Preliminary geotechnical characterization of a site in southwest Nigeria using integrated electrical and seismic methods

Geophysical investigation using Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT) and Seismic Refraction at a proposed conference center site along Ajibode-Labani road, Ibadan, southwestern Nigeria has been carried out. The investigation aims at characterizing and delineating the subsurface strata to understand the weathered profile at the site. Understanding the weathered profile is essential in determining the suitability of the site for engineering construction of the future conference center. A total of 25 VES and 10 ERT profiles were acquired in a systematic grid pattern using both Schlumberger andWenner configurations with Allied omega terrameter. TheVES data were processed and analyzed using WinResist and the ERT data were inverted using RES2DINV. The data were combined to form a 3-D data set of the site and RES3DINV was used to produce the depth slices. Seismic refraction data were also acquired with an ABEM seismograph and processed using SeisImager and Fajseis software. Seismic data were used in understanding the velocity distribution and thickness. The results of VES, ERT and seismic refraction show good correlation. Four sub-surface layers were delineated: top layer of reworked sand, clayey sand/ lateritic hard pan, clay/ sandy clay and fracture/ fresh basement. The 3-D model permits a pictorial view of the sub-surface in relation to materials that overlie the basement. The thickness of unconsolidated materials to bedrock varies from 2.7 m to 12.2 m which revealed inhomogeneity in weathering under the shallow sub-surface. It is found that the integrated geophysical tool is well suited to characterize and delineate sub-surface structure (weathered profile) for engineering site characterization

Statistička analiza obrade podataka u nekim seizmičkim refrakcijskim metodama: Primjer sintetičkih podataka

The delay time method has gained attention in shallow seismic refraction survey because it has the capability to map the lateral thickness of overburden and relief of bedrock. This study addresses the comparison between the performances of the plus-minus and conventional reciprocal methods using a synthetic data. The interpretations obtained from both methods are reasonably comparable to the actual geophysical models. This suggests that either of the methods can be used to construct a geologic section. However, the result of randomized complete block design (RCBD) experiment shows a significant difference in the type of method used and this necessitate the need for further test. The pairwise comparison suggests that the plus-minus method produces a model that better mimics the actual data than the conventional reciprocal method.Metoda vremenskog kašnjenja često se koristi u istraživanju plitke seizmičke refrakcije jer ima mogućnost kartiranja lateralne debljine nadsloja i reljefa stijena. Ovaj rad bavi se usporedbom svojstava plus-minus i konvencionalnih recipročnih metoda korištenjem sintetičkih podataka. Interpretacije dobivene objema metodama razumno su usporedive sa stvarnim geofizičkim modelima. To sugerira da se bilo koja od metoda može koristiti za konstrukciju geološke sekcije. Međutim, rezultat eksperimenta randomiziranog cjelovitog projekta blokiranja (RCBD) pokazuje značajne razlike ovisno o korištenoj metodi, a to nameće potrebu za daljnjim ispitivanjima. Uparena usporedba sugerira da plus-minus metoda daje model koji bolje oponaša stvarne podatke u odnosu na konvencionalnu recipročnu metodu

Comparison of Conventional Schlumberger and Modified Schlumberger Arrays in Estimating Aquifer Parameters in A Typical Basement Complex, Southwestern Nigeria

This study compared the interpretation results of the Vertical Electrical Sounding (VES) data acquired using the conventional Schlumberger and modified Schlumberger arrays with a view to assessing the effectiveness of the modified Schlumberger arrays of VES as alternatives to the conventional Schlumberger array at sites with space constraint during geophysical exploration. A total of thirty (30) VES data for both conventional Schlumberger and modified Schlumberger arrays were collected across different rock units within Federal University Oye-Ekiti campus and Irare estate in Oye-Ekiti metropolis, south-western Nigeria, with electrode spacing

Assessing the suitable electrical resistivity arrays for characterization of basement aquifers using numerical modeling

Numerical modeling analysis was used to assess the suitable electrical resistivity arrays for the characterization of geological structures, including dyke, horst, graben, sub-vertical, and vertical structures. These geological structures usually make up the aquifers interested in the hydrogeological evaluation of crystalline basement terrains. Six electrode configurations, including Wenner alpha (α), Wenner beta (β), Wenner gamma (γ), Schlumberger array, dipole-dipole array, and pole-dipole array, were used to assess the geological structures for groundwater exploration. The synthetic models of the geological structures were generated using RES2DMOD code, and 5% noise was added to all the models. The generated models were inverted using the RES2DINV code. The results show that the most suitable arrays for dyke and graben structures are Wenner alpha, while Wenner beta is the most suitable for the horst structure. The Schlumberger array was the best for both sub-vertical and vertical structures. This study has demonstrated the efficacy of numerical modeling in assessing the best resistivity arrays for 2D electrical resistivity imaging for groundwater exploration prior to geophysical field investigation

Hydrocarbon potential evaluation of Middle Benue Basin, East Central Nigeria using high resolution aeromagnetic data

Relatively cheap and non-intrusive magnetic geophysical technique has been employed to reduce the hydrocarbon exploration uncertainty in the Middle Benue Basin which straddles the northeastern and southeastern parts of Nigeria. Various derivatives and magnetic attributes extracted from the filtered, transformed and enhanced aeromagnetic data of the basin highlighted valuable information employed to appraise the hydrocarbon generating and preservation potentials of the basin, thus delineating the prospective regions where further search light of expensive and intrusive geophysical exploration tools could be focused. The trough delineated from the analyzed aeromagnetic data comprised of a massive 350 km NE-SW trending fan shaped sedimentary body bounded to the northwest and southeast by Basement Complex rocks. Active structural framework with dominant NE-SW fault systems which appeared to be the landward extension of the Chain and Charcot offshore fractures characterize the basement rocks that underlay the sediments. Sedimentary thickness of up to 3.68 km in some parts of the basin obtained from depth weighting analyses, indicate the possibility of hydrocarbon generation through the maturation of organic matter from available source rock(s). Analytic signal grids confirmed the occurrence of already documented volcanic intrusions in addition to newly delineated ones. These volcanics which are often renowned for compromising the hydrocarbon potential of a sedimentary basin are widespread in the western and southern parts the basin. Combination of adequately thick sedimentary units which overly structurally active basement rocks in regions of limited (very few/non) occurrence of volcanic intrusions delineate the north central region for further petroleum exploration prospect evaluation

Interpretation of High Resolution Aeromagnetic Data for Hydrocarbon Exploration in Bornu Basin, Northeastern, Nigeria

Hydrocarbon exploration in Bornu basin, Northeastern Nigeria commenced due to the discovery of gigantic hydrocarbon reserves in extended basins in neighboring countries. This study was carried out to map the Mungono and Marte parts of the Bornu basin for geologic structures that could guide in exploring new hydrocarbon fields using aeromagnetic method. Magnetic Intensity grids and their derivatives were used for mapping these structures while depths of magnetic sources (basement) and corresponding sedimentary thickness were estimated using Euler deconvolution and Source Parameter Imaging (SPI) methods. Anomalies show characteristic high positive polarity in the south in contrast to low magnetic signature of northern domain. Features such as faults, folds and intrusive rock bodies were identified as geological structures that could serve as hydrocarbon entrapment. Lineament analysis shows that fractures generally trend NE-SW following the dominant rift system of Bornu basin and Benue Trough. Three basement depressions with thick sediments (more than 3000 m) were also delineated which should be investigated for further petroleum exploration. The outcome of this study would help in delineating promising areas for detail hydrocarbon prospecting in the area

Seismic interpretation and petrophysical evaluation of SH field, Niger Delta

Abstract Three out of the 14 hydrocarbon-bearing sands (A, B and I) in the ‘SH’ field onshore Niger Delta which contain bulk of the hydrocarbon reserves in the field were considered as development candidates. Seismic interpretation and petrophysical evaluation of logs of 13 wells were integrated with the aim of verifying and ascertaining the hydrocarbon reserves prior to field development which involves enormous financial commitment. Results show that the field is structurally controlled by sets of northwest–southeast-trending synthetic faults which dip southwest. Hydrocarbon traps at the three sand levels are rollover anticlinal closures that are generally sealed by a major listric fault that demarcated the field into northwest and southeast blocks. The southern fault block is hydrocarbon bearing; wells drilled in the field targeted these closures and encountered a number of stacked hydrocarbon-bearing sand levels. Reservoir-A developed a hanging-wall rollover anticlinal structure sealed by a major listric fault forming a trap with oil–water contact (OWC) of 1222 m TVDSS. Reservoir-B also shows similar structure as reservoir-A, but it is partitioned into two hydrocarbon compartments by a sealing fault; these two compartments have different OWCs. Reservoir-I exhibits similar structure to reservoir-A. The evaluation of the petrophysical characteristics revealed that the reservoirs are of good quality with average net to gross, porosities, water saturation and hydrocarbon saturation ranging from 0.774 to 0.980, 0.220–0.339, 0.133–0.367 and 0.633–0.867, respectively. Variation in the petrophysical parameters and the uncertainty in the reservoir structure of the three reservoirs were considered in calculating range of values of gross rock volume and in-place volume. The study shows oil-in-place volume in the range of 243.83–357.90 MMstb in reservoir-I, whereas reservoir-A contains 148.98–241.14 MMstb, reservoir-B1 31.31–50.36 MMstb and reservoir-B2 67.79–108.98 MMstb of oil. Conclusively, this study has further confirmed the high productivity and commercial viability of the wells within the field of study to be able to adequately compensate for the cost of development

Geostatistical modeling of porosity data in ‘oba’ field, onshore Niger Delta

A geostatistical approach was used to model porosity of OBA field in onshore area of Niger Delta using simulation technique. The objective is to understand the spatial distribution of porosity and characterize the degree of heterogeneity of underlying formation. Porosity data from twenty-two wells were loaded into SGeMS software. Univariate statistical analysis, experimental semivariogram and Sequential Gaussian Simulation (SGS) were applied on the data. The data was close to normal approximation of Gaussian based of the results of univariate statistics. However, to construct and model horizontal and vertical semivariograms, the data was log-normalized to reduce the coefficient of variation and to get good fit of the model. Parametric semivariogram model shows the range of 72–6480 m, nugget effect of 0.006 and sills of 0.0095, 0.0099 and 0.0111. Six realizations were generated using SGS algorithm and the results suggest that any one of the realizations can independently represents the true picture of the subsurface geology within the study area. Ranking of realizations shows realization 6 as the best and realization 2 as the lowest. This model could be used as an initial condition for simulation of flow

A Resistivity Survey of Phosphate Nodules in Oshoshun, Southwestern Nigeria

This geophysical study was carried out to determine the occurrence of phosphate nodules in the Oshoshun Formation of the Dahomey Basin, Southwestern Nigeria. The electrical resistivity method, comprising 1D vertical electrical sounding (VES; using Schlumberger array) and 2D geoelectrical imaging (using Wenner array), was used to determine the nature and depth of occurrence of the phosphate nodules. Six profile lines were established within the study area, and inverted sections were generated from the apparent resistivity data using DIPRO inversion algorithm. Five VES points were also acquired in the study area, and Win- Resist programme was used to process and interpret the field resistivity data. Four pits were dug along the profiles to verify the interpreted results. The results obtained by both techniques reveal similar geoelectric units: the top soil, clay, clayey sand and clay at different depths. These layers host pockets of phosphate nodules (78-≥651 Ωm) with varying thicknesses. The strong correlation between the lithology profiles obtained from the pits and the interpreted results of the inverted apparent resistivity sections demonstrates the efficacy of the electrical resistivity method in characterising phosphate occurrence within the formation