Geophysical assessment of subsurface conditions at proposed building sites: implications for foundation failure and building collapse

Building collapse has been a recurrent environmental hazard in Nigeria in the last two decades. This is a corollary of inadequate foundation investigation prior to construction, poor government policies, and general lack of awareness on the importance of geophysical and geotechnical investigations. In this study, geological mapping and detailed geophysical investigation using Electrical Resistivity Imaging (ERI) and Vertical Electrical Sounding (VES) were carried out to understand the suitability of proposed building sites at the main campus of the Olabisi Onabanjo University (OOU), Ago-Iwoye, Nigeria for construction. Both Wenner array and dipole-dipole were used for profiling and Schlumberger for sounding. Four transverses and VES were used in each of the three areas investigated. Our results show that the subsurface of the study areas is underlain by Precambrian basement rock of Nigeria. Rocks in the study area include banded gneiss, porphyroblastic gneiss, biotite-hornblende granite and quartzite schist. The sounding stations across the three areas and 2D resistivity imaging revealed three principal geoelectric layers, the topsoil, the weathered layer and the fractured/fresh basement with varied resistivity values for each layers. At the VES stations, the three geoelectric layers have resistivity values of 62 to 1182 Ωm, 3.2 to 1360Ωm and 87 to 4680 Ωm. On the 2D resistivity imaging profiles, the resistivity of the three layers varies from 2 to 1182 Ωm, 30to 1360 Ωm, and 40 to 2904 Ωm for the topsoil, the weathered basement, and fractured/fresh bedrock. Our work demonstrates that some of the proposed sites are structurally incompetent for engineering or foundation purposes. Excavation of the topsoil and reinforcement are required to sustain the proposed structures

Integration of magnetic residuals,derivates and located euler deconvolution for structural and geologic mapping of parts of the precambrian gneisses of Ago-Iwoye, Southwestern Nigeria

Ground based magnetic survey conducted between longitude 06O 55I 51IIN –06O 55I 54IIN and latitude 03O 52I 06IIE –03O 52I 4.8IIE (Olabisi Onabanjo University) remarkably revealed a consistent subsurface NW -SE structural azimuth of localized discontinuities within the shallowly buried heterogeneous basement rocks, which at exposed locations are composed of strongly foliated granite gneiss and migmatite-gneiss with veins and veinlets principally orientated in NNW –SSE direction.Magnetic survey of the area was preceded by site inspection to avoid metallic objects interferences. Field procedure in the area involved Cartesian gridding, base station establishment, data acquisition at gridded points, and repeated bihourly diurnal checksat the base station. At the processing stage, diurnal variation effect was aptly removed before subjection to Kriging (gridding). The gridded data was then prepared as input for Forward Fourier Filter Transform (FFT), which upon definition and implementation enabled Butterworth filtering of isolated ringing effects, reduction to the equator (RTE) for geomagnetic correction, and the use of Gaussian and Upward Continuation filtering for regional magnetic intensity trend determination. Removal of the regional magnetic intensity (RMI) from the total magnetic intensity (TMI) resulted in the derivation of the residual anomaly. Enhancement filters adopted for better resolution of the residual magnetic gradient include analytical signal (AS), tilt-angle derivative (TDR), vertical derivative deconvolution (VDD), and the first vertical derivatives (FVD).TMI and RMI values range between 32925nT –33050nT and 32935nT –333050nT respectively, while the residual gradient ranges between 15nT/m and10nT/m; AS ranges between 0.28nT/m and4.1nT/m; and TDR ranges from-1.4nT/m to 1.4nT/m. Source depth calculation estimated from power spectrum analysis and Euler deconvolution ranges between 1m and15m. Composite overlay of magnetic maps revealed jointed and faulted zones within the area; exhibiting a NW-SE principal azimuth of Liberian orogenic impress, which are in consistence with the foliation direction of the jagged foliated bedrock with an estimated maximum overburden of about 15m.The structural significance of this area as a prospective hydro-geological centre, and as an undesirable spot for high-rise building has been accurately evaluated from research findings. Application of integrated geophysical approach, complemented by detailed geological studies may furnish greater information about the subsurface structural architecture.Keywords:Gneisses; Ground Magnetic Surveying;RTE;Structural discontinuities;TDR.1INTRODUCTIONStructuralmapping is an integral part of geologic surveys. It involves measurements, analyses, interpretation and recognition of geometrical features (structures) generated by rock deformations [1]. These structures often serve as fountains of environmental challenges or unparalleled opportunities depending on their modesof occurrences, which in most cases are imminently controlled by the dynamic interplay of differential stress distributions within the earth interior. In line with the principle of uniformitarianism, a broad understanding about Earth’s paleo processes and internal workingsare deductible from the various deformation types for diverse applications. Deductible inferences from brittle deformationsinclude the kinematics of crustal blocks, orientation of principal axes of regional and local stresses, and geometry. Deeper insights indeep seated stresses, regional movements and block motions are obtainable from ductile deformations

Integrated geophysical assessment of a municipal waste disposal site for its geological suitability in terms of the underlain material

Dumpsites are major sources of groundwater pollution as a result of leachates that drain out of the decomposed waste. If there are no underlain materials that could serve as a seal to stop the percolation of leachate, it finds its way to the groundwater. A properly designed landfill is expected to have a high leachate curtailment capacity to limit groundwater pollution. A suitable landfill is expected to have a specific thickness of clay which acts as a natural filter. This study aims to determine the subsurface material and the leachate curtailment of Oke–Diya dumpsites. Very low frequency-electromagnetic method was adopted as a reconnaissance survey, after which electrical resistivity and multichannel analysis of surface waves (MASW) methods were carried out. The resistivity values obtained were used to determine the lithological units of the study area while the MASW was employed to determine the seismic wave arrival times which was processed to obtain the shear wave velocities of the subsurface. The rigidity moduli were also obtained from the shear wave velocities, from which the lithological units of the subsurface were inferred. The integrated method appeared to be the ideal tool to characterize the dumpsite and adjudge the leachate curtailment capacity. The methods corroborated each other. Oke-Diya dumpsite, from the results, revealed the study area had low leachate curtailment capacity and should be evacuated

Geophysical evaluation of groundwater potential in part of southwestern Basement Complex terrain of Nigeria

Abstract The geophysical assessment of groundwater in Awa-Ilaporu, near Ago Iwoye southwestern Nigeria was carried out with the aim of delineating probable areas of high groundwater potential. The area falls within the Crystalline Basement Complex of southwestern Nigeria which is predominantly underlain by banded gneiss, granite gneiss and pegmatite. The geophysical investigation involves the very low frequency electromagnetic (VLF-EM) and Vertical Electrical Sounding (VES) methods. The VLF-EM survey was at 10 m interval along eight traverses ranging between 290 and 700 m in length using ABEM WADI VLF-EM unit. The VLF-EM survey was used to delineate areas with conductive/fractured zones. Twenty-three VES surveys were carried out with the use of Campus Ohmega resistivity meter at different location and at locations areas delineated as high conductive areas by VLF-EM survey. The result of VLF-EM survey along its traverse was used in delineating high conductive/fractured zones, it is, however, in agreement with the delineation of the VES survey. The VES results showed 3–4 geoelectric layers inferred as sandy topsoil, sandy clay, clayey and fractured/fresh basement. The combination of these two methods, therefore, helped in resolving the prospecting location for the groundwater yield in the study area