Geophysical Investigations and Remote Sensing Techniques for Groundwater Exploration in Wadi Almilk Area, North Kordofan State, Sudan

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1 American Journal of Earth Sciences 2015; 2(2): Published online February 20, 2015 ( Geophysical Investigations and Remote Sensing Techniques for Groundwater Exploration in Wadi Almilk Area, North Kordofan State, Sudan Khalid A. Elsayed Zeinelabdein 1, *, Abdalla E. M. Elsheikh 2 1 Department of Geology, Al Neelain University, Khartoum, Sudan 2 Department of Hydrogeology, Al Neelain University, Khartoum, Sudan address kalsayed2001@yahoo.com (K. A. E. Zeinelabdein) To cite this article Khalid A. Elsayed Zeinelabdein, Abdalla E. M. Elsheikh. Geophysical Investigations and Remote Sensing Techniques for Groundwater Exploration in Wadi Almilk Area, North Kordofan State, Sudan. American Journal of Earth Sciences. Vol. 2, No. 2, 2015, pp Abstract North Kordofan State represents the world s main supplier of Gum Arabic in addition to considerable amount of livestock. Nevertheless, the area suffers from acute shortages of fresh water supply for both human and animal consumption. The overall objective of the present study is the investigation of groundwater occurrence through an integrated methodology incorporating remote sensing, lineament mapping, structural analysis and geophysical studies via electrical resistivity methods. The study area is located in semi-arid zone with annual average rainfalls range between 150 to 250 mm. Topographically, the area is flat with slight slope to the northeast. Wadi Almilk and its tributaries represent the main source of groundwater recharge in addition to the direct precipitations. The alluvial, fractured and weathered basement are the main yielding groundwater source. The area is a part of a basement terrain, where the structural setting represents the main controlling factor of the aquifers distribution. The relatively low resistivity values were interpreted as conductive zone within the hard rock terrains. The relatively higher resistivity values were attributed to the hard basement rocks. The conductive zones show resistivity values range between 40 to 100Ωm, which may represent the weathered or saturated fractured zones within the basement rocks. The relatively higher resistivity values of 200 Ωm and more, represent the compact hard rocks. The lower resistivity values 30 Ωm or less may represent the weathering products of schistose rocks that distorted to clayey materials. Analysis of the lateral profiling data allowed the determination of the best points to be measured utilizing Vertical Electrical Sounding technique. The subsequent VES data analysis enabled the determination of the most potential points among the measured VESs. The electrical resistivity curves reflected different resistivity curve types such as the Q-type; the H-type and the KH-type. These types are attributed to the existence of the alluvial, weathered basement, fractured basement and hard basement rocks. Based on the current investigations, 14 out of 30 measured points are considered as potential sites concerning the groundwater occurrence. Keywords Geophysical Investigations, Remote Sensing, Fractured Basement, Wadi Almilk, North Kordofan, Sudan 1. Introduction North Kordofan State occupies an effective role in the economy of Sudan. It is the world s main supplier of Gum Arabic. Moreover, considerable amount of the livestock come from this area. Nevertheless, the area suffers from acute shortages of fresh water supply for both human and animal consumption. The present study is carried to investigate the groundwater potentiality in Wadi Almilk area, North Kordofan State for the benefit of the local community s water supply. This is achieved through the assessment of groundwater resources occurrences within the study area. Since the area is predominantly covered by crystalline basement rock, groundwater occurrences are restricted to the fractured and weathered basement. The overall objective of the present study would involve

2 16 Khalid A. Elsayed Zeinelabdein and Abdalla E. M. Elsheikh: Geophysical Investigations and Remote Sensing Techniques for Groundwater Exploration in Wadi Almilk Area, North Kordofan State, Sudan the investigation of groundwater occurrence in the study area. To this end, an integrated methodology will be adopted that incorporate remote sensing investigations and lineament mapping, structural analysis and geophysical studies through both electrical resistivity profiling and vertical electrical sounding. Investigations for groundwater in basement terrains in Sudan have been conducted using a variety of methods and techniques. [1] investigated the potentiality of groundwater in the northwest Butana area. [2] studied the groundwater occurrences in Darfur Region through remote sensing techniques and the structural analysis. [3] assessed the groundwater potentialities of the River Atbara alluvial sediments through hydrogeological and geophysical techniques. Basement-sedimentary transitional zones have been investigated through hydro-geophysical techniques north of Khartoum by [4]. amount of rainfall. The vegetation is, therefore, confined to the courses of the seasonal streams. The drainage system of the area is irregular, have dendritic drainage pattern and seems to be structurally controlled by shear zones, as well as faults, major joints and zones of weakness such as foliation plains (Fig. 2). Wadi Almilk and its tributaries represent the main seasonal stream that bisects the study area flowing mainly from the southwest to the northeast direction. Most of the remaining seasonal streams are tributaries of Wadi Almilk which come from hills and high relief areas. 2. The Study Area The study area is located in North Kordofan State along Wadi Almilk watershed. It is bounded by longitudes: E and latitudes: N (Fig. 1). Fig. 2. Digital Elevation Model and drainage system of the study area. Fig. 1. A map showing the location of the study area. The topography of North Kordofan State is characterized by low laying gently undulating surface with few scattered moderately elevated hills that reach up to 700 m above sea level in the northern half of the study area (Fig. 2). These hills have varying shape from tabular to elongate with general trends in NE-SW and E-W directions. The eastern part of the study area is dominantly covered by sandy plains and sand dunes with scattered low laying outcrops of the basement rocks. The study area lies within the dominantly prevailing arid and/or semi-arid climate, with dry hot summer, dry cold winter and relatively short rainy season extended from July to September with average annual rainfall ranges from 150 to 250 mm. The area is sparsely vegetated as a result of the low Wadi Almilk bosses enormous watershed, upon which large amounts of water precipitate during the short rainy season. The total catchment in the study area is more than 6x 10 9 m 2. However, not all this catchment is of importance to the study area. The eastern micro watersheds drain to the north and join Wadi Almilk out of the study area. For this reason, the effective catchment area is approximately 4x10 9 m 2. Considering the mean annual precipitation, the average yield will be of about 7x10 8 m 3. 85% of this amount is lost through evaporation (about 6x10 8 m 3 ). The amount of about 10x10 7 m 3 is remaining for runoff, soil moisture, vegetation and recharge to groundwater. The runoff is calculated as 10% of the average yield, while the last 5%, i.e. 3.5x 10 7 m 3 infiltrates to recharge the groundwater. Although the evaporation and runoff are significantly high, the remaining amounts for groundwater recharge are considerably huge.

3 American Journal of Earth Sciences 2015; 2(2): The high runoff encourages the construction of micro dams to harvest stream water and to enhance the groundwater recharge. Tectonically, the late Proterozoic crust in the Arabian Nubian Shield (ANS) was built by the growth and coalescence of several intra-oceanic island arcs and Andeantype magmatic arcs to form large continental masses [5]. The Precambrian basement of Sudan can be divided into two major geodynamic systems, namely gneisses with interfolded supra-crustal metasediments and a dominantly low-grade juvenile ophiolitic island-arc assemblage [6]. Two distinct high-grade and low-grade basement sequences are exposed in the foreland region of the Arabian Nubian Shield (ANS) west of the River Nile. The high-grade sequence, which is probably middle to lower Proterozoic in age, is overlain by low-grade late Proterozoic Pan African sequences of metasediments, metavolcanics/volcanoclastic rocks and associated calc-alkaline intrusive rocks [7]. Relatively intact ophiolite complexes have so far been reported from the Jabel Rahib to the north of the study area [8] and from the Nuba Mountains in southern Kordofan [9]. Mesozoic (Carboniferous-Triassic) alkaline ring complexes affected both of the high-grade and low-grade sequences in the region [10]. The general structural trend of the low-grade Pan-African sequences is NE-SW parallel to other Pan-African structures east of the Nile. The NNE-SSW shearing, which is known as Sodari Shear Zone represents the most common structural feature in the north Kordofan region. The Cretaceous Sandstone unconformably overlies the basement complex rock represented by Jebel Abu Zaima as distinguishable outcrop in the area [11], [12]. The superficial deposits in the form of sand dunes and alluvial sediments cover all the above-mentioned rock units. 3. Methodology To achieve the objectives of the present study, an integrated methodology was adopted. The area was studied through the interpretation of remotely-sensed data of Landsat 8 OLI multispectral image. Different digital image processing procedures were applied to the Landsat image in order to enhance its quality and increase the visual interpretability for improved identification of various land forms and structural elements in the study area. A number of band combinations were prepared within the frame of the present study. The colour composite of OLI bands 7, 5, 3 in RGB, respectively was chosen to detect the surficial geological and hydrogeological guides for groundwater occurrence. The Shuttle Radar Topography Mission Data (SRTM) with 90m spatial resolution were used for topographic features identification including elevation, slopes and drainage system in addition to major faults. Lineament mapping was carried out to portray the main fractures. The structural analysis was applied based on the finding of the remotely sensed data and on the field measurements and observations concerning the structural elements to determine the directions of paleo-stress and their expressions on rocks mass as folds and ductile shearing. The fractures were classified using the stress strain analysis to determine their capability of groundwater occurrences and movement within the hard rock terrain. A 10-day field trip was conducted to the study area, during which thirty one electrical resistivity profiles and thirty vertical electrical sounding measurements were executed. The VES curves were interpreted with an aid of IPI2WIN computer Software of the Moscow State University (2003). The program provides an option of inverse modeling for the field curve as well as other options for direct interpretation. In this work, a model for each field curve is proposed based on the type of the curve and its resistivity variation. The program on iteration computes a theoretical curve based on an input model and continues to adjust it until a good fit with the field curve is obtained. The validity of the resulting model is checked against the prevailing lithological and hydrological conditions in the vicinity of the VES point. VES data were processed and analyzed to produce geo-electrical sections (profiles) that were further interpreted as (hydro) geological sections. 4. Results and Discussion 4.1. Remote Sensing Investigations The Landsat 8 OLI images were enhanced using various digital image processing procedures. The spatial filtering was utilized to improve the appearance of the linear features that facilitated the production of lineament map. This map was later classified according to the structural analysis, which helped in the delineation of the open fractures that are suitable for groundwater accumulations. Different colour composite images were prepared and used as base maps during the field work. These images were also used in the GIS framework for overlay analysis Structural Analysis North Kordofan region has been affected by multideformational phases, pre and during the Pan-African Orogeny. D1: is a pre Pan-African episode forming the foliation in the old gneisses accompanied by the high-grade metamorphism reflecting implementation and deformation of these rocks under high PT condition. D2 is defined by the foliation of the low grade meta-volcanosedimentary sequence. D3 resulted from the compressional forces during the East African Orogeny in the Pan-African Era ( ma). As a result the tight upright isoclinal folds were developed in the meta-volcanosedimentary rocks, dominated by NNE-SSW folding axial trend. The refolding (D4) in open forms are observed in the area with almost E-W folding axes. The interference of the folding patterns has been manifested in the satellite imageries as ductile structural features, where the common folding systems are in NNE- SSW trends (Fig. 3). The foliation plains are of high angles with dip between degrees manifested in the tight isoclinal folds in the low-grade meta-volcano-sedimentary

4 18 Khalid A. Elsayed Zeinelabdein and Abdalla E. M. Elsheikh: Geophysical Investigations and Remote Sensing Techniques for Groundwater Exploration in Wadi Almilk Area, North Kordofan State, Sudan rocks (Fig. 3). These types of folds are very common structural style in the area, which used to understand the behavior of the related fracture systems that controlling the groundwater occurrence in the hard rocks. Fig. 3. Structural map of the study area superimposed over decorrelation stretched Landsat image and Digital Elevation Model. The stress/strain ellipsoid was used to classify the generated lineaments (fractures). The maximum principal stress axis σ1 is illustrated in ESE-WNW, the least principal stress axis σ3 is oriented perpendicularly in NNE-SSW direction, while the medium principal stress axes σ2 (+/-) that represent the shear arms inclined from the (σ1) by 30 o -45 o clock and anticlockwise, respectively [13]. Accordingly, the open extensional fractures are oriented parallel to σ1 in ESE- WNW direction and the open release fractures being parallel to σ3 and perpendicular to the extensional factures in the NNE-SSW direction. Therefore, the fractures systems that generated during the deformation D3 are the most favorable fractures for groundwater occurrences. Based on this structural analysis the lineaments delineated from the enhanced satellite images were then classified into extensional, release and shear fractures. The open fractures including the release and extensional fractures are presented in a map (Fig. 4). This map was used to guide the subsequent geophysical survey in the study area. Fig. 4. Location of geophysical measurements the classified lineament map superimposed over Landsat OLI colour composite of bands 765 in RGB, respectively Geophysical Investigations The Electrical Resistivity methods were applied in this study as ones of the common geophysical methods used in groundwater investigations. The methods are based on the transmission of the electrical current to measure the apparent resistivity for different lithological units using the Winner configuration for profiling techniques and Schlumberger arrangement for the Vertical Electrical Sounding (VES) Profiling Technique Measurements The resistivity profiles were planned to cut across the intersection of the open fractures deduced from the interpretation of remote sensing data, structural analysis and the consideration of the watershed elements. Most of the

5 American Journal of Earth Sciences 2015; 2(2): conducted profiles satisfy the mentioned criterion, with exception of a number of a few profiles that cross only one of the open fractures. The intent of the profiling technique in the present study is to locate the exact locations of the open water-bearing fractures which will guide the positioning of the VES measurements. Accordingly, a number of 31 profiles were conducted, the locations of which are presented in Figure (4) Interpretation of Profiling Data The profiling measurements were carried out adopting Winner configuration. In this configuration, the four electrodes A, M, N, and B are placed on the ground surface along a straight line, so that AM equal MN equal NB equal (a), i.e. the electrode spacing (a) is kept equal. In the present study, the space was chosen to be 40m in order to investigate the resistivity variation at a depth equal to this space. This depth was determined during the well inventory phase of study conducted in the study area in quest of getting general understanding about the hydrogeologic conditions of the area. The profiles were conducted across the detected open fractures as deduced from the structural analysis. Examples of the measured profiles are presented in Figure (5). Fig. 5. Plots of the measured profiles No.: A =2, B = 12, C= 13 and D= 27. In general, the relatively low resistivity values were interpreted as conductive zone within the hard rock terrains, while the relatively higher resistivity values were attributed to the hard basement rocks. In most of the profiles, the conductive zones show resistivity values range between 40 to 100Ωm which may represent the weathered or saturated fractured zones of the basement rocks. The relatively higher resistivity values of 200 Ωm and more such as in profiles P12 and P13 (Fig. 5), represent the compact hard rocks. The lower resistivity values of 30 Ωm or less may represent the weathering products of schistose rocks that are distorted to clayey materials such as in profiles P2 and P27 (Fig. 5). On the bases of these interpretations, the locations of the most promising targets from the profiling technique were determined so as to be measured using VES technique for investigating the resistivity variations with depth Vertical Electrical Sounding (VES) Measurements The study area is covered by 30 points of Vertical Electrical Sounding (VES) measurements (Fig. 4). The measurements were made using ABEM SAS 4000 Terrameter instrument adopting Schlumberger arrangement. The selection of measurement points was based on the geological and structural interpretation as well as on the findings of the geo-electrical profiles conducted in this study. The distribution of the geophysical measurement points were chosen so as to cover the whole area under investigation in a manner of checking the different rock units with special emphasis on the expected most potential targets for groundwater accumulation. According to the structural analysis, the potential targets are the fractures trending in 115 to 135 (ESE-WNW). The second potential fractures are those trending in 15 to 35 (NNE-SSW). However, the intersections between these two main directions represent the most potential fractures. Locations the measured VESs are presented in Figure (4) Interpretation of VES Data The VES data generally reflects the resistivity of the subsurface layering. The Cretaceous sediments are preserved as thin layer or isolated outcrop in the area, such as in J. Abu Zaima. The rest of the area is predominantly underlain by basement rocks and covered by superficial deposits. The

6 20 Khalid A. Elsayed Zeinelabdein and Abdalla E. M. Elsheikh: Geophysical Investigations and Remote Sensing Techniques for Groundwater Exploration in Wadi Almilk Area, North Kordofan State, Sudan Static Water Level (SWL) in the area, as obtained from the nearest wells, is around m in the shallow aquifer. The relation between the groundwater occurrence and the resistivity of the water-bearing formation is well established in the literature. However, the main task of the sounding method in the present study is to identify resistivity zones corresponding to the occurrence of good quality groundwater and of considerable amounts. The curves of the measured VESs (Fig. 6) revealed that the resistivity values for the area are variable, depending on the different types of lithologies. Basement trend readings are registered within the surveyed area. Moreover, most of the curves tend to indicate relatively high resistivity range at the maximum separation attained. The processing, manipulation and interpretation of the data involve the calculation of the apparent resistivity and detection of depth associated with different lithological varieties. It was assumed that the depth is equal to AB/4 based on previous experience calibrated with drilled wells in and around the study area. Fig. 6. Curves of selected measured Vertical Electrical Sounding showing the major lithological varieties in the study area. The curves reflected different resistivity curve types that interpreted as: the Q-type in VES 1 and VES 3 that show the relatively clayey soil overburden overlying the fractured basement rocks. The H-type curve (VES 7) is attributed to the existence of the alluvial sediments over the clayey products of the weathered basement and then hard basement. The KH curve type appears in VES 9 and VES 29 shows the succession of alluvial sediments of considerable thickness overlying the saturated weathered basement above the hard basement rocks. The existence of the hard basement rocks near the earth surface is represented in VES 12, in which the curve is ascending upward by about 45 below the clayey overburden. 5. Conclusions The study area is located in semi-arid zone with annual average rainfalls range between 150 to 250 mm. Topographically, the area is flat with slight slope to the northeast. Wadi Almilk and its tributaries represent the main source of groundwater recharge in addition to the direct precipitations. The alluvial, fractured and weathered basements are the main yielding groundwater sources. The area is a part of a basement terrain, of which some parts are potential groundwater zones, where the structural setting represents the main agent behind the aquifers distribution. An integrated methodology was adopted in the present investigations. This incorporates remote sensing, lineament mapping, structural analysis and the field measurements of structural elements and geophysical data. The relatively low resistivity values were interpreted as conductive zones within the hard rock terrains. The relatively

7 American Journal of Earth Sciences 2015; 2(2): higher resistivity values were attributed to the presence of the hard basement rocks. In most of the profiles, the conductive zones show resistivity values ranging between 40 and 100Ωm, which may represent the weathered or saturated fractured zones of the basement rocks. The relatively higher resistivity values of 200 Ωm and more, represent the compact hard rocks. The lower resistivity values 30 Ωm or less may represent the weathering products of schistose rocks that were distorted to clayey materials. Analysis of the lateral profiling data allowed the determination of the best points to be measured utilizing the Vertical Electrical Sounding technique. The subsequent VES data analysis enabled the determination of the most potential points among the measured VESs. The electrical resistivity curves reflected different resistivity curve types such as the Q-type; the H-type and the KH-type. These types represent alluvial, weathered basement, fractured basement and hard basement rocks. Based on the current investigations, 14 out of 30 measured points are considered as potential sites concerning the groundwater occurrence. Acknowledgment The authors wish to thank Dubai Mining Company, Sudan for providing the logistics and accommodation during the field work and for financial support. References [1] Elsayed Zeinelabdein, K.A., Elsheikh, A.E.M. and Abdalla, N.H Assessment of groundwater potentiality of northwest Butana Area, Central Sudan. Nile Water Science and Engineering Journal, vol. 5 issue 2, pp [2] Elsheikh, A.E., El Khidir, S.O.H., Elsayed Zeinelabdein, K.A., Gachet, A The application of remote sensing and structural analysis in groundwater exploration in basement terrains, Darfur Region, western Sudan. Arabian Journal of Geosciences. Vol. 6, Issue 6, pp DOI: /s [3] Elsheikh, A.E.M., Nayl, K.E.A., Elsayed Zeinelabdein, K.A. and Babikir, I.A.A Groundwater potentialities assessment of the River Atbara alluvial sediments El Girba- New Halfa area, eastern Sudan. American Journal of Science and Technology, 1(4): [4] Elsayed Zeinelabdein, K.A., Elsheikh, A.E.M. and Bireir, F.A Hydro-geophysical study in Al-Khogalab basementsedimentary basin transition area using Vertical Electrical Sounding method, Khartoum State, Central Sudan. Open Transactions on Geosciences, Vol. 1, No. 2. pp. 1-10, DOI: /GEOS [5] [5] Greenwood, W.R., Hadley, D.G., Anderson, R.E., Fleck, R.J. and Schmidt, D.L., Late Proterozoic cratonization in southwestern Saudi Arabia. Phil. Trans. R. Soc. Lond. A, 280, [6] Abdelrahman, E.M., Geochemical and geotectonic controls of the metallugenetic evolution of selected ophiolite complexes from the Sudan. Berl. Geowiss. Abh. A 145, 175. [7] Geological Research Authority of Sudan, Geological map of Sudan, 1: 2,000,000. [8] Abdelrahman, E.M., Harms, U., Schandelmeier, H., Franz, G., Darbyshire, D.P.F., Horn, P. and Miller, Sohnius, D., A new ophiolite occurrence in NW Sudan -constraints on Late Proterozoic tectonism. Terra Nova, 2, [9] Abdelsalam, M.G. and Dawoud, A.S., The Kabus ophiolitic mélange, Sudan, and its bearing on the western boundary of the Nubian Shield. Journal of the Geological Society, London, 148, [10] Müller S.D. and Horn, P., K-Ar dating of ring complexes and fault systems in Northern Kordofan, Sudan: evidence for independent magmatic and tectonic activity. Geol. Rundsch, 83: [11] Vail, J.R., Outline of the geochronology and tectonic units of the basement complex of north-east Africa. Proc. R. Soc. London, A350, [12] Vail, J.R., Outline of the geology and mineral deposits of the Democratic Republic of the Sudan and adjacent areas. Overseas Geol. and Miner. Resour. No pp. [13] Park, R.G Foundation of structural geology, 3rd ed. Chapman & Hal. Pp. 202.