Geophysical Methods Research Articles

Multi-parameters approach to assessment of soil liquefaction vulnerability in wetland areas of Lagos, Southwestern, Nigeria

With respect to geology, most coastal terrains are underlain by problematic soils, some of which are liquefiable in nature and may cause sudden failure of engineering infrastructures. Against this background, this study was carried out to investigate the subsurface geology of some Lagos coastal areas and their engineering implications using geophysical and geotechnical methods. To achieve this purpose, the Multichannel Analysis of Surface Waves, Cone Penetration Test, and Standard Penetration Test were deployed. Surface waves measurements were collected using a 24-channel seismograph to which 4.5 Hz twenty-four vertical geophones were connected via the takeouts of the two cable reels. CPT soundings were carried out with a 10-tons motorized cone penetrometer and boring with SPT were carried out as well. The results of the Multichannel Analysis of Surface Waves measurements showed that the shear waves velocity (Vs) ranges from 160 to 470 m/s. The very loose to loose sand delineated have Vs in the range from 170 to 250 m/s. The tip resistance and sleeve resistance values spanned between 4.0 and 72.0 kg/cm2 and 6.0–94 kg/cm2 respectively. The thickness of the liquefiable sands in the study area varied between 2.5 and 18.0 m. At Ikoyi site, owing to the prevalence of loose silty sand, corroborated by the available borehole data and the Liquefaction Potential Index, it is classified as having a high-risk liquefaction and could be responsible for the periodic damages to structural infrastructures such as roads and buildings. The sediments mapped at Okun-Ajah and Badore sites are mainly saturated loose sands with high likelihood to liquefaction with very-high to high risk severity. The study concludes that the presence of these sediments and other factors that could induce ground motion making the study sites potentially susceptible to liquefaction. Hence, an urgent attention must be given to early monitoring measures to address the trend.

Defining Mesoscale Eddies Boundaries From In‐Situ Data and a Theoretical Framework

AbstractMesoscale eddies play an important role in transporting water properties, enhancing air‐sea interactions, and promoting large‐scale mixing of the ocean. They are generally referred to as “coherent” structures because they are organized, rotating fluid elements that propagate within the ocean and have long lifetimes (months or even years). Eddies have been sampled by sparse in‐situ vertical profiles, but because in‐situ ocean observations are limited, they have been characterized primarily from satellite observations, numerical simulations, or relatively idealized geophysical fluid dynamics methods. However, each of these approaches has its limitations. Many questions about the general structure and “coherence” of ocean eddies remain unanswered. In this study, we investigate the properties of seven mesoscale eddies sampled with relative accuracy during four different field experiments in the Atlantic. Our results suggest that the Ertel Potential Vorticity (EPV) is a suitable parameter to isolate and characterize the eddy cores and their boundaries. The latter appear as regions of finite horizontal extent, characterized by a local extremum of the vertical and horizontal components of the EPV. These are found to be closely related to the presence of a different water mass in the core (relative to the background) and the steepening of the isopycnals due to eddy occurrence and dynamics. Based on these results, we propose a new criterion for defining eddies at the mesoscale. We test our approach using a theoretical framework and explore the possible magnitude of this new criterion, including its upper bound.

3D geophysical image translated into photorealistic virtual outcrop geology using generative adversarial networks

Outcrop analogues play a pivotal role in resolving meter-scale depositional facies heterogeneity of carbonate strata. Two-dimensional outcrops are insufficient to decipher the 3D heterogeneity of carbonate facies. Near-surface geophysical methods, notably ground-penetrating radar (GPR), can be employed to step into 3D and extend the dimensionality of the outcrops to behind the outcrop. However, interpreting geophysical images requires specific geophysical expertise, often unfamiliar to field geologists who are more familiar with the actual rock than the geophysical data. A novel generative adversarial network (GAN) application is presented that constructs a photorealistic 3D virtual outcrop behind-the-outcrop model. The method combines GPR forward modeling with a conditional generative adversarial network (CGAN) and exploits the apparent similarities between outcrop expressions of lithofacies with their radargram counterparts. We exemplified the methodology and applied it to the open-source GPR data acquired from the Late Oxfordian-Early Kimmeridgian Arabian carbonate outcrop. We interpret a 4 km long outcrop photomosaic from a digital outcrop model (DOM) for its lithofacies, populate the DOM with GPR properties, and forward model the synthetic GPR response of these lithofacies. We pair the synthetic GPR with DOM lithofacies and train them using CGAN. Similarly, we pair the DOM lithofacies with outcrop photos and train them using CGAN. We chain the two trained networks and apply them to construct an approximately 2 km long 2D and an approximately 60 m2 3D volume of photorealistic artificial outcrop model. This model operates in a visual medium familiar to outcrop geologists, providing a complementary instrument to visualize and interpret rock formation instead of geophysical signals. This virtual outcrop replicates the visual character of outcrop-scale lithofacies features, such as the intricate bedding contacts and the outline of reef geobodies.

Delineation of Nickel Laterite Deposits in “DCV” Block Southeast Sulawesi Based on Data Analysis of Ground Penetrating Radar (GPR) Method

The need for nickel is very intensive in the development of upstream to downstream industries so that domestic consumption of nickel ore will increase in the coming years, therefore domestic downstream is increasingly being improved. Overcoming the problem of the high demand for nickel in Indonesia can be done by conducting exploration and exploitation to support the downstream of nickel. Exploration of nickel laterite can be accomplished with geophysical methods that aim to obtain subsurface data from nickel laterite deposits. The Ground Penetrating Radar (GPR) geophysical method is one of the non-destructive methods that can be applied to determine the subsurface conditions of nickel laterite deposits based on physical properties in the form of dielectric constants and reflection patterns and amplitude contrasts produced on radargrams. The nickel laterite deposits can be separated into 4 layers based on Ground Penetrating Radar (GPR) data analysis: bedrock, saprolite, limonite, and topsoil. Based on GPR measurements, the average dielectric constant value of topsoil is 6.8 mS/m, the limonite layer is 10.87 mS/m, the saprolite layer is 12.37 mS/m, and bedrock is 7.87 mS/m. It can be seen that the saprolite layer has a high conductivity so that the dielectric constant value is also high, this is influenced by the very high nickel content in this layer. The depth of bedrock in the research area varies from 20 - 40 meters, the thickness of topsoil is dominated in the value range of 3.6 - 5 meters, while the thickness of the laterite layer which is the main target of nickel laterite mining includes saprolite and limonite layers which have varying values of 15 - 40 meters where the distribution of the thickness of this layer is in the eastern and central parts of the research area.

Integrated application of geophysical methods in Earth dam monitoring

Integrated application of geophysical methods in Earth dam monitoring

Subsurface Utility Identification at ITERA Campus Using Multi-frequency Ground Penetrating Radar

Infrastructure development at the Sumatra Institute of Technology is growing rapidly, hence, it requires new utilities installation network such as cables and pipes to supply electricity and clean water. Installing new utility line, it is necessary to secure and know the existence and depth of the previously embedded utility network to avoid damage that could hamper the construction process or cause large losses. Accessing the sub-surface information, geophysical method, such as Ground Penetrating Radar (GPR) can be utilized for identifying buried utility networks, ensuring the safe installation of new utilities and preventing damage to existing embedded utilities. In this study, a GPR device with a frequency range 40 MHz-3.4 GHz comprising 5 tracks was employed. The acquired data underwent processing using GPR Insights software, generating a radargram cross-section that provides information about the subsurface.The results from the radargram reveal a hyperbolic reflection anomaly, signifying the presence of a utility network beneath the surface, suspected to be pipe and cable utilities. The utility network is detected at varying depths of 0.5 m, 0.8 m, 1 m, and 1.2 m at different distances. Furthermore, several anomalies in the form of hyperbolas are observed, suggesting potential utilities due to their continuity with other paths.

An assessment of seismicity and near surface geophysical characteristics of potential solid waste landfill sites in the Eastern Black Sea Region of Türkiye.

This study aimed to assess the suitability of the potential solid waste landfill sites in seven provinces (Samsun, Ordu, Giresun, Trabzon, Gümüşhane, Bayburt and Artvin) in the Eastern Blacksea Region of Türkiye. The earthquake hazard analysis for two major earthquakes which occurred in the region was first carried out. Then, the geophysical methods including seismic refraction tomography (SRT), electrical resistivity tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) were conducted to find out the structural and physical properties of the subsurface which include the layering, soil classification based on VS30 and the groundwater content at 25 locations of 13 in target provinces. The integrated interpretation of whole data sets demonstrates that Işıktepe, Esence, Çamburnu and Kazantaş which are characterized by VP > 1200m/s, VS30 ≥ 400m/s, ρ > 70 Ohm-m, low earthquake hazard and seismicity are more suitable among others. Vezirköprü, Şebinkarahisar, Yenice, Bayburt-Center, Balkaynak and Murgul will be suitable after a geotechnical reclamation due to moderate seismic velocities and electrical resistivity which are 900 < VP ≤ 1200m/s, 200 < VS30 < 400m/s and 10 < ρ ≤ 70 Ohm-m representing stiff and wet soils. In addition, Bafra, Ağalık and Ovacık were considered to be unsuitable due to the presence of thick, water-saturated soft soil and extremely weathered rocks. Finally, this study shows that the joint interpretation of seismicity and geophysical data in potential waste landfill sites, extremely important for the planning and development of a city, can provide the valuable information which will enable to prevent possible deformations, environmental problems and economic losses after waste landfill.

Elimination of seismic characteristics of solid-filled in ultra-deep fractured-vuggy reservoirs

A large number of high-quality fractured-vuggy reservoirs (FVRs) are buried in ultra-deep (6500–9000 m) carbonate layers in the Tarim Basin, represented by the strong “beaded” reflection (SBR) on seismic profiles. This characteristic corresponds to the vast majority of high yield oil and gas wells, but there have also been a number of instances where the reservoir was filled with solid, leading to exploration failure. To avoid drilling failure, it is crucial to accurately identify fake reservoirs using geophysical methods. This paper proposes a novel method to eliminate the seismic characteristics of solid-filled in FVRs. Based on the waveform component decomposition (WCD) data, combined with drilling and logging data, an adaptive component reconstruction (ACR) can be completed to distinguish whether the reservoir is filled with solid or not, achieving the goal of eliminating the seismic characteristics of filling. Firstly, supervised WCD is performed on the target seismic data. Then, the waveform characteristics, which indicate the difference in the FVRs filled with solids, are combined accordingly with the sensitive attribute characteristics (instantaneous amplitude, heterogeneous anomalies, arc length, etc.) of each component data, and the feature templates of filled and non-filled FVRs are defined. Finally, the information entropy method is applied to optimise the component set that can distinguish the solid filled and non-filled FVRs and complete the component reconstruction. Forward modelling shows that this method can effectively distinguish the characteristics of FVRs that have been filled by solids from those that not. This method has achieved favourable results in the M block in the Tarim Basin. Successfully removed some or all the characteristic information of solid filled reservoirs confirmed by drilling from seismic data. The effectiveness of the method in identifying solid filled reservoirs has been confirmed by drilling, and the prediction rate of FVRs non-filled with solids has increased from 78% to 95%. This method can be used for reference in blocks with similar geological characteristics.

Construction of Buildings in Soft Clay Deposits: A Case Study in a Public School in Florianópolis/SC, Brazil

The gradual increase in the risks of constructions without structural planning in hillside areas characterizes the expansion of urbanization in Brazil. In this process of territorial occupation, constructions on soft clay soils represent significant structural hazards. This study aimed to evaluate structural instabilities in a public school in Florianópolis (SC, Brazil), using integrated geology, geotechnics, and geophysics data. This is a case study in an area characterized by the urbanization process of the West Side of the Costeira do Pirajubaé Massif. Geologically, the study area is composed of granitic rocks, soft clay deposits, and colluvial soils, in an environment of the upper third to the middle of the slope. For data analysis, geophysical, geotechnical, and geological methods were used. In the results, it was identified upstream colluvium, evidenced by the wide variety of soils (sandy, yellowish silt clay, of medium consistency), as well as gray clay deposits, ranging from very soft to hard, with distinct characteristics of the residual soils of the granitoid downstream, which are normally medium consistency brown to yellowish clays, expected for the B horizon. The results indicate a potential slip surface, which corresponds to a layer of soft clay deposits. Thus, it can be concluded that the observed instabilities, characterized by footing displacement, cracks, and subsidence demonstrate subsoil fragilities related to human interventions and the insufficiency of the drainage system, even when existing.

Improvements in Rock Mass Description for Stope Design by Geophysical and Geochemical Methods

Stope design is an important part of mine planning, and it aims to balance ore recovery, ore dilution, and production costs without compromising the safety aspects. This paper summarizes the main results from the research, which aims to introduce new techniques to describe the ore body and surrounding rock mass at the tunnel face prior to stope excavation. The research comprises a literature review and a survey among mining professionals to assess current stope design practices. The study identifies geotechnical data, software improvements, and integration of design into mine planning as the most critical areas for improvement. The empirical part of the study proposes new techniques for fast data acquisition. The laser-induced breakdown spectrometry (LIBS) technique is developed for measurements at the tunnel face and from core boxes to provide mineralogical and geometallurgical data. Ground-penetrating radar (GPR) studies are conducted to improve discontinuity characterization, and rapid photogrammetric methods are proposed for efficient tunnel geometry characterization. The techniques discussed in this paper already have many industrial applications. This study reveals their potential to be adopted and further developed to serve ore and rock mass characterization for stope design.

Exploring Shear Wave Velocity—NSPT Correlations for Geotechnical Site Characterization: A Review

Shear wave velocity (Vs) is a critical parameter in geophysical investigations, micro-zonation research, and site classification. In instances where conducting direct tests at specific locations is challenging due to equipment unavailability, limited space, or initial instrumentation costs, it becomes essential to estimate Vs directly, using empirical correlations for effective site characterization. The present review paper explores the correlations of Vs with the standard penetration test (SPT) for geotechnical site characterization. Vs, a critical parameter in geotechnical and seismic engineering, is integral to a wide range of projects, including foundation design and seismic hazard assessment. The current paper provides a detailed analysis of the key findings, implications for geotechnical engineering practice, and future research needs in this area. It emphasizes the importance of site-specific calibration, the impact of geological background, depth-dependent behavior, data quality control, and the integration of Vs data with other geophysical methods. The review underlines the continuous monitoring of Vs values due to potential changes over time. Addressing these insights and gaps in research contributes to the accuracy and safety of geotechnical projects, particularly in seismic-prone regions.

Research on the Stability and Water Isolation of Waterproof Coal Pillars between Adjacent Working Faces under the Influence of Water Ponding Goaf—A Case Study

Retaining a waterproof coal pillar is an important measure to defend against water inrush accidents in mining areas and guarantee the safe mining of the next working face. In this paper, the mechanical model of the coal pillar is established and the calculation formula of the waterproof coal pillar width is derived. Then, the development of the water-conducting fracture zone of the overlying rock layer under different coal pillar widths is analyzed using numerical simulation and finally, the integrity of the coal pillar is detected using the geophysical survey method. The main conclusions are as follows: (1) According to the mechanical failure characteristics of the coal pillar, it can be divided into the plastic zone, elastic zone, and water pressure damage zone. The mechanical calculation model for each zone was established, and the formula for calculating the width of the waterproof coal pillar was obtained. (2) Numerical simulation was employed to investigate the development condition of the water conducting fracture zone in the overlying rock strata under the actual width of the waterproof coal pillar; the simulation results indicated that the water conducting fracture zone of two working faces was not connected, which can effectively prevent the accumulation of water in the 2303 goaf. (3) On-site geophysical surveys determined that the influence of water-logged goaf on the coal pillar is between 5 to 15 m; the integrity of the waterproof coal pillar is good, which effectively prevents water accumulation in the previous working face goaf and ensures safe mining in the next working face.

Litho-Lineament Mapping of Rocks in Ila Orangun Area Southwestern Nigeria Using Remote Sensing and Aeromagnetic Data: Implications for Mineral Exploration

Ila Orangun is located north of Okemesi and falls within latitudes 7054l and 8000l N and longitudes 40 53l and 50 00l E respectively. Field studies revealed six lithological units in the study area namely quartzites, granites, granite gneiss, porphyritic granites, amphibolite schist and pegmatites. The aim of the research is to elucidate the geology and structure as well as evaluate the metallic mineral potentials of the study area. An integrated multi-technique study approach was adopted for reconnaissance survey and structural interpretation which involved the use of acquired remotely sensed satellite imageries such as Landsat-8 OLI, Shuttle Radar Topographic Mission, Radar Sentinel-2A and a geophysical method involving the use of aeromagnetic data. Results obtained from interpretation of remote sensing data was used to produce different lineament maps which displayed fractures of varying lengths and trend dominantly in the NE-SW directions and subsidiary fractures also orientate in the NW-SE, E-W and N-S directions. Results from the stereographic projection plots showed that the dominant orientation of foliation planes is in the NE-SW direction with the NE and SE sections of the study area recording the highest cluster of the foliation planes. Hence, the eastern section spotted as the mineralization zone.

Investigation of petrophysical and hydrogeological parameters of the transboundary Nubian Aquifer system using geophysical methods

The recent research aims to investigate the petrophysical and hydrogeological parameters of the Nubian aquifer system (NAS) in Northern Khartoum State, Sudan, using integrated geophysical methods, including surface electrical resistivity and geophysical well-logging. The Nubian aquifer is a transboundary regional aquifer that covers vast areas in Sudan, Egypt, Libya and Chad. The well-logs, including self-potential (SP), natural gamma ray (GR), and long normal resistivity (RS), are integrated with Vertical Electrical Sounding (VES) measurements to delineate the hydrostratigraphical units. As a result, two aquifers are detected. An upper aquifer comprises coarse sand with an average thickness of 50 m and a lower aquifer of sandstone with more than 200 m thickness. For a thorough evaluation of the aquifers, in the first stage, the petrophysical and hydrogeological parameters, including formation factor, total and effective porosity, shale volume, hydraulic conductivity, and transmissivity, are measured solely from geophysical well-logs. In the second step, the results of geophysical well logs are combined with VES and pumping test data to detect the spatial variation of the measured parameters over the study area. As a result, the hydraulic conductivity of the Nubian aquifers ranged from 1.9 to 7.8 m/day, while the transmissivity varied between 120 and 733 m2/day. These results indicated that the potentiality of the Nubian formation is high; however, in some regions, due to the sediment heterogeneity, the aquifers have intermediate to high potential. According to the obtained results, it can be concluded that the Nubian Aquifer in Khartoum state is ideal for groundwater development. This research discovered that geophysical approaches can be used to characterize moderately heterogeneous groundwater systems by comparing the Nubian aquifer with similar aquifer systems that have similar hydrogeological settings. This study emphasized the application of universal principles in extrapolating hydraulic parameters in hydrogeophysical surveys. This approach aims to reduce the costs and efforts associated with traditional hydrogeological approaches.

Application of Integrated Geophysical Methods in Mapping the Location of Basaltic Intrusions in Mambilla Plateau and Environs, Northeast, Nigeria

Integrated geophysical investigation on the Mambilla Plateau and its environs was necessary to map the locations of basaltic intrusions and their patterns of distribution. The Landsat-8 ETM+ false colour composite image (bands 7, 4, 2 in R, G, and B) was used to identify rocks such as younger basalt (yb), porphyritic granite (pg), granite gneiss (Gn), and migmatite gneiss (Mg). Similarly, rocks like granite gneiss (Mg), porphyritic granite (pg), and migmatite gneiss (Mg) were recognised by the Principal Component Analysis of the Landsat 8 ETM+ picture (PC2, PC1, and PC4 in R, G, and B). The lowest and maximum values for the total magnetic intensity map were 33030.343 nT and 33172.792 nT, respectively, along with mean of value 31.50nT to 33068.89nT.The primary magnetic field map showed an average value of 33293.4495 nT, with values ranging from 33228.522nT to 33358.377nT. The crustal magnetic field map resulting from ore bodies or intrusions had an average value of -159.50 nT, with values ranging from -397.669 nT to -80.699 nT. The RTE values, which range from -82.382nT to -392.122nT, indicate regional geological intrusions, fractures, and faults. Values were shown on the magnetic equator field map, which was further upward continued to 15 km, with an average values ranging from -279.141nT to -157.160nT. The pseudo-gravity map of the region showed two low densities, ranging from -0.11 to -0.01 mGal, depending on the location, and three high densities, indicating moderate to intermediate densities, ranging from 0.25 mGal to 0.01 mGal. RTE anomaly data produced from subsurface rocks typically has the greatest horizontal gradient along the object's border. This means that the maximum gradient is directly localised to the map boundaries of the distinct lithologies. By examination of the abrupt changes in rock magnetism in a lateral direction, this led to the determination of contact limits of the basaltic intrusion, faults, and mineralization of the underlying rocks

Application of Geophysical Methods for Investigation of Groundwater Potential in Layunrun Village, Ogun State, Southwestern Nigeria

An investigation of the groundwater potential of the area for tube well construction has been conducted in Layunrun village in a typical basement complex of Ogun State using electromagnetic and electrical resistivity techniques in geophysical exploration. Six (6) electromagnetic profiles were used, and twenty-eight (28) Vertical Electrical Soundings (VES) were performed along some of the profiles' potential regions. The typical partial curve matching utilized to create the VES curves was examined, and the results were used to create themed maps of the region. Within the study area, between four and six geoelectric layers were found. These layers are Topsoil, weathered layer (clay), weathered layer, fractured basement, and fresh basement. According to the findings, a bigger percentage of the land has an overburden thickness ranging from 9 to 17 m, with a maximum overburden of about 23 m. It also showed how well fractured the research area is, with thicknesses of the fractures varying from about 12 m to more than 30 m. The study area has a good chance of accumulating groundwater and, consequently, developing groundwater through digging boreholes. The outcomes also suggested that a community with similar geological settings may use the electromagnetic method in addition to the electrical resistivity method to identify places with high groundwater potentials.

Site-Specific correlation between geotechnical and geoelectrical properties of soil deposits in India

The combined application of geotechnical and geophysical methods provides an effective means of subsurface exploration. The insufficient correlation between geotechnical and geophysical methods prevents the conversion of geophysical data to geotechnical information and vice versa. In the present study, a correlation has been proposed between the geotechnical and geoelectrical properties of two soil deposits in India. Eleven boreholes along with a Standard Penetration Test (SPT) and different laboratory tests have been conducted to obtain the different engineering and physical properties of the soil layers. Twelve Electrical Resistivity Tomography (ERT) profiles have also been conducted to obtain the resistivity profile of the subsurface layers. The variation of the resistivity of the soil layers is analyzed and explained. Results show a strong linear correlation between resistivity and SPT blow counts (N′) of the soil layer for one site while a weak correlation for another site. A strong linear correlation between transverse resistivity (T) and N′ values has been observed for both sites. A comparison with the other existing correlation between resistivity/transverse resistivity and N′-values has also been presented. The range of resistivity values and engineering properties of soil layers are mentioned. The study is useful for the development of similar correlations and provides an approximate estimation of different geotechnical parameters for infrastructure design.

Ambient noise tomography of El Hierro island (Canary Islands)

El Hierro island is one of the most active islands in the Canary Islands from a volcanological point of view. This is the reason why the imaging of the internal crustal structure is of huge importance. The geophysical exploration methods employed on El Hierro Island, such as gravimetry and seismic tomography, allowed obtaining the high-resolution characterization of the crust’s deep part. However, these methods did not yield significant information about the surface and the shallower part of the crust. To gain a deeper insight into the shallow geological structure of El Hierro island, we employed Ambient Noise Tomography to construct a 3D S-wave velocity model. Our investigation revealed the presence of seven significant seismic velocity anomalies, partly identified by previous studies. We identified two high-velocity anomalies located in the eastern and western parts of the island at a depth between 0 and 3 km below sea level (b.s.l.). We interpreted these anomalies as dense intrusive complexes of dikes, possibly linked to the Tanganasoga volcano and the formation of the Tiñor edifice. Additionally, we observed two high-velocity anomalies in the northern and southern parts of the island at a depth between 3 and 4 km b.s.l., which we related to the accumulation of solidified igneous rocks. On the other hand, a low-velocity anomaly was observed in the Golfo valley, between 0 and 0.5 km b.s.l., and we interpreted it as megalandslide deposits. This anomaly was evidenced for the first time in the present study. Finally, two low-velocity anomalies were observed in the southern part of the island at different depths, between 0–0.5 km b.s.l. and 0–2 km b.s.l. These were interpreted as fractures generated by Quaternary volcanism along the SSE Rift. Also, one of them was evidenced for the first time in this study, corresponding to the zone of the fractures produced during the Quaternary volcanism. This study has allowed us to gain a more detailed understanding of the shallow geological structure of the island. Even if most of the anomalies had been evidenced previously, we could observe the existence of two low-velocity zones in the shallow crust that have not been observed before.

Geophysical and geotechnical investigations of the site of a collapsed two-storey building in Modomo, Ile-Ife, Southwestern Nigeria

Geophysical and the geotechnical methods were used to study the site of a collapsed two-storey building in orderto determine the sequence and competence of the subsurface layers and determine the cause of the collapse of the building. The geophysical method employed the electrical resistivity method involving one dimensional (1D) Vertical Electrical Sounding (VES) and the two dimensional (2D) imaging techniques. Three VES stations were occupied using Schlumberger electrode configuration. 2D imaging data were acquired along four traverses using dipole-dipole electrode configuration. The geotechnical method involved Cone Penetration Test (CPT). CPT data was acquired at two points with the aid of 2.5 ton Dutch CPT machine. The results showed four subsurface layers based on the 1D VES results and 2D imaging results namely: topsoil, weathered layer, fractured basement rock and fresh basement rock. The topsoil resistivity varied from 116 Ωm to 191 Ωm and has thickness of about 1.5 m. It was classified as moderately competent. The second layer is weathered rock with resistivity varying between 92 Ωm and 327 Ωm and of about 8 m to 13 m thickness. This layer is also moderately competent. The third layer is fractured basement rock. It underlies the weathered rock and have resistivity varying from 391 Ωm to 405 Ωm. Its thickness varied from about 2 m to 20 m. This layer is competent. The fourth layer is the fresh basement rock having resistivity varying from 1365 Ωm to 12348 Ωm and is highly competent. The subsurface materials in the area are competent and can sustain the foundation of the building. Hence, the study concluded that the building collapse did not result from incompetent subsurface materials. Factors other than incompetent subsoil material that is human factors are believed to be responsible. This study has demonstrated the effectiveness of geophysical and geotechnical investigations in determining the competence of subsurface materials at engineering sites.

ESTIMATION DES TASSEMENTS DIFFÉRENTIELS DES SOLS DE FONDATION DE BÂTIMENT : CONTRIBUTION DES SONDAGES DE RÉSISTIVITÉ ÉLECTRIQUE

To estimate differential settlements of building foundation soils, the site on which the structure is based must be geotechnically surveyed in order to determine the deformation parameters and the thickness of the compressible soil layers in various places. This involves conducting destructive surveys. In this study, the use of geophysical methods (electrical soundings) as a substitute for the conventional methods used was considered. The Schlumberger electrodes array was deployed at the level of a control borehole and at three projected footings. The combination of the results of the electrical soundings with the lithological section of the borehole reveals a significant correlation between the variations of cumulative resistivities and the lithological section of the subsoil. The thickness of the compressible layers at the level of the projected footings is estimated to vary between 5 and 6 m. Œdometric differential settlements have an estimated maximum value of 4.56 cm