Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan

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1 American Journal of Earth Sciences 2015; 2(6): Published online October 23, 2015 ( Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan Kheiralla K. M., Salma E. E., Al-Imam O. A. O., Elzien S. M. * Faculty of Petroleum and Minerals, Al Neelain University, Khartoum, Sudan address siddigzien@gmail.com (Elzien S. M.) To cite this article Kheiralla K. M., Salma E. E., Al-Imam O. A. O., Elzien S. M.. Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan. American Journal of Earth Sciences. Vol. 2, No. 6, 2015, pp Abstract The purpose of this geophysical investigation is to delineate existence of buried channels filled mainly with natural aggregate (gravel) in the area of proposed dams of Upper Atbara River and Setait River (Fashaga Area). The River Atbara is characterized by a special land platform (badland topography) called the (Karab). The saturation zone in the study area extended under this Karab which needs more investigations. The main objective of the geophysical engineering survey is to determine the surface and subsurface conditions at the proposed site assess the geological engineering condition of proposed site and provide geological engineering data and basis to detailed design. Total of five resistivity profile and three seismic lines were collected and processed. The resistivity imaging coinciding to some extent with the available borehole data drilled in the investigated area, but there is some interference with overlying top soil which has high resistivity value in some profiles. It noticed that from the resistivity imaging investigations the resistivity of gravels or very coarse material can be expected to lie in the range of 300 to 1500 Ohm.m, depending on moisture conditions, and the resistivity values of the Basaltic intrusion lie in the range of 500 to 5000 Ohm.m.so there are some difficulties to differentiate between basaltic intrusion and gravels. Seismic Refraction in Fashaga area shows that the depth to the top of bedrock ranges from 15m to 30m. The interpreted geological section had been made according to geophysical investigation, available borehole data and the geological setting of the area these lines are the dam axis of the Fashaga Area GF02. Keywords Karab zone, Fashaga Area, Resistivity Imaging, Seismic Refraction 1. Introduction The purpose of this paper is to delineate buried channels filled mainly with natural aggregate (gravel) in the area of proposed dams of Upper Atbara River (Fashaga and Bardana Areas) using multi geophysical techniques. The River Atbara is characterized by a special land form (badland topography) called the Karab. The saturation zone in the study area which extends under this Karab needs more investigations. The objective of the study is to determine the surface and subsurface conditions at the proposed dam site to assess the geology and engineering conditions to provide geological engineering data as basis for detailed design. The specific objectives are to carry out in-situ Seismic Refraction measurements and Soil Electrical Resistivity using the resistivity imaging techniques and performing engineering analysis of the field findings and developing geotechnical conclusions. 2. Study Area The study area lies in eastern Sudan in the Gadaref State, around 100 km northeast of Gadaref town, El showak and Khashm El Girba are the most important towns in the area; they are well connected with different towns and villages through a network of paved and dry seasonal roads. The major road in the area is via Portsudan Kassala Khartoum

American Journal of Earth Sciences 2015; 2(6): 164-170 165 Highway. The estimated population in the study area according to 2008 census is about 15000, with estimated increasing rate of 2.5% per year.

The pattern of distribution and settlement is mainly determined by the permanent availability of water, other resources, infrastructure and services in the field of health and education.

Circularly in the form of heavy density cells around water points and service centers. Seasonally, especially among nomads, dispersion in the wet season and concentration in the dry season.

The winter is dry and pleasantly cool. The winds usually come from the North. The summer is hot and dusty, and the wind turns direction and very often violent dust storm (haboob) occur.

2 American Journal of Earth Sciences 2015; 2(6): Highway. The estimated population in the study area according to 2008 census is about 15000, with estimated increasing rate of 2.5% per year. The population is concentrated at the western bank of the River Atbara. Avery small numbers of people are living at the eastern bank of River. The pattern of distribution and settlement is mainly determined by the permanent availability of water, other resources, infrastructure and services in the field of health and education. The distribution takes place according to several patterns: Linearly along rivers and irrigated schemes, the asphalt road and the railway. Circularly in the form of heavy density cells around water points and service centers. Seasonally, especially among nomads, dispersion in the wet season and concentration in the dry season. Climatologically, the study area is located in the arid zone. The main seasons are winter (November - February), summer (March-June) and the rainy season (July- September). The winter is dry and pleasantly cool. The winds usually come from the North. The summer is hot and dusty, and the wind turns direction and very often violent dust storm (haboob) occur. The rainy season starts in July and lasts until September. The maximum average temperature in summer (April) is around 41 C, and the Maximum temperature rises up to 43 C in April and May with the minimum temperature is around 15 C in January. The annual average rainfall is around 228 mm and the annual average evaporation is around 185 mm. The pressure in the morning, at 6.00 am, which is generally, higher than in the evening, at 18.00pm, which it reaches the values of 18.3 and 16.2 mb, respectively. The highest value is in August (24.2 mb; Saeed, 1969). The highest value of evaporation is in June where it reaches 20.0 mm/d while the lowest is that in January of about 11.2 mm/d. Fig. 1. Location map of the study area. The vegetation in this area consists of a variable mixture of grasses and herbs, frequently associated with scrub bushes up to 6.5 feet (2meters) high. The decrease in rainfall in recent years has produced species more tolerant to dry climate conditions. These species include Laot (Acacia nubica) and Kitir (Acacia mellifera). The natural vegetation is mainly Talih trees (Acacia seyal) and Kitir trees (Acacia mellifera), with tall grasses in the semi-arid zone. Samor trees (Acacia tortillis) dominate in the arid zone (Fadull et al., 1999). 3. Geology and Tectonic Setting The geological succession based on the field observation, geophysical investigations and boreholes data include the following geological units (Table1): Table 1. The geological units of the study area. No. Geological term Age 5 The superficial deposits Lower Pleistocene 4 Unconsolidated Karab formation Lower Pleistocene 3 River Atbara sediments Late tertiary to early Quaternary 2 Cenozoic Basalts Oligocene/Miocene-Pliocene 1 The basement complex Late Proterozoic (Precambrian- Cambrian) The basement complex, which forms lower impervious boundary of the aquifer, is of Precambrian age. It consists mainly of slates, schist, granitic gneiss, quartzite and pegmatitic dykes. It is generally, crops out in the form of scattered hills in the western bank of River Atbara in the plain area near new Halfa town (Fig. 2). The basaltic rocks occur in the form of dykes, sills, and flows. Most of this rock is fine to very fine grained. Miocene to Pliocene age was assigned (Ibrahim et al., 1992). Through the study survey, the basaltic rocks has been found only at the southern part of the study area (Fig. 2). They occurred as a thin layer intruded within the loose formation and sometimes overlie the basement complex. A shallow graben developed, possibly as a result of a combination of tectonic and erosional activities from the Late Tertiary to Early Quaternary. This shallow graben is filled with River Atbara sediments. The deposits of the River Atbara mainly consist of sands, silt; intercalated layers of clay and gravelly layers dominate this sequence. These deposits are always underlain by basement complex. The total thickness of this formation varies between m, the gravel and sands of this sequence seem to form the lower zone of aquifer and seem to lie unconformably on the basement complex. What's very much striking the existence of these sands and gravel on the river banks? These sands possibly appear at the surface as a result of a combination of tectonic and erosional activities. Some authors related this deposit to old wadi sediments. This formation still needs verification and proper geological investigations. The River Atbara is characterized by a special land form (badland topography) called the Karab, which is always attached to the banks of the river covering almost a distance

$: Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan of 2-3 km from each bank, Plate (1).$ The word (Karab) corresponds to a rugged topography limited to deposits in proximity to River Atbara and Setait. The age of this formation is Neogene to Quaternary, (Ibrahim et al., 1992).

The word (Karab) corresponds to a rugged topography limited to deposits in proximity to River Atbara and Setait. The age of this formation is Neogene to Quaternary, (Ibrahim et al., 1992).

Masdar I991 defined Karab as sloping land, severely dissected and eroded between the clay plain and the alluvial flood plain bordering streams and watercourses.

Karab occurred only in River Atbara may be, the Karab is a part of the old sediments of River Atbara and overlain of this sediment as a result of the leaching.

These clays are forming extensive huge plain; sand dunes are existing in some parts of the area.

the basement complex. They are mainly made up of clay, silt, sandy clay clayey sand and pebbly materials. The thicknesses and extension of such soils depend on the slope, shape and rock type.

$The geological structures are represented by folds, faults, fractures and joints.$

3 166 Kheiralla K. M. et al.: Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan of 2-3 km from each bank, Plate (1). The word (Karab) corresponds to a rugged topography limited to deposits in proximity to River Atbara and Setait. The age of this formation is Neogene to Quaternary, (Ibrahim et al., 1992). Karab is a name referring to typical bad land terrain that has developed and is characterized by extensive gullying along the rivers draining from the Ethiopian highlands. (Fadull et al., 1999). Masdar I991 defined Karab as sloping land, severely dissected and eroded between the clay plain and the alluvial flood plain bordering streams and watercourses. Karab phenomena occurred as a result of erosional activities according to the large variation in the slope between the banks and the river bed (badland topography). Karab occurred only in River Atbara may be, the Karab is a part of the old sediments of River Atbara and overlain of this sediment as a result of the leaching. The superficial deposits are mainly elluvial, colluvial and alluvial deposits of Pleistocene to Recent age. These are all pediment deposits mainly formed of clays. These clays are forming extensive huge plain; sand dunes are existing in some parts of the area. The huge plain, which directly starts from the boundary of the Karab zone, is characterized by the presence of residual soils which are mainly derived by the weathering processes from the rocks of the basement complex. They are mainly made up of clay, silt, sandy clay clayey sand and pebbly materials. The thicknesses and extension of such soils depend on the slope, shape and rock type. These soils appear to be suitable for agriculture. This plain area formed by a dark cracking cotton soil mainly composed of clay minerals (Ibrahim et al., 1992). The geological structures are represented by folds, faults, fractures and joints. In the study area, some minor faults distributed in the study area controlling the streams scattered in the study area. The fractures and joints of basaltic rocks which is concentrated at the southwest of the study area, that make rich water bearing within the river basin not far of 500 m from the river in each bank (Fig. 2). 4. Methodology Plate 1. The Karab formation zone. Fig. 2. Regional Geological map of El Girba area (Bedri et al., 2010). Two geophysical methods were chosen to achieve the objectives of this study: seismic refraction and electricalresistivity (imaging) methods Resistivity Imaging Resistivity Imaging is one of the geophysical techniques. It has been chosen in most of the environmental and engineering studies because this technique can monitor the internal structure, water content, depth of bedrock, and layer thickness from subsurface image. Electrical-resistivity surveys have been used for locating and mapping buried gravel deposits since the 1950s (Jakosky, 1950; Welkie and Meyer, 1983). In a more recent survey, Nowroozi et al., (1997) employed electrical sounding and profiling to create contour maps of apparent and true resistivity, from which aggregate volume could be estimated. In general, the applicability of electrical methods to gravel exploration is based on the high resistivity of coarse-grained materials, in contrast to surrounding clay, silt, or soil. The study area consist of five parallel lines running from almost east to west namely GF01, GF02,GF03,GF04andGF05. The lengths of these lines vary from 400m to1200m Fig. (3).In this study we explore the possibility of using a 24- electrode, automated DC (direct-current) imaging equipment for rapid reconnaissance surveys for buried channels. We use the SAS 4000 Terrameter imaging system with the multielectrode selector, manufactured by ABEM Company. All apparent-resistivity data were collected using the line of 40 electrodes spaced at 10 m, to give a total length of the line of 400 m. Multi-electrode data collection was done in the Wenner Schlumberger modification. The multi-electrode apparent-resistivity readings are written on the disk memory

American Journal of Earth Sciences 2015; 2(6): 164-170 167 of the instrument and transferred to an external PC.

The program uses a nonlinear least squares optimization technique to obtain the inversion of apparent resistivities (Griffiths and Barker, 1993; Loke and Barker, 1996).

The inversion program allows control of the accuracy of final inverted sections; this accuracy is defined as the root-mean-square (RMS) difference between the observed and calculated pseudo sections.

SeismicLines (in Red color)location in Fashaga Area. 5. Results 5.1.

Most of these deposits occurs as lenses or pockets and may consist of a variety of mixed materials including sand, gravel, silt, and clay.

$4.2. Seismic Refraction Seismic Refraction is a geophysical method used in the fields of engineering geology, geotechnical engineering and exploration geophysics.$

4 American Journal of Earth Sciences 2015; 2(6): of the instrument and transferred to an external PC. The files are then inverted using the commercial interpretation software RES2DINV (Loke, 1997, 1999). The program uses a nonlinear least squares optimization technique to obtain the inversion of apparent resistivities (Griffiths and Barker, 1993; Loke and Barker, 1996). The total time to obtain a complete inversion of one 40- electrode file does not exceed a few seconds on a Pentium IV processor. The inversion program allows control of the accuracy of final inverted sections; this accuracy is defined as the root-mean-square (RMS) difference between the observed and calculated pseudo sections. The inversion stops when this difference decreases below a user defined tolerance level, given in percent. All the subsequent inverted sections have the RMS error of less than 7%. Fig. 4. SeismicLines (in Red color)location in Fashaga Area. 5. Results 5.1. Resistivity Imaging The main source of natural aggregate in the area is a transported deposits associated with flooding time of these two rivers (Atbara and Setait). Most of these deposits occurs as lenses or pockets and may consist of a variety of mixed materials including sand, gravel, silt, and clay. In general, the high resistivity value in the studied area resulted mainly from the gravels and basaltic rocks and some of the high resistivity resulted from transported or unconsolidated dry material which is found in the surface Table (2). Table 2. Typical range of Resistivity for some Rocks. Fig. 3. Locations of resistivity imaging lines (Blue color) and boreholes in the Fashaga Area Seismic Refraction Seismic Refraction is a geophysical method used in the fields of engineering geology, geotechnical engineering and exploration geophysics. The method utilizes the refraction of seismic waves on geologic layers and rock/soil units in order to characterize the subsurface geologic conditions and geologic structure. The tomographic method, involves the creation of an initial velocity model, and then iteratively tracing rays through the model, comparing the calculated travel times to the measured travel times, modifying the model, and repeating the process until the difference between calculated and measured times is minimized. The Seismograph used in this survey was 48 channel ABEM instrument ABM TERRALOC MK6 system. A total of three seismic lines (GF01, GF02 and GF03) were collected and processed (Fig. 4). Each seismic spread consisted of two cables of 24 geophones, with 48 geophones station per spread. Geophone interval spacing was 6 m for all lines. Rock /material type Igneous Limestone Sandstone Sand and gravel Clay Unconsolidated wet clay 20 Soil 1-10 Fresh water Drill mud, hydraul-ez 4.5 Sea water Copper(native) 2.0 x 10-7 Resistivity range (Ωm) A total of five lines were investigated at Fashaga area, the electrical profiles were done from west to east in all lines (Fig. 5). All of them showed significant accumulation of high resistive material on or below the surface. Profile GF01 This profile shows three pockets of high resistivity seen in the eastern side of the profile having resistivity value above1500 Ohm.m and another pocket in the almost the center of the profile with resistivity value of 500 ohm.m (Fig. 5a).

5 168 Kheiralla K. M. et al.: Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan Fig. 5a. The apparent resistivity pseudo section for the GF01 profile. Profile GF02 This profile represents the dam axis of upper Atbara dam, which shows a high resistive layer on the surface that may be due to the transported material along the dam axis (Fig. 5b). Fig. 5b. The apparent resistivity pseudo section for the GF02 profile. Profile GF03 This profile shows high resistivity value varying from 400tomorethan 1500 Ohm.m (Fig. 5c). Fig. 5c. The apparent resistivity pseudo section for the GF03 profile. Profile GF04 This profile shows high resistivity value in the eastern side of the section. The resistivity value ranges from 500 to more than 4500Ohm.m (Fig. 5d), the lengthofthishighresistivepocketisabout120m and the maximum thickness is about20m. Fig. 5d. The apparent resistivity pseudo section for the GF04 profile.

6 American Journal of Earth Sciences 2015; 2(6): Profile GF05 This profile shows high resistivity pocket in the middle of the profile with resistivity value of more than1200ohm.m, the depth to this pocket is about15m (Fig. 5e). Fig. 5e. The apparent resistivity pseudo section for the GF05 profile Seismic Refraction The seismic refraction survey was carried out to determine the thickness of overburden and the situation of bedrock. Bedrock elevation determine from the seismic refraction were plotted in section form. The seismic records is transferred from the data recording instrument to a computer, then the processing and interpretation performed using computer software (Seisimager package) to provide travel time curve and velocity model. The processing results for each seismic profile are presented in the appendix containing the velocity models and travel time curves. Generally, various techniques are being used for seismic refraction data interpretation; in this survey we have applied the tomography method. The results of Fashaga seismic record as following: I. Line GF01: the velocity range varies from 0.3km/sec to more than 3km/s, the bedrock from 25 to more than40 m. II. LineGF02: the velocity range varies from 0.3km/s to more than3km/s, depth to the top of bedrock varies from15minthe center of the line to 40 m in the both sides of the line. III. Line GF03: the velocity ranges from 0.3 km/sec to more than 3km/sec.depthofthebedrockvariesfrom25mtomorethan 30 m in the point from 430m to 460m. Fig. 6a. FashagaLineGF01SeismicSection. Fig. 6b. Fashaga Area, LineGF02SeismicSection. Fig. 6c. FashagaArea,LineGF03SeismicSection.

7 170 Kheiralla K. M. et al.: Geology and Geophysical Conditions of Land Platform (Karab) Using Resistivity Imaging and Seismic Refraction Survey, Fashaga Area-Gadaref State, Sudan The summary of Fashaga area result shown in Table (3), and Figure (7), shows the interpreted geological cross section from geophysical investigation and borehole data for Fashaga center line. Table 3. Thesummaryof Fashaga seismic results. Line Velocity Range ( km/s) Depth to bedrock top(m) GF01 0.3to 3 25 to 40 GF to 3 15 to 40 GF to 3 25 to 30 Fig. 7. Interpreted geological cross section from geophysical investigation and borehole data for Fashaga center line. 6. Conclusions The resistivity imaging coinciding to some extent with the available borehole data drilled in the investigated area, but there is some interference with overlying top soil which has high resistivity value in some profiles. It is noted that from the resistivity imaging investigations the resistivity of gravels or very coarse material can be expected to lie in the range of 300 to1500vm, depending on moisture conditions, and the resistivity values of the basaltic intrusion lie in the range of 500 to 5000 Vm. so there are some difficulties to differentiate between basaltic intrusion and gravels. Seismic Refraction study in Fashaga shows that the depth to the top of the bedrock from 15-30m in Fashaga area Finally, one interpreted geological section had been made based on the geophysical investigations, available borehole data and the geological setting of the area. This line is the dam of the Fashaga Area. Due to open water-filled fissures, the resistivity within a fracture zone is in general lower than the resistivity of the host rock even for dry fracture zones due to enhanced weathering at open fissures. Acknowledgements Thanks and appreciations are due to: the Dam Implementation Unit (DIU) for providing the data, advice and support all over the period of this study. I wish to thank the Faculty of Petroleum and Minerals, especially appreciate the assistance of the staffs of the geophysical department. References [1] Bedri O, Abbate E, Albianelli A, Awad A, Billi P, Bruni P, Delfino M, Ferretti M P,Filip P O, Gallai G, Ghinassi M, Lauritzen S E, Verto D L,Navarro B M, Martini F, Napoleone G, Papini M, Rook L, Sagri M Pleistocene Enviroments And Human Presence In The Middle Atbara Valley (Khashm El Girba Eastern Sudan). Palaeogeograph, Palaeoclimatology, Palaeoecology 292 (2010) [2] Fadull, H. M., Salih, A. A., Ali, I. A. & Lnanagaz, S., Use of remote sensing to map gully erosion along Atbara River, Sudan JAG l Volume 1 - issue pp1-6. [3] Griffiths, D. H., Barker, R. D., Two-dimensional resistivity imaging and modeling in areas of complex geology. Journal of AppliedGeophysics29, [4] Ibrahim, K. E., Hussein, M.T. & Giddo, I. M., Application of combined geophysical and hydrogeological techniques to ground water exploration: a case study of Showak Wad Elhelew area, Journal of Africa Earth Science, Vol. No.1, pp.1-10, Printed in Great Britain. [5] Jakosky, J. J., ExplorationGeophysics, 2 nd edn. Trija Publishers, Los Angeles, 1195 pp. [6] Loke, M. H., Electrical imaging surveys for environmental and engineering studies. A practical guide to2- Dand3-Dsurveys, Penang, Malaysia, 57pp. [7] Loke, M. H., RES2DINV-rapid2-D resistivity and IP inversion using the least-squares method. Software manual, 81pp. [8] Loke, M.H., Barker, R. D., Rapid least-squares inversion of apparent resistivity pseudo sections by a quasi-newton method. Geophysical Prospecting 44, [9] Masdar, Land use plan interim report for southern Kassala Agricultural Development Project, Sudan. U.K. Ltd, unpublished report. [10] Nowroozi, A. A., Whittecar, G. R., Daniel, J. C., Estimating the yield of crushable stone in an alluvial fan deposit by electrical resistivity methods near Stuart Draft, Virginia. Journal of AppliedGeophysics38, [11] Saeed, E. M., Hydrogeology of Kassala district, Kassala province, Demogratic Republic of Sudan. Geol. Miner. Resour. Dep., Khartoum.Bull.17, 95 pp. [12] Welkie, C. J., Meyer, R. P., Geophysical evidence that the Haeger Tillmember underlies southern western Lake Michigan.GeosciencesWisconsin8,

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