Vertical Electrical Sounding Locations Research Articles

Geo-electrical investigation for groundwater reserves in the Boranakanive Reservoir Catchment in Tumkur district, Karnataka, India

The groundwater resources have been under significant constraints due to excessive exploitation and climate change. It is imperative to comprehend these dwindling resources' characteristics, given their distribution and occurrence variability across the space and time. A semi-arid climate characterizes the study area and agriculture is the primary occupation, their reliance on groundwater is indispensable due to the lack of surface water sources and erratic rainfall patterns. Consequently, the study employed the Schlumberger array of 200 m-spaced half-current electrodes Vertical Electrical Sounding (VES) survey was conducted at 100 VES locations to identify groundwater potential zones. Further, curve-matching and correlation of lithologies have been carried out from various VES-soundings, and also generated pseudo-cross sections of the subsurface geometry. The results revealed the existence of five geoelectrical resistivity layers as follows layer 1–6 to 422 Ωm, layer 2–8 to 7708 Ωm, layer 3–4 to 37117 Ωm, layer 4–37 to 95059 Ωm, and layer 5–205 to 83142 Ωm. The iso-apparent resistivity maps of the study area reveal values ranging for the first layer 11Ωm - 333 Ωm, the second layer 23Ωm - 5347Ωm, the third layer 11Ωm - 26734Ωm and 10Ωm - 67250Ωm for the fourth layer. The present research determined that the third and fourth layer constitute the principal aquifers due to their fractured structures. As a result, these layers produce a significant quantity of groundwater in the north-eastern (NE) and southern (S) regions of the study region. As a consequence of these findings, further exploration and management of groundwater resources in the study areas can be carried out with valuable insight.

GEOPHYSICAL SURVEY FOR GROUNDWATER IN PART OF OBIO/AKPOR LOCAL GOVERNMENT AREA OF RIVERS STATE, NIGERIA

This research is aimed at carrying out geophysical survey for groundwater in parts of Obio/Akpo Local Government Area of Rivers State, Nigeria, using the Schlumberger electrode configuration of vertical electrical sounding (VES) technique. The field data were obtained using ABEM terrameter SAS1000 which was applied on eight VES location, a maximum distance of 300m current electrode spacing was used in the eight vertical electrical sounding points. Data acquired were interpreted using IPIN2Win 2022 version and Surfer13 softwares. The results obtained showed a 4-layers type A curve for Rumuepirikom and Rumuolumini VES locations, 5-layers type A curve for Rumuosi VES location, 4 and 5-layers type H curve for Ozuoba and Eneka VES locations respectively, 5-layers type K curve for Egbelu VES location and 4-layers type HK curve for Rukpoku and Rumuola VES locations. The apparent resistivity values range from 29.3344 – 252898.2075Ωm for Egbelu, 22.6195 – 674.0601Ωm for Rukpoku, 1.0631 – 230.9071Ωm for Rumuepirikom, 29.5833 – 600.8296Ωm for Rumuosi, 32.9867 – 561.5283Ωm for Rumuola, 29.0597 – 458.4212Ωm for Ozuoba, 29.0597 – 134.0413Ωm for Rumuolumini and 71.2094 – 338.1611Ωm for Eneka VES locations, and shows that Rumuepirikom and Rumuolumini are the most groundwater potential zones, thus will yield sufficient groundwater to wells, while Egbelu and Rukpoku are the least groundwater potential zones, thus will not yield sufficient groundwater to wells. The knowledge of this study can be applied in groundwater resources management, hydrological studies and regional town planning programs.

Determination of aquifer position, aquifer protective capacity and groundwater quality in selected dumpsite in obubra local government area of south eastern Nigeria

At Obubra Urban in Cross River State, aquifer position, aquifer protective capacity, and groundwater quality assessment were carried out. With a maximum electrode spacing of 480 meters, 12 vertical electrical sounding (VES) surveys were conducted. The data collected were processed using the IPI2win program. employing five locations for a 2D survey A resistivity map of the dumpsites was also produced using electrical resistivity tomography (ERT). While traditional laboratory techniques were utilized to examine the groundwater, tritium tagging investigations were also conducted to determine the extent of infiltration in the study area. Four to five geoelectric sections are shown in the study. The aquifer is located at depths of 51.7, 56.4, 34.5, and 56.4 meters in the third, fourth, and fifth layers, respectively. While other VES locations have values between 0.06g and 015 mhos and are classed as weakly protected, the over burden protection capacity has values of 0.3, 0.2, 0.18, and 0.17 mhos at VES 1, 4, 7, and 8, respectively. The area's aquifer system is likely extremely permeable with strong storativity because of the area's high porosity and transmissivity characteristics. The area has an infiltration value between 7 and 21 percent, according to the tritium tagging technique used to measure the extent of infiltration from the dumpsite. The aquifer system in the research area is porous, according to the results of the procedures described above, and toxins will circulate through it quickly if they are contaminated. The investigation reveals that while magnesium and cadmium were present in quantities beyond the WHO-approved limits for portable water, all other psychochemical markers were within acceptable limits.

Hydrogeophysical investigation for the aquifers in part of Ilorin, Central Nigeria: Implication on groundwater prospect

Hydrogeophysical study involving the use of Vertical Electrical Sounding (VES) was carried out in part of the basement complex rocks of Ilorin, central Nigeria, with the aim of determining its geoelectric parameters and groundwater potential. A total of thirty (30) VES were carried out using Schlumberger electrode configuration, with half electrode separation (AB/2) varying from 1m to 100m. Information on the subsurface lithologies, overburden thickness and aquiferous layers were obtained from the different VES locations in the study area. From the quantitative interpretations of the data collected, using the method of curve matching with the Orellana-Mooney master curves and 1-D forward modeling with WinResist 1.0 version software, three to five lithologic units were identified in the study. These include: the topsoil, sandy/lateritic clay/laterite, the weathered basement, the fractured basement and the fresh bedrock which are predominantly of the ‘KH’ curve type (30%), followed by ‘H’ type (26.7%), other type curves include ‘QH’ (16.7%), ‘HKH’, ‘HA’ and ‘A’ (6.7% each) and KQ and KQH (3.3% each). The weathered layer and the fractured basement constitute the main aquifer units. The aquifers are of generally low resistivity values (mostly below 100 Ω-m). The depths to dry bedrock at the chosen VES locations vary from 2.7 to 62.7 m with a mean value of 13.02 m in the study area. The geoelectrical interpretations of data obtained in these areas have permitted the delineation of the study area into low and moderate groundwater potential zones. This study is expected to assist in future planning for groundwater resources.
 Keywords: Hydrogeophysical, Basement Complex, Groundwater, Electrical Soundings, Weathered, Fractured

An integrated electrical resistivity and geochemical approach to delineate groundwater contamination due to seawater intrusion in the southern part of Mangalore, Karnataka, India

In this paper, integration of electrical resistivity survey and chemical analyses of groundwater has been executed in Mangalore’s southern part, Karnataka to delineate seawater intrusion phenomena. About 11 locations representing different hydrogeological and geomorphological setups were selected for the vertical electrical sounding (VES) to identify the subsurface formations and the extent of seawater intrusion. With the VES, the contaminated groundwater zones and the depth to fresh and saline water interface have been demarcated. Overall, 59 groundwater samples were subjected to various geochemical analysis viz; pH, electrical conductivity (EC), total dissolved solids (TDS), salinity, total hardness, alkalinity, calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), sodium (Na2+), potassium (K+)and sulfate (SO4 2-) to understand the nature of salinity in the aquifer. The geochemical analyses show that good groundwater conditions characterize the study area except for the southern part. The southern part is under the moderate to high salinity hazard risk, possibly due to its proximity to the sea and hydraulic connection with the seawater through the tidal creek. Two among the VES locations situated in this region show a saline water trend towards the bottom, and in the rest of the VES locations, a freshwater trend is encountered towards the depth.

HYDROGEOPHYSICAL INVESTIGATION FOR GROUNDWATER RESOURCE POTENTIAL IN MASAGAMU, MAGAMA AREA, FRACTURED BASEMENT COMPLEX, NORTH-CENTRAL NIGERIA

Vertical Electrical Sounding (VES) was conducted in Salbi farm in the fractured Basement Complex, North- Central Nigeria to determine the groundwater resource potential to serve for agricultural purposes. Four VES stations using Schlumberger electrode configuration with a maximum current electrode spread of 300m were employed for data acquisition. ZHODY software was employed in computing resistivities, depths and thicknesses of the various layers and curve types. Results indicate that the area is characterized by 3 distinct geoelectric layers inferred differently at the VES locations. One potential groundwater aquifer zone was delineated at VES 1, 2, and 4 within the fractured/weathered basement columns having depths ranging between 48.8 – 59.60m and resistivities ranging between 213 – 513 Ωm. These results suggest that boreholes for sustainable groundwater supply in Salbi farm should be sited either at VES 1, 2 or 4 location and screened at a depth ≥60.0m. Wells to develop this resource should be drilled to an effective depth of 40 to 60 m for optimum yields. It is recommended that pumping test be done in order to further determine the aquifer efficiency and productivity in the area. However, the aquifers at these locations have potentials for groundwater but may be vulnerable to contamination.

Hydrochemical, geophysical and multivariate statistical investigation of the seawater intrusion in the coastal aquifer at Phetchaburi Province, Thailand

Seawater intrusion in coastal regions is an imperative ecological issues, and results in an undesirable consequence on future groundwater resources. To assess and mitigate seawater intrusion, the affected aquifers need to be characterized. By integrating geophysical investigation and multivariate statistical analysis of hydrochemical data, seawater intrusion into coastal aquifers at Cha-Am District in Phetchaburi Province (Thailand), as a model area that is adversely affected by this problem, was evaluated. The vertical electrical sounding (VES) survey study was conducted at 80 locations, and then 47 VES locations were used to create four pseudo cross-section lines in a west-east direction. The geophysical results were described together with the hydrochemical analysis of 57 groundwater samples revealed that seawater intrusion occurred in the Quarternary colluvial sediment aquifer at an average depth of 50–60 m. A resistivity value of <5 Ωm represented areas that were highly impacted by seawater intrusion, while a range between 5 and 10 Ωm represented moderately contaminated areas. The hydrochemical characteristics, divided the groundwater into three groups according to the level of impact of seawater intrusion: (i) Ca2+-Na+-HCO3- and (ii) Ca2+-HCO3--Cl-; (slightly impacted) and (iii) Na+-Cl- (highly impacted). The hydrochemical facies evolution diagram (HFED) revealed that most of the samples fell close to the mixing line, demonstrating mixing between seawater and fresh water and that some samples fell in the intrusion phase. Multivariate analysis was in agreement with the HFED. Furthermore, the northern part of the study area faced seawater intrusion with a relatively higher impact than other areas.

Integrated geophysical methods and techniques for siting productive boreholes in basement complex terrain of southwestern Nigeria

Electrical resistivity survey involving 2D Dipole-Dipole subsurface imaging and 1D Vertical Electrical Sounding (VES) and an integrated magnetic and electrical resistivity survey involving magnetic profiling and 1D VES/2D electrical imaging techniques were carried out at two sites underlain by grey gneiss and pegmatised schist, respectively, within the Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. The surveys were carried out with a view to delineating the subsurface layers and determining the geoelectrical characteristics; identify geological structures such as faults, fractured, jointed and sheared basement zones that are favourable to groundwater accumulation and transmission and estimate hydrogeologically significant borehole drill depth in an area with previous experience of failed borehole drilling attempts. At the two sites (A&amp;B) investigated, the 2D resistivity images identified suspected fractured basement zones as low resistivity vertical discontinuities within high resistivity fresh basement host rock. The magnetic profiles at site B displayed typical magnetic anomaly of a thick dyke (two magnetic lows with central high in low magnetic latitude) over each vertical resistivity discontinuity. The follow-up VES delineated maximum of five subsurface layers which include the topsoil/laterite, weathered layer, partly weathered/fresh basement, fractured basement and the fresh basement. The weathered/fractured basement constitutes the main aquifer unit with thicknesses varying from 17.7 - 125 m. Two boreholes, one per site, were drilled to 80 m at site A and 70 m at site B. Both boreholes encountered the predicted fractured basement zones and the borehole logs correlated well with the interpretation models of the drilled VES locations. The boreholes were very productive (yields of about 1.5 l/s and 2.2 l/s at sites A &amp; B respectively). The study demonstrated the effectiveness of integrated geophysical methods and techniques in providing information on the subsurface sequence and the structural disposition required for a successful groundwater development through borehole drilling, in a borehole-failure-prone basement complex terrain.Keywords: Integrated Geophysical Methods, Geophysical Techniques, Productive Borehole, Basement Complex Terrain

Site Characterization for Construction Purposes at FUNAAB using Geophysical and Geotechnical Methods

Abstract Geophysical and geotechnical techniques were used to investigate the sub-surface information of a proposed site for a hostel construction at Federal University of Agriculture, Abeokuta. Ten vertical electrical sounding (VES) stations were adopted. Typical sounding curves obtained include the HA, KH, AKH and KQH types, of which the AKH-type consists of 40% of the survey points, and a maximum of five geo-electric sub-surface layers were delineated. Laboratory analyses were performed to investigate particle size distribution, Atterberg limit, compaction limit, California bearing ratio (CBR) and specific gravity. The CBR revealed that all soil samples, except L4, are mechanically stable and have high load-bearing capacity. The Atterberg limit test and the geo-electric section showed that the second layer of VES 4 is composed of sandy clay with high plastic index and low liquid limit, which may pose a threat to the foundation of any engineering structure. VES locations 5, 6 and 8 were identified as high groundwater potential zones suitable for optimum groundwater abstraction. The study area is suitable for both shallow and deep foundations, however VES 4 and VES 5 require reinforcement.

Prediction of transmissivity of aquifer from geoelectric data using artificial neural network

We have used artificial neural network (ANN) for the prediction of transmissivity (T) from geoelectric parameter in an aquifer of a typical basement complex, Southwestern, Nigeria. The study area is composed of migmatite-gneiss, charnockite and granite gneiss. A geophysical investigation involving Vertical Electrical Sounding (VES) was carried out in the study area. Twenty VES were acquired using Campus Ohmegaresistivity meter. VES curves were interpreted quantitatively by using the technique of partial curve matching and with 1D forward modeling by means of computer using WinResistsoftware. The interpreted data were used to determine transverse resistance, TR. Transmissivity were measured in the boreholes drilled at the 20 VES locations. TR and measured T were subjected to ANN analysis using MATLAB 2017a software in order to predict T of the aquifer. Root mean square error (RMSE) was used to test the performances of our model. The results show that TR range from 133.00 -381.20 Ωm2, T also vary from 0.4 to 4.1 m2/day. However, ANN model was able to predict T values with coefficient of correlation (R) values of 0.97, 1.00, 0.99 and 0.94 for training, test, validation and all network models respectively. RMSE value for the ANN model was found to be 0.085 which implies high performance of our model. A linear relationship was suggested for the ANN analysis to predict T. It can therefore be concluded that with ANN model, it is possible to predict T of aquifer in the study area where geoelectric data such as TR is known and T values unknown.

Geo-electrical approach to determine the lithological contact and groundwater quality along the KT boundary of Tamilnadu, India

A geophysical survey involving electrical resistivity method has been carried out at Ariyalur region to delineate the geo-electric characteristics of the subsurface formation and evaluate the groundwater quality variation along the lithological contact. A total of 15 vertical electrical sounding (VES) stations were established along the various geological formation such as 3 in Archean, 7 in Cretaceous, 2 in Tertiary and 3 in Quaternary formation. The data acquisition was carried out by adopting Schlumberger configuration. The layer parameters like apparent resistivity (ρa) and thickness (h) of different layers were arrived. The results of interpretation revealed three distinct geoelectric layers, which is comprised of weathered, fractured and massive layer in hard-rock region and topsoil, sandy layer and clayey layer in sedimentary region. Sounding curves obtained in the area are mostly of A, H, K type. The 1st layer thickness shows a decreasing trend in the following order Tertiary > Alluvium > Achaean. However, for the 2nd layer, the trend is Achaean > Tertiary > Alluvium. The higher 1st layer thickness along the southeastern part of the study area is due to the overexploitation of the groundwater represented by alluvium. Resistivity values and their corresponding depth for some selected VES locations were used in producing three resistivity cross sections (A–A′, B–B′ and C–C′), which shows the geoelectric distribution of the subsurface. The observed high resistivity in a shallow depth in the western part is due to the variations in lithology. The relationship between the resistivity and conductivity are also taken into consideration to identify the shallow contaminated zones of the study area.

HYDROGEOPHYSICAL SURVEY FOR UNDERGROUD WATER IN FUGAR EDO STATE NIGERIA

Twelve (12) vertical electrical sounding (VES) was done using the Schlumberger electrode array configuration to obtain data and the Schlumberger automatic analysis method of interpretation was adopted. The lithological section of the study area revealed that the area is a basement complex with a thick fresh basement beneath the surface. The weathered basement is a good accumulation for underground water due to its low resistivity values. The interpretation of the data showed a resolution of nine geoelectric layers for the VES locations and a total cumulative thicknesses of 203.55m (677.82ft), and 272.29m (906.73ft) and 27l.43m (903.83ft) depth. From the geologic explanation of the subsurface materials, these values indicate a deeper probing into the aquifer zone. This study would give a guide on effective aquifer layer position in the studied locations (Fugar, Etsako Central Local Government of Edo State).

A study on landslides and subsurface piping, facilitated by dykes, using vertical electrical sounding and δO18 and δH2 stable isotopes

A combination of vertical electrical sounding (VES) and δO18 and δH2 stable isotope geochemistry is used in this study to trace out the extension of a dyke and for deciphering the subsurface piping phenomena in a landslide-affected hamlet, Pasukadavu, in the Western Ghats of Kerala, India. VES was successful in extracting three to four different subsurface layers characterized by differing resistivity. Two VES sections were prepared from 24 different VES locations, each one for understanding the dyke extension and for delineating subsurface conduits. The dyke was characterized by high resistivity of 800–5000 Ω.m and shows varying thickness. In the second profile, the void zone, which is characteristic of piping, is delineated through a low resistivity zone (75 to 350 Ω.m). δO18 and δH2 stable isotopes collected along the second VES profile show the same chemistry, indicating that it is the same water which flows all along the VES profile 2. A four-stage conceptual model was developed to illustrate and narrate the sequence of development of the piping phenomena and landslide activity.

Combining geophysical techniques and multi-criteria GIS-based application modeling approach for groundwater potential assessment in southwestern Nigeria

A new approach of modeling very low frequency–electromagnetic (VLF–EM) and vertical electrical sounding (VES) data with a view of evaluating groundwater resources potential via application of GIS-based multi-criteria technique is investigated in this study. On eight VLF–EM traverses established in the site, 40 VES locations were combed. The acquired geophysical data (VLF–EM and VES) were processed applying Fraser/Karous–Hjelt filter and Win-Resist program geophysical software to determine the area subsurface geophysical parameters. Five hydrogeologic maps were produced based on the results of the interpreted geophysical parameters. The produced hydrogeologic maps were assigned suitable weights and different rankings to the individual classes boundary within the maps using the standard Saaty’s scale principle in the context of analytical hierarchy process (AHP) data mining technique. A raster-based empirical GIS model was developed for integrating the hydrogeologic maps to compute the groundwater potential index (GWPI) values in the range of 1.02–2.82 for the study area. Based on the estimated GWPI results, a final map zoning the area into low (0.0930–1.3922), medium (1.3922–1.9109) and high (1.9109–2.8173) groundwater potential classes was produced in GIS environment. The prediction accuracy of the produced potential map was established via cross-validation and in situ well correlation analysis. The results of the study established a new approach of modeling geophysical data for exploring groundwater productivity potential in the study area.

The use of geoelectrics in the study of a shallow quaternary alluvial aquifer in Yenagoa, southern Nigeria

Fifteen (15) electrical surveys (VES) were carried out and results used to delineate the shallow lithostratigraphic units and evaluate the electrical and hydrogeological characteristics of the aquifer zone of the alluvial deposits in Yenagoa metropolis and environs. The Schlumberger electrode configuration was adopted and maximum current electrode halfspacings (AB/2) ranged from 100 to 200m. The VES data were interpreted using 1D inversion technique software (1X1D, Interpex, USA). A maximum of four geoelectric layers were delineated and the aquifer is characterised by loose sand with resistivity ranging from 136.1 – 828.69m and thickness of 8.3-97.6m. The hydraulic conductivity (K = 41.3m/day) value measured in a reference borehole was combined with the normalized electrical conductivity (σ1) from a nearby vertical electrical sounding data. The resulting relation was interpreted with Dar Zarrouk parameters to infer the transmissivity variations at other vertical electrical sounding locations, where K values are unknown. The results showed that the transmissivity is linearly related to the normalized transverse resistance and hydraulic conductivity is also linearly related with normalized aquifer resistivity. Transmissivity values in the aquifer vary from 342.8 to 4030.9 m2/day, and hydraulic conductivity K varies from 12.0 – 88.0m/day throughout the study area.KEYWORDS: Porosity, hydraulic conductivity, transmissivity, transverse resistance, Yenagoa

Estimation of aquifer hydraulic parameters from geoelectrical method—a case study of Yenagoa and environs, Southern Nigeria

Knowledge of water transmitting properties of an aquifer is crucial for successful groundwater development and management practices in an area. In this study, the determination of aquifer hydraulic characteristics at Yenagoa and environs from Dar Zarrouk parameters was carried out using 14 vertical electrical sounding data (VES) and information from existing boreholes. The Schlumberger electrode configuration was employed for the field data acquisition with a maximum half electrode separation (AB/2) ranging between 100 and 350 m. The VES data were interpreted using 1D inversion technique software (1X1D, Interpex, USA). A maximum of four geoelectric layers were delineated and the aquifer is characterized by loose sand with resistivity ranging from 136.1 to 828.6 Ωm and thickness of 8.3–97.6 m. Using the VES results and water resistivity, porosity was determined using modified Archie’s equation. The hydraulic conductivity (K) values obtained from three reference boreholes were combined with the electrical resistivity obtained from the respective nearby VES stations to determine a diagnostic constant (Kρ). The resulting relation was interpreted with Dar Zarrouk parameters to infer the hydraulic conductivity (K) and transmissivity (T) variations at vertical electrical sounding locations, where K values are unknown. The results indicate that porosity varies between 0.09 and 0.32. The hydraulic conductivities range from 0.8 × 10−4 to 6.6 × 10−4 m/s while the transmissivity values vary between 1.1 × 10−3 and 29.6 × 10−3 m2/s. The high transmissivity values are consistent with the aquifer being unconsolidated with fine–medium–coarse sands.

Compositional Trend in the Specks of Tantalite, Tourmaline and Beryl Hosted within Complex Basement Rocks Using Geophysical and Geochemical Methods of Exploration

Vertical Electrical Sounding (VES) using Schlumberger array was employed in data collection. Data acquisition was carried out along Oyan river bank with a total of twenty-three (23) soundings. This location was divided into two zones: VES 01 -09 and VES 10 -23. Rock samples hosting specks of tantalite, tourmaline and beryl were collected from the artisan pits within the study area and taken for geochemical analysis in the laboratory. The result of geochemical analyses revealed the quality of the solid minerals in terms of mineral compositions (major, trace and rare earth elements) while geophysical field measurement revealed the resistivity values, vertical and lateral distributions and depth of the solid minerals. The range of electrical resistivity values of these solid minerals as revealed by laboratory measurement is 19.4 - 31.1Ωm while that of the host rock is 10.0 - 100.0Ωm as obtained from field measurements. Specks of tantalite, tourmaline and beryl were suspected to be sparsely distributed in other VES locations except at VES02 and 09, where vast deposits were suspected. Two litho-facies changes were observed and this revealed the economic values of these solid minerals. Black tourmalines, beryl and tantalite samples collected as specks within the study area were of low quality while pink tourmaline is of moderate quality on the basis of the chemical constituents contained.

Geoelectric investigation of Owuruwuru Dam site, Ikere Ekiti, Southwestern Nigeria

Geophysical investigation involving the electrical resistivity method has been undertaken along three proposed dam axes trending NNW-SSE (335° WCB) across ENE to WSW flowing Owuruwuru River at Ikere Ekiti Southwestern Nigeria. The study is aimed at evaluating the feasibility of the area for establishing a small earth dam and reservoir. The electrical resistivity method involving the Wenner and Schlumberger arrays for horizontal profiling and Vertical Electrical Sounding (VES) techniques was adopted. The study area is underlain by the Precambrian Basement Complex rocks of Southwestern Nigeria. Three Wenner profilings were undertaken along each axis with electrode separations of 10, 20 and 30 m respectively. Forty-four VES locations were occupied within the study area. The resistivity profiles showed that the northern flank of the stream (right abutment) is characterized by relatively lower resistivity values (77 to 327 I©-m) with respect to those obtained at the central and southern (left abutment) flanks (83 to 1077 I©-m). The resistivity profiles thus present a general morphology of the concealed basement in form of low relief on the northern flank with respect to high relief on the southern flank. Geoelectric sections generated from VES results showed that the dam site is underlain by clayey sand topsoil, sandy clay weathered basement, partially weathered/fractured basement and the presumably fresh bedrock. Materials of the overburden (topsoil + weathered basement) are generally thicker (3.4 to 19.3 m) on the northern flank of the stream channel and thinner (0.9 to 10.3 m) on its southern flank. The fractured bedrock delineated on the southern flank is confined and poses minor threat to the proposed dam. However the thicker overburden beneath the northern flank and thin overburden on the southern flank present incompatible structure that may initiate uneven settlement of the dam embankment thus making the site unsuitable for earth dam construction. Key words: Dam axes, horizontal profiling, vertical electrical sounding, overburden, fractured bedrock.

Estimation of aquifer hydraulic parameters from surface geophysical measurements: a case study of the Upper Cretaceous aquifer, central Sinai, Egypt

The integration of geophysical data with direct hydrogeological measurements can provide a minimally invasive approach to characterize the subsurface at a variety of resolutions and over many spatial scales. The field of hydrogeophysics has attracted much attention during the last two decades. In this domain, the geophysical data inverted to geophysical models are interpreted in terms of the hydrogeology to serve as a basis for the definition of hydraulic models in the areas of interest. The hydraulic conductivity (K) value measured in a reference borehole has been combined with the electrical conductivity obtained from nearby geo-electromagnetic sounding data in the Cenomanian (Upper Cretaceous) aquifer, central Sinai, Egypt. The resulting relation was interpreted with Dar Zarrouk parameters to infer the transmissivity variations at other vertical electrical sounding locations, where K values are unknown. Coincident transient electromagnetic data have been adopted to increase accuracy while interpreting the aquifer geoelectrical properties. The results indicate that the transmissivity values in the aquifer of interest vary from 2,446 to 9,694 m2/day, and K varies from 12.9 to 57.0 m/day throughout the studied area.

The application of geophysics in environmental impact assessment: A case study in Jeddo, Delta State, Nigeria

Geophysical study using the Schlumberger vertical electrical sounding (VES) was conducted with half current electrode spacing ranging from 1-215m. Also five boreholes were drilled to the depths, 15.2-30m close to five of the VES locations for the purpose of comparing the derived geoelectric sections from VES curves with the geologic sections from the boreholes. The results from VES curves showed the presence of clay of thicknesses, 15.2-26.4m at depths 0-4.4m in two VES locations, while sands of thicknesses,12.2-116.1m were exposed in seven VES locations. Also the logs derived from soil samples collected from the boreholes showed clay presence of thicknesses, 15.2-23.9m at depths 0-3m in the two boreholes close to the VES locations where thick clay presence was detected, while three boreholes showed exposed sands of thicknesses, 12.2-30m. The application of geophysics for the purpose of subsurface study in environmental impact assessment has been discussed. Journal of the Nigerian Association of Mathematical Physics Vol. 10 2006: pp. 77-82