Geological Discontinuities Research Articles

An extended finite element method for the prediction of acid-etched fracture propagation behavior in fractured-vuggy carbonate reservoirs

Acid fracturing is an important method of achieving high-efficiency economic development of carbonate reservoirs. The propagation of acid-etched fractures is affected significantly by geological discontinuities such as natural fractures and limestone caverns. Therefore, acid-etched fractures in carbonate reservoirs also differ greatly in morphology to those inhomogeneous reservoirs. However, no clear propagation mechanism has been available to describe and predict the propagation behavior of acid-etched fractures. In order to investigate the process, based on the extended finite element method, a set of acid-etched fracture propagation models for fracture-vuggy reservoirs, including mechanical, hydro and acid-rock reaction modules, were established. The effect of vuggy, natural fracture and acid on the initiation and propagation of induced fracture, the surface morphology of the acid-etched fractures, the effects of the number of perforation clusters, and natural limestone caverns, were considered. The calculated geometric size of the fracture was compared with the results based on the CGD (Christianovich-Geertsma-Daneshy model) model, for validating the reliability of the present model. The effects of the injection fracturing liquid type and the number of perforation clusters on fracture initiation position, propagation path, and surface morphology were evaluated. The calculation results also were compared with large-scale tri-axial physical experiment data. Finally, the fracture propagation behavior in rock samples containing natural fractures and limestone caverns was simulated in order to examine examining the effects of the limestone caverns with different sizes at different positions on the propagation path of acid-etched fractures. The interaction between induced fractures and limestone caverns was discussed. The induced fractures priority was given to activating natural fractures around the wellbore and extending along the natural fractures and tended to be deflected from the maximum horizontal principal stress direction due to the existence of caverns arrested fractures to a certain degree.

Coherence algorithm with a high‐resolution time–time transform and feature matrix for seismic data

ABSTRACTTraditional coherence algorithms are often based on the assumption that seismic traces are stationary and Gaussian. However, seismic traces are actually non‐stationary and non‐Gaussian. A constant time window and the canonical correlation analysis in traditional coherence algorithms are not optimal for non‐stationary seismic traces and cannot describe the similarity between adjacent seismic traces in detail. To overcome this problem, a new coherence algorithm using the high‐resolution time–time transform and the feature matrix is designed. The high‐resolution time–time transform used to replace the constant time window can produce a frequency‐dependent time local series to analyse non‐stationary seismic traces. The feature matrix, constructed by the frequency‐dependent time local series and the related local gradients, defines a new correlation metric that enhances more details of the geological discontinuities in seismic images than does the canonical correlation analysis. Additionally, the Riemannian metric is introduced for related calculations because the feature matrices are not defined in a Euclidean space but rather in a manifold space. Application to field data illustrates that the proposed method reveals more details of structural and stratigraphic features.

Characterization, Modelling and Engineering of Rocks and Rockmasses

Rocks are heterogeneous materials usually associated with geological discontinuities making them weak for sustaining the loads due to civil and mining engineering structures. Characterization details of different isotropic, anisotropic and weathered rocks are presented along with the newly formulated classification system for weathered rockmass. Physical modelling of jointed rockmass under uniaxial, biaxial and polyaxial stress state through newly designed polyaxial system yielded significant results. Critical role of intermediate principal stress on strength and deformation for jointed rockmass is highlighted. New polyaxial failure criteria are proposed and compared. New observations on brittle fracture propagation and interlocking phenomena in crystalline rocks are modelled under polyaxial stress state. Extensive large-scale test results on rock joints under constant normal load and constant normal stiffness in static and cyclic modes were presented along with new predictive models. A new closed-form analytical solution is provided for circular openings in isotropic, homogeneous and elastoplastic rockmass for assessing the wellbore stability and openings. Results of physical modelling tests conducted on shallow tunnels subjected to impact loads and new predictive models developed for squeezing behaviour are also included in this paper. Understanding gained from the extensive characterization and physical modelling of materials in appropriate stress regime used effectively to solve real-time cases. The case studies included are: stability analysis of jointed rock slopes of Chenab bridge abutments, Subansiri power house slope stability, Tindharia landslide in Darjeeling, evaluation of bored tunnelling practices in rocks for Delhi metro and coupled hydromechanical modelling for the slopes in Beas catchment.

Improved seismic interpretation of a salt diapir by utilization of diffractions, exemplified by 2D reflection seismics, Danish sector of the North Sea

Development of salt diapirs affects the hydrocarbon trapping systems in the Danish sector of the North Sea, where the reservoirs mainly consist of chalk. Seismic imaging and interpretation of the salt structures are challenging, primarily due to the complex geometry of the salt bodies and typically strong velocity contrast with the neighboring sediment layers. The quality of seismic imaging in the North Sea is highly dependent on the quality of the estimated velocity model. We have studied diffracted arrivals originating from the salt flanks and adjacent sedimentary structures using a diffraction imaging technique. The diffracted waves carry valuable information regarding seismic velocity and the location of geologic discontinuities, such as faults, fractures, and salt delimitations. We apply a plane-wave destruction method to separate diffractions from our stacked data. We optimize imaging based on diffraction analysis by using a velocity continuation migration technique, which leads to an estimation of the optimum focusing velocity model. We determine that the diffraction-based approach significantly improves the seismic imaging adjacent to the salt diapirs and the neighboring layers when compared with a standard approach in which we mostly ignore the diffractions. The new poststack time-migrated results provide detailed information that optimizes our interpretation of the salt diapir itself (e.g., the width of the salt neck) as well as the sediment layers related to the rim synclines. Processing schemes such as prestack depth migration and full-waveform inversion may potentially provide high-resolution images of the salt structures. We only account for diffractions in nonmigrated stacked data to better constrain seismic velocity and improve imaging around the salt diapir. The obtained results are critical for reservoir characterization.

The Influence of an Interlayer on Dual Hydraulic Fractures Propagation

Multi-cluster hydraulic fracturing of long-range horizontal wells is an approach for enhancing the productivity of low-permeability shale reservoirs. In this study, RFPA-Petrol (rock failure process analysis on petroleum problems) is applied for modeling hydraulic fracture propagation in multilayered formations. RFPA-Petrol based on coupled hydraulic-mechanical-damage (HMD) modeling was first tested by modeling a laboratory scale experiment on a physical (cement) model with a single completion. The modeling demonstrated the capability of RFPA-Petrol for simulating hydraulic fracture propagation. Then, we used RFPA-Petrol to investigate how the difference in material properties between oil-bearing layers and interlayers and the fracturing fluid properties influence the propagation of dual fractures in multilayered laboratory-scale models. In this case, the models with geological discontinuities in the vertical direction are strongly heterogeneous and RFPA-Petrol simulations successfully modeled the fracture configurations.

A Generalized Coherence Algorithm Based on Kernel Correlation

Traditional coherence algorithms are mostly based on the assumption that seismic traces are Gaussian and linear correlative. However, for nonGaussian seismic traces, the linear correlation analysis cannot accurately describe the similarity between adjacent seismic traces. To solve this issue, we adopt the kernel correlation instead of the linear correlation to improve coherence-based algorithms. Note that the kernel correlation is a generalized correlation with various kernel functions. It provides a framework for expanding the coherence algorithm based on linear correlation. Moreover, we discuss how to choose an appropriate kernel function in detail. To testify the validity of the kernel correlation, we apply it to field data using different kernels. The results demonstrate the effectiveness of the proposed algorithm to describe geological discontinuity and heterogeneity, such as fluvial channels and faults.

3D diffraction imaging method using low-rank matrix decomposition

Seismic weak responses from subsurface small-scale geologic discontinuities or inhomogeneities are encoded in 3D diffractions. Separating weak diffractions from a strong reflection background is a difficult problem for diffraction imaging, especially for the 3D case when they are tangent to or interfering with each other. Most conventional diffraction separation methods ignore the azimuth discrepancy between reflections and diffractions when suppressing reflections. In fact, the reflections associated with a specific pair of azimuth-dip angle possess sparse characteristics, and the diffractions adhering to Huygens’ principle behave as low-rank components. Therefore, we have developed a 3D low-rank diffraction imaging method that uses the Mahalanobis-based low-rank and sparse matrix decomposition method for separating and imaging 3D diffractions in the azimuth-dip angle image matrix. The advantages of our 3D diffraction imaging method not only includes the handling of interfering events but also includes ensuring a better protection of weak diffractions. The numerical experiment illustrates the good performance of our method in imaging small-scale discontinuities and inhomogeneities. The field data application of carbonate reservoirs further confirms its potential value in resolving the masked small-scale cavities that can provide storage spaces and a migration pathway for petroleum.

Identifying and mapping potentially adverse discontinuities in underground excavations using thermal and multispectral UAV imagery

Geotechnical mapping, rock mass characterization, and inspections of underground excavations are critical to ensuring that excavation geometry and ground support design are appropriate for stable conditions. A lack of access, poor timing, or hazardous conditions are all factors that can prohibit engineering personnel from collecting high-quality data in the field. Advances in unmanned aerial vehicles (UAV) and portable sensor technology now allow engineering personnel to remotely capture data from underground workings that can be used for developing designs, generating inputs for analyses, and making more informed decisions. Ground falls are a source of many accidents in the underground environment, and they are the focus of many geotechnical investigations and analyses. In this study, the authors investigated how portable UAV-mounted thermal and multispectral imagers could be used to detect and quantify adverse geological discontinuities in hard rock masses that are structurally controlled. Multiple thermal, multispectral, RGB (red, green, and blue), and LiDAR (light detection and ranging) data sets were captured from 5 flights in sub-level open stopes at the Barrick Golden Sunlight Mine in the state of Montana in the western United States of America. Using off-the-shelf software, the thermal, RGB, and multispectral images were processed to create individual three-dimensional point clouds and meshes, which were georeferenced using the LiDAR data. Discontinuities identified in the three-dimensional point clouds and meshes were mapped using tools found in the freeware CloudCompare. One of the thermal models identified wedges that had developed in the crown pillar of an open stope, and 4 of the multispectral models contained enough detail for mapping individual discontinuities. The results of this investigation indicate that these portable imagers are viable tools that can be used to aid engineering personnel in identifying and mapping adverse geological discontinuities and unique rock mass composition. The techniques for capturing, processing, and interpreting thermal and multispectral imagery captured in underground excavations are described in this paper and can be used as the basis for future investigations at individual underground sites.

Análise derivativa de testes de bombeamento em aquíferos fissurais no município de Jundiaí/SP

A água subterrânea extraída de aquíferos cristalinos é a principal fonte de água em diversas partes do mundo, particularmente em regiões em desenvolvimento, como África subsaariana, Brasil e Índia. Décadas de pesquisas permitiram constatar uma forte complexidade do regime de fluxo nestes aquíferos, notadamente heterogêneos e anisotrópicos. Tradicionalmente, no Brasil, é assumido que a água se encontra armazenada em descontinuidades geológicas verticalizadas, associadas a zonas pretéritas de falhamentos. Tendo em vista o elevado potencial dos testes de bombeamento para diagnosticar o regime de fluxo, o presente trabalho consistiu na análise derivativa de testes de bombeamento de três poços situados no munícipio de Jundiaí/SP, com vistas ao aprimoramento da compreensão dos modelos conceituais de fluxo em aquíferos fissurais. Tentativas de ajustes com soluções para fraturas verticais e horizontais simples não produziram resultados satisfatórios. Embora nenhuma solução testada tenha se ajustado perfeitamente às curvas de rebaixamento obtidos nos testes de bombeamento, verificou-se que os melhores ajustes foram obtidos com o emprego da solução para aquíferos fraturados com dupla porosidade, compatível com o contexto geológico da área estudada. Novos estudos, similares ao descrito neste trabalho, devem ser conduzidos para o aprimoramento do entendimento das condicionantes de fluxo neste tipo de aquífero.

Hydromechanical modeling of unrestricted crack propagation in fractured formations using intrinsic cohesive zone model

Production from unconventional reservoirs has revolutionized the oil and gas industry strongly through the development of innovative applications, such as horizontal drilling and shale stimulation by hydraulic fracturing. However, the presence of geological discontinuities such as natural fractures (NFs) complicates the hydraulic fracturing treatment and affects the general geometry of the hydraulic fracture (HF). For that reason, it is essential to develop and implement robust numerical methods capable of simulating the phenomena present during hydraulic fracturing in a naturally fractured formation. This paper presents a novel mesh fragmentation technique to simulate unrestricted hydraulic fracture propagation in fractured media. This method is based on the hydromechanical zero thickness interface element combined with the cohesive zone model (CZM). An algorithm for inserting the especial triple-noded interface elements into conventional 2D finite element meshes is described. We introduce a simple but effective topological data structure that considers each finite element independent of others. The proposed topological structure ensures the generation of new nodes and faces which are necessary for the insertion of interface elements. The implementation is validated by simulating some cases of fracturing experiments in laboratory. The numerical procedure provides very good agreement with the laboratory tests and shows the robustness of the proposed computational approach. Some advantages and limitations of the proposed methodology are discussed.

Multispectral Phase-Based Geosteering Coherence Attributes for Deep Stratigraphic Feature Characterization

The deep exploration has become the focus of attention in the field of earth sciences. Coherence is a routine measure to identify structural and stratigraphic anomalies, such as faults, channels, and fractures in subsurface. However, deep seismic data typically suffer from a low signal-to-noise ratio and a weak reflection amplitude, thus it may not provide a better insight for seismic attribute analysis. The phase information has the ability to detect subtle changes in subsurface but it is sensitive to noise, thereby masking some stratigraphic features in the full-bandwidth data. To address these two issues, we propose a multispectral phase-based geosteering coherence method by combining coherence and spectral decomposition for deep stratigraphic feature characterization. The proposed method can effectively select and utilize the phase components of favorable spectral bands, which can detect different scale geologic discontinuities and reduce or avoid the effect of random noise in deep seismic data. Furthermore, corendering the coherence images of three different frequency components using red–green–blue blending can detect more geologic details in subsurface. The examples including 3-D physical modeling data and real seismic data set of carbonate reservoir from western deep formation are employed to demonstrate the effectiveness of the proposed method. The coherence attributes obtained from the proposed method can detect the weak or hidden geologic details clearer than the geosteering coherence calculated from the broadband seismic data, and it may serve as a future tool for detecting the distribution of geologic abnormalities in deep exploration.

Delineation of fault segments in mines using seismic source mechanisms and location uncertainty

The identification and quantification of faults or other geological discontinuities is an important task in managing seismic hazard in mines. Unstable slip along these faults may lead to seismic events with fatal or major economic consequences. Current approaches for delineating faults that are employed in mines rely on the interpretation of geological mapping. This mapping, however, may be sparse and can miss structures of potential concern. Clustering techniques are often used to associate seismic events to a common source process, but as previously used make no connection to the underlying physical processes.The Expectation Maximisation Algorithm is used in this study to identify probabilistic kernels representing active segments of faults. This soft assignment clustering method describes the kernels according to the seismic source mechanism, location and location uncertainty of the microseismic events. The method is tested on synthetic data and real data from an underground mine with the aim to delineate a previously unknown structure. Results using synthetic data illustrate how the incorporation of the seismic source mechanism improves the association of events to kernels in the case of scattered events with large location uncertainty. Application of the method to the real data indicates that the results can be interpreted at different levels, a smaller number of kernels provides good and robust description of the overall seismic behaviour, while using more kernels can provide insight into local variations in the location and orientation of faults.The resultant kernels have physical meaning in terms of the location and orientation of the structures. This provides geotechnical engineers at mines with improved tools to identify potentially hazardous areas in the mine and therefore manage these risks.

Probabilistic approach for open pit bench slope stability analysis – A mine case study

The geotechnical slope design of an open pit wall starts at the bench scale configuration. At this scale, the rock slope stability is governed primarily by the geological discontinuities within the rock mass and as a result, structurally-controlled failures (e.g. planar, wedge or toppling) are most likely to occur. The probabilistic approach offers a major advantage over the traditional deterministic method in that it accounts for the different degrees of variability and uncertainty often encountered in rock properties. This paper presents a bench slope stability assessment for an open pit mine in Peru using a probabilistic-based approach by coupling a kinematic analysis based on stereographic projection techniques followed by a kinetic analysis by means of the limit equilibrium method. Finally, these two probabilities are combined to provide an overall measure of the probability of failure (PoF) of the bench slope system. The case study is characterized by significant scatter in the geometrical and mechanical properties of the joints. Extensive surface mapping was conducted at 36 different sites following the ISRM suggested procedures. Several direct shear tests were carried out. It is shown that by combining field and laboratory measurements and engineering judgment, the probability density functions (PDF) of the discontinuity parameters can be obtained. These are then used in a Monte Carlo simulation process to compute both kinematic and kinetic probabilities of failure. The overall probability of failure aims to provide the design engineer with a tool to critically evaluate the bench performance from a geotechnical risk perspective and to provide a basis for future bench design optimization.

A Coherence Algorithm for 3-D Seismic Data Analysis Based on the Mutual Information

Coherence algorithm is widely used to describe geological discontinuity and subtle features of seismic data. Traditional coherence algorithms often use the linear correlation measurement to measure the relationship between two seismic traces. It does not work well because seismic data do not obey the normal distribution. To describe the coherence measurement of seismic data, we propose an improved coherence algorithm by combining the mutual information (MI) and third-generation coherence (C3) algorithm. The MI is a measurement of the general dependence and used to describe nonlinear relationships between two variables. Note that, the MI does not require that variables obey specific distribution (e.g., the normal distribution). Note that, we calculate precise MI values using the copula function. In addition, we introduce the information divergence to save calculation time by replacing the eigenvalue decomposition of the C3 algorithm. To demonstrate the effectiveness of the proposed algorithm, we apply it to field data. Field data experiments demonstrate the effectiveness of the proposed algorithm to describe geological discontinuity and heterogeneity, such as channels with different thicknesses.

Investigating Hydraulic Fracturing Complexity in Naturally Fractured Rock Masses Using Fully Coupled Multiscale Numerical Modeling

Naturally fractured rock mass is highly inhomogeneous and contains geological discontinuities at various length scales. Hydraulic fracture stimulation in such a medium could result in complex fracture systems instead of simple planar fractures. In this study, we carried out fully coupled multiscale numerical analysis to investigate some key coupled processes of fluid-driven fracture propagation in naturally fractured rock mass. The numerical analysis follows the concept of the synthetic rock mass (SRM) method initially developed in the discrete element method (DEM). We introduce a total of five case study examples, including fracture initiation and near wellbore tortuosity, hydraulic fracture interaction with natural fractures, multi-stage hydraulic fracturing with discrete fracture network (DFN), in-fill well fracturing and frac hits after depletion-induced stress change, and induced seismicity associated with fault reactivation. Through those case studies, we demonstrate that with an advanced numerical modeling tool, the complex fracturing associated with hydraulic fracturing in naturally fractured rock mass can be qualitatively analyzed and the extent of various uncertainties can be assessed.

Urban geophysical approach to characterize the subsoil of the main square in San Benedetto del Tronto town (Italy)

Most historic urban areas have a lack information regarding subsoil and buried structures (such as old utilities, private cellars, etc.). Therefore, when planning new engineering interventions, limited information of subsoil composition leads to greater risk when conserving and reclassifying historic urban areas. Obtaining information about potential sources of risk for infrastructures is important for civil engineering. However, only high-resolution information improves the management of important activities involving historic urban infrastructure. Although direct measurements can alleviate a lack of information concerning the subsoil, they are, generally, expensive, invasive and interferes with everyday use of the site for long periods. Therefore, the use of low or non-invasive technologies to quickly and accurately analyse the subsoil is preferable and strongly recommended. For this purpose, a new discipline, recently termed Urban-Geophysics, is developing rapidly. This paper describes an application of Urban-Geophysics in a historically important Italian town. In order to characterize the subsoil and identify the presence of natural voids and unknown anthropic underground structures such as cellars, an extensive geophysical investigation based on use of Electrical Resistivity Tomography and Ground Penetrating Radar to perform a survey in the town of San Benedetto del Tronto (Marche region, Italy) with a total time frame of around two days was carried out. Previously unknown buried structures and geological discontinuities were highlighted by using these geophysical techniques. In order to support and confirm the interpretation of the geophysical data, geotechnical drillings were carried out. Finally, direct data validated and supported the non-invasive geophysical results. The combined results have provided valuable information to the local authorities and engineers involved in the decision-making process for the construction of new underground structures.

A comparative evaluation of vertical fractures using different azimuthal electrical resistivity survey arrays

ABSTRACTThe identification of geological discontinuities such as fractures and faults is fundamental to geotechnical and hydrogeological characterization of rock masses. It is difficult to characterize these discontinuities, even by means of drill cores. Electrical resistivity has been historically employed as an alternative tool for the characterization of discontinuities, and new equipment for data acquisition and modern interpretation techniques have increased this possibility. However, when the characterization of vertical discontinuities is to be carried out in small areas, such as urban zones or for engineering works, traditional surveying may be impracticable, because it requires larger areas for proper acquisition. In these situations, azimuthal electrical resistivity surveying can be a good option, thanks to faster data acquisition and the possibility of reaching greater depths of investigation. There are several types of azimuthal arrays, but comparative analyses of their efficiency are still scarce. The objective of this study is to compare the applicability of several types of azimuthal resistivity surveys to identify vertical discontinuities in rock masses. The study was carried out in the laboratory using a scale model (tank), where boundary conditions could be well‐defined. We tried to replicate the natural conditions of a karstic area in Brazil, known to have hydrogeological and geotechnical problems associated with subvertical discontinuities. The simulations involved a limestone rock mass crosscut by a set of discontinuities with varied apertures. The selected arrays (square, equatorial dipole–dipole, Wenner and Schlumberger) encompassed varied inter‐electrode spacings and three different thicknesses of a saturated isotropic overburden, so as to represent field conditions. The azimuthal resistivity surveys, especially the Wenner array, proved to be promising in order to detect vertical discontinuities. The non‐collinear arrays are preferable when it comes to discerning the depth of investigation (square) and detecting electric anisotropy (equatorial dipole–dipole), despite small azimuthal distortions that were observed in relation to the strike of the discontinuities.

Understanding basement fracture architecture in Padra Field, South Cambay Basin, India, through full-azimuth imaging

Detection and characterization of fractures in reservoirs is of great importance for maximizing hydrocarbon productivity and recovery efficiency. Fractures play an important role in the producibility of unconventional reservoirs such as basement reservoirs. Basement reservoirs are typically found within metamorphic and igneous rock underlying a sedimentary basin, where faulting and tectonic uplift has led to creation of a fracture network. For fracture characterization, integration of information from seismic and nonseismic data such as cores and/or formation microimaging (FMI) logs is essential. Various seismic attributes such as coherency and curvature that are derived from reflection seismic data have been used for more than a decade to detect faults and fractures. In advanced seismic fracture detection technology, automatic fault extraction (AFE) from diffraction seismic data (discontinuity volume) more effectively detects finer scale features in seismic data. We demonstrate the utility of this methodology with an application to seismic data from the Padra Field, South Cambay Basin, India, where the basaltic Deccan Trap forms the basement, and hydrocarbons are produced from basement fractures. Diffraction imaging was applied during processing of the full-azimuth 3D-3C seismic data that cover this field. Using wavefield decomposition in the subsurface local angle domain, separate reflection (specular) and diffraction data volumes were produced. The high-resolution specular stack data imaged a prominent reflector well below the trap top, which is not visible in conventional seismic reflection data. Diffraction stack data also provided higher resolution fault definition and enhanced imaging of spatially consistent geological discontinuities. Subsequent application of the AFE technique to diffraction-imaged data yielded sharp and crisp definition of faults and fractures. We also performed velocity variation with azimuth analysis of 3D angle-azimuth reflection gathers to generate a fracture orientation map. Both sets of results were validated by fractures detected in FMI logs from wells in the field.

Mechanism of mine water inrush from overlying porous aquifer in Quaternary: a case study in Xinhe Coal Mine of Shandong Province, China

Mine water inrush from coal seam roof is one of the serious disasters that threaten safe production of coal mines. Identification of water inrush source and water inrush pathway is a key task for preventing and controlling such mine water hazards. A water inrush accident at approximately 1316 m3/h occurred at no. 3301 working face of Xinhe Coal Mine in Shandong Province, China. Multiple lines of evidence including characteristics of mine water inflow rate, dynamic monitoring data of water levels in different aquifers, geochemical fingerprinting, and drill hole core examination suggest that the water inrush source originated from the overlying Lower Quaternary porous aquifer with calcite cementation. Structural analysis and numerical simulation with FLAC3D indicate that a low-angle fault, DF49, and mining-induced fractures provided the pathway for the water inrush. Mining activities made the fault hydraulically conductive and connected to the Quaternary aquifer. The numerical simulation demonstrated that the water-conducting fracture zone in the coal seam roof extended to the fault. Groundwater gushed into the mining area from the Quaternary aquifer via the combined pathway of the activated fault and mining-induced fractures. Presence of the fault in the overlying formations played a critical role in occurrence of the water inrush. Results from this case study can be of reference to all coal mines with faults or other geological discontinuities present in the overlying formation. These discontinuities may significantly extend the height of the mining-induced water-conducting fracture zone in the roof and thus increase water inrush risks.

Revisiting theLepetodrilus elevatusspecies complex (Vetigastropoda: Lepetodrilidae), using samples from the Galápagos and Guaymas hydrothermal vent systems

The current distribution ranges of vent species result from the complex tectonic history of oceanic ridges. A growing number of DNA barcode studies report the presence of cryptic species across geological discontinuities that offset ridge systems and have gradually helped to draw a more precise picture of the historical migration pathways of vent fauna. We reexamined the phylogeny of species within the Lepetodrilus elevatus complex along the East Pacific Rise (EPR) ridge system in the light of new samples from the Galapagos Rift and the Guaymas Basin. Our analyses of mitochondrial cytochrome c oxidase subunit I gene sequences, coupled with morphological data, highlight the occurrence of a distinct lineage along the Galapagos Rift and offer new insight into the current distribution range of this species complex. Due to the absence of clear morphological diagnostic criteria and the potential overlap of these lineages at key locations, we recommend reassigning the taxon L. galriftensis to the subspecies level and maintaining the name L. elevatus for all clades along the EPR/Galapagos Rift system.