Complex Geological Structures Research Articles

A Comparative Study for Evaluating the Groundwater Inflow and Drainage Effect of Jinzhai Pumped Storage Power Station, China

Various hydrogeological problems like groundwater inflow, water table drawdown, and water pressure redistribution may be encountered in the construction of hydraulic projects. How to accurately predict the occurrence of groundwater inflow and assess the drainage effect during construction are still challenging problems for engineering designers. Taking the Jinzhai pumped storage power station (JPSPS) of China as an example, this paper aims to use different methods to calculate the water inflow rates of an underground powerhouse and evaluate the drainage effect caused by tunnel inflow during construction. The methods consist of the analytical formulas, the site groundwater rating (SGR) method, and the Signorini type variational inequality formulation. The results show that the analytical methods considering stable water table may overestimate the water inflow rates of caverns in drained conditions, whereas the SGR method with available hydro-geological parameters obtains a qualitative hazard assessment in the preliminary phase. The numerical solutions provide more precise and reliable values of groundwater inflow considering complex geological structures and seepage control measures. Moreover, the drainage effects, including a seepage-free surface, pore water pressure redistribution, and hydraulic gradient, have been accurately evaluated using various numerical synthetic cases. Specifically, the faults intersecting on underground caverns and drainage structures significantly change the groundwater flow regime around caverns. This comparative study can not only exactly identify the capabilities of the methods for cavern inflow in drained conditions, but also can comprehensively evaluate the drainage effect during cavern construction.

Elastic least‐squares reverse time migration from topography through anisotropic tensorial elastodynamics

AbstractLeast‐squares reverse time migration is an increasingly popular technique for subsurface imaging, especially in the presence of complex geological structures. However, elastic least‐squares reverse time migration algorithms face significant practical and numerical challenges when migrating multi‐component seismic data acquired from irregular topography. Many associated issues can be avoided by abandoning the Cartesian coordinate system and migrating the data to a generalized topographic coordinate system conformal to surface topology. We introduce a generalized anisotropic elastic least‐squares reverse time migration methodology that uses the numerical solutions of tensorial elastodynamics for propagating wavefields in computational domains influenced by free‐surface topography. We define a coordinate mapping assuming unstretched vertically translated meshes that transform an irregular physical domain to a regular computational domain on which calculating numerical elastodynamics solutions is straightforward. This allows us to obtain numerical solutions of forward and adjoint elastodynamics and generate subsurface images directly in topographic coordinates using a tensorial energy‐norm imaging condition. Numerical examples demonstrate that the proposed generalized elastic least‐squares reverse time migration algorithm is suitable for generating high‐quality images with reduced artefacts and better balanced reflectivity that can accurately explain observed data acquired from topography in a medium characterized by arbitrary heterogeneity and anisotropy. Finally, the computational cost of our method is comparable to that of an equivalent Cartesian elastic least‐squares reverse time migration numerical implementation.

Seismic imaging of deep‐seated gold deposit and host rocks through a reappraisal of legacy seismic data in the Fochville mining area, South Africa

AbstractReappraisal of legacy reflection seismic data has shown to deliver value in mineral exploration, particularly in brownfield settings. In this work, we demonstrate how the reappraisal and processing of legacy reflection seismic data can be advantageous in the mineral exploration industry. We use today's standard seismic processing tools to improve the imaging of deep and complex geological structures that host mineral deposits. The recovered and processed 25.3 km long legacy seismic profile in this study was acquired in 1983 by the Gold Division of Anglo‐American as part of the Witwatersrand Gold Fields exploration program. This study aims to improve the imaging of the Ventersdorp Contact Reef gold‐bearing horizon (termed reef), a world‐class gold deposit (2 m thick) situated at depths between ∼2400 and ∼4100 m below the ground surface near South Deep Gold Mine in Fochville, South Africa. The final processing results from the pre‐stack time and phase‐shift migration approaches clearly reveal a dipping reflection associated with the gold‐bearing horizon and major steeply dipping faults that crosscut and displace the deposit. The final results are integrated with borehole information, 1D synthetic modelling and aeromagnetic data to constrain the structural interpretation. In particular, 1D synthetic simulation and borehole data constrain the depth position of the gold deposit. The magnetic data provides additional constraints on the complex faulted blocks of the host rocks such as the intrusions that may have a direct impact on ore resources and evaluation. The mining companies, such as South Deep Gold Mine, operating closer to the seismic profiles can use this new structural information to update the current geological models and improve future mine planning and designs, thus providing some insight into the prospectivity of unmined ground.

Surface geochemistry as a key to understanding the petroleum system in regions of complex geology - Polish Outer Carpathians

The goal of the study is to investigate the relationships between soil gas composition and geological structures in a structurally complex area of the Carpathian fold and thrust belt. Surface geochemical surveys, aimed at determining the molecular and isotopic composition of soil gas samples, were carried out in the central part of the Polish Outer Carpathians in two major tectonic units, the Magura Unit and the Fore-Magura Group of units, which outcrop at the front of the former and within it as a tectonic window (the so-called Mszana Dolna Tectonic Window, MDTW). The tectonic structure of the area has numerous deformations, such as folds, thrusts, and strike-slip faults, that are accompanied by broad zones of mélange. In all 572 soil gas samples, the concentrations of light alkanes, gaseous alkenes as well as non-hydrocarbon (e.g. hydrogen) components were determined, and part of the samples were analysed to test the carbon isotope composition of methane and carbon dioxide.The results of the surface geochemical survey showed significant differences in the recorded microconcentrations of hydrocarbons within the tectonic units of the Outer Carpathians. Significantly higher concentrations of total alkanes C2-C5 were mainly recorded in the MDTW, although the distribution of total alkane values within the Window shows strong connections with the arrangement of strata and the strike of faults and thrusts. Generally, the sealing is insufficient within the MDTW to support the existence of conventional economically viable hydrocarbon accumulations. Our data confirms the sealing role of the Magura Unit in the Carpathian oil system. Therefore, hydrocarbon sources (i.e. potential hydrocarbon accumulations) may be expected within the Fore-Magura Group beneath the cover of the Magura Unit.Our results are also of key importance for the interpretation of surface geochemistry data in other oil and gas provinces with a complex geological structure.

Geomechanical methods for pore pressure prediction in complex geological structures: a case study of a field in southwest of Iran

Geomechanical methods for pore pressure prediction in complex geological structures: a case study of a field in southwest of Iran

Improving fluid identification in well logging using Continuous Wavelet Transform and Vision Transformers: An innovative approach

Well logging fluid prediction is one of the key steps in assessing oil and gas reserves. By analyzing downhole logging data, different types of fluids contained in underground rocks, such as crude oil, natural gas, and water, can be determined. This information is crucial for assessing the abundance and recoverable reserves of oil and gas resources and helps guide oil and gas exploration and development work. We have introduced a novel model called CWT (Continuous Wavelet Transform)-ViT (Vision Transformer). CWT can simultaneously provide frequency information at different scales, enabling the model to analyze downhole logging data more comprehensively and accurately at different scales. Underground rock structures often exhibit features at multiple scales, and CWT can effectively capture these features, aiding in better differentiation of different types of fluids. The ViT model utilizes the Transformer architecture, allowing for global attention over input sequences without being limited by sequence length. This enables the model to comprehensively understand the overall information of downhole logging data and extract richer features. For complex geological structures and fluid distributions in geological exploration, the global attention mechanism helps the model better grasp the overall situation, thereby improving the accuracy of fluid prediction. When we used the CWT-ViT method for well logging fluid prediction, we achieved a high accuracy rate of 97.50% in the first dataset, which further improved to 97.77% in the second dataset. These results demonstrate the significant robustness and efficiency of the CWT-ViT method in lithology prediction using well logging data. We also conducted blind well experiments, and our CWT-ViT model outperformed other models, achieving a blind well prediction accuracy of 97.36%. Therefore, the experiments indicate that the key to improving accuracy in well logging fluid prediction with CWT lies in its multiscale analysis capability, effectively capturing different fluid characteristic frequencies. Additionally, CWT enhances signal features and removes noise, increasing the precision of fluid identification. Finally, the integration with ViT further optimizes fluid prediction performance, making it outstanding in complex geological environments. The advantages of ViT in fluid prediction include its excellent sequence modeling capability, effective handling of long-distance dependencies, and enhanced ability to capture fluid characteristics in complex well logging data.

Application of Experimental Configurations of Seismic and Electric Tomographic Techniques to the Investigation of Complex Geological Structures

The main purpose of this study is the subsurface investigation of two complex geological environments focusing on the improvement of data acquisition and processing parameters regarding electric and seismic tomographic techniques. Two different study areas, in central–east Peloponnese and SE Attica, were selected, where detailed geological mapping and surface geophysical survey were carried out. The applied geophysical survey included the application of electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and ground penetrating radar (GPR). The geoelectrical measurements were acquired with different arrays and electrode configurations. Moreover, various types of seismic sources were used at seventeen shot locations along the seismic arrays. For the processing of geoelectrical data, clustered datasets were created, increasing the depth of investigation and discriminatory capability. The seismic data processing included the following: (a) the creation of synthetic models and seismic records to determine the effectiveness and capabilities of the technique, (b) spectral analysis of the seismic records to determine the optimal seismic source type and (c) inversion of the field data to create representative subsurface velocity models. The results of the two techniques successfully delineated the complex subsurface structure that characterizes these two geological environments. The application of the ERT combined with the SRT are the two dominant, high-resolution techniques for the elucidation of complex subsurface structures.

Nonparametric and Continuous Variable-Based Stratigraphic Modelling from Sparse Boreholes using Signed Distance Function and Bayesian Compressive Sensing

An accurate stochastic interpretation of subsurface stratigraphy with quantified uncertainty can benefit the subsequent risk management of geotechnical infrastructure. Traditional approaches to developing geological cross-sections from sparse boreholes typically require the calibration or definition of empirical model parameters and functions, which may introduce subjectivity and bias. In this study, a non-parametric and continuous variable-based spatial predictor that leverages the signed distance function and Bayesian compressive sensing (BCS) is proposed for subsurface stratigraphic modelling. The proposed method transforms sparse categorical borehole data from a low-dimensional space into continuous variables in a high-dimensional space, enabling a comprehensive representation of more implicit characteristics of intricate geological patterns. This transformation facilitates the use of the continuous-variable-based BCS for non-parametric spatial prediction. The most probable geological cross-section and uncertainty qualification plot are derived after transforming spatially interpreted fields of continuous variables back into soil types. The performance of the proposed method is demonstrated using synthetic and real-world cases. Results indicate that the proposed approach can handle intricate stratigraphic scenarios characterized by complex geological structures, such as crossed-inclined, folded, inclined-folded, and interbedded strata, in a data-driven and non-parametric manner. The advantages of the proposed method over existing spatial predictors for developing geological cross-sections are also demonstrated.

Monitoring and Evaluation of Debris Flow Disaster in the Loess Plateau Area of China: A Case Study

The Loess Plateau area, with complex geomorphological features and geological structure, is highly prone to geologic disasters such as landslides and debris flow, which cause great losses. To investigate the initiation mechanism of landslide and debris flow disasters and their spreading patterns, historical satellite images in the Laolang gully were collected and digitized to generate three-dimensional topographic and geomorphological maps. Typical landslides were selected for landslide thickness measurement using a standard penetrometer and high-density electrical method. Numerical models were established to simulate the occurrence and development of landslides under different working conditions and to evaluate the spreading range based on the propagation algorithm and friction law. The results show that the 10 m resolution DEM data are well matched with the potential hazard events observed in the field site. The smaller the critical slope threshold, the greater the extent and distance of landslide spreading. The larger the angle of arrival, the greater the energy loss, and therefore the smaller the landslide movement distance. The results can provide scientific theoretical guidance for the prevention and control of rainfall-induced landslide and debris flow disasters in the Loess Plateau area.

METHOD FOR ASSESSING THE IMPACT OF GEOLOGICAL HETEROGENEITY ON OIL SATURATION DISTRIBUTION

This article examines a method for assessing the impact of reservoir heterogeneity on the distribution of oil saturation in pay zones using the example of the Bajossian and Bathonian stages of the Jurassic system of the Kalamkas field. Within this field, the Jurassic oil and gas stratum under consideration lies on the eroded surface of various horizons of the Middle and Lower Triassic and is represented by uneven alternation of sand-siltstone and clay rocks, as a result of which these deposits have a complex geological structure, due in the lithological plan to uneven alternation of sand-siltstone and clay rocks. As a result, a rather complex distribution of oil (water) saturation may occur in the formation, which differs from the standard ideas about the geological structure of the object under study, i.e. the presence of such «unconventional» reservoir boundaries (oil-bearing contour) as capillary barriers of the first kind is possible. As a result of this approach, the following is obtained. For the conditions of occurrence of productive formations of the Bajossian stage of the field in question, in order to ensure that the dimensions of the capillary barrier in the reservoir do not exceed the height of 1 m at an average value of the porosity coefficient Kп = 27,6 % it is necessary to fulfill the inequality of Kпр ≥ 6,1∙10−3 μm2. For the conditions of occurrence of productive formations of the Bathonian stage, in order for the dimensions of the capillary barrier in the reservoir not to exceed the height of 1 m with an average value of the porosity coefficient Kп = 28,0 % it is necessary to Kпр ≥ 7,5∙10−3 μm2. Also, on the basis of this method, various ratios of Kпр permeability coefficients and porosity were obtained Kп at which the dimensions of the capillary barrier in the deposit will exceed the heights of 1.0, 1.5 and 2.0 m above the level of the accepted oil-water contact (OWC) for the conditions of occurrence of the Kalamkas field in question. It was noted that this approach makes it possible to assess uncertainties in the position of fluid contacts when justifying the boundaries of the oil potential contour and the distribution of oil reserves in formations with a wide range of reservoir properties.

APPROACH TO CREATION OF GEOLOGICAL AND RESERVOIR SIMULATION MODEL AND METHODS FOR HISTORY MATCHING OF RESERVOIR ENERGY STATE

Under conditions of complex geological structure and substantial share of hard-to-recover oil reserves, reservoir simulation is a key tool in reservoir management. Direct problems solved by reservoir simulation include forecasting of the production performance, control and management of reservoir development, and localization of residual oil reserves. Inverse problem solution is useful for updating the knowledge of geological structure, reservoir quality, and historical production data by means of history matching. The quality of history matching is fundamental to ensure reliable future forecasts and localization of residual oil zones. The main approaches to appropriate history matching rely on analysis and adjustment of well logging and geological parameters, as well as checking and correction of errors, inadequate historical data, application of physics-based and reasonable approaches and tools. The prerequisite for adequate history matching and predictive ability of the resultant reservoir simulation model is the compliance of simulated and actual energy state of the reservoir. In this paper, the initial stage of reservoir simulation is performed, the approach to history matching of reservoir energy state aimed at improving the quality of further predictions is described as exemplified by one of the production areas of Russian oil field. The main causes of poor history match of formation and bottomhole pressures are revealed. These causes are grouped as follows: — geological causes/reservoir quality, which include permeability field distribution, reservoir connectivity, and determination of rock and fluid compressibility;— technical causes (during reservoir simulation), that is model limitations in case of larger areas, the need to use sector models, consideration of buffer region;— technological causes, including inadequate historical data or lack of data, detection of behind-the-casing flows and production casing leaks in wellbore, etc. For each group of factors that have a substantial impact on reservoir pressure, recommendations and methods of energy state history matching on geological and reservoir simulation models are provided, examples are given.

Application of CSAMT Geophysical Method in Geothermal Exploration of Beijing

Abstract Geothermal resources are abundant in the southeast urban area of Beijing, but the complex geological structure and high geomagnetic interference in the urban area make it difficult to detect the distribution characteristics of geothermal resources by the electromagnetic exploration method from natural sources. This paper presents an integrated physical exploration method combining the controlled source acoustic magneto-telluric (CSAMT) and gravity exploration for middle-deep geothermal exploration in the southeast Beijing urban area. The method is verified by examining the recommended boreholes, which have produced water at temperatures of 62.5°C and 63°C. This is a highly beneficial and socially impactful outcome, and it serves as a valuable reference for future exploration and utilization of geothermal resources in this area. It also has significant implications for future exploration and utilization of geothermal resources not only in the southeastern urban area of Beijing but also in similar urban environments around the world. The methodologies and findings from this research offer a valuable blueprint for future geothermal energy prospecting and development, contributing to the global transition towards cleaner and more sustainable energy alternatives.

Mountain building process of the Taiwan orogeny.

The Taiwan orogeny, an example of arc-continental collision, exhibits complex geological structures and rapid exhumation. Many models have tried and failed to fully capture the dynamics of these processes. We developed a comprehensive thermomechanical model that considers the transition from brittle to ductile behavior with depth, lithology-dependent erosion and observed decollement and backstop geometries. This model successfully reproduces the intricate structures observed within the Taiwan orogeny, aligns with structural complexities, metamorphic temperature profiles, thermochronological records, strain distributions, and the rates of exhumation and cooling and elucidates the roles of ductile deformation and ramp structures in forming the Hsuehshan Range and the Western fold and thrust belt. The insights from this model offer potential applicability to other orogenic wedges worldwide.

Prospects for the allocation of Paleorousels in the subsalt complex of the south-east of the Caspian sedimentary basin

development and features of the formation of the sedimentary complex of the south of the Caspian sedimentary basin are presented. The complex geological structure of the region is mainly due to the influence of tectonic, volcanic and denudation processes on sedimentation. The introduction of terrigenous material deep into the sedimentary basin (Matken-Ushmolinsk, Biikzhal and South Embin zones) by paleorubic flows contributed to the emergence of accumulative shafts and interbranch uplifts composed mainly of clay rocks with alternating siltstone-sandstones. In the paleorusls themselves, coarser sediments were deposited – gravelites and sandstones. The reconstruction of the conditions of formation of sand bodies made it possible to predict their facies composition, morphology and identify patterns of spatial placement, and are also of great interest in the search for oil and gas deposits in deep-lying lithological traps. The prospects of these structures and their resource potential are evidenced by the world experience in identifying large deposits of hydrocarbon raw materials. This is a new highly promising direction of large accumulations of hydrocarbons for the Caspian Basin. The results of drilling deep wells conducted in recent years have made it possible to refine the internal structure, substantiate new facilities in the Late Paleozoic sedimentary complex, and reassess the prospects of oil and gas potential and forecast oil and gas resources of the south of the Caspian Basin.

Creep deformation characteristics and control technology in deep mine soft rock roadway

In order to control the strong ageing creep and large deformation of deep soft rock roadway effectively, with the 61–71 track on the uphill of the mining area in Suzhou, Anhui as the research background, the triaxial creep test of mudstone was conducted using the TYJ-1500 M rock mechanics testing system. The creep deformation and failure characteristics of mudstone were analyzed. Additionally, the creep deformation characteristics of deep soft rock roadways were obtained through FLAC3D numerical simulation experiment, and the control techniques for deep soft rock roadway was proposed. The results showed that the axial strain and lateral strain of the specimen were mainly instantaneous strain and creep strain under triaxial stress conditions, and the both confining pressure and the axial pressure have a significant impact on the deformation and creep failure strength of the specimen. Under the condition of high ground stress and complex geological structure, the high stress concentration of roadway roof and floor and two bottom angles is the main cause of creep failure of soft rock roadway, and the large degree of surrounding rock fragmentation and unreasonable support mode reduce the bearing capacity of surrounding rock and aggravate the creep failure of roadway. The 'anchor net cable shotcrete + floor and two corners in floor bolt-grouting + deep and shallow hole grouting + secondary reinforcement support' combined support method was proposed and industrially tested, with average deformation of the roof, floor, and two sidewalls being 111.9 and 62.5 mm, respectively, representing 13.2 and 10.3% of the deformation under the original support scheme.

Blind spectral inversion of seismic data

AbstractReflectivity inversion is a key step in reservoir prediction. Conventional sparse‐spike deconvolution assumes that the reflectivity (reflection coefficient series) is sparse and solves for the reflection coefficients by an L1‐norm inversion process. Spectral inversion is an alternative to sparse‐spike deconvolution, which is based on the odd–even decomposition algorithm and can accurately identify thin layers and reduce the wavelet tuning effect without using constraints from logging data, from horizon interpretations or from an initial model of the reflectivity. In seismic processing, an error exists in wavelet extraction because of complex geological structures, resulting in the low accuracy of deconvolution and inversion. Blind deconvolution is an effective method for solving the problem mentioned above, which comprises seismic wavelet and reflectivity sequence, assuming that the wavelets that affect some subsets of the seismic data are approximately the same. Therefore, we combined blind deconvolution with spectral inversion to propose blind spectral inversion. Given an initial wavelet, we can calculate the reflectivity based on spectral inversion and update the wavelet for the next iteration. During the update processing, we add the smoothness of the wavelet amplitude spectrum as a regularization term, thus reducing the wavelet oscillation in the time domain, increasing the similarity between inverted and initial wavelets, and improving the stability of the solution. The blind spectral inversion method inherits the wavelet robustness of blind deconvolution and high resolution of spectral inversion, which is suitable for reflectivity inversion. Applications to synthetic and field seismic datasets demonstrate that the blind spectral inversion method can accurately calculate the reflectivity even when there is an error in wavelet extraction.

Evaluation of Key Development Factors of a Buried Hill Reservoir in the Eastern South China Sea: Nonlinear Component Seepage Model Coupled with EDFM

The HZ 26-B buried hill reservoir is located in the eastern part of the South China Sea. This reservoir is characterized by the development of natural fractures, a high density, and a complex geological structure, featuring an upper condensate gas layer and a lower volatile oil layer. These characteristics present significant challenges for oilfield exploration. To address these challenges, this study employed advanced embedded discrete fracture methods to conduct comprehensive numerical simulations of the fractured buried hill reservoirs. By meticulously characterizing the flow mechanisms within these reservoirs, the study not only reveals their unique characteristics but also establishes an embedded discrete fracture numerical model at the oilfield scale. Furthermore, a combination of single-factor sensitivity analysis and the Pearson correlation coefficient method was used to identify the primary controlling factors affecting the development of complex condensate reservoirs in ancient buried hills. The results indicate that the main factors influencing the production capacity are the matrix permeability, geomechanical effects, and natural fracture length. In contrast, the impact of the threshold pressure gradient and bottomhole flow pressure is relatively weak. This study’s findings provide a scientific basis for the efficient development of the HZ 26-B oilfield and offer valuable references and insights for the exploration and development of similar fractured buried hill reservoirs.

3D modeling of deep borehole electromagnetic measurements with energized casing source for fracture mapping at the Utah Frontier Observatory for Research in Geothermal Energy

AbstractWe present a 3D numerical modelling analysis evaluating the deployment of a borehole electromagnetic measurement tool to detect and image a stimulated zone at the Utah Frontier Observatory for Research in Geothermal Energy geothermal site. As the depth to the geothermal reservoir is several kilometres and the size of the stimulated zone is limited to several 100 m, surface‐based controlled‐source electromagnetic measurements lack the sensitivity for detecting changes in electrical resistivity caused by the stimulation. To overcome the limitation, the study evaluates the feasibility of using a three‐component borehole magnetic receiver system at the Frontier Observatory for Research in Geothermal Energy site. To provide sufficient currents inside and around the enhanced geothermal reservoir, we use an injection well as an energized casing source. To efficiently simulate energizing the injection well in a realistic 3D resistivity model, we introduce a novel modelling workflow that leverages the strengths of both 3D cylindrical‐mesh‐based electromagnetic modelling code and 3D tetrahedral‐mesh‐based electromagnetic modelling code. The former is particularly well‐suited for modelling hollow cylindrical objects like casings, whereas the latter excels at representing more complex 3D geological structures. In this workflow, our initial step involves computing current densities along a vertical steel‐cased well using a 3D cylindrical electromagnetic modelling code. Subsequently, we distribute a series of equivalent current sources along the well's trajectory within a complex 3D resistivity model. We then discretize this model using a tetrahedral mesh and simulate the borehole electromagnetic responses excited by the casing source using a 3D finite‐element electromagnetic code. This multi‐step approach enables us to simulate 3D casing source electromagnetic responses within a complex 3D resistivity model, without the need for explicit discretization of the well using an excessive number of fine cells. We discuss the applicability and limitations of this proposed workflow within an electromagnetic modelling scenario where an energized well is deviated, such as at the Frontier Observatory for Research in Geothermal Energy site. Using the workflow, we demonstrate that the combined use of the energized casing source and the borehole electromagnetic receiver system offer measurable magnetic field amplitudes and sensitivity to the deep localized stimulated zone. The measurements can also distinguish between parallel‐fracture anisotropic reservoirs and isotropic cases, providing valuable insights into the fracture system of the stimulated zone. Besides the magnetic field measurements, vertical electric field measurements in the open well sections are also highly sensitive to the stimulated zone and can be used as additional data for detecting and imaging the target. We can also acquire additional multiple‐source data by grounding the surface electrode at various locations and repeating borehole electromagnetic measurements. This approach can increase the number of monitoring data by several factors, providing a more comprehensive dataset for analysing the deep‐localized stimulated zone. The numerical analysis indicates that it is feasible to use the combination of the energized casing and downhole electromagnetic measurements in monitoring localized stimulated zone at large depths.

Shear wave velocity model using HVSR inversion beneath Bandar Lampung City

The horizontal-to-vertical spectral ratio (HVSR) method has been used to characterize site-effect parameters that are indispensable in seismic hazard and risk-reduction studies in urban areas and rapid land-use planning. This method is widely used because it is the cheapest and simplest geophysical method for the acquisition and processing stages. In subsequent developments, the HVSR method has been widely used to determine elastic rock parameters, particularly shear wave velocity (vS), through the HVSR curve inversion process. Furthermore, the vS structural model can be used to delineate the presence of complex geological structures, particularly faults and sedimentary basins. Bandar Lampung is a city in Lampung Province with many fault structures and groundwater basins to the south. There are 83 HVSR measurement points around Bandar Lampung for delineating the presence of fault structures and groundwater basins. We produced the HVSR curve from the measurement results and then performed an inversion process using the particle swarm optimization algorithm to obtain vS for the depth profile. Subsequently, from this profile, we produced a two-dimensional (2D) lateral and vertical model. The mean vS value was calculated from all the measurement points, and we found stiff soil layers reaching depths of approximately 5 m, with a value of vS < 330 m/s. A bedrock layer with a velocity exceeding 1250 m/s was visible at a depth of 100 m. Based on the 2D model, the vS structure shows that the city of Bandar Lampung is divided into two zones, with a NW-SE boundary. The north-middle-eastern part of the city consists of harder rocks. This harder rock is characterized by extremely high vS values, starting from a depth of 50 m. In contrast, the south-middle-west exhibits a low-moderate vS anomaly associated with groundwater basins SW of the city. From the 2D vS structural model, fault structures can be found along the city, characterized by a contrast of vS values from low to medium and from medium to high.

Investigating subsurface structural lineaments of the northwest Ethiopian plateau using gravity data

This study provides advanced knowledge of the subsurface structural characteristics of the northwest Ethiopian plateau. With its complex geologic structure, the Northwest Ethiopian plateau is being studied for density boundary mapping using geophysical methods. However, its application to the mapping of geological structures such as faults and contacts which appear as lineaments in gravity data has not been studied so far. Thus, the goal of this research is to increase our understanding of the subsurface structural lineaments and their depth beneath the Ethiopian plateau in the northwest using global gravity model GGMplus2013. These subsurface structural lineaments and their depths were investigated using the LTHG, EHGA, and tilt depth techniques. The results indicate that the main subsurface structural lineaments identified in the study region are dipping between 3.2 and 8 km and are developing in the NE-SW, N–S, E-W, and NW-SE orientations.