Hydrogeophysical investigations for assessing the groundwater potential in part of the Peshawar basin,Pakistan

A large number of valleys and basin systems are present in the northwestern part of the Himalayas in Pakistan which form significant aquifers in the region. Hydrogeophysical investigations in the western part of Nowshera District, a part of the intermontane Peshawar basin, were undertaken to help to determine the availability of groundwater resources in the region. Thirty vertical electrical resistivity soundings (VES) were acquired using a Schlumberger expanding array configuration with a maximum current electrode spacing (AB/2) of 150 m in delineating the groundwater potential in the study area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling methods suggest that the area is underlain by 3 to 5 geo-electric layers. The interpretation results showed that the geo-electrical succession consists of alluvium comprising of alternating layers of clay, silty clay, fine to coarse sands, sand with gravels and gravels of variable thickness. High subsurface resistivity values are correlated with gravel–sand units and low resistivity values with the presence of clays and silts. The modeled VES results were correlated with the pumping tests results and lithological logs of the existing wells. The pumping test suggests the transmissivity of the aquifer sediments is variable corresponding to different sediments within the area. The gravel–sand intervals having high resistivity value show high transmissivity values, whereas clay–silt sediments show low transmissivities. It is concluded that majority of the high resistive gravel–sand sediments belong to an alluvial fan environment. These gravel–sand zones are promising zones for groundwater abstraction which are concentrated in the central part of the study area.     

Geoelectrical method in the investigation of groundwater resource and related issues in Ophiolite and Flysch formations of Port Blair, Andaman Island, India

Geoelectrical resistivity investigation using Vertical Electrical Sounding (VES) technique was conducted at Port Blair, South Andaman Island, to locate the fractures in different formations and to decipher its groundwater potential. A total of 40 VES were carried out covering the entire study area using Schlumberger electrode configuration out of which 34 VES fall in Andaman Flysch formations and the remaining VES in Ophiolite formations. The interpreted resistivity results were integrated with nine borehole lithologs for the subsurface analysis. The combination of VES with borehole litholog data has provided a close correspondence on subsurface hydrogeological conditions. The interpreted VES data of various formations showed drastic variations in the resistivity ranging from higher in Ophiolite, moderate in Andaman Flysch and very low in valleys of Andaman Flysch formations. The study further revealed that the weathered and fractured volcanics of Ophiolite groups of rocks and sandstone that occur in the Andaman Flysch formations constitute the productive water bearing zones categorized as good groundwater potential zone. Based on the geoelectrical parameters, viz., thickness of layers and the layer resistivity values, a groundwater potential map was prepared, in which good, moderate, and poor groundwater zones were demarcated. Further, numerical, spatial and litho-geoelectric models of resistivity were analyzed in terms of groundwater potential and these models have thus enabled to prepare a comprehensive groundwater development and management plans proving its efficacy in this art of exploratory investigations.     

Geoelectric resistivity sounding of riverside alluvial aquifer in an agricultural area at Buyeo,Geum River watershed,Korea: an application to groundwater contamination study

Twenty profiles of vertical electric soundings (VES) were obtained in a riverside alluvium at the Buyeo area, South Korea, to examine the variations of subsurface geology and associated groundwater chemistry. The combination of the VES data with the borehole data provided useful information on subsurface hydrogeologic conditions. The vestige of an ancient river channel (e.g. oxbow lake) was identified on the resistivity profiles by the lateral continuation of a near-surface perched aquifer parallel to the river. Such a perched aquifer is typically developed in the area with a clay-rich silty surface alluvium which prohibits the infiltration of oxygen. Therefore, groundwater below the oxbow lake shows a very low nitrate concentration and Eh values under the strong anoxic condition. The distribution of water resistivity is correlated with that of measured total dissolved solids concentration in groundwater, while the earth resistivity of the aquifer shows a significant spatial variation. It is interpreted that the earth resistivity of the aquifer is mainly controlled by the soil type rather than by the water chemistry in the study area.     

Geophysical characteristics of Wadi Hanifah water system, Riyadh, Saudi Arabia

Integrated geophysical techniques including resistivity image, vertical electrical sounding (VES), and seismic refraction have been conducted to investigate the Wadi Hanifah water system. The groundwater in Wadi Hanifah has problems caused by the high volumes of sewage water percolating into the ground. The combination of VES, resistivity image, and seismic refraction has made a valuable contribution to the identification of the interface between the contaminated and fresh water in Wadi Hanifah area. The contaminated groundwater has lower resistivity values than fresh groundwater due to the higher concentration of ions which reduces the resistivity. Resistivity image and sounding in this area clearly identified the nature of the lithological depth and proved useful at identifying water-bearing zones. Fresh groundwater was found in the study area at a depth of 100 m within the fractured limestone. Water-bearing zones occur in two aquifers, shallow contaminated water at 10 m depth in alluvial deposits and the deeper fresh water aquifer at a depth of about 100 m in fractured limestone. The interface between the contaminated water (sanitary water) and fresh water marked out horizontally at 100 m distance from the main channel and vertically at 20 m depth.     

The use of vertical electrical sounding resistivity method for the location of low salinity groundwater for irrigation in Chaj and Rachna Doabs

A geoelectrical resistivity survey using vertical electrical sounding (VES) was conducted at Chaj Doab (land between rivers Jhelum and Chenab, Pakistan) and Rachna Doab (land between rivers Chenab and Ravi, Pakistan), with the objective of investigating groundwater conditions. A total of 90 sites were selected with 43 sites in Chaj and 47 sites in Rachna Doabs. The resistivity meter (ABEM Terrameter SAS 4000, Sweden) was used to collect the VES data by employing a Schlumberger electrode configuration, with half current electrode spacings (AB/2) ranging from 2 to 180 m and the potential electrode (MN) from 1 to 40 m. The field data were interpreted using the Interpex IX1D computer software and the resistivity versus depth models for each location was estimated. The outputs of subsurface layers with resistivities and thickness presented in contour maps and 3-D views by using SURFER software were created. A total of 102 groundwater samples from nearby hydrowells at different depths were collected to develop a correlation between the aquifer resistivity of VES and the electrical conductivity (EC) of the groundwater and to confirm the resulted geophysical resistivity models. From the correlation developed, it was observed that the groundwater salinity in the aquifer may be considered low and so safe for irrigation if resistivity >45 Ω m, and marginally fit for irrigation having resistivity between 25 and 45 Ω m. The study area has resistivities from 3.9 to 2,222 Ω m at the top of the unsaturated layer, between 1.21 and 171 Ω m, in the shallow aquifers, and 0.14–152 Ω m in the deep aquifers of the study area. The results indicate that the quality of groundwater is better near the rivers and in the shallow layers compared to the deep layers.     

A survey of the repository of groundwater potential and distribution using geoelectrical resistivity method in Itu Local Government Area (L.G.A), Akwa Ibom State, southern Nigeria

Vertical electrical sounding (VES), employing a Schlumberger electrode configuration, was used to investigate the sediments and aquifer repositories in Itu Local Government Area of Akwa Ibom state, southern Nigeria. This was done in sixteen (16) locations/communities with the maximum current electrode spread ranging between 800–1000m. The field data were interpreted using forward and iterative least square inversion modeling, which gives a resolution with 3–5 geoelectric layers. The observed frequencies in curve types include 31.25% of AKH, 18.8% of AAK and HK and 6.25% of K, QHK, AKH, KA and KHQ, respectively. These sets of curves show a wide range of variabilities in resistivities between and within the layers penetrated by current. The presence of K and H curve types in the study area indicates the alteration of the geomaterials with limited hydrologic significance to the prolific groundwater repository. A correlation of the constrained nearby borehole lithology logs with the VES results shows that the layers were all sandy formations (fine and well sorted sands to gravelly sands or medium to coarse-grained sands as described by nearby lithology logs) with some wide ranges of electrical resistivity values and thicknesses caused by electrostratigraphic inhomogeneity. The geologic topsoil (motley topsoil) is generally porous and permeable and as such the longitudinal conductance (S) values for the covering/protective layer is generally less than unity of Siemens (S < 1Ω^?1), the value considered for efficient protection of the underlying aquifers by the topmost and overlying layer. The spatial orientations and the leveling patterns of the most economically viable potential groundwater repository within the maximum current electrode separations has been delineated in 2-D and 3-D contoured maps. The estimated depth range for the desired groundwater repository is 32.6–113.1m and its average depth value is 74.30m. The thickness of this layer ranges from 27.9–103m while its average depth has been evaluated to be 63.02m. Also, its resistivity range and average value have been estimated to be 507–5612m and 3365.125Ωm     

Electrical imaging of the groundwater aquifer at Banting,Selangor, Malaysia

A geophysical study was carried out in the Banting area of Malaysia to delineate groundwater aquifer and marine clay layer of the alluvial Quaternary deposits of Beruas and Gula Formations. The Beruas Formation is formed by peat and clayey materials as well as silt and sands, whereas the Gula Formation consists of clay, silt, sand and gravels. Both Formations were deposited on top of the Carboniferous shale of the Kenny Hill Formation. A 2-D geoelectrical resistivity technique was used. Resistivity measurement was carried out using an ABEM SAS 4000 Terrameter. The 2-D resistivity data of subsurface material for each survey line was calculated through inverse modelling and then compared with borehole data. The resistivity images of all the subsurface material below the survey lines show similar pattern of continuous structure of layering or layers with some lenses with resistivity ranging from 0.1 to 50 Ωm. The upper layer shows resistivity values ranging from 0.1 to 10 Ωm, representing a clay horizon with a thickness up to 45 m. The second layer with depth varies from 45 to 70 m below surface and has resistivity values ranging from 10 to 30 Ωm. Borehole data indicate coarse sand with some gravels for this layer, which is also the groundwater aquifer in the study area. The lowermost layer at a depth of 70 m below ground level shows resistivity values ranging from 30–50 Ωm and can be correlated with metasedimentary rocks consisting of shale and metaquartzite.     

Delineation of shallow resistivity structure in the city of Burdur,SW Turkey by vertical electrical sounding measurements

The city of Burdur, which is built on an alluvium aquifer, is located in one of the most seismically active zones in southwestern Turkey. The soil properties in the study site are characterized by unconsolidated and water-saturated sediments including silty, clayey and sandy units, and shallow groundwater level is the other characteristic of the site. Thus, the city is under soil liquefaction risk during a large earthquake. A resistivity survey including 189 vertical electrical sounding (VES) measurements was carried out in 2000 as part of a multi-disciplinary project aiming to investigate settlement properties in Burdur city and its vicinity. In the present study, the VES data acquired by using a Schlumberger array were re-processed with 1D and 2D inversion techniques to determine liquefaction potential in the study site. The results of some 1D interpretations were compared to the data from several wells drilled during the project. Also, the groundwater level map that was previously obtained by hydrological studies was extended toward north by using the resistivity data. 2D least-squares inversions were performed along nine VES profiles. This provided very useful information on vertical and horizontal extends of geologic units and water content in the subsurface. The study area is characterized by low resistivity distribution (<150 Ωm) originating from high fluid content in the subsurface. Lower resistivity (3–30 Ωm) is associated with the Quaternary and the Tertiary lacustrine sediments while relatively high resistivity (40–150 Ωm) is related to the Quaternary alluvial cone deposits. This study has also shown that the resistivity measurements are useful in the estimation of liquefaction risk in a site by providing information on the groundwater level and the fluid content in the subsurface. Based on this, we obtained a liquefaction hazard map for the study area. The liquefaction potential was classified by considering the resistivity distributions from 2D inversion of the VES profiles, the types of the sediments and the extended groundwater level map. According to this map, the study area was characterized by high liquefaction hazard risk.     

Groundwater investigation in Kuala Selangor using vertical electrical sounding (VES) surveys

Integrated geoelectrical and hydrochemical surveys were used to investigate and delineate different types of groundwater in the Kuala Selangor alluvial aquifer. Previous hydrogeological borehole investigation showed that this aquifer contains several types of groundwater in relation to its salinity. The high salinity of the groundwater in some areas is believed to be due to either saltwater intrusion from the nearby sea or river infiltration during high tide season. The vertical electrical sounding (VES) method was employed to study and map the subsurface variation of resistivity in the area. For each sounding measurement, a total spread length of 300 m was obtained with a vertical depth penetration of about 60 to 75 m. Chemical analysis of the groundwater samples taken from both shallow and deep boreholes was carried out for the water quality determination. A total of 45 VES stations were succesfully established along three parallel roads with a direction almost perpendicular to the coastal line. The distance between stations varies from 1 to 2 km with a maximum length of about 60 km surveyed line. Results of the vertical electrical soundings as well as the hydrochemistry of the groundwater samples show that the soil and groundwater in the study area can be grouped into fresh and brackish water zones. The subsurface resisitivity sections derived from the VES study suggest that the area is dominated by brackish soil and groundwater zones, especially in the area towards the coast. This result appears to agree well with the groundwater pumped from boreholes scattered around in the area. Water drawn from boreholes near the coast showed higher salinity compared to the water pumped from inland boreholes. Chloride values greater than 250 mg/L are considered to represent the brackish zones whilst values less than 250 mg/L represents zones of fresh soil and groundwater.     

Delineation of saltwater intrusion into the freshwater aquifer of Lekki Peninsula,Lagos, Nigeria

Recently, the deterioration of water quality in the coastal zones of Lekki Peninsula area of Lagos due to saltwater infiltration into the freshwater aquifer has become a major concern. With the aim of providing valuable information on the hydrogeologic system of the aquifers, the subsurface lithology and delineating the groundwater salinity, vertical electrical resistivity (VES) sounding survey was carried out utilizing surface Schlumberger electrode arrays, and electrode spacing varying between 1 and 150 m. The DC resistivity surveys revealed significant variations in subsurface resistivity. Also, the VES resistivity curves showed a dominant trend of decreasing resistivity with depth (thus increasing salinity). In general, the presence of four distinct resistivity zones were delineated viz.: the unconsolidated dry sand (A) having resistivity values ranging between 125 and 1,028 Ωm represent the first layer; the fresh water-saturated soil (zone B) having resistivity values which correspond to 32–256 Ωm is the second layer; the third layer (zone C) is interpreted as the mixing (transition) zone of fresh with brackish groundwater. The resistivity of this layer ranges from 4 to 32 Ωm; while layer four (zone D) is characterized with resistivities values generally below 4 Ωm reflecting an aquifer possibly containing brine. The rock matrix, salinity and water saturation are the major factors controlling the resistivity of the formation. Moreover, this investigation shows that saline water intrusion into the aquifers can be accurately mapped using surface DC resistivity method.     

Application of electrical resistivity method in estimating geohydraulic properties of a sandy hydrolithofacies: a case study of Ajali Sandstone in Ninth Mile,Enugu State,Nigeria

Geoelectric investigation using vertical electrical sounding (VES) (Schlumberger electrode configuration) was carried out in 14 locations at Ninth Mile area, southeastern Nigeria to determine the variations and interrelationship of some geoelectric and geohydraulic parameters of a sandstone hydrolithofacies. The measured resistivity data were interpreted using manual and computer software packages, which gave the resistivity, depth, and thickness for each layer within the maximum current electrodes separation. The aquifer resistivity values range from 86.56 to 4753.0 Ωm with 1669.40 Ωm average value. The values of water resistivity from borehole locations close to the sounding points range from 79.49 to 454 .55 Ωm and averaging about 264.7 Ωm. Porosity values of the sandy aquifer range from 30.19 to 34.20%. Fractional porosity values range from 0.3019 to 0.3292, while the tortuosity values vary between 2.91 and 22.85. The geohydraulic parameters estimated vary across the study area. Formation factor ranges from 0.28 to 15.29, hydraulic conductivity ranges from 1.21 to 66.54 m/day which, however, influences the natural flow of water in the aquifer while tortuosity values range from 2.91 to 23.27. The contour maps clearly show the variation of these parameters in the subsurface and the plots show their relationship and high correlation coefficients with one another. The results of this study have revealed the geological characteristics of the subsurface aquifer, established the influence on the amount of groundwater, and proposed a strategy for the management and exploitation of groundwater resources in the area and other aquiferous formations.     

Environmental applications of geophysical and geochemical methods to map groundwater quality at Tuticorin, Tamilnadu, India

Coastal aquifers can become polluted due to natural and human activities, such as intrusion of saline water, discharge of effluents in industrial areas and chemical weathering of natural geological deposits. The present study is aimed mainly at understanding the geophysical and chemical characteristics of groundwater near Tuticorin, Tamilnadu, India by studying the electrical resistivity distribution of the subsurface groundwater by applying the Schlumberger vertical electrical sounding (VES) technique followed by chemical analysis of water samples. A total of 20 VES soundings were carried out to understand the resistivity distribution of the area and 21 water samples were collected to analyze the chemical quality. The interpretation and analysis of the results have identified different hydrogeologic behaviors, a highly saline coastal aquifer and freshwater locations. The results obtained from geophysical and geochemical sampling are in good agreement with each other. The approach shows the efficacy of the combination of geophysical and geochemical methods to map groundwater contamination zones in the study area.     

Electrical resistivity surveys to delineate groundwater potential aquifers in Peddavanka watershed,Anantapur District,Andhra Pradesh,India

Vertical electrical resistivity soundings were conducted in order to delineate groundwater potential aquifers in Peddavanka watershed, which is a catchment of about 398 km² in Anantapur District, Andhra Pradesh, India. The main lithologic units in the watershed are quartzite, limestone, shale, and alluvium. Ninety-nine vertical electrical soundings were conducted using the Schlumberger configuration, covering the entire watershed. The data were interpreted with the help of master curves and auxiliary point charts. Interpretations of VES were used to generate a top layer apparent resistivity contour map and longitudinal conductance map. Isoresistivity contour maps were prepared and interpreted in terms of resistivity and thickness of various sub-surface layers using computer software (SURFER), and isocontour diagrams depicting the depth to bedrock were prepared. Resistivity results were correlated with the existing lithology. Based on the depth to bedrock, the thickness of the saturated layer and the resistivity of the second layer, a groundwater potential map has been prepared, in which good, moderate, and poor zones are classified. The study reveals that the weathered and fractured portions in shale and limestone that occur in the southernmost and central portions of the watershed area constitute the productive water-bearing zones categorized as good groundwater potential aquifers.     

Approximate relationship between frequency-dependent skin depth resolved from geoelectromagnetic pedotransfer function and depth of investigation resolved from geoelectrical measurements: A case study of coastal formation,southern Nigeria

The task involved in the interpretation of Vertical Electrical Sounding (VES) data is how to get unique results in the absence/limited number of borehole information, which is usually limited to information on the spot. Geological and geochemical mapping of electrical properties are usually limited to direct observations on the surface and therefore, conclusions and extrapolations that can be drawn about the system electrical characteristics and possible underlying structures may be masked as geology changes with positions. The electrical resistivity study pedotransfer functions (PTFs) have been linked with the electromagnetic (EM) resolved PTFs at chosen frequencies of skin/penetration depth corresponding to the VES resolved investigation depth in order to determine the local geological attributes of hydrogeological repository in the coastal formation dominated with fine sand. The illustrative application of effective skin depth depicts that effective skin depth has direct relation with the EM response of the local source over the layered earth and thus, can be linked to the direct current earth response functions as an aid for estimating the optimum depth and electrical parameters through comparative analysis. Though the VES and EM resolved depths of investigation at appropriate effective and theoretical frequencies have wide gaps, diagnostic relations characterising the subsurface depth of interest have been established. The determining factors of skin effect have been found to include frequency/period, resistivity/conductivity, absorption/attenuation coefficient and energy loss factor. The novel diagnostic relations and their corresponding constants between 1-D resistivity data and EM skin depth are robust PTFs necessary for checking the accuracy associated with the non-unique interpretations that characterise the 1-D resistivity data, mostly when lithostratigraphic data are not available.     

Integrated geophysical interpretation for the area located at the eastern part of Ismailia Canal, Greater Cairo, Egypt

The integrated geophysical interpretation for the different geophysical tools such as resistivity and gravity is usually used to define the structural elements, stratigraphic units, groundwater potentiality, and depth to the basement rocks. In the present work, gravity and resistivity data were utilized for detecting the groundwater aquifer and structural elements, as well as the upper and lower surfaces of the subsurface basaltic sheet in an area located at the eastern side of Ismailia Canal, northeastern Greater Cairo, Egypt. Two hundred and ten gravity stations were measured using an Autograv instrument through a grid pattern of 50?×?50 m. The different required corrections were carried out, such as drift, elevation, tide, and latitude corrections. The final corrected data represented by the Bouguer anomaly map were filtered using high- and low-pass filters into regional and residual gravity anomaly maps. The resulting residual gravity anomaly map was used for gravity modeling to calculate the depths to the upper and lower surfaces of the basaltic sheet. The resulting gravity models indicated that the depths to the upper surface of the basaltic sheet are ranged between 26 and 314 m, where the shallower depths were found around the southern and eastern parts. The depths to the lower surface of the basaltic sheet are varied from 86 to 338 m, and the thickness of the basaltic sheet is ranged from 24 to 127 m, where the biggest thicknesses were found around the southern and northern parts of the study area. Forty-two vertical electrical soundings (VES) were carried out using Schlumberger configuration with AB/2 spacings ranged from 1.5 to 500 m. 1D quantitative interpretation was carried out through manual and analytical interpretations. The VES data were also inverted assuming a 3D resistivity distribution. The results from the 3D resistivity inversion indicated that the subsurface section consists of sand, sandstone, and sandy–clays of Miocene deposits overlying the basalts. Such basaltic features (of Oligocene age) are underlain by Gabal Ahmar Formation of Oligocene deposits, which are composed of sand and sandstone. Therefore, two aquifers were deduced in the area. The first is the Miocene aquifer (shallower) and the other is the Oligocene aquifer (deeper).     

Resistivity imaging survey to delineate subsurface seepage of hydrocarbon contaminated water at Karbala Governorate,Iraq

This research is an attempt to accomplish a 3-D resistivity imaging survey, which was carried out near a water well contaminated with hydrocarbon materials in Karbala governorate. Two-dimensional resistivity imaging measurements were collected along four parallel profiles, using a Wenner array with electrode spacing of 1 m. The RES3DINV program was used to invert the apparent resistivity data. The results displayed a resistivity distribution of the subsurface in a three-dimensional volume. Thus, both the horizontal and vertical extents of the contaminated zone were displayed. This technique revealed a low resistivity zone at depth ranges from 3 to 6 m in the investigation area, but the seepage starts at depth ranges between 2 and 3 m and continues down depth (may be to the groundwater level). This low resistivity zone is the most likely location for a subsurface seepage of contaminated water. It is clear that the sufficient measurement points along 2-D lines in a small area can increase the 3-D imaging resolution, and nearly real 3-D imaging can be achieved, when the size of subsurface anomaly compared with the electrode spacing (a) of the Wenner array is taken into consideration.     

Assessment of groundwater occurrence in Olomoro,Nigeria using borehole logging and electrical resistivity methods

The objective of this study was to assess the subsurface strata and groundwater situation of Olomoro, Nigeria using borehole logging and electrical resistivity techniques. The borehole logging consisting of resistivity and spontaneous potential logs were conducted by using the Johnson Keck logger on a drilled well in the study area. The electrical resistivity survey involving 17 vertical electrical soundings (VES) with a maximum current electrode spacing of 100 to 150 m was conducted using the Schlumberger electrode configuration. Analysis of the well cuttings revealed that the lithology of the subsurface consist of topsoil, clay, very fine sand, medium grain sand, coarse sand and very coarse sand. Results of the downhole logging also revealed that the mean electrical conductivity and the total dissolved solid of the groundwater was obtained as 390 μS/cm and 245 mg/cm³ respectively. These values are within the acceptable limit set by the Standard Organization of Nigeria (SON) for drinking water. The result of the vertical electrical sounding interpreted using the computer iterative modeling revealed the presence of four to five geoelectric layers which showed a close correlation with result from the lithology and downhole logging. Results further showed that the resistivity of the subsurface aquifer ranged between 1584 and 5420 Ωm while the aquifer depths varied between 27.8 and 39.3 m. Groundwater development of the area is suggested using the depth and resistivity maps provided in this study.     

Hydrogeophysical investigation for groundwater potential of Ishara/Ode-Remo geological transition zone,southwestern Nigeria

Geological transition zones are noted to be problematic in groundwater potential and development, due to their erratic and complex nature as well as characteristic of the subsurface lithologies. There were several occurrences of reported borehole failures and dry wells in these zones in Nigeria as a result of very scanty information that could serve as database for studying its groundwater potential. This study was therefore designed to generate hydrogeophysical data that could serve as baseline information on the groundwater potential in the study. In addition, to also delineate various subsurface lithologies present. Electrical resistivity survey for geophysical investigation was carried out using vertical electrical sounding (VES) technique. A total of 150 VES stations were purposively probed using Schlumberger electrode array. The interpreted data were used to produce geoelectric subsurface lithologies and to draw the geological section across the entire area. Various subsurface lithologies with their resistivities (Ωm) were delineated for basement complex (BC), transition zone (TZ), and sedimentary terrain (ST). In BC were topsoil, weathered zone, and fresh bedrock and in TZ were topsoil, sandy, laterite/clay, dry sand, sandstone, and fresh bedrock delineated while in the ST, topsoil, lateritic and sandy clay, dry sand, and the sandstone were delineated. In conclusion, the groundwater potential of the study area is largely been affected by the topography and the nature/composition of the Abeokuta Group that underlie the sedimentary part of the study area and the presence of thick laterite/clay unit of the basement complex portion of the study area.     

Combining TEM/resistivity joint inversion and magnetic data for groundwater exploration: application to the northeastern part of Greater Cairo,Egypt

This paper presents the combination of two complementary methods, magnetic and joint inversion of resistivity/TEM data, as an effective approach to characterize groundwater reservoirs. Twenty stations of transient electromagnetic (TEM) and vertical electrical soundings (VES) were measured and interpreted using a joint inversion technique to evaluate the subsurface low resistivity zones connected to the groundwater reservoir. A complementary survey including 871 land magnetic stations was carried out at the same area to detect the upper surface of the basaltic sheet, which represents the bottom of the Miocene aquifer in the study area. The geological log from one borehole drilled in the zone was used to partially calibrate the calculated models. The results revealed that the study area consists of five different geological units with ages ranging from Paleogene (Oligocene) to Quaternary. The methodology provides good results at a very low cost when compared with drilling boreholes.     

Vertical Electrical Sounding Investigation in East River Nile Area （Khartoum State）, Sudan

Vertical electrical sounding (VES) was carried out in the east of the River Nile. The main objectives of the resistivity survey are to determine the types and thicknesses of sedimentary units in the area, to define the contact separating the sediments from the crystalline basement complex, and to determine the structural features of the subsurface formations. Several local depressions, whose maximum depth to the Imsement surface is about 160 m, are revealed as an outcome of the VES method, and suggested to have been infilled with undifferentiated units of the Nubian Group in particular Omdurman Formation. Thus, a depth to the basement complex is calculated and the associated structural map of the east of the River Nile is drawn. The map is useful for the groundwater drilling, as far as the presence or absence of the aquifer is concerned.