer, the values of apparent resistivity ranged between (9.5 - 1318) Ω-m, with an average of 193.5 Ω-m. The spatial distribution map of resistivity in this layer, Figure 7, showed heterogeneity in values of resistivity and increasing towards the south and south-east. The heterogeneity reflects the distribution of subsurface rock
Resistivity and thickness of geo-electrical layers within Dhabaa region
Formations, different soils resulting from erosion in the valleys and karsts. The resistivities of Ubaid Formation including marls, marly limestones and silty sandy clastic sediments of Naher Umer Formation are low and ranged between (10 - 170) Ω-m in the northern and north west parts, while the resistivities of dolomites and dolomitic limestone in Maudod Formation and sandstones of Rutba Formation are high and ranged between (170 - 1318) Ω-m, in the eastern part of region as shown in Figure 8. Thickness of the First geo-electrical layer ranged beween
(22 - 119) meters, with an average of 48.5 meters. The iso-thickness map of this layer Figure 9, showed that there is a variation in the thickness of the layer increased in Dhabaa basin and the outcrop of Naher Umer Formation. Increasing of the thickness along Dhabaa valley basin, reflects elongated subsidence with maximum in (VES-7), and may be reflect Dhabaa fault zone intersecting the study area in the direction of north-south. Three dimensional model of the first geo-electrical layer Figure 10, showed that there is variation in the levels of the lower contact, oscillating between (468 - 586) m·asl, with slope ranging between 1 m/10 km - 95 m/1 km in Dhabaa downstream. It is symmetry to some extent with topography of land’s surface and this confirms the structural control on the terrain of the study area.
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