Basement Bedrock Research Articles

GEOELECTRIC SIGNATURES FROM A LEACHATE PLUME MAPPED FROM A BASEMENT COMPLEX TERRAIN IN AKURE, NIGERIA

In this study, we investigated the implications of a leachate plume within an aquifer system, unravelling the intricate dynamics that govern contaminant transport and dispersion. An integrated approach of geophysical investigations is used to establish the impact of an open-waste disposal site around Aromed, in a part of Akure, in the Precambrian Basement Complex of Southwestern Nigeria. Investigations using eight (8) Vertical Electrical Sounding (VES) and Double-Dipole Resistivity Tomography (ERT) along two (2) traverses were conducted. The geoelectric interpretation and the inverted two-dimensional electrical resistivity tomography (ERT) images provided insights into the underlying geological composition, identifying three distinct units: the topsoil, the weathered column, and the fractured Basement/fresh Basement bedrock. The geoelectric tomography structures in the 2-D interpretation reveal that the dumpsite area exhibits low apparent resistivity estimated to be between 12 to 71 ohm-m in both the topsoil and the weathered column aquiferous zone. These values closely agree with results from the geoelectric sections, which range between 11 - 68 ohm-m, respectively. The relatively low apparent resistivity results are suggestive to be due to the presence of leachate’s chemical composition from the open waste disposal sites, which are suspected to be generated from the dissolution of ions of iron and other conductive minerals producing the leachate plume. The probable leachate depth of migration as revealed by the 2-D tomography structures varies from about <1 – > 14 m beneath the dumpsites. The leachate migration, which is structurally controlled, has a southward flow, majorly in the orientation with the observed structures. The subsurface depression-relief is also attributed to influence the direction of leachate flow. In conclusion, it is suggested that groundwater in the aquiferous zones in the vicinity of the dumpsites have entered a significant pollution level; hence water in the aquifer is not safe for consumption.

The Great Boundary Fault in the Indo-Gangetic foreland basin: A geomorphological perspective

Identifying active faults in alluvial plains with varying sedimentation rates, erosion, and extensive anthropogenic modifications is challenging. In tectonically active foreland basins, tectonics outweighs other controlling factors and produces a set of key fluvial geomorphic markers that are critical for precisely locating active faults. This study employs several key tectonic fluvial morphometric parameters, seismically induced soft-sediment deformation structures (SSDS) as seismites, and the displaced rocks of the Siwalik mountains to mark the surface positions of a buried basement fault system, namely the Great Boundary Fault (GBF) beneath the alluvial cover of the Indo-Gangetic foreland basin.The Great Boundary Fault (GBF) is a NE-SW trending, oblique-slip, branching reverse basement fault running from peninsular India to the Sub-Himalayas. About 400 km length of its NE segment is buried under thick sediments of the Indo-Gangetic foreland basin. This fault's surfaceward propagation deforms the overlying sediments via a network of subsidiary faults, resulting in 20 km wide broad fault zones. The extreme flatness of the Ganga plain conceals the primary faulting evidence but tectonic fluvial geomorphic signatures, the SSDS, and the laterally displaced Siwalik rocks, including south-verging thrusts at the frontal Himalayas, provide collective evidence of neotectonic activity along both fault branches. The primary objectives of this study are to characterise the GBF's buried segment in the Ganga plain through the geomorphic signatures and SSDS, to elucidate its extension up to the Main Boundary Thrust (MBT), and to understand the surfaceward propagation mechanism of its reverse faults through the alluvial-fills.Tectonic geomorphic parameters such as sinuosity spectra and knickpoints were analysed using ArcGIS and Matlab. Meander scar mapping was carried out using Google Earth Pro and Landsat imagery within ArcGIS to characterise the pattern of avulsions and adjustments of the floodplain linked to the tilting directions of the fault blocks.Fourteen prominent knickpoints of 4 to ≥6 m height were derived from the river profiles across the fault zone. Rivers on the up-thrown block possessed low sinuosity (as low as 1.1), whilst those on the down-thrown block had higher sinuosities (up to 3.1), indicating active uplift and rapid subsidence. The deflection of multiple river courses (Ganga, Ramganga, Burhiganga, Sot, Pangaili, and Deoha) and the presence of asymmetrical meander-belts demonstrate an eastward tilting of the tectonic block. The occurrence of liquefaction-induced SSDS close to the GBF corroborates its recent seismogenic nature. Field observation suggests these SSDS are likely related to the Usawan paleo-earthquake (estimated magnitude of 6.5–7.2), epicentered near the GBF. Based on the geometry and the venting characteristics, a new type of SSDS has been recognized, and the term “liquefaction vents” has been proposed for it. Their association with the other types of SSDS that are characteristically formed by >6.5 magnitude earthquakes suggests it is the minimum magnitude of earthquake required for the formation of liquefaction vents. The regional seismic record suggests a significant number of both large and moderate earthquakes around this fault.Finally, we synthesise the results to propose a growth model for the propagation of reverse faulting from the underlying basement bedrock through to the alluvial sediments and the surface of the Ganga plain.

The influence of multiple impedance contrasts on mHVSR site period estimates in the Canterbury Plains of New Zealand and implications for site classification

An experimental study of the fundamental period of vibration of geologic strata above multiple impedance contrasts is presented for the Canterbury Plains, a deep, complex sedimentary basin on the South Island of New Zealand. Microtremor Horizontal-to-Vertical Spectral Ratio (mHVSR) analysis was used at 152 locations to investigate the fundamental period of the strata above three significant impedance contrasts in the region: the Riccarton Gravel, the Banks Peninsula volcanic (BPV) rock, and the deep graywacke basement bedrock. Up to three mHVSR peaks were observed in many of the mHVSR curves, with the long-period (2–7 s) peaks controlled by the BPV and/or the underlying bedrock. As the thickness of the BPV increases, the response of the geologic strata above the basement bedrock was masked by the shallower BPV, effectively changing the definition of what constitutes the reference condition (i.e. engineering bedrock) for site response analyses. The mHVSR peaks matched the long-period amplification peaks observed in the response spectra calculated from ground motions recorded during the Canterbury Earthquake Sequence events, underscoring the importance of the identification and consideration of deep impedance contrasts in site classification.

Mapping and Delineation of Hard Rock Aquifers in Parts of the South-western Nigeria Basement Complex Using Integrated Geophysical Techniques

Abstract: An integrated geophysical survey involving three different techniques; namely, VLF-EM, Seismic Refraction and Vertical Electrical Sounding (VES) has been carried out in parts of Eruwa, Ibarapa East LGA, Oyo State with a view of determining the structural characteristics of the subsurface geology and identifying suitable sites where viable borehole could be sited to reduce the problem associated with acute shortage of groundwater in the area. The area lies within the crystalline basement rock terrain of South-Western Nigeria and bounded by Latitude 7° 32' 59"N and Longitude 3° 25' 59"E. The VES results reveal that the area is made up of four major lithologic layers with the topmost layer being majorly laterites, the second layer being sandy clay, the third is weathered basement, while the fourth layer is fractured basement. The aquifer in the study area is mostly confined with curve responses of HA- and HKH-types. The 2-D pseudosection also showed that the fresh basement is very extensive laterally with undulating topography. The result of VLF-EM revealed high-amplitude anomalies. These anomalies are indicative of high conductive zones within the basement bedrock of the study area. The results of the seismic inversion reveals three lithologic layers; namely, the topmost layer / topsoil which is sandy (moderately loose) with an average velocity of 500 m/s, the middle layer can be sandy clay/laterite/caked clay with an average velocity of 1,149.33 m/s and the third layer can be said to be the beginning of the weathered/fresh basement with a velocity of 2,719 m/s. In conclusion, the three geophysical techniques have shown that the area is a hard-rock terrain with complicated geology with aquifers confined and the water bearing formation with low total longitudinal conductance. Keywords: Basement terrain, Granitic masses, Aquifer, Topography, Curve types.

Assessment of geoelectric properties and localized geology as indicators on cocoa (Theobroma cacao) yield in a part of southwestern Nigeria

The need to establish a sound scientific basis for subsoil studies using a geoelectric method for locating sites best suited for cocoa (Theobroma cacao) production is highly expedient considering recent scientific trends. An integration of geoelectrical and geological investigation was carried out with a view to assessing the relationship between subsoil parameters and cocoa production output on fifteen (15) selected farm-sites across different communities in Ondo State, currently ranking as highest producer of Cocoa in Nigeria. This is with the aim of delineating the subsurface geological sequences, determining their geoelectric parameters, identifying the composition of subsurface layers and soils best related for high yield of cocoa. The studied sites which are located on the Precambrian Basement complex and the Dahomey Basin, southwestern Nigeria, showed from the geoelectric sections a total of between three to five (3–5) subsoil layers comprising the topsoil, the weathered or fractured basement/cretaceous sediments layers, and imfinite depth basement bedrock. The well-drained clayey loam/loamy and clayey sand/sandy clay weathered/partially weathered subsoil groups coincided with high cocoa produce data zones. Granite Gneiss - Grey Gneiss and Charnockites rock groups make up the geology in these areas. While the weathered - partially weathered columns that are characterized as sands and plastic clay subsoils within the Schist/Granite Gneiss rock groups coincided with areas where cocoa are reported to have short life span and cocoa produce data is low. We concluded that fairly thick subsoil spectrum from the topsoil to weathered - partially weathered column, formed from the weathered products from rock units including Grey Gneiss - Charnockite rocks, make up the most important underlying lithologic units most suitable for optimal cultivation of cocoa crop in the study area.

Evaluation of Uncertainties in Site Response Analysis of Deep Soil Profiles in South Carolina Coastal Plain

ABSTRACTThe South Carolina Coastal Plain consists of deep soil sediments over basement bedrock. The depth of basement bedrock varies from being present at the surface to a depth of more than 1200 m at the southern tip of the state. A large variation exists in the thickness of the sediment, which impacts the seismic site response analyses of the Coastal Plain, particularly in areas where the availability of deep shear-wave velocity profiles is limited. This study evaluates the impact of variations in the shear-wave velocity profiles for two sites in the South Carolina Coastal Plain. The shear-wave velocity profiles were measured using different geophysical methods, including a combined multichannel analysis of surface waves and microtremor array measurement (MASW-MAM) method and P–S suspension logging. The equivalent-linear site response analyses were conducted by applying a synthetic earthquake motion at the depth of the B–C boundary (a depth of competent rock in which the shear-wave velocity is 760 m/s). The results are presented in terms of the amplification factor and its standard deviation. Results show that the average shear-wave velocity at the first 30 m (VS30), the shear-wave velocity contrast at the interface of the base layer and the B–C boundary, and the depth to the B–C boundary have a significant impact on the amplification factor and its variability, particularly for the amplification factor at periods higher than 0.1 s. The MASW-MAM method provided significantly lower VS30 values than the P–S suspension logging method at one of the two sites. Consequently, an additional peak in the amplification factor was observed for the site that had a low VS30, and the corresponding period was close to the resonant period of the loose, surface deposit.

Geophysical assessment of open dumpsite nearby Khamis Mushait industrial zone, Southwestern Saudi Arabia

Vertical electrical soundings, seismic refraction tomography, and ground magnetic measurements were taken or conducted close to an open dumpsite near Khamis Mushait, southwestern Saudi Arabia, which is characterized by shallow groundwater conditions. Structurally, intersected series of north- to northwest-trending faults affect the area. A new urban expansion and the Khamis Mushait industrial zone are close to the open dumpsite. This study evaluates the potential for leachate intrusion into groundwater aquifers. The results of electrical soundings reveal four geoelectric units: the topsoil, weathered layer, partly weathered/fractured basement, and fresh basement bedrock. At the dumpsite, relatively low resistivity values that are less than 30 Ωm characterize the topsoil, weathered layer, and partly weathered/fractured basement because of conductive leachate from the dumpsite. Besides, the results of ground magnetic data indicate that the deformation of varying intensities affects the dumpsite area, resulting in fractures developing in the N–S, NE–SW, NW–SE, and, to a lesser extent, E–W directions. Moreover, geoseismic tomographic models illustrate a low P-wave velocity zone corresponding to a low resistivity zone, and a NE–SW fault was identified from geoseismic cross-sections. Finally, these fractures and faults could become contaminant pathways for shallow aquifers around the dumpsite. To minimize the impact of such leachate, from a dumpsite, on groundwater quality and the environment in general, the dumpsite must be properly designed, constructed, and managed using engineering principles.

Delineation of Groundwater Potential Using GIS, Hydrogeological, Geophysical and Analytical Hierarchy Process (AHP) Technique in Olorunda-Abaa, Ibadan, Southwest Nigeria

Integration Geographic Information System (GIS), hydrogeological, geoelectrical method involving Vertical Electrical Sounding (VES) technique, coupled with the Multi-Criteria Decision Analysis (MCDA) data mining technique were utilized with the aim to delineate the groundwater potential zones of Olorunda-Abaa area, Ibadan, southwestern Nigeria. Fifty-three (53) Vertical Electrical Sounding (VES) measurements using Schlumberger electrode array, depth to Water Level (DWL) estimation and determination of depth to the bottom of Hand dug wells were carried out across the study area. Multi-criteria Decision Analysis (MCDA) using Analytical Hierarchy Process (AHP) technique was applied to the factors controlling groundwater accumulation in the area. Weights were assigned and subsequently integrated in the ArcGIS environment using Arc Map 10.1 to develop the groundwater potential map of the investigated area. The geoelectric sections developed delineated four subsurface geological units consisting of the topsoil, weathered layer, partly weathered/fractured basement and the fresh bedrock. The VES gave depths to basement bedrock which generally range from 5.6 - 59.4 m. The groundwater conceptual model developed delineated five groundwater potential zones classified as low, medium, high and very high and validated with the thickness of water column obtained from wells over the entire study area. The groundwater potential map generated for the study area show that the medium to very high groundwater potential zones indicates the favourable area where groundwater development is feasible in the study area. This study concludes that the characterization of the groundwater potential zones in the study area can be adopted for future allocation of social amenities, planning, location, development and management of groundwater resources. Keywords: Hydrogeology, Geophysical, Multi-criteria Decision Analysis, Groundwater Potential, Basement Complex, Ibadan. DOI: 10.7176/JEES/11-5-08 Publication date: May 31 st 2021

Subsurface Investigation of Basement Structures Using Electrical Resistivity Methods at Zainawa Village Kano, Nigeria

Electrical Resistivity Methods involving Schlumberger Vertical Electrical Sounding (VES) and Wenner Electrical Profiling (EP) were carried out to map the Geological features of the earth subsurface in Zainawa Area of Kano State, Nigeria. Five profiles were established; consist of six (6) VES points at each profile. GEOPULSE resistivity meter (SAS 300) was used for the data acquisition. The field data obtained have been analyzed using computer software (IPI2win) which gives an automatic interpretation of the apparent resistivity. A maximum of three geoelectric subsurface layers were delineated from the VES master curves. The geoelectric section beneath the study area was composed of top soil (clayey-sandy and sandy-lateritic), weathered layer, partly weathered (fractured basement) and fresh basement. The resistivity value for the topsoil layer varies from 20 Ωm to 600 Ωm with thickness ranging from 0.5 to 7.2 m. The weathered basement has resistivity values ranging from 15 Ωm to 593 Ωm and thickness of between 2.75 to 33.04 m. The fractured basement has resistivity values ranging from 201 Ωm to 835 Ωm and thickness of between 11 to 20.4 m. The fresh basement (bedrock) has resistivity values ranging from 1161 Ωm to 3115 Ωm with infinite depth. The depth to basement map was produced to give a good picture of the basement topography within the study area. The depth to basement ranges from 11 m around VES 01 to 85 m around VES 25 m. The map also reveals linear structures (VES 05, 21, 22 and VES 23) which trends in the NE-SW direction. These structures suggest a basement depression at these points. However, the depth from the topsoil to the bedrock surface varies between 2.5 to 37.75 m.

A systematic analysis of basin effects on surface ground motion

“Basin effects” refers to trapped and reverberating earthquake waves in soft sedimentary deposits overlying convex depressions of the basement bedrock, which significantly alter frequency content, amplitude, and duration of seismic waves. This has played an important role on shaking duration and intensity in past earthquakes such as the Mw 8.0 1985 Michoa'can, Mexico, Mw 6.9 1995 Kobe, Japan, and Mw 7.8 2015 Gorkha, Nepal. Although the phenomenon has been understood and addressed in the literature, it has not been fully incorporated in seismic hazard analysis, and Ground Motion Prediction Equations (GMPE). In this study, we perform an extensive parametric study on the characteristics of surface ground motion associated with basin effects using finite element simulations. We use an elastic medium subjected to vertically propagating SV plane waves and utilize idealized basin shapes to examine the effects of basin geometry and material properties. We specifically study the effects of four dimensionless parameters, the width-to-depth (aspect) ratio, the rock-to-soil material contrast, a dimensionless frequency that quantifies the depth of the basin relative to the dominant incident wavelength, and a dimensionless distance quantifying distance of the basin edges relative to the dominant wavelength. Our results show that basin effects can be reasonably characterized using at least three independent parameters, each of which can significantly alter the resultant ground motion. To demonstrate the application of dimensional analysis applied here, we investigate the response of the Kathmandu Valley during the 2015 Mw 7.8 Gorkha Earthquake in Nepal using an idealized basin geometry and soil properties. Our results show that a simplified model can capture notable characteristics of the ground motion associated with basin effects which suggests that such studies can provide useful insights, relevant to the parameterization of basin effects in GMPEs and design code provisions.

An integrated geophysical approach for post foundation studies at Edunabon town, southwestern Nigeria

An integration of geophysical methods including electrical resistivity and seismic refraction was used to investigate the possible causes of the cracks observed on some buildings at Edunabon, southwestern Nigeria. A total of three 2D electrical resistivity imaging (ERI) traverses and eight vertical electrical sounding (VES) were acquired using dipole–dipole and Schlumberger electrode configurations, respectively. The 2D ERI and VES data were processed and inverted to obtain the subsurface models. Seismic refraction data were also acquired along eight traverses using a 24-channel ABEM seismograph with geophone spread of 2 m, and integrated with electrical resistivity data, to adequately delineate the geomaterials responsible for the localized structural defects in the area. The results of electrical resistivity method revealed four geoelectric layers including topsoil, clayey material, weathered basement and fractured/fresh basement. However, the geosection models of seismic refraction revealed two geophysical layers (overburden and basement bedrock) with characteristics that corroborate the electrical resistivity results. The overburden has an average velocity and thickness of 405 m/s and 7.1 m, respectively, which is diagnostic of weak/unconsolidated materials, presumably clayey material. The integrated results revealed that the observed cracks on the buildings may be due to the foundation soil that is composed of incompetent materials (clayey materials) which may compress under differential effective stress. This study shows that the integration of geophysical methods can identify the causes of the differential subsidence of engineering structures.

Evaluation of Aquifer Protective Capacity and Groundwater Potential in Part of Iju, Akure-North, Ondo State, Nigeria

Groundwater potential and aquifer protective capacity of the overburden unit was evaluated in part of Iju, Akure North, Ondo State using integrated geophysical methods involving Very Low Frequency Electromagnetic (VLF-EM) profiling and Vertical Electrical Sounding (VES). Four major traverses were established of varying length extents. The VLF-EM measurements were taken along the four major profiles of 10 m station interval.Forty two (42) Vertical Electrical Soundings were also conducted with half electrode spacing varying between 1 and 100 m and interpretation was done using the partial curve matching techniques and computer aided iteration. Five subsurface geological units were identified from geoelectric sections, consisting of the top soil, lateritic, weathered, partly weathered and fresh basement layers consecutively. For the first layer, resistivityranges between 23 and 323 Ωm with values of thickness ranging between 0.5 and 2.2 m. The resistivity and thickness of the second (lateritic) layer range from 132 to 430 Ωm and 1.6 to 4 m respectively. The resistivity of the weathered layer ranges from 4 to 94 Ωm and variable thickness between 10 and 24.4 m. The fourth layer has a resistivity value range of 65 to 120 Ωm and thicknesses between 20 and 30 m. The basement bedrock (fifth layer) has resistivity values between 770 and 820 Ωm. The depth to bedrock ranges from 1.8 to 31 m. The geophysical data and the basement aquifer delineated were then used to evaluate the hydrogeological setting and aquifer protective capacity of the study area. The observation from the results shows that close to 70 % of the study area falls within the zones of low groundwater potential, 25 % falls within medium potential zones while only 5 % make up the high potential zones. 75 % of the study area constituted the weak to poor protective capacity zones.

Gold in Irish Coal: Palaeo-Concentration from Metalliferous Groundwaters

Gold grains, up to 40 μm in size and containing variable percentages of admixed platinum, have been identified in coals from the Leinster Coalfield, Castlecomer, SE Ireland, for the first time. Gold mineralisation occurs in sideritic nodules in coals and in association with pyrite and anomalous selenium content. Mineralisation here may have reflected very high heat flow in foreland basins north of the emerging Variscan orogenic front, responsible for gold occurrence in the South Wales Coalfield. At Castlecomer, gold (–platinum) is attributed to precipitation with replacive pyrite and selenium from groundwaters at redox interfaces, such as siderite nodules. Pyrite in the cores of the nodules indicates fluid ingress. The underlying Caledonian basement bedrock is mineralised by gold, and thus likely provided a source for gold. The combination of the gold occurrences in coal in Castlecomer and in South Wales, proximal to the Variscan orogenic front, suggests that these coals along the front could comprise an exploration target for low-temperature concentrations of precious metals.

Geohazard characterization of subsurface materials using integrated geophysical methods for post foundation studies: a case study

Different geophysical approaches including seismic refraction, electromagnetic resistivity with very low frequency and magnetics prospecting method were used to gain a richer and more complete understanding of subsurface environment in delineating the origin of the cracks on the buildings of the Faculty of Social Sciences and Humanities, Federal University Oye- Ekiti. Three (3) profiles of seismic refraction, Five (5) Very Low Frequency-Electromagnetic traverses, twenty-one (21) Vertical Electrical Soundings and three (3) magnetic profiles were occupied and investigated around the distress building. Three major layers were delineated in the study region comprising the topsoil, the weathered/fractured layer and the fresh basement. The VES curve types obtained were A, H, K and Q curves. The buildings were found to be situated within the area of a fairly high concentration of fractures that can aid subsidence in the area and relatively low resistivity zones typical of incompetent clay formation. The geovelocity layers’ characteristics substantiate the electrical resistivity imaging and vertical electrical sounding results as it revealed a 3-layer model namely: topsoil and basement bedrock as the weathered layer is blinded. The topsoil has average velocity 363 m/s which is diagnostic of weak/unconsolidated materials presumably clayey materials and average thickness of 1.7 m. The sub-weathering/ is blinded while the third layer is the bedrock with average velocity of 1183 m/s and characterized with a displaced parallel time segment indicating fault within the bedrock. From the result, the building failure observe as cracks and foundation subsidence may have been caused by the foundation soils that made up of incompetent materials (clay) which could compress on imposing loads by differential settlement. It is noteworthy that geological features such as fractures/faults delineated within the bedrock is also inimical to the building foundation.

Subsoil Characterization for Foundation Stability Using Geophysical and Geotechnical Methods

An Integrated geophysical and geotechnical subsoil characterization of parts of the new stadium complex, Akure, southwestern Nigeria had been carried out with a view to characterize the subsoil materials for stability of proposed engineering foundation. Five (5) magnetic and three (3) dipole-dipole profiling were carried out at station separation of 10 m along the traverses. Twenty one (21) Vertical Electrical Sounding (VES) measurements were made at selected locations along the traverses. A total of five (5) soil samples were collected from a dug pit at a depth of 1.0 m for laboratory analysis. The magnetic profiles delineated characteristic one to two negative peak amplitude anomalies that are typical of thin dipping dyke models (suspected to be fractures, shear zones, faults or geological contacts) along Traverses 1-5. The 2-D geomagnetic section delineated overburden thicknesses ranging from 5 – 15 m along the Traverses. The subsurface geologic units delineated by the geoelectric sections and the 2-D resistivity images consisting of the topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. Part of the topsoil and the weathered layer are characterized by low resistivity values suggestive of the presence of clay and or weak geomaterials at a depth range of 1 – 4 m within which civil engineering foundation are usually placed. These zones and the fractured/fault zones delineated by the magnetic methods constitute weak geomaterials that are considered to be inimical to civil engineering structures within the study area. Results of geotechnical analysis of soil samples adjudged plasticity indices of samples A, C and D to be of high index and were characterized by high plasticity/compressibility and consequently of low engineering competence. Samples B and E were classified as low-medium plasticity/compressibility, and are rated moderate–high engineering competence. However, it was observed that soil with higher liquid limits or plasticity index have lower electrical resistivity values and hence were adjudged low in engineering foundation competence. Based on the analysis of the results obtained from this study, the engineering foundation suitability of the soil were generally classified as good (B and E), fair (D) and poor (A and C). It can therefore be concluded that the subsoil in the investigated area are generally of low to high civil engineering competence. Keywords: Geophysical; Geotechnical; Subsoil; Competence; Characterization; Foundation, Stability. DOI: 10.7176/JEES/10-6-08 Publication date: June 30 th 2020

Integrated geophysical investigation of aquifer and its groundwater potential in phases 1 and 2, Federal University Oye-Ekiti, south-western basement complex of Nigeria

Electrical resistivity method is frequently used in groundwater studies. It is one of the many geophysical methods used in finding, locating, and defining subsurface water. The geoelectrical survey was done in Federal University Oye-Ekiti, phases 1 and 2, south-western Nigeria, to assess the groundwater potential. Fifteen very low frequency electromagnetic traverses, three dipole–dipole profiles and three Wenner profiles and 25 vertical electrical soundings were occupied and investigated with maximum current electrode, were arranged with an equal potential spacing (AB/2) of 1–100 m with the aid of ABEM SAS 300C resistivity meter and presented as VES curves. Twenty-five VES location were evenly distributed within the study area. The VES interpretation results are of different VES curves types, namely QA, HA, KH, AH, H, KHK, KQ and AK types. The geoelectric sections delineated three to four geoelectric layers, which are the topsoil with resistivities and thickness values ranging between 113.4 Ωm and 753.1 Ωm and 0.4 and 4.4 m, respectively. The weathered layer has resistivity and thickness values ranging between 37.4 Ωm and 278.1 Ωm and 1.3 and 9.8 m, respectively. The partially weathered layer resistivity and thickness values ranging 495.1–651.1 Ωm and 10.2–22.2 m; for lateritic zones, it shows the resistivity and thickness values ranging 280.6–730 Ωm and 0.1–6.3 m. The geosections show a relatively thin overburden of less than 2.1 m. The third layer is the fractured basement. The fresh basement has a resistivity ranging between 930.5 and 4660 Ωm with depth starting from about 10 m to infinity (∞). Based on relatively thin overburden and the moderate to high fracture density of the basement bedrock, it was concluded that the groundwater potential of the study area is generally low to moderate. However, the aquifer in the north-eastern and south-eastern parts of the study area was determined to be of good quality. It can yield reasonable quantities of water that can serve the university community.

Geoelectrical Investigation of Subsurface Structures for Mapping Groundwater Potential of Joseph Ayo Babalola University Campus Environment, Ikeji Arakeji, Osun State, Southwestern Nigeria

Owing to fast increase in number of staff and students of Joseph Ayo Babalola University, Ikeji Arakeji, Southwestern Nigeria, it is therefore very important to carry out this research in order to recommend the actual locations where boreholes can be sunk for good potential yields of groundwater when the time comes. The aim of this research is to carry out vertical electrical sounding geophysical survey at study area with a view to determining good aquifers that are good for the accumulation of groundwater. The study area is located between latitude 0820225 m to 0820345 m (UTM) Northings and longitude 717320 m to 717450 m (UTM) Eastings. Schlumberger electrode array was employed for the study using Ohmega Resistivity meter for the data acquisition. The geoelectric survey of the study area comprised of twelve vertical electrical soundings, with maximum current electrode spacing (AB) of 100 m. The modeled curves are mainly KH-type. The geoelectric sections generated from the sounding curves revealed 4 major layers earth models. The topsoil is made up of clay, clayey sand/lateritic sand with resistivity and thicknesses varying from 54.7 – 210.1ohm-m and 0.2 –0.8m respectively. The second layer is the lateritic clay with resistivities and thicknesses varying from 334 – 963ohm-m and 1.5 – 10.8m respectively. The third layer constitutes the clay / sandy fractured quartzite and it serves as the aquifer unit. The resistivity values lie between 71.7 and 498 ohm-m while the thicknesses vary from 2.1 – 76.3 m. The fourth layer is the fractured/presumably fresh basement bedrock with the resistivity varying from 1879 – 13991.8 ohm-m. Areas characterized with fractured basement of low resistivity with appreciable thickness are therefore recommended for the siting of boreholes. It is concluded that the vertical electrical sounding points of the study area are good aquifers for groundwater accumulation.

The basement and tectonism of the river Jama’are floodplain: deduction from DC resistivity data

The analysis of DC resistivity data of part of the River Jama’are Floodplain, Northeastern Nigeria was carried out with a view to mapping the basement and the subsurface structures. One hundred and six Schlumberger Vertical Electrical Sounding (VES) stations located at the corners of a 225 x 225m square grid network were occupied. The interelectrode spacing (AB/2) was varied from 1-225m. Results identified four subsurface geologic layers; topsoil, alluvial sand, Chad Formation/weathered column and basement bedrock as undulating and basal unit at different depths. Four major parallel basement depressions suspected to be regional faults striking approximately NW-SE were delineated. This trend correlated with the general trend of the regional structures in the Chad Basin. The isopach maps of the topsoil and alluvial sand mirrored a more recent NE-SW trend that correlated with the Tibesti-Cameroon trough. However, this recent induced tectonics was adjudged significant to hydrogeology in the area. KEYWORDS : Resistivity, floodplain, basement, sounding, tectonics, hydrogeology

Seismic Refraction and Electrical Resistivity Investigation of Building Failure: A Case Study

Geophysical methods involving electrical resistivity and seismic refraction were adopted to investigate the cause(s) of failure of main library complex of the Olabisi Onabanjo University, Ago-Iwoye, South-Western Nigeria. The geoelectric section generated from geosounding data revealed four geologic layers. The topsoil has resistivity values varying from 128Ωm to 220Ωm and thickness between 0.9m and 2.4m. The second layer is composed of lateritic clay with resistivity values varying from 238Ωm to 410Ωm and thickness between 2.5m and 5m. The third layer is composed of weathered basement with resistivity values ranging from 56Ω-m to 88Ω-m with thickness between 9.1m and 13.5m. The fourth layer is fresh basement with resistivity values between 1110Ωm and 1200Ωm. The depth to the rock head is between 14.1m and 18.6m. The 2-D subsurface imaging revealed that the foundation soil is composed of clayey materials with resistivity values ranging from 4.3Ωm to 80Ωm, between 0.7m to 5m which correlate well with the geoelectric section. The geovelocity layers’ characteristics substantiate the electrical resistivity imaging and vertical electrical sounding results as it revealed a 4-layer model namely: topsoil, laterite and basement bedrock as the weathered layer is blinded. The topsoil has average velocity 486m/s which is diagnostic of weak/unconsolidated materials presumably clayey materials and average thickness of 2.2m. The sub-weathering/ laterite has average velocity of 1506m/s and average thickness of 7m. The third layer is the bedrock with average velocity of 2292m/s and characterized with a displaced parallel time segment indicating fault within the bedrock. Geotechnical analysis of the subsoil sample revealed that the soils have plasticity index and strength values that are typical of clay. From the result, the building failure observed as cracks and foundation subsidence may have been caused by incompetent foundation soils. It is noteworthy that such faulted zone delineated within the bedrock is also inimical to the building foundation.

Application of Geophysics in Post Foundation Study: A Case Study of the Faculty of Social Science Building, Phase I, Federal University, Oye-Ekiti, Southwestern, Nigeria

Post foundation study of the causes of distress on the Faculty of Social Science building, Phase I, Federal University, Oye-Ekiti was carried out using geophysical approach. The geophysical methods used were the magnetic and electrical resistivity. The magnetic measurements were taking at an interval of 5 m along the established three traverses. The magnetic data were presented as profiles and interpreted using Euler deconvolution software. The Vertical Electrical Sounding data were quantitatively interpreted using partial curve matching technique and 1-D forward modelling with Win Resist software. The dipole-dipole data were inverted using DIPPROTM software. The magnetic profiles delineated two subsurface layers; the overburden and the Basement bedrock. Series of geological linear structures were delineated by the ground magnetic. The VES results reveal three subsurface geoelectric layers. These include; the topsoil with resistivity ranging from 20-290 Ωm and thickness ranging from 0.4-2.5 m; the weathered layer with resistivity ranging from 2.6-122Ωm and thickness ranging from 0.9-23 m; and the Basement bedrock with resistivity ranging from 83-∞Ωm. The low resistivity values of the Basement bedrock in places indicate an intense fractured basement rock. The 2-D resistivity structure also delineated three geoelectric sequences; the topsoil, the weathered layer and the Basement rock which correlate with the geoelectric sections. Geological linear features were also delineated by the 2-D resistivity imaging which was correlated with the magnetic results. The depth on which the foundation of the investigated building was hosted is not known but it is suspected to have been hosted within the weathered layer. This layer predominantly composed of clayey formation which are of high swelling potential and may be subjected to subsidence under heavy load. The causes of distress on the investigated building may have being precipitated by the clayed nature of the layer where the structure is founded and the presence of linear structure suspected to be fractured or fault within the basement.