Groundwater Exploration using Integration of Electrical Resistivity Data with Remote Sensing and GIS Data, Northern State Sudan

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1 736 Groundwater Exploration using Integration of Electrical Resistivity Data with Remote Sensing and GIS Data, Northern State Sudan Dafalla Siddig Dafalla Department of Geology Khalid Mustafa Kheiralla Department of Geophysics Muhammed Ali Hassan Dahab Department of Hydrogeology Abstract Remote sensing and GIS studies have been utilized and aimed to reveal the geological, topographical aspects, as well as to decipher the main lineaments, structural and paleo-drainage patterns. Electrical Resistivity measurements have been carried out in form of Vertical Electrical Sounding (VES) to show water bearing formations and the aquifer thickness. Remote sensing data revealed general elevation range between m (a.m.s.l.), general slope is from North to the south and several primary paleo-drainage systems are clear depicted, the drainage system runs from the south-east to north-west. Resistivity data showed that aquifers are of good permeability and of regional recharge. The recommended depths of the boreholes to be drilled are ranging from 80 m to 240 m. zero degree in some nights. The prevailing wind direction is a northern and northeastern cold winds that reach up to 7.5 mile/hour in the winter and very high speed hot winds in summer between April and July. The overall objective of this study is to investigate the groundwater potentiality in the area under consideration. Whereby, the specific objectives are: To conduct geological and topographical studies. To determine the extent, lithological and geological structures of the water-bearing formations. To assess the groundwater occurrences. To locate favorable sites for high productive well drilling. Keywords Resistivity, Remote sensing, GIS, Aquifer, Borehole. I. INTRODUCTION This study has been carried out in area lies in southern part of Northern State -Sudan. It is a part of the Nubian (Cretaceous) sedimentary basin, which is engulfed by the second big loop of the River Nile and represents by latitudes Northing and longitudes Easting (Fig. ). The area characterized by undulating topographic surface that formed of moderate and low elevation sandstone ridges. Numerous sandy peneplains are extended in which paleo- wadies and Khors took their courses. The surface land is covered by unconsolidated sandy soil of two types; the fine-grained aeolian while the second type is the weathering product of the Nubian Sandstone Formations, which composed of silty sand and clayey sandy soil. However, a few transported longitudinal. The study area is characterized by a typical desert climate, where the rainfall could be zero in the northern parts of Atmour desert and increase gradually to the south to reach about 25mm annually. The area is distinguished by two dominant seasons; a very hot summer with 40 C and a dry cold winter with almost Fig.. The location of the study area where located at about 7 kilometers south of Korti town, Northern state. II. MATERIALS AND METHODS To achieve the objectives of this work; general geological information have been obtained from the regional geological maps of Sudan, Remote sensing studies have been utilized, whereby the optical satellite imageries of Landsat ETM+7 in addition to the Shuttle Radar Topographic Mission (SRTM) data have been used, compilation of sub-subsurface geological and

2 737 hydro-geological information from borehole data within the surrounding were collected, the available information of reconnaissance trips within the area was confirmed to locate borehole sites, geographic features and geologic outcrops using GPS, vertical electrical sounding (VES) measurements along predetermined traverses (or sites) were carried out to investigate the sedimentary sequences and the related groundwater condition. VES data were analyzed to produce geo-electrical sections that can further be interpreted into (hydro) geological sections, aquifer and water- bearing geological units based on well inventory data and VES results was evaluated. III. RESULTS. Geology The Cretaceous Sandstone (Nubian) rocks are the main lithological units that crop out in the study area. These sedimentary rocks are a part of the sedimentary Nubian Saharan Basins (NSB) that extends in northern Sudan (Fig. 2). The NSB formed in Palaeozoic and Mesozoic era as a result of the reactivation of the old Central Shear Zone by extensional deformation. The subsidence led to formation of the interior rift basins in central and northern Sudan in which these sedimentary rocks are preserved. The NSB formed of various lithologies of the Nubian Sandstone Supergroup (Formations) and exposed on the surface in most southern Atmour desert and overlain the Basement Complex rocks. The nearest crystalline rocks in the area crop out at Merowe Dam, which is located at 60Km east of the study area. The Nubian Sandstone rocks are product of fluvial and lake deposits (fluviatile and lacustrine environments) of Upper Cretaceous age. The Nubian Sandstone Formations represent the most potential groundwater aquifers in central and northern Sudan. 2. Remote Sensing and GIS Investigation Remote sensing studies have been utilized in this study, whereby the optical satellite imageries of Landsat ETM+7 in addition to the Shuttle Radar Topographic Mission (SRTM) data have been used that represented by: Landsat ETM+7, path 74, row 47, acq. Date: 3/0/200. Landsat ETM+7, path 75, row 47, acq. Date: 20/0/2000. SRTM DATA (Shuttle Radar Topography Mission Data). Remote sensing studies aimed to reveal the geological, topographical aspects of the area, as well as to decipher the main lineaments, structural and paleo-drainage patterns. Fig. 2. General geological map of the project area (source GRAS 2005) The digitally processed data of Landsat imageries paved the way for the geological studies in area. The processes data exemplified by the color composite of Deccorrelation Stretched bands 7, 4 and (as R, G and B) revealed that ferruginous Sandstone rocks exposed in most the study area and shown with blue to cyan hues in the image due to the high content of ferrous and ferric oxides. The argillaceous sandstone exposed in fine texture and dark blue color as in the central area of the scheme. The high content of quartz in the gravels of the pebbly conglomeratic sandstone led to depiction of these rocks with unique dark brown and reddish hues as in the east northern and eastern parts inside and outside the scheme area, respectively. However, the alluvial wadi deposits display faint pinkish tone but in a finer texture than that of the pebbly conglomeratic sandstone. The aeolian sandy deposits represented by yellowish green hues around the scheme area, indicating that most of the soils are not transported one. The digitally processes and analyzed satellite imageries have been used in GIS framework. GIS analysis permits the production of various products such as lineaments and drainage maps.

3 738 The produced lineament map depicts the linear features on the surface. The linear feature could be natural fracture and fault systems or man-made feature as roads and canals. The produced lineaments have been spatially and structurally analyzed. The results show that the dominant lineaments are extensional fractures trending NW-SE trend. A subdominant two fracture systems are classified as tensional fractures trending E-W and NNE- SSW (Fig. 2). The Digital Elevation Model derived from the Radar Interferometer SRTM data shows the general topography of the area. The model illustrate that the undulating topography of general elevation range between m (a.m.s.l.) with average value of 37 m (a.m.s.l.). The DEM image also demonstrates that the general slope of the area is from North to the south, as well as it represent the inconsiderable variation in elevation with the adjacent surrounding areas (Fig. 3). The analyzed remotely sensed data also revealed the general paleo- drainage pattern in the area of consideration. Several primary paleo- drainage systems are clear depicted, the drainage system runs from the south-east to north-west dissecting the eastern and western parts of the scheme area (Fig. 3). Fig. 3. Digital Elevation Model and drainage pattern in the project area 3. Geophysical Investigation 3. Vertical Electrical Sounding Measurements Geo-electrical survey had been applied in this study using Vertical Electrical Sounding (VES) with Schlumberger, amount of 8 VES's were carried out in the project area, along 6 profiles lines perpendicular to the main fractures system in NW-SE direction to be in NE-SW trend, (Fig. ). 90 % of the conducted VESs give real-able data while 0% of the VESs data were disturbed due to the present of un conducted conglomeratic sandstone and due the existence of Iron bands at the surface that effect the vertical current transmission. The geophysical readings using SAS 0 were analyzed using computer software ResixPLUS. 3.2 Vertical Sounding Curves All VES curves observed reveal relatively thick multilayered earth. The range of resistivity for the area is generally moderate ranging between 34 and 30 Ω.m. The dominant types of curves are QKH-type, KHKHtype and HKH-type indicating 5 geo-electric layers reflecting thick and low resistivity ground. No Basement trends are revealed within the surveyed area however, most of the curves tend to indicate relatively high resistivity range (60-7 Ω.m) at the maximum separation attained at 800-0m (Fig 4). The work was conducted in the form of Vertical Electrical Sounding Measurements (table ). The VES data generally reflect the resistivity layering of the subsurface. The area is known to be a basin in which relatively thick Nubian sediments of varying grain size occur below the static water level (SWL) in the area. The SWL in the area as observed in the nearest wells to the area amounts to m. The relation between the groundwater occurrence and the resistivity of the waterbearing sediments is well established in the literature however, the main task of the sounding method in this study is to identify resistivity zones which are related to the occurrence groundwater of good quality and quantity. 3.3 Interpretation of VES Data The VES curves are interpreted with aid of a IP2WIN. 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. This is commonly realized through a process of

4 A pparent R esistiv ity (ohm -m ) D epth (m ) 739 calibration in which a known lithological model is fit to the observed field curve at the same locality so that the resulting resistivities of the layers of the model are adopted for further interpretation of the neighboring VES data. Table : Results of quantitative interpretation of VES data Pivot VES Layer Resistivity [Ω.m] Thickness [m] Depth [m] Postan Based on previous results of calibration analyses in the region, the following resistivity ranges and the corresponding lithologies are adopted for the area: Lithology Upper gravels Surface clays Subsurface sandstones (saturated) Subsurface fine sandstones (saturated) Deep mudstones Resistivity (Ω.m) The depth correcting factor (CF) is calculated from C6 VES one carried out near new borehole and another D6 VES in the project area using the formula: C.F = Ht / Happ Where: Ht is the depth and Happ is the apparent depth. The values of correction factor are given in Table (2). Table 2: Correction Factor Location Correction VES Correction Pivot 30 Factor 0.4 Factor 0.37 Pivot Pivot 24 V E S 0 0 Spacing (m ) U nregistered V ersion 0 R esistiv ity (ohm -m )

5 A pparent R esistiv ity (ohm -m ) D epth (m ) Pivot 26 V E S 0 0 Spacing (m ) U nregistered V ersion 0 0 R esistiv ity (ohm -m ) Fig. 4. Typical Measured Resistivity Curves 3.4 Geo-electric Sections The above ranges of resistivity are used to construct geoelectrical sections for the project area. The surface layer in the area is reflected as widely very high resistivity zone of ranging from Ω.m to more than 500 Ω.m. This layer is generally thin (-2 m thick) and generally reflects conglomerate of the ground surface. The main intermediate layer is a relatively thick low resistivity zone having a range of ( Ω.m). It is practically dominant in all sections with minor interruptions by higher resistivity lenses of sediments. This layer generally occurs below the static water level in the area. Obviously such a layer is a sandstone member of the Nubian formation which is water bearing. The third layer is an apparently thick, resistive and continuous zone dominating the whole area. It marks the maximum penetration of the sounding beneath the VES points. It has a low resistivity (40 50 Ω.m) and generally reflects fine sandstone, which is saline water-bearing formation. The final layer is an apparently thick, resistive and continuous zone dominating the whole area. It marks the maximum penetration of the sounding beneath the VES points. It has a resistivity less than 5 Ωm. which is attributed to non saturated deep mudstone of the Nubian Formation of different cementing materials. 3.5 Hydrogeological investigations The project area is lies within the Nubian sedimentary basins of the regional extension through Sudan, Egypt, Libya and Chad. The basins are consisting of sedimentary rocks of wide aquifers extensions. Due to displacement of many people to Amri villages (2, 3, 4 & 5) many boreholes were drilled in the surrounding project area. The wells depths are dominantly range from 500 to 830 ft deep and main aquifer (screen emplacement) lies between ( ) from surface with recommended pumping rates range from 45,000 to 82,000 gallon per hour. In this study the water tables measurements the values are range from 25 to 35 meters from the surface. The water flow regime map in the study area are grading from 235m at the eastern part to 228m (a.m.s.l.) at the western side of the study area, with a hydraulic gradient of IV. CONCLUSION This study combined with the previous studies done in the Nubian basin aquifers evident that the aquifers are of good permeability and of regional recharge. For the sustainable groundwater production for the large agricultural schemes the dating using the isotopes to study the recharge sources is important. The drilling shall start with the following recommended sites in table (3). Table 3: proposed sites for boreholes drilling. Pivot VES Drilling depth [m] Remarks well well well Low Productive Postan 2 80 well REFERENCES [] Andrew, F. M. (948): Geology of the Sudan, In Tothill, J. D. (Ed). Agriculture in the Sudan, Oxford Univ. Press, London pp [2] Dobrin, M. B. (98): Introduction to geophysical prospecting Me Graw-Hill, Singapore, Third edition, pp [3] Geological map of sudan, done by Geological Research Authority of Sudan, (2005). [4] Klitzsch, E. H. and Squyres, C. H. (990): Palaeozoic and Mesozoic Geological History of Northeastern Africa based upon new interpretation of Nubian Strata, AAPG Bulletin, V.74, 8, pp [5] Mallet, J. L., Geomodeling. Oxford University Press, [6] Mallet, J. L.,992. Discrete Smooth Interpolation in Geometrics Modelling, Computer Aided Journal, Vol.24, pp [7] Vail, J. R. (978): Outline of the geology and mineral deposits of the Democratic Republic of the Sudan and adjacent areas. Overseas Geol. Miner Resources,London, 49.