alexa Geoelectrical Investigation at Ain Al-Faras Spring Site of Gadames City, Libya | Open Access Journals
ISSN: 2157-7587
Hydrology: Current Research
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Geoelectrical Investigation at Ain Al-Faras Spring Site of Gadames City, Libya

Fathi Salem Elburas*
Department of Geophysics, Faculty of Sciences, Tripoli University, Tripoli, Libya
Corresponding Author : Fathi Salem Elburas
Assistant Professor
Department of Geophysics
Faculty of Sciences
Tripoli University, Tripoli
P.O Box 13410, Libya
Tel: +218913223944
E-mail: [email protected]
Received October 19, 2012; Accepted November 22, 2012; Published November 24, 2012
Citation: Elburas FS (2012) Geoelectrical Investigation at Ain Al-Faras Spring Site of Gadames City, Libya. Hydrol Current Res S12:003. doi: 10.4172/2157-7587.S12-003
Copyright: © 2012 Elburas FS. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

Geoelectrical resistivity techniques are based on the response of the earth to the flow of electrical current. With an electrical current passed through the ground and two potential electrodes to record the resultant potential difference between them, we can obtain a direct measure of the electrical impedance of the subsurface material. The resistivity of the subsurface, a material constant, is then a function of the magnitude of the current, the recorded potential difference, and the geometry of the electrode array.

Depending upon the survey geometry, the data are plotted as 1-D sounding or profiling curves or in 2-D crosssection in order to look for anomalous regions. In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of resistivity is, in general, a measure of water saturation and connectivity of pore space. Resistivity measurements are associated with varying depths relative to the distance between the current and potential electrodes in the survey, and can be interpreted qualitatively and quantitatively in terms of a lithologic and/or geohydrologic model of the subsurface.

This technique conducted at fifteen vertical electrical sounding (V.E.S) locations; were carried out, employing the expanding Schlumberger configuration, covering a radial distance of 5km from Ain Al-Faras spring area, of Gadames city, Libya, as a part of UNDP sustainable Human Development for Rehabilitation of the Old City of Gadames.

The qualitative and quantitative results of the field curves and space sections together with the available
geological borehole information were used to construct geo-electrical sections for the study area. The most common features characterizing these sections were the appearance of a fault or barrier zone. clearly shows that some of these faults or barrier zones were laterally displaced for a few meters these faults can be divided in two systems oriented in different directions; the first one, trending North-east South- west, whilst, the other one trending nearly in East-west direction. The main obvious feature demonstrated was the intersection between those different systems, in which a horizontal displacement had been occurred, and the trend and delineation of the topographic features of the investigated site strongly reflect the presence of such fault system, and according to the mentioned horizontal displacement (i.e. the intersection) of the fault system, Ain Al Faras spring had been developed, together with Ain
Tala spring few tens of meters to the North-West.

Keywords
Geoelectrical; Ain Al Faras spring; Gadames city; Field curves and space sections; Geo-electrical sections
Introduction
Geophysical resistivity techniques are based on the response of the earth to the flow of electrical current. In these methods, an electrical current is passed through the ground and two potential electrodes allow us to record the resultant potential difference between them, giving us a way to measure the electrical impedance of the subsurface material. The apparent resistivity is then a function of the measured impedance (ratio of potential to current) and the geometry of the electrode array. Depending upon the survey geometry, the apparent resistivity data are plotted as 1-D soundings, 1-D profiles, or in 2-D cross-sections in order to look for anomalous regions.
The old city of Gadames is located in the Western Libya about 600Km southwest of Tripoli at a major cross-roads of trade routes linking sub-Sahara Africa with the Mediterranean (Figure 1). Gadames is an outstanding example of historic human settlement in a desert oasis. Its architecture and social organization are adapted to the arid climate, water supply, and agricultural activities. The life support of the city was provided by Ain Al Faras (AAF) spring water. Reduction of natural flow of AAF spring resulted in disturbance of the traditional irrigation system and historic water allocation controls. The arid desert climate dominates the area. It characterized by extremely hot summer and mild winter with occasionally rain storms [1]. The aquifer system;based on three main groundwater aquifers; shallow (Zamam-Mizdah) aquifer, intermediate (Nalut) aquifer, and deep (Kiklah) aquifer [2-4].
Electrical Resistivity Investigation
The electrical resistivity method has been used as one of practical solution of hydrogeological problems [5,6]. This leads to obtain details about the depth, thickness, resistivity and the possible location of underground water table. Recent advances in equipment and computer interpretation techniques make the electrical resistivity method highly effective and economic for obtaining data for groundwater modeling studies [7]. The principle of this method is based upon the difference of electrical resistivity measured in ohm-meter corresponding to various geological media. The direct current is sent into the ground between two stain leas steel electrodes (A &B) and the electrical potential measured between two other electrodes (M &N), driven into the ground in the middle of (AB) at small distance. The apparent electrical resistivity (ρa) is determined from the following equation [8,9]:
ρ? = (ΔV/I)K
Where;
ΔV = the potential difference in volt, I = the current intensity in amperes,
K = Schlumberger geometrical factor, calculated from the given formula.
K= (π/2MN)(AB² - MN²)
Equipment and Field Operation
Fifteen vertical electrical sounding (V.E.S) were carried out, employing the expanding Schlumberger configuration, covering a radial distance of 5km from Ain Al-Faras spring area. The fifteen VESs were distribution around the AAF spring taking into consideration the topography of the site, and the site condition (Figure 2). Measurements were made using the updated SAS 300b terrameter system.
Electrical Field Data Processing and Presentation
Fifteen VESs were constructed from electrical resistivity field measurements in the investigation area. They have been plotted on a logarithmic scale, with apparent resistivity (ρa (ohm-m)) as an ordinate and half current electrode spacing (AB/2 (m)) as an abscissa, Figure 3, according to the interpretation standard curves of Schlumberger configuration. It should be noted that the apparent resistivity curves obtained, consist of a few segments that correspond to the different values of the measured electrode spacing due to the following:
1. The measurements were made with asymmetrical electrode array in which the ratio (MN/AB) has a finite value. Changing this distance between (MN) entails a change in the ratio of the arrangement, then the apparent resistivity for a given layer stratification differs depending on the ratio of electrode arrangement. A correction can be applied based on the linear filter method given by Ghosh, [10] a, b. more reliable results are available within very short time. [11-14].
2. Lack of the tie-in between these segments; are due to the occurrence of near- surface non-homogeneities in the ground. However, the adjustment of the lack of tie-in between the segments of an apparent resistivity curve, caused by near surface non-homogeneities in the ground is less well defined.
To reduce such scattering effect, several numerical techniques have been presented in published literature. Tang Muoi, [15], proposed an adequate combination of sweeping a direct current around a common sounding center keeping the electrode spacing constant to reduce the lateral change in electrode spacing, Habberjam, et al. [16,17], proposed a ladder network to display the lateral changes and an adjustment method to the potential difference to reduce them. These methods were adapted in the present work.
Moreover, the field data were subjected to a computer program (RESIST) in order to yield a theoretical apparent resistivity value from a specified earth model [18]. The corrected field data were presented in two modes; as field curves and as apparent resistivity space sections (to show the behavior of the current flow within the earth model and then the response of the electrical resistivity within the same model).
Analysis and Interpretation of the Electrical Survey
The field curves where subject to qualitative and quantitative analysis interpretation whereas the space section subjected to the qualitative analysis only. Interpretation of the 15 VESs curves (Figure 3), had been done by utilizing two-layer master curves with an auxiliary point charts [14] and Orellana & Moony for determining and giving the preliminary information about the geo-electrical unit as (thickness and resistivity) of the geological model (Table 1; appendix), to be used as a base to the later stages of interpretation. Further, the apparent resistivity space sections (Figure 4); were constructed to show the response of the subsurface geological structure of the distributed apparent resistivity with depth.
Qualitative Interpretation
Field curve analysis
It is very useful to get an idea about the number of earth layers model and its physical characteristics. The qualitative analysis of these curves with its type and physical contrast characteristics has shown in table 1. in the appendix, as below.
Space section analysis
These sections have been plotted between the sounding location as an abscissa and the half distance of the current electrode as an ordinate (Figure 4) [19,20]. In general, the main objective of these sections is to reveal the presence and effect of the subsurface structure on the behavior and shape of the contour lines in a vertical plane; (i.e. it can be said that, it demonstrated the distribution of the apparent resistivity versus the apparent depth). Most of these space sections have shown the same phenomena, in which the behavior of contour lines, showed discontinues and bending towards the down direction. This phenomenon has strongly reflected the presence of discontinuous surface feature, representing mostly a fault structure. It is worth noting that; this behavior will be taken into consideration when constructing the geo-electrical sections.
Quantitative Interpretation
The preliminary quantitative interpretation of the 15 VESs curves were carried out by using the two layer master curves in conjunction with auxiliary point charts. A software (RESIST) was applied to yield more reliable and accurate geological earth model (i.e. more accurate geological parameters). This process was applied to all field curves. However, the interpreted results were tabulated in (Table 1 and 2, appendix), which illustrate the physical properties and the thickness of the Geo-electrical unit.
Geo-electrical Sections
The qualitative and quantitative results of the field curves and space sections together with the available, piezsometers and geological borehole information were used to construct seventeen geo-electrical sections (L1 – L17) for the study area (Figure 5). These seventeen sections represent the final results of the subsurface geological structure appearance (i.e. layering stratification, thickness, resistivity values, dipping layers and discontinuity surfaces). The most common features characterizing these sections were the appearance of a fault or barrier zone. Figure 6 clearly shows that some of these faults or barrier zones were laterally displaced for a few meters these faults can be divided in to two systems oriented in different directions; the first one, trending North-East South- West (i.e. F1, F2 and F3), whilst, the other one trending nearly in East-West direction (i.e. F4 and F5). The main obvious feature demonstrated in this map was the intersection between those different systems (F2& F3 with F4), in which a horizontal displacement had been occurred. Figure 6 clearly shows that the trend and delineation of the topographic features of the investigated site strongly reflect the presence of such fault system. Never the less, and according to the mentioned horizontal displacement (i.e. the intersection) of the fault system, Ain Al Faras spring had been developed, together with Ain Tala spring few tens of meters to the North-West (Figure 7).
Underground Cavity Location
As we have mentioned before, that the geo-electrical method is a very useful tool in detecting the subsurface geological features which could be represented either as isolated cavities or as channels. An attempt has been made aiming to locate the position of such features and their depth. Various directions of profiles were taken in order to reach the mentioned objectives (Figure 8). Based on the analyses of these profiles, the general delineation of existing underground channels was revealed (Figure 9). Table 1 shows the approximate depth to the top surface of these cavity or channels along each profile.
Conclusion and Recommendations
A fieldwork using a geo-electrical method in the Gadames area at specified measuring points had been carried out. Schlumberger sounding technique was utilized for measurements with a maximum current spread of 500 m. it is believed that such distance can yield useful information about the subsurface geology within a good depth of penetration.
The collected data has been processed and presented as field curves and space sections. These modes of data have been subject to both qualitative and quantitative analysis. The interpreted data in conjunction with available geological information has been used to construct geo-electrical sections along the pre determine surveyed profiles. In general these sections show successive geo-electrical units, with an obvious fault or barrier zones of various system directions. Moreover, an underground cavity and channel location were detected at a depth ranging from 1.5 to 6.5 meters. It should be noted that AAF and Ain Tala springs occurred due as a result of the faulting in this area.
Based on the obtained results of this investigation and other parallel studies were conducted as part of this project (seismic reflection survey and hydrogeological investigation); Practical recommendations made to the UNDP office at Gadames city, aimed to remediate the AAF,such as cleaning the spring site by removing all the dirt surrounding the spring, lining the depression walls with impermeable material to prevent leakage of the spring water into the shallow aquifer, diverting the artesian well WG-2 (20 meters east of the spring pole) to recharge directly into the depression.
Acknowledgment
The author would like to express his thanks to the UNDP, Gadames office, for their support during the field work. Also would like to thank, the hydrogeological team supervisor; Dr. Mohamed Belaed on his editing and scientific discussion.
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Tables and Figures at a glance

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Table 1

 

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Figure 1   Figure 2   Figure 3   Figure 4   Figure 5   Figure 6

 

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Figure 7   Figure 8   Figure 9
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