Subsurface Geological Features Research Articles

A neural network approach for elucidating fluid leakage along hard-linked normal faults

Increasing displacement and strain accumulation in normal faults can result in the formation of hard-linked structures that are preferred loci for fluid leakage. Three-dimensional seismic data from offshore New Zealand reveals Miocene geological units that were structurally deformed to form several hard-linked fault zones. Fluids are observed to migrate through these breached zones into younger strata. Here, we use an automated approach by designing two different meta-attributes, the Thinned Fault (TFC) and Fluid (FC) Cubes, to capture the detailed geometry of hard-linked fault zones, and of fluid flowing through these same structures. The two meta-attributes are prepared through an amalgamation of different seismic attributes, which are trained based on the interpreter's skills and experience following a supervised scheme of neural learning. The meta-attributes enhance sub-surface geological features and reveal their structural geometries. Faults in the study area are therefore observed to strike to the NE with different geometries, e.g. forming curved shapes (F1 and F2), sigmoid shapes (F3), and Y shapes (F4). Relay ramps between these faults are intensively breached, allowing for important fluid migration through hard-linked structures. We demonstrate that our interpretation approach does not only honour the interpreter's knowledge about key geological processes, but also adds value in revealing the 3D architecture of hard-linked normal faults.

The statistical eigenvector analysis technique (SEAT) for dip data analysis

Abstract Dip data acquired from image logs (i.e. the dip/azimuth of planar features) are extensively used in the oil and gas industry to assist in interpreting subsurface geology. Gradual and/or abrupt variations in dip data with depth may indicate different structural features (e.g. faults, folds, etc.) or changes to sedimentary systems (e.g. unconformity). The statistical eigenvector analysis technique (SEAT) and accompanying computer application are useful in statistically calculating the orientation and geometry of subsurface structures from dip data. Poles to planes on a spherical projection over a given depth interval may reveal cluster or girdle distributions, which can be interactively analyzed using statistical distribution functions on the unit sphere. SEAT provides a mathematical solution for as few as two data points, and has the potential to be applied to dip data at the meter-scale. The orientation and geometry of various subsurface geological features (e.g. folds, fault drag deformation, etc.) can be interactively calculated over a chosen depth interval. SEAT is applied to a dip dataset acquired from a modern, 8-padded electronic borehole image log and a previously studied dipmeter data from the discovery well of the Railroad Gap Field to illustrate its higher accuracy in determining the orientation and geometry of subsurface structures compared to older methods. SEAT allows for a vastly improved interpretation of subsurface geological features when combined with high-resolution borehole image logs and accurate dip data. Improved understanding of subsurface geometries may allow for better business decisions in future exploration and appraisal of existing assets.

Characterization of a Proposed Quarry Site using Multi-Electrode Electrical Resistivity Tomography

This research focuses on the delineation of subsurface basement granitic structures suitable for engineering construction materials for the sitting of quarry industry in the area. The key objective of the study was to locate and delineate the depths of burial to the subsurface granite rock bodies and the regolith thickness overlain the bedrock unit. 14 resistivity profile lines with a surveyed length of 200 m and electrode spacing of 5 m, were carried out with the application of electrical resistivity tomography software, to image the subsurface structural units around this area, utilizing pole-dipole electrode configurations method towards assisting the Engineers in obtaining information on the subsurface geological features in this part of the Peninsula Malaysia. The focus is on characterizing engineering construction materials suitable for sitting the quarry industry, determination of the longitudinal conductance and coefficient of anisotropy of subsurface lithological units that determines the competency of the bedrock underneath the area from the geoelectric parameters obtained through the interpretations of the RES2DINV ERT images. The depth of bedrock unit as delineated from the results ranged from about 5 m to 100 m while the resistivity values recorded was greater than 6000 Ω-m in most of the profiles. Groundwater bearing channels that would serve the factory needs was delineated alongside the granitic rock unit. These results make the subsurface granitic bedrock unit to be adjudged competent and suitable enough as quarry construction materials for sitting the factory in the area.

Land subsidence triggered by groundwater withdrawal under hyper-arid conditions: case study from Central Saudi Arabia

Natural and human-induced groundwater dynamics in hyper-arid aquifers play a crucial role in the evolution of the landscape. The area overlying the Saq Aquifer, in particular the Al-Qassim area within the central part of the Kingdom of Saudi Arabia, has witnessed numerous land deformation (land subsidence and fissures) events throughout the last two decades. An integrated remote sensing and hydrogeologic approach is adapted to identify areas affected by the land deformation and also to better understand the role of human-induced groundwater dynamics in the formation of these deformation features. A fourfold approach was implemented including: (1) conducting field surveys to collect observations and validate the reported deformation features, (2) applying a spatial correlation in a GIS environment for the reported damaged locations together with surface and subsurface geological features and groundwater extraction. (3) extracting the subsidence rates using SBAS radar interferometric technique using ENVISAT data sets, and (4) correlating these subsidence rates spatially and temporally with GRACE mass variations data. The results show that high subsidence rates of − 5 to − 12 mm/year along a northwest–southeast direction coincident with areas witnessing a significant drawdown in the fossil groundwater levels (up to 150 m) and a depletion (−10.1 ± 1.2 mm/year) in GRACE-derived terrestrial water storage. Findings from the present study draw attention to the quick responses of landscapes to human-induced groundwater dynamics under hyper-arid conditions.

Geophysical inversion of geologic structures of Oyo Metropolis, Southwestern Nigeria from airborne magnetic data

High-resolution airborne magnetic data of Oyo Metropolis has been used to map subsurface geological features which in most cases, corresponds to the area of interest for mineral expedition. The airborne magnetic data was first reduced to the magnetic equator (RTE); this is normally done to make the data simpler to interpret. The RTE map was subjected to Fast Fourier Transform to separate the regional and residual magnetic anomalies and estimate their depths. An average depth of 2500 m and 500 m were obtained for the top of regional and residual sources respectively. 3D-Euler deconvolution and tilt derivative interpretation techniques were combined and used on the RTE map, to locate and estimate depths to geologic contacts and pegmatite intrusions attributed to lineaments. The resulting 3D-Euler depth solutions were plotted on the tilt derivative map of the study area. A very good correlation was highlighted, indicating that the methods are good interpretative techniques for mapping magnetic lineaments. The estimated depths obtained from 3D-Euler deconvolution method are in the range of 108.1–544.3 m for structural index 1.0 corresponding to intrusions (dykes), and the result of structural index 0.0 corresponding to geologic contacts are in the range of 5.9–295.2 m. The estimated depth parameters coupled with known geological information, were further used to produce the best image view of the subsurface geological features of the study area, using 2-D profile modeling technique. The result obtained from the 2-D profile model is in good agreement with the results obtained from the 3D-Euler deconvolution and tilt derivative methods, indicating locations and depths of various intrusions (dykes) and geologic contacts. The obtained results from this study will help shed more light into the structural framework of the study area and can help potential explorer target new areas for mineral exploration.

Geophysical Characterization of Abandoned Dumpsite Soil Properties for Pre- Foundation Delineation at Bowen University Campus, Iwo, Nigeria

Geophysical investigation and microstudy of soil properties were carried out within Bowen University, for the characterization of the subsurface pattern in the area using vertical electrical sounding (VES) survey and evaluation of soil properties with a view to mapping subsurface geological features, such as weak and competent zones and to determine the overburden thickness of the area for pre-foundation studies for a proposed high-rise building. Four (4) VES was carried out in the study area employing schlumberger electrode configuration. The data collected were used to curve match and the results used for subsequent interpretation to reveal the depth, thickness and resistivity of the study area. In addition, soil samples were collected from three (3) selected points located at the entrance, centre and the end of the study area at about a depth 0-60 cm. Consequently, the VES results were presented as resist graph, which revealed the range of values for the depth (1.3 – 24.5), thickness (1.3 – 15.4 m) and resistivity (357.4 - 6311.6 ohms), which was later used to generated the geoelectric maps of the study area. It was also observed that the result of the soil properties revealed that all the samples taken from the study area have low bulk density (1.41 g/cm 3 , 1.26 g/cm 3 , 1.36 g/cm 3 ), high particle density (2.81 g/cm 3 , 2.94 g/cm 3 , 3.16 g/cm 3 ), and high porosity (49.64%, 57.22%, 57.03%). In conclusion, it was observed that erection of high-rise building is not advisable within the study area since the results showed that the overburden (depth to basement) is generally thick mostly greater than 15 m. Also revealed from the microstudy of soil properties is that foundation stability should be properly managed, since a porous soil does not accommodate engineering activities except when an artificial basement is put in place. Keywords : Geophysical Investigation, Soil Analysis, Foundation Studies, Subsurface Competency

Efficient Data-Driven Geologic Feature Characterization from Pre-stack Seismic Measurements using Randomized Machine-Learning Algorithm

Conventional seismic techniques for detecting the subsurface geologic features are challenged by limited data coverage, computational inefficiency, and subjective human factors. We developed a novel data-driven geological feature detection approach based on pre-stack seismic measurements. Our detection method employs an efficient and accurate machine-learning detection approach to extract useful subsurface geologic features automatically. Specifically, our method is based on kernel ridge regression model. The conventional kernel ridge regression can be computationally prohibited because of the large volume of seismic measurements. We employ a data reduction technique in combination with the conventional kernel ridge regression method to improve the computational efficiency and reduce memory usage. In particular, we utilize a randomized numerical linear algebra technique, named Nystr\"om method, to effectively reduce the dimensionality of the feature space without compromising the information content required for accurate detection. We provide thorough computational cost analysis to show efficiency of our new geological feature detection methods. We further validate the performance of our new subsurface geologic feature detection method using synthetic surface seismic data for 2D acoustic and elastic velocity models. Our numerical examples demonstrate that our new detection method significantly improves the computational efficiency while maintaining comparable accuracy. Interestingly, we show that our method yields a speed-up ratio on the order of $\sim10^2$ to $\sim 10^3$ in a multi-core computational environment.

SH-Mode Seismic-Reflection Imaging of Earthfill Dams

Abstract Assessing subsurface characteristics and imaging geologic features (e.g., faults, cavities, low-velocity layers, etc.) are typical problems in near-surface geophysics. These questions often have adverse geotechnical engineering implications, and can be especially acute when associated with high-hazard structures such as large earthen flood-control dams. Dam-related issues are becoming more frequent in the United States, because a large part of this major infrastructure was designed and constructed in the early- to mid-twentieth century; these dams are thus passing into the latter stages of their design life, where minute flaws that were overlooked or thought to be insignificant in design/construction are now proving problematic. The high-hydraulic heads associated with these structures can quicken degradation of weak areas and compromise long-term integrity. Addressing dam-related problems solely with traditional invasive drilling techniques is often inadequate (i.e., lack of lateral resolution) and/or economically exorbitant at this scale. However, strategic geotechnical drilling integrated with the broad utility of near-surface geophysics, particularly the horizontally polarized shear-wave (SH-mode) seismic-reflection technique for imaging the internal structural detail and geological foundation conditions of earthfill embankment dams can cost-effectively improve the overall subsurface definition needed for remedial engineering. Demonstrative evidence for this supposition is provided in the form of SH-wave seismic-reflection imaging of in situ and engineered as-built components of flood-control embankment dams at two example sites in the central United States.

Application of Electrical Resistivity Method in Site Characterization along Ado – Afao Road, Southwestern Nigeria

A geophysical study was carried out at a proposed location for the construction of a structure along Ado-Afao road, Southwestern Nigeria. The aim of the study is to evaluate the electrical properties of the soil for Founding of Engineering structures. The geophysical investigation involved the Vertical Electrical Sounding (VES) technique using the Schlumberger configuration with a total of twenty-one (21) VES within the investigated area. The electrode separation varies from 1 to 100 m. The geoelectric sections identified three to five geoelectric/geologic subsurface layers along the traverses. The topsoil comprising of clay, clayey sand and sandy clay with the resistivity values range from 28 to 800 Ω-m with its thickness varying from 0.4 to 1.9 m. The second layer was found to be lateritic with resistivity ranging between 200 to 800 Ω-m and thickness ranges from 1 to 7.5 m while the weathered layer comprising of clay, clayey sand and sandy clay with resistivity varies from 30 to 220 Ω-m and its thickness varies from 1.2 to 54 m. The fractured basement with resistivity value of 763 Ωm and thickness value of 8m while the fresh basement has a resistivity value ranging from 365 to 2964 Ωm with depth to basement ranging from 8 to 58 m. The resistivity values of the topsoil are indicative of clay, sandy clay and clayey sand. This layer may not be of any special interest since topsoil is normally excavated. Hence, foundation of the proposed structures cannot be found on this layer. Based on the investigation, the subsurface of the study area can be generally classified as incompetent. There is a presence of lateral inhomogeneity of the subsurface layers and geologic features such as fractures and faults. The construction in the area should be founded on the lateritic layer or fresh basement layer coupled with pile foundation to ensure the stability of the building. The choice of foundation material, clay content and topography elevation should be put into consideration.

Geoimaging of subsurface fabric in Awgbagba, Southwestern Nigeria using geomagnetic and geoelectrical techniques

The heterogeneous nature of the earth has given rise to variations experienced in the subsurface. Some parts are good for hydrological exploration while others are good for civil engineering activities. These variations experienced in the subsurface could not be detected except through geophysical survey and analysis. Integration of ground magnetic and Vertical Electrical Sounding (VES) techniques were carried out in Awgbagba to image the subsurface fabric with a view to mapping subsurface geological features, such as the weak and competent zones, to determine the overburden thickness, and suitability of Awgbagba for civil engineering and hydrogeological purposes. Ten ground magnetic traverses were established in W-E and N-S azimuths. Twenty VES points were randomly sounded for the study in order to cover the entire study area. The magnetic residual field anomaly values ranged from -1, 600 to 700 nT. The study area is grouped into low, average, and high magnetic zones. The depth to magnetic sources ranged from 8.1 to 48.9 m with a mean value of 17.4 m. Three planar feature orientations in the study area are in NE-SW, NW-SE, and SE-NW orientations. VES results showed 7 QH-curve type, 6 HA-curve type, 4 KH-curve type, 1 AA-curve type, 1 HK-curve type and 1 QQ-curve type respectively. The ratio of thin-to-thick overburden and fresh-to-fractured bedrock are 3:7 and 2:3 respectively. It is concluded that the study area cannot withstand high-rise building constructions. However, the mapped fracture zones in the study area would serve as promising zones for borehole development.

Stochastic porous media modeling and high-resolution schemes for numerical simulation of subsurface immiscible fluid flow transport

This paper proposes stochastic petroleum porous media modeling for immiscible fluid flow simulation using Dykstra–Parson coefficient (VDP) and autocorrelation lengths to generate 2D stochastic permeability values which were also used to generate porosity fields through a linear interpolation technique based on Carman–Kozeny equation. The proposed method of permeability field generation in this study was compared to turning bands method (TBM) and uniform sampling randomization method (USRM). On the other hand, many studies have also reported that, upstream mobility weighting schemes, commonly used in conventional numerical reservoir simulators do not accurately capture immiscible displacement shocks and discontinuities through stochastically generated porous media. This can be attributed to high level of numerical smearing in first-order schemes, oftentimes misinterpreted as subsurface geological features. Therefore, this work employs high-resolution schemes of SUPERBEE flux limiter, weighted essentially non-oscillatory scheme (WENO), and monotone upstream-centered schemes for conservation laws (MUSCL) to accurately capture immiscible fluid flow transport in stochastic porous media. The high-order schemes results match well with Buckley Leverett (BL) analytical solution without any non-oscillatory solutions. The governing fluid flow equations were solved numerically using simultaneous solution (SS) technique, sequential solution (SEQ) technique and iterative implicit pressure and explicit saturation (IMPES) technique which produce acceptable numerical stability and convergence rate. A comparative and numerical examples study of flow transport through the proposed method, TBM and USRM permeability fields revealed detailed subsurface instabilities with their corresponding ultimate recovery factors. Also, the impact of autocorrelation lengths on immiscible fluid flow transport were analyzed and quantified. A finite number of lines used in the TBM resulted into visual artifact banding phenomenon unlike the proposed method and USRM. In all, the proposed permeability and porosity fields generation coupled with the numerical simulator developed will aid in developing efficient mobility control schemes to improve on poor volumetric sweep efficiency in porous media.

Wave equation datuming applied to S-wave reflection seismic data

S-wave high-resolution reflection seismic data was processed using Wave Equation Datuming technique in order to improve signal/noise ratio, attenuating coherent noise, and seismic resolution and to solve static corrections problems. The application of this algorithm allowed obtaining a good image of the shallow subsurface geological features. Wave Equation Datuming moves shots and receivers from a surface to another datum (the datum plane), removing time shifts originated by elevation variation and/or velocity changes in the shallow subsoil. This algorithm has been developed and currently applied to P wave, but it reveals the capacity to highlight S-waves images when used to resolve thin layers in high-resolution prospecting. A good S-wave image facilitates correlation with well stratigraphies, optimizing cost/benefit ratio of any drilling. The application of Wave Equation Datuming requires a reliable velocity field, so refraction tomography was adopted. The new seismic image highlights the details of the subsoil reflectors and allows an easier integration with borehole information and geological surveys than the seismic section obtained by conventional CMP reflection processing.In conclusion, the analysis of S-wave let to characterize the shallow subsurface recognizing levels with limited thickness once we have clearly attenuated ground roll, wind and environmental noise.

Rayleigh waves from correlation of seismic noise in Great Island of Tierra del Fuego, Argentina: Constraints on upper crustal structure

In this study, the ambient seismic noise cross-correlation technique is applied to estimate the upper structure of the crust beneath Great Island of Tierra del Fuego (TdF), Argentina, by the analysis of short-period Rayleigh wave group velocities. The island, situated in the southernmost South America, is a key area of investigation among the interaction between the South American and Scotia plates and is considered as a very seismically active one. Through cross-correlating the vertical components of ambient seismic noise registered at four broadband stations in TdF, we were able to extract Rayleigh waves which were used to estimate group velocities in the period band of 2.5–16 s using a time-frequency analysis. Although ambient noise sources are distributed inhomogeneously, robust empirical Green's functions could be recovered from the cross-correlation of 12 months of ambient noise. The observed group velocities were inverted considering a non-linear iterative damped least-squares inversion procedure and several 1-D shear wave velocity models of the upper crust were obtained. According to the inversion results, the S-wave velocity ranges between 1.75 and 3.7 km/s in the first 10 km of crust, depending on the pair of stations considered. These results are in agreement to the major known surface and sub-surface geological and tectonic features known in the area. This study represents the first ambient seismic noise analysis in TdF in order to constraint the upper crust beneath this region. It can also be considered as a successful feasibility study for future analyses with a denser station deployment for a more detailed imaging of structure.

Nonlinear gray-level co-occurrence matrix texture analysis for improved seismic facies interpretation

Seismic texture analysis is a useful tool for delineating subsurface geologic features from 3D seismic surveys, and the gray-level co-occurrence matrix (GLCM) method has been popularly applied for seismic texture discrimination since its first introduction in the 1990s. The GLCM texture analysis consists of two components: (1) to rescale seismic amplitude by a user-defined number of gray levels and (2) to perform statistical analysis on the spatial arrangement of gray levels within an analysis window. Traditionally, the linear transformation is simply used for amplitude rescaling so that the original reflection patterns could be best preserved. However, the seismic features of interpretational interest often cover only a small portion of its amplitude histogram. For representing such features more effectively, it is helpful to perform a nonlinear rescaling of the amplitude distribution between different seismic features. To achieve such an objective, this study proposes a nonlinear GLCM analysis based on four types of nonlinear gray-level transformation (logarithmic, exponential, sigmoid, and logit) and investigates their implications for seismic facies interpretation. Applications to the 3D seismic data set from offshore Angola (West Africa) demonstrate the added values of the generated nonlinear GLCM attributes in better characterizing the channels, fans, and lobes in a deep-marine turbidite system.

Geologic influence on induced seismicity: Constraints from potential field data in Oklahoma

AbstractRecent Oklahoma seismicity shows a regional correlation with increased wastewater injection activity, but local variations suggest that some areas are more likely to exhibit induced seismicity than others. We combine geophysical and drill hole data to map subsurface geologic features in the crystalline basement, where most earthquakes are occurring, and examine probable contributing factors. We find that most earthquakes are located where the crystalline basement is likely composed of fractured intrusive or metamorphic rock. Areas with extrusive rock or thick (>4 km) sedimentary cover exhibit little seismicity, even in high injection rate areas, similar to deep sedimentary basins in Michigan and western North Dakota. These differences in seismicity may be due to variations in permeability structure: within intrusive rocks, fluids can become narrowly focused in fractures and faults, causing an increase in local pore fluid pressure, whereas more distributed pore space in sedimentary and extrusive rocks may relax pore fluid pressure.

High-resolution seismic complex trace analysis by adaptive fast sparse S-transform

Complex trace analysis provides seismic interpreters with a view to identify the nature of challenging subsurface geologic features. However, the conventional procedure based on the Hilbert transform (HT) is highly sensitive to random noise and sudden frequency variations in seismic data. Generally, conventional filtering methods reduce the spectral bandwidth while stabilizing complex trace analysis, whereas obtaining high-resolution images of multiple thin-bed layers requires wideband data. It is thus a challenging problem to reconcile the conflict between the two purposes, and a powerful signal processing device is required. To overcome the issue, I first introduced the fast sparse S-transform (ST) as a powerful time-frequency decomposition method to improve the windowed Hilbert transform (WHT). Then, in addition to the mixed-norm higher resolution provided by the fast sparse ST, I have developed a novel sparsity-based optimization for window parameters. The process adaptively regularizes sudden changes in frequency content of nonstationary signals with the same computational complexity of the nonoptimized algorithm. The performance of the proposed windowing optimization is compared with those of available methods that have so far been used for adaptivity enhancement of Fourier-based spectral decomposition methods. The final adaptive and sparse version of WHT is used to achieve high-resolution complex trace analysis and address the above-mentioned conflict. The instantaneous complex attributes obtained by the proposed method for several synthetic and real data sets of which multiple thin-bed layers contain wedges, trapped gas reservoirs, and faults are superior to those obtained by WHT via adaptive sparse STFT, robust adaptive WHT, and conventional HT. Potential applications of the adaptive double-sparse ST as a new spectral decomposition method were also evaluated.

A Sparse Bayesian Imaging Technique for Efficient Recovery of Reservoir Channels With Time-Lapse Seismic Measurements

Subsurface reservoir flow channels are characterized by high-permeability values and serve as preferred pathways for fluid propagation. Accurate estimation of their geophysical structures is thus of great importance for the oil industry. The ensemble Kalman filter (EnKF) is a widely used statistical technique for estimating subsurface reservoir model parameters. However, accurate reconstruction of the subsurface geological features with the EnKF is challenging because of the limited measurements available from the wells and the smoothing effects imposed by the $\ell _{2}$ -norm nature of its update step. A new EnKF scheme based on sparse domain representation was introduced by Sana et al. (2015) to incorporate useful prior structural information in the estimation process for efficient recovery of subsurface channels. In this paper, we extend this work in two ways: 1) investigate the effects of incorporating time-lapse seismic data on the channel reconstruction; and 2) explore a Bayesian sparse reconstruction algorithm with the potential ability to reduce the computational requirements. Numerical results suggest that the performance of the new sparse Bayesian based EnKF scheme is enhanced with the availability of seismic measurements, leading to further improvement in the recovery of flow channels structures. The sparse Bayesian approach further provides a computationally efficient framework for enforcing a sparse solution, especially with the possibility of using high sparsity rates through the inclusion of seismic data.

Integration of seismic refraction tomography and electrical resistivity tomography in engineering geophysics for soil characterization

The use of both seismic refraction tomography (SRT) and electrical resistivity tomography (ERT) techniques have commonly been used to detect physical properties in the subsurface materials in order to map the subsurface geological features and soil characterization in the site investigations. Using of both techniques increases confidence in interpretation to limit inaccurate interpretation due to the large amount of heterogeneity in the near surface. Twelve shallow seismic refractions and ten profiles for electrical resistivity were conducted in the Teaching Hospital Project site in Mosul University, Iraq. The linear arrays by using 12 geophones with 10 Hz frequency are used to SRT while ERT traverses were conducted with a total length (280 m) by using Wenner array with an initial electrode spacing of 3 m. Both SRT and ERT data were acquire and interpreted using SRT method and ERT methods to create tomogram velocity models and two dimension resistivity images by using the SeisImager/2D and RES2DINV software, respectively. The seismic velocity values show that the site has three layers, just as follows: the first one corresponded to recent superficial deposits and is characterized by low velocity ranging from about 340 to about 700 m/s. The second one corresponded mostly to the river deposits composed especially of river terraces and clays are characterized by relatively highest velocity values ranging from about 840 m/s to about 1700 m/s. The third one corresponded to the upper part of Fat’ha (Lower Fars) formation which is mostly composed of marl layers and is characterized by high velocity ranging from about 1900 m/s to about 2800 m/s. Interpretations of 2D resistivity profiles indicated alteration zones at depth. It was determined that the material could be classified into two main zones. The first zone has a relatively high true resistivity value ranging from about 80 to about 320 Ω m with a thickness ranging from about 1 to about 25 m, which is mainly consisted of conglomerate, gravel and sand (weathered layer). The second zone has a relatively low true resistivity value reached to 80 Ω m which represents upper part of Fat’ha (Lower Fars) formation deposits which mainly consisted of clays. The two geophysical methods were used to be enjoined the better way to aid the interpretation and evaluate the significance and reliability of the results obtained in each single method. It is demonstrated that engineering geophysics is able to provide solutions for determining subsurface properties and that different prospection techniques are necessary for developing a reasonable model of the subsurface structure. Hence, the third layer is suggesting for engineering and foundation purposes.

Groundmagnetic Method for Subsurface Structural Analysis around Ogbagba,Southwestern Nigeria

The results of Groundmagnetic survey around Ogbagba near Iwo part of southwestern Nigeria were presented for evaluation of the subsurface structural analysis in the area, with a view to determine the promising area for hydrological purposes and also competent area for engineering activities. Total field magnetic data was recorded using proton precession magnetometer. A total of ten traverses were established and presented as ground magnetic profiles of varying magnetic intensity which were subjected to both qualitative and quantitative interpretation. The result of the investigation revealed information on the subsurface geological features which were interpreted as the competent and the weak zones within the study area.

Crustal structure beneath the central Tien Shan from ambient noise tomography

AbstractThrough analysis of seismic ambient noise recorded by the GHENGIS array, we constructed a high‐resolution 3‐D crustal shear‐wave velocity model for the central Tien Shan. The obtained shear‐wave velocity model provides insight into the detailed crustal structure beneath the Tien Shan. The results obtained at shallow depths are well correlated with known subsurface geological features. Low velocities are found mainly beneath sedimentary basins, whereas high velocities are mainly associated with mountain ranges. At greater depths of ~43–45 km, high velocities were observed beneath the Tarim Basin and Kazakh Shield; these high velocities extend forward in opposite directions and tilt down towards the central Tien Shan to a depth of in excess of 50 km, most likely reflecting lateral variations in crustal thickness beneath the Tien Shan and surrounding platforms.