Geoelectrical Soundings Research Articles

Using Geoelectric Soundings for Estimation of Hydraulic Characteristics of Aquifers in the Coastal Area of Lagos, Southwestern Nigeria

Electrical resistivity investigation was carried out at Ibeju Lekki, Southwestern Nigeria. The thrust of this study is to determine the geoelectrical parameters of the shallow aquifer and estimate the hydraulic characteristics of this aquifer unit from the surface geophysics. The area falls within the Dahomey basin of the Nigeria sedimentary terrain. Twenty-one VES were conducted using Shlumberger array with a maximum half current electrode (AB/2) of 100 m giving total spread of 200 m. Data were interpreted using partial curve matching technique and assisted 1-D forward modeling with WINRESIST software. The qualitative interpretation revealed KQ curves (ρ1 < ρ2 > ρ3 > ρ4) and KH curve (ρ1 < ρ2 > ρ3 < ρ4). The geoelectric section generated from the results of the VES revealed a four geo-electric layers; these include topsoil with resistivity ranging from 213-5404 Ωm, dry sand with resistivity values vary from 301to 17178 Ωm, saturated sand with resistivity varying from 110 to1724 Ωm and sand (saline water content) with resistivity values of between 8 and 97 Ωm. The major aquifer in the area occurs at the third geoelectric layer. The depth to this aquifer is of between 0.7m and 6.0m and the layer thickness is between 0.2 m and 19.9 m. The hydraulic characteristics of the aquifer estimated from the geoelectric parameters reveal that the aquifer has porosity values of between 29.4 % and 57.7 %, protective capacities of between 0.00013 and 0.015 mhos, transverse resistance ranges from 345-18502 Ωm2, transmissivity values vary from 13 to 310 m2/day and hydraulic conductivity ranges from 0.8-65 m/day. The results show that the aquifer is characterized by high porosity and low protective capacities of overburden layers indicating that it is highly vulnerable to surface contamination. It has high transverse resistance, high transmissivity, and high hydraulic conductivity indicating that the aquifer can transmit water at higher rate and sustain the need of the community. This study has demonstrated the efficacy of surface geophysics in estimating hydraulic characteristics of an aquifer where pumping test data are not available and also to determine its vulnerability to surface contaminants.

An investigation of the potential environmental contamination from the leachate of the Rasht waste disposal site in Iran

This research deals with detecting the underground layer geometrics and conditions, probable faults and crushed zones, thickness of alluvium, determination of pollutant plume development, and groundwater quality in the study area. The Visual HELP and HYDRUS programs were used to evaluate hydrologic properties and contaminant transport, respectively. All required data which consisted of the geotechnical study (drilling the borehole and dug well), the geophysical study (geoelectrical sounding), and geochemical analysis of water and leachate samples were collected in the field operations. According to the results of ten years of hydrologic data evaluation of this site by using the Visual HELP program, the mean rate of leachate percolation from this site was equal to 1.095 m per year. From the interpretation of the vertical electrical resistivity soundings curves which was calibrated by the geotechnical borehole log, a four-layer geoelectrical model was recognized. These layers consisted of surface layer (loose and fine grained topsoil), medium grained alluvium, saturated layer, and high resistivity bedrock. The physico-chemical analysis of ten surface and groundwater sample indicated that these samples were not contaminated by leachate yet. Based on 1D, 2D and 3D modeling of contaminant transport using the HYDRUS 1D and the HYDRUS 2D/3D program, during the ten years of the onset of leachate seepage, contaminant plume migrated a short distance (<200 m) due to high compaction of fine grain sediments. In addition, the high compaction of a third layer (M3) in a depth of more than 40 m introduces a natural barrier to infiltration and movement of contaminant to the deep subsurface.

Using Geoelectric Soundings for Estimation of Hydraulic Characteristics of Aquifers in the Coastal Area of Lagos, Southwestern Nigeria

Electrical resistivity investigation was carried out at Ibeju Lekki, Southwestern Nigeria. The thrust of this study is to determine the geoelectrical parameters of the shallow aquifer and estimate the hydraulic characteristics of this aquifer unit from the surface geophysics. The area falls within the Dahomey basin of the Nigeria sedimentary terrain. Twenty-one VES were conducted using Shlumberger array with a maximum half current electrode (AB/2) of 100 m giving total spread of 200 m. Data were interpreted using partial curve matching technique and assisted 1-D forward modeling with WINRESIST software. The qualitative interpretation revealed KQ curves (ρ1 &lt; ρ2 &gt; ρ3 &gt; ρ4) and KH curve (ρ1 &lt; ρ2 &gt; ρ3 &lt; ρ4). The geoelectric section generated from the results of the VES revealed a four geo-electric layers; these include topsoil with resistivity ranging from 213-5404 Ωm, dry sand with resistivity values vary from 301to 17178 Ωm, saturated sand with resistivity varying from 110 to1724 Ωm and sand (saline water content) with resistivity values of between 8 and 97 Ωm. The major aquifer in the area occurs at the third geoelectric layer. The depth to this aquifer is of between 0.7m and 6.0m and the layer thickness is between 0.2 m and 19.9 m. The hydraulic characteristics of the aquifer estimated from the geoelectric parameters reveal that the aquifer has porosity values of between 29.4 % and 57.7 %, protective capacities of between 0.00013 and 0.015 mhos, transverse resistance ranges from 345-18502 Ωm2, transmissivity values vary from 13 to 310 m2/day and hydraulic conductivity ranges from 0.8-65 m/day. The results show that the aquifer is characterized by high porosity and low protective capacities of overburden layers indicating that it is highly vulnerable to surface contamination. It has high transverse resistance, high transmissivity, and high hydraulic conductivity indicating that the aquifer can transmit water at higher rate and sustain the need of the community. This study has demonstrated the efficacy of surface geophysics in estimating hydraulic characteristics of an aquifer where pumping test data are not available and also to determine its vulnerability to surface contaminants.

The application of geoelectrical attributes based on crossed square array resistivity sounding data at 'Pudarske Kolibe' site in eastern Serbia

Most of modern geophysical investigations routinely utilize seismic exploration method. 3D seismic and multiple seismic attributes enable the most complete definition of the local geological setting. This paper deals with utilization of crossed-square geoelectrical sounding method, which can be used for a 3D interpretation of resistivity sounding data as well as for defining additional geoelectrical attributes. With crossed-square array, we get a directional independent measurement and multiple geoelectrical attributes (ρm, ρx, n, θ and ξ). A more complete approach towards better interpretation is achieved with maps of five attributes: geometric mean apparent resistivity (ρm), average apparent resistivity (ρx) apparent anisotropy coefficient (n), azimuth of the plane beddings (θ) and a ratio of deviation from homogeneous anisotropic subsurface model (ξ). Multiple geoelectrical attribute maps correspond to depth of investigation of the array, like time and depth slices in seismic method. Horizontal resolution and the depth of investigation are controlled by the size and position of the array. Vertical resolution is, however, limited just like for the usage of any other electrode array in geoelectrical sounding. Combination of depth maps and profile maps enables a 3D interpretation similar to 3D cubes in seismic and recent software enables 3D modelling. The measurements using crossed square array were conducted at site 'Pudarske kolibe' in Eastern Serbia. The goal was to determine and separate 3D calcarinte bodies created in the depositional process, but also the eventual existence of electrical anisotropy in fluvial and shallow see water sediments. Results of investigations revealed the existence of electrical anisotropy with apparent anisotropy coefficient (n) up to the value of 2 and, on the other hand, successfully delineated calcarenite bodies embedded in sedimentary formations present at investigated site.

Identification of formations of soil and subsoil using finite elements modeling

The Soil and subsoil survey are performed by direct and indirect methods. The direct methods are the investigations of field and laboratory while indirect methods are the geophysical methods. These allow, from one or more physical properties, to find out the underground's structure nondestructively. Geophysical methods for characterizing subsurface have long been used, in geotechnical, hydrogeological and environmental studies. For our study, we used the electrical resistivity's method. 52 geoelectric soundings VES was performed with a mesh of 500 m and electrodes distance between 1000 m and 3000 m. The interpretation of these VES allows the determination of the vertical succession of formations in place. In order to have a lateral facies variation, we used geostatistical methods for having adequate data that can be used in the input file of inversion's code Res2Dinv. The 2D images of the subsoil obtained confirm and complement the results of the correlations of oil drilling that are in this region. Thus, the study area also has a horst and graben structure. This study also allowed us to locate the sandstone formations that extend in two places along a North-South direction.

Hydro-geoelectric study of Ijare town, southwestern Nigeria

Geoelectric study involving fifty four (54) Vertical Electrical Soundings (VES) has been undertaken within Ijare, southwestern Nigeria with the aim of evolving the hydrogeologic setting of the town. Ijare is underlain by the Precambrian Crystalline Basement Complex rocks with inhabited parts of the town underlain by migmatite gneiss and fine-grained biotite granite while inselbergs of older granites suites adjoins the town on all sides. Based on the hydro-geoelectric characteristics obtained from geoelectric sections and maps, the study area is classified into low, moderate and high groundwater potential zones. Four geoelectric subsurface layers delineated from the study are the topsoil, weathered basement, fractured basement and the presumably fresh bedrock. The concealed ‘basement complex’ morphology is rugged in the town with concealed ridges underlying the central areas while the eastern and northwestern areas are underlain by irregular concealed depressions. The zones of concealed basement depressions with resultant significant overburden thickness constitute target areas for groundwater development in the town. Arising from the general classifications undertaken using the results of the geoelectric soundings, sustainable groundwater development schemes for Ijare is considered feasible on the eastern and northwestern flanks where groundwater potential rating varies from moderate to high. Key words: Geoelectric, groundwater, basement, ridge, depression, overburden, fracture.

Geo-electrical Resistivity Survey in the High Barind for the Delineation of Aquifer Geometry

Geo-electrical resistivity techniques have been employed in the High Barind as a reconnaissance investigation for the subsurface lithology. Out of 13 upazillas, comprising the Barind tract, Sapahar and Porsha Upazillas, situated in the High Barind are badly affected by water shortage. Vertical electrical soundings (VES) were executed using Schlumberger configuration in different areas taking the current electrode separation up to 170 meters. Direct method technique was employed to evaluate the geo-electric layer such as resistivity and thickness of the discrete layers. The results of the resistivity survey give the subsurface information consisting of three to four layers. The results of geo-electric sounding have been compared with the geological section wherever available. DOI: http://dx.doi.org/10.3329/rujs.v39i0.16540 Rajshahi University J. of Sci. 39, 27-37 (2011)

Geoelectrical studies for the delineation of potential groundwater zones at Oduma in Enugu State, Southeastern Nigeria

This work evaluates the use of geoelectrical method in the delineation of potential groundwater zones at Oduma in Enugu state, Southeastern Nigeria. Oduma lies within latitudes 6°02Ei N to 6°07Ei N and longitudes 7°35Ei E to 7°41Ei E with an area extent of about 102.6 km2. The area is underlain by Awgu Shale group with its lateral arenaceous facie; Owelli Sandstone outcropping south of Oduma. Thirteen (13) vertical electrical soundings (VES) were carried out within the study area. Interpretated VES data shows predominance of Q and H curve type, indicating a fracture-shale subsurface. Contour maps of iso-resistivity, depth, transverse resistance, longitudinal conductance, aquifer transmissivity and hydraulic conductivity were constructed. Computed aquifer transmissivity from VES data values indicates a low yield aquifer. The latter was used to delineate the potential groundwater zones based on Gheorge aquifer transmissivity classifications. Comparisons of aquifer hydraulics estimated from geoelectrical sounding and the pump test analysis indicates a fairly good match. Three potential groundwater zones were delineated; the low, very low and negligible potential zones. The various contour maps and potential groundwater zone map will serve as a useful guide for groundwater exploration in the study area. Key words: Resistivity, transmissivity, hydraulic conductivity, groundwater zones, aquifer yield.

Internal structure and permafrost characteristics of the rock glaciers of southern carpathians (romania) assessed by geoelectrical soundings and thermal monitoring

Six rock glaciers in the outhern arpathians have been investigated by means of geoelectrical soundings in order to detect their internal stratigraphy and the existence of frozen sediments. In the case of three relict rock glaciers, the electrical resistivity measurements indicated a typical internal structure. Low resistivity values (<10-kΩm) which are typical of unfrozen fine‐grained materials were obtained, but high resistivity values (25–240-kΩm) measured in the ietroasa, Ieşu and ietrele rock glaciers denote the presence of sediments cemented by interstitial ice and ice lenses. Based on the moderate resistivity values, the ice content is probably low to medium in the upper portion of these rock glaciers, that is, above 2040 m.At two sites (ietroasa and ăiuga rock glaciers), ground surface temperature evolution was monitored using digital dataloggers. Mean annual ground surface temperature and regime throughout the winter were extracted from the recordings and confirmed the probability of permafrost occurrence in ietroasa rock glacier. In the Ieşu and ietrele rock glaciers, measurements of bottom temperatures of the winter snow cover were performed in March 2012. Considering the thick active layer, the reduced ice content and the presence of scarce vegetation on their surface it could be assumed that the permafrost exists in marginal conditions in the outhern arpathians. The ground ice in permafrost is produced by the groundwater freezing or by snow banks buried by coarse angular boulders following large rockfalls.

English

&nbsp; Hydrogeophysical studies have been undertaken at Irepodun/Ifelodun local government area of Ekiti State, Nigeria. The area is underlain by the Basement Complex rocks. The study involved very low frequency electromagnetic (VLF-EM) and electrical resistivity methods. The results are presented as profiles, sections, curves, charts and maps. VLF-EM anomalies were evaluated using geoelectric soundings. Geoelectric sounding results enabled the determination of overburden thickness, transverse unit resistance, longitudinal unit conductance and coefficient of anisotropy (l). Overburden thickness zones of 45 to 55 m were classified as high, 15 to 45 m as moderate and values less than 15 m as low. Afao, Obo, Iyin, Araromi, Iketun and Igede are of moderate to high groundwater potential, while other towns are of low groundwater potential. Longitudinal unit conductance showed that the area is characterized by sequence of good, moderate and weak groundwater protective capacities. Computed &lambda; values revealed that the northern flank (Esure and Iropora) are underlain by charnockite of low &lambda; (1.0 - 1.1) with other areas of high &lambda; (1.1 - 2.1) underlain by granite and migmatite gneiss. Groundwater development is feasible in Araromi, Afao, Iyin, Obo, Iketun and Igede but marginally feasible in the other towns. Thus, groundwater potential of Irepodun/Ifelodun local government area is generally low. &nbsp; Key words:&nbsp;Electromagnetic, geoelectric sounding, longitudinal unit conductance, coefficient of anisotropy, groundwater potential.

The estimation and significance of Dar-Zarrouk parameters in the exploration of quality affecting the Gulf of Aqaba coastal aquifer systems

The subsurface resistivity layer parameters of 47 vertical geoelectrical soundings are analyzed for a rectangle shaped area of 70 km2 from the Gulf of Aqaba coast. For this purpose, the Dar-Zarrouk parameters i.e., (i) total longitudinal unit conductance, (ii) total transverse unit resistance and (iii) average longitudinal resistivity are used to determine the resistivity regime of the water bearing formations. The significance of these parameters is to establish a vision about the occurrence and distribution of water bodies, while dealing with complicated situations of resistivity ranges intermixing for saline, brackish and fresh water aquifers. The results obtained from the study area suggest that the Dar-Zarrouk parameters proved useful in providing confident solution to delineate water-bearing formations. The behavior of the Dar-Zarrouk parameters and its patterns in space over large areas with respect to the occurrence of water aquifer systems in the deltaic coastal area has been demonstrated.

Subsurface 2D Image Analyses of the Uyangha Basement Area, South-Eastern Nigeria

Geo-electric soundings were made in Stella Maris Secondary School, in Uyangha, Nigeria to image the subsurface and obtain thicknesses and resistivities of different layers. A quantitative interpretation of the data obtained clearly reveals the presence of four (4) geo-electric sections which are interpreted to be dry laterite, moist laterite, weathered basement, and saturated basement. The depth probed is about 100m. The saturated basement is the aquifer unit. Depth to aquifer unit in the area is at about 65m to 80m.The thickness of the aquifer unit ranges from 20m to 35m. For ground water exploitation, boreholes in the area should therefore be drilled to the depth of 91m, for reasonable groundwater yield. The lateritic layer makes the study area suitable for building construction in the area. Keywords: 2-D imaging, electrical resistivity, vertical electrical sounding, pseudo-section, saturated basement. I. Introduction Geophysical surveys are aimed at detecting and locating subsurface geological structures and to measure their dimensions and their physical properties. Geophysical exploration methods are many, which can be classed according to the source of energy used in their measurement. To select a suitable method to be used for any survey, a comprehensive knowledge of the geology of the area and site conditions are necessary. This is because in some instances, site conditions may preclude the successful use of most if not all geophysical techniques. The resistivity study requires the introduction of an artificial current into the ground through point electrodes or long line contacts. The potential in the earth (ground) due to the introduction of the current into the ground is measured using two potential electrodes which are planted on the ground surface. Since the current is measured as well, it is possible to determine the apparent resistivity of the subsurface. This however makes the resistivity method more superior (at least theoretically) to all the other electrical methods (Keary and Brooks 1991). It is important to note that, deviations from the pattern of potential differences expected from homogenous medium, provides information on the form and electrical properties of subsurface inhomogenities. The depth soundings or a vertical electrical sounding (VES) is also known as electrical drilling. It is the process by which depth investigations are made (Zohdy, et al., 1974). The technique is best adopted to determining depth and resistivity for flat laying layered rock such as found in sedimentary terrains. The Schlumberger configuration is mostly commonly used for VES investigations. The technique further finds application in geotechnical survey to determine overburden thickness and also in hydrogeology, to define depth and thickness of porous strata which are potential aquifers (Burger, 1992). The objectives of the study are to attempt to identify the different rock types in the area and their vertical thickness using the electrical resistivity method. This result will help to detect saturated layers for ground water exploitation in the area. The apparent resistivity of the ground is a function of the measured impedance (ratio of potential to current) and the geometry of the electrode array (Burger, 1992). II. Geologic Setting

Interpretation of the results of geoelectric sounding based on a mathematical model of double-layered soil

This paper presents the soil resistivity testing results carried out at the location designated for the construction of substation 110/35/10 kV 'Zlatibor'. Testing results show that the tested soil is double-layered. Interpretation of the results of geoelectric sounding based on a mathematical model of double-layered soil shows good matching with the test results.

Geoelectric Sounding for Evaluating Soil Corrosivity and the Vulnerability of Porous Media Aquifers in Parts of the Chad Basin Fadama Floodplain, Northeastern Nigeria

A geoelectrical survey was carried out in parts of the Chad Basin Fadama Floodplain as a means of evaluating both the soil corrosivity and protective capacity. One hundred and six Schlumberger Vertical Electrical Sounding data were collected at the corners of a 225 x 225 m square grid network. Topsoil resistivity and topsoil longitudinal unit conductance maps were generated from the first and second order geoelectric parameters respectively. Areas considered as high corrosivity are the north central, southwestern, southern and northern parts with (r<180w-m). Part of the study area characterized by materials of poor to weak protective capacity has longitudinal conductance values of less than 0.1 and (0.1 - 0.19) mhos respectively. Values between (0.2 - 0.79 mhos - sandy clay cover) and (0.8 - 4.9 mhos - clay cover) correspond to moderate and good protective capacity respectively. It can thus be concluded that the flanks of the floodplain underlain by appreciable clayey topsoil thickness columns are susceptible to corrosion tendency. These same flanks are characterized by materials of moderate to good protective capacity and serve as sealing potential for the underlying hydrogeological system in the area.

GEOELECTRIC EVALUATION OF SUBSURFACE CONTAMINATION AT A GAS AND SERVICE STATION, ALAGOINHAS, BA, BRAZIL

This work contains the results of geoelectrical soundings performed around the Lubrijau Gas and Service Station, in Alagoinhas County, Bahia State, Brazil. A total of 56 vertical electrical soundings (VES) were conducted along five parallel lines using a Schlumberger electrode array with a maximum AB/2 spacing of 300 m. The aim was to define the structure of the Marizal-São Sebastião aquifer system found below the area, as well as to evaluate the status of eventual contamination, caused either by fuel leaks or improper sewage disposal practices. Data were interpreted as one-dimensional sounding curves and as two-dimensional resistivity sections using both public domain (RESIST 1.0 and RES1D) and commercial (RES2DINV) softwares. The interpreted results have shown that: (i) the groundwater flow under the area is directed from NW to SE; (ii) the static water table lies from 30 to 45 m in depth; (iii) the depth to the contact between the Marizal and SËœao SebastiËœao formations exceed 45 m; and (iv) there is a conductive plume (ρ &lt; 300.m) in the subsurface, outlined as an ellipsoidal body extending to SE, having a length of about 850 m and a width of 600 m along its maximum spread. This plume has not yet reached the water well situated about 550 m from the gas station.

Geophysics Contribution for the Determination of Aquifers with a Case Study

The determination and monitoring of aquifer formations on the eastern border of Moroccan Gharb basin are very difficult because of their spatial and temporal variation. To delimit these formations, a geophysical survey of 52 geoelectric soundings was performed with a mesh of 500 m and electrodes distance between 1000 m and 3000 m. Geoelectric sections and resistivity maps show a horst and graben structure. The correlation of existing oil drillings shows that the Jurassic and Neogene formations are both affected by normal faults causing Jurassic deposits collapse with local thickening of the Miocene deposits, and reverse faults delimiting tectonic slices due to tension caused by prerifaine nappe advance. This fact confirms the generated structure by the resistivity method. The isobath map of resistant formations's roof show average depths extending from 400 to 800 m for calcareous sandstone that are potential aquifers while oil drillings indicate over 1000 m depths.

Investigation of Soil Aggressiveness towards Underground Fuel Storage Tanks and Water Pipelines in Parts of Bayelsa State, Southern Nigeria

Structural failure of buried cast/ductile iron water mains and tanks due to corrosion attacks manifests in leaks and is common in most cities throughout Nigeria. The appropriate corrective action, which aims to restore pipe/tank integrity is usually based on proper understanding of the degree of corrosiveness of the soil. In an attempt to determine the potential corrosiveness of the soil to buried metallic structures in Bayelsa State, surface geoelectrical sounding was carried out. Twenty-five Schlumberger Vertical Electrical Soundings (VES) was carried out in the freshwater and meander belt geomorphic zone and the salt water mangrove swamp and estuary complex of the state using a maximum current electrode separation ranging from 200 - 400 m. The data obtained was interpreted by computer iterative modeling using a 1D inversion technique software (1X1D, Interpex, USA). The results show a high degree of heterogeneity, both laterally and vertically, which is typical of a complex depositional environment. Generally, the sub-soil condition within the expected depth of installation of water mains and storage tanks (0 - 10 m) is slightly or moderately aggressive (effective aggressivity) in the freshwater and meander belt geomorphic zone but is very strongly aggressive in the salt water mangrove swamp and estuary complex. Corrosion cells which may lead to significant corrosion failures may occur in the vicinities of strongly aggressive stations. This poses a significant corrosion risk to metallic water pipes and storage tanks. Current day design should therefore either mandate the use of a non-metallic piping product (water mains) or cathodic protection system. Prediction of potential corrosiveness of a soil and thus the application of proper corrosion control measures will not only protect the environment from spillages but will also avert cost of repair, clean-up and replacement.

Geoelectric Sounding for the Determination of Aquifer Transmissivity in Parts of Bayelsa State, South South Nigeria

The application of geophysical methods in combination with pumping tests provides a cost-effective and efficient alternative to estimate aquifer parameters. In this study, nineteen Schlumberger vertical electrical soundings (VES) were occupied in parts of Bayelsa State using a maximum current electrode separation ranging beweeen 300 - 400 m with the aim of estimating the transmissivity of the alluvial aquifer in areas where no pumping test has been carried out. Four of the soundings were carried out near existing boreholes in which pumping test had been carried out. The VES data obtained was interpreted, and layer parameters such as true resistivities and thickness were determined. The geoelectric parameters were used to generate the Dar Zarrouk parameters. Correlating the Dar Zarrouk parameter (e.g longitudinal unit conductance) with transmissivity derived from pumping test data, a constant was found which translate longitudinal unit conductance to transmissivity in a hydrogeological setting where effective porosity is the primary control on resistivity and hydraulic conductivity. Transmissivity determined from the pumping test data range between 1634.0 - 5292.0 m2/day while transmissivity values estimated from the longitudinal unit conductance (Lc) range between 721 - 8991 m2/day. The transmissivity estimated from the pumping test (Tp) data and transmissivity estimated from the longitudinal conductance (Lc) on comparison show excellent correlation (R2 = 0.92). The high transmissivity values agree with the geology of the Benin Formation (Coastal Plain sands) consisting of fine-medium-coarse sands. The results give a useful first approximation of the transmissivity and could be used to site exploratory boreholes.

Determination of groundwater potential in Asaba, Nigeria using surface geoelectric sounding

Providing potable water has been a major concern to the people of Asaba the capital city of Delta State, Nigeria. This study which is aimed at identifying formations that present fresh and healthy aquifer water for the people employed the use of the geoelectric sounding to probe the subsurface. Data collected were interpreted in terms of the true resistivity and thickness of subsurface layers and result correlated with record from existing borehole. Results indicate that there are four to five distinct formations in the study area. Analyses of the data suggest that water bearing formation exist in the third formation having resistivity range of 318 to 567 Ωm. The thickness of the aquifer ranges from 20.4 to 33.9 m, while the depth range is from 43.5 to 52.8 m. The study therefore indicates the presence of thick and highly prolific aquifer zone which is an indication of adequate water resource in the area. It is recommended that boreholes for good drinking water and water scheme be drilled to the third layer of the various locations investigated. Key words: Schlumberger array, groundwater, geoelectric sounding, electrical resistivity, aquifer, electrode separation.

Geoelectric resistivity sounding for deciphering hydrogeology and locating deep tubewell installation sites in Puroshava area of Bagerhat, Bangladesh

The study area includes Bagerhat Pouroshava and its surroundings under Bagerhat Sadar Upazila covering an area of 7.53 sq. km with a population of about 50,000. Bagerhat is a coastal district and the subsurface geology is complicated. As in other areas of the coastal belt the quality of ground water in the area is also variable. For ground water development in the study area the shallow aquifer is not suitable as the water is mostly saline to brackish except some isolated fresh water pockets of limited yielding capacity. The deep aquifer is also not very homogeneous in water quality. In the northwestern part it bears fresh water but water quality deteriorates south-southeast with higher depth of occurrence. A comprehensive study is carried out to demarcate the aquifers and to judge the water quality to find the suitable location of the deep tube wells in Bagerhat Pouroshava area.&#x0D; Fifteen geoelectric soundings have been executed in the study area using Schlumberger configuration with maximum spreading of 1200 m. Based on the vertical electrical sounding interpretation results the subsurface sequence is divided into following geoelectric units: The top unit has resistivity less than 5.0 Ωm with a thickness of 1.5 to 20 m and represents the top clay­ silty/sandy clay layer. The second geoelectric unit represents a very fine to medium sand with thin clay lenses and resistivity varying from 5.0 Ωm to more than 100.0 Ωm with a thickness of 16 to 135 m. The resistivity of the following unit ranges from l.40 Ωm to 4.8 Ωm and thickness varies from 100 m to more than 300 m. The deepest geoelectric unit shows resistivity from 8.0 Ωm to 18.0 Ωm and represents the deep aquifer. The depth to the aquifer varies from 235 m to 355 m. The most suitable site for groundwater development from the deep aquifer is in the vicinity of East Saira of Shatgambuj union.