Geological Discontinuities Research Articles

Real-time monitoring and FEMLIP simulation of a rainfall-induced rockslide

Abstract. Rockslides are a common and devastating problem affecting mining and other engineering activities all over the world; consequently, there have been many studies into their prediction and prevention. This study focused on a recent rockslide in an open-pit mine in Liaoning Province, China. The stability of the rock slope under excavation and rainfall conditions was monitored using an efficient real-time monitoring system. A further numerical analysis was performed using the finite element method with Lagrangian integration points (FEMLIP), and two forms of the normalized global second-order work were calculated to analyze the stability of the rock slope. In fact for the future it would be very interesting to compare measurements and simulations in real time, and not only to develop back computations after failure. The numerical results indicate that the rock slope remained stable during excavation, yet lost stability after subsequent rainfall. Water infiltration, along with a major geological discontinuity, degraded the strength of the weak zone and induced the rockslide. The monitoring approach presented its robustness and generality, and was worth being generalized. The numerical approach proposed the evolution of the safety factor, the monitoring data were compared, and the mechanism of the rockslide was determined. It could be used as an assistant tool for disaster prediction.

Insight on the western Mediterranean crustal structure from GOCE satellite gravity data

GOCE-derived gravity anomalies are analyzed over the western Mediterranean in order to evaluate the information brings by these satellite observations on the complex lithospheric structure of this region. Free-air and Bouguer anomalies deduced from the latest GOCE model are analyzed and compared with the theoretical response computed from the CRUST1.0 and EPcrust models. Semi-quantitative and qualitative approaches are performed to highlight major geological domains and discontinuities. We show that GOCE gravity anomalies in Western Mediterranean are mainly caused by the thickness of the crust. We also observe two GOCE gravity anomalies located in the centre of the Mediterranean basin that could be related to ocean spreading process, but their nature is unclear and an inhomogeneity of the GOCE gravity response of continental shelves that are not explain by regional 3D models. Our analyses also suggest the presence of two major regional crustal-scale North-East-trending lineament in the western Mediterranean: the well-known North Balearic transform zone and the Cassidaigne-Asinara lineament that separates the Sardinia and Corsica blocks. This later lineament is associated with an apparent sinistral offset of gravity anomalies in the western Mediterranean suggesting strike-slip displacement during opening of the Mediteranean basin. A normal-sense displacement along the Cassidaigne-Asinara lineament between Corsica and Sardinia is alternatively proposed and could be more consistent with geodynamic reconstructions from previous tectonic and paleomagnetic studies. Although additional local geological and geophysical investigations are required to establish the possible separation of Corsica and Sardinia, the homogeneous observations provided by GOCE on both terrestrial and marine areas brings new views on lithospheric scale structure and evolution of this region.

Numerical investigation of the impact of rock mass properties on propagation of ground vibration

Ground vibrations induced by the blasting of explosives can cause damage to the slope stability of mines. The important indicators of rock mass such as the geological discontinuities, physical and mechanical properties of rock can affect the creation and prediction of blast damage. This paper aims to assess the simultaneous effects of the above properties. For that purpose, peak particle velocity as the indicator of damage at two rock slopes was examined through a three-dimensional distinct element code (3DEC). The developed model simulated the dynamic pressure of detonation and provided useful results for analysis. The qualitative analysis demonstrated how significant it is to incorporate the rock mass properties in the model. Moreover, a statistical analysis was performed to quantify the effects of these properties. Therefore, the distance from blast, dip/direction of discontinuities, blast damage factor (Hoek–Brown failure criterion) and Geological Strength Index have the most influences on the slope damage, respectively. The moment failure analysis of 3DEC explained the Shear–Tension and Shear–Shear failures associated with the rock mass damage caused by blasting. The dip/direction of discontinuities plays a major role in the control or propagation of dynamic pressure and should be considered in blast damage control. If necessary, artificial discontinuities such as pre-splitting should be employed for better control and use of the favorable effects of geological discontinuities.

Reprocessing legacy three-dimensional seismic data from the Halfmile Lake and Brunswick No. 6 volcanogenic massive sulphide deposits, New Brunswick, Canada

We reprocessed legacy three-dimensional (3D) seismic data from the Halfmile Lake and Brunswick areas, both of which were acquired for mineral exploration in the Bathurst Mining Camp, New Brunswick. Each 3D seismic survey was acquired over known volcanogenic massive sulphide deposits and covered areas with strong mineral potential. Most improvements resulted from a reduction of coherent and random noise on prestack gathers and from an improved velocity model, combined with re-imaging with dip moveout corrections and poststack migration or prestack time migration. At Halfmile Lake, the new imaging results show the Deep zone and a possible extension of the sulphide mineralization at greater depth. True amplitude processing has shown that this anomaly has strong amplitudes and is offset from the Deep zone by a shallowly dipping fault (<15°). With the clearer geological context provided by our results, this anomaly, which appears as a stand-alone anomaly on an original image obtained by Noranda Exploration Ltd., becomes a defendable exploration target. Nonorthogonal acquisition geometry and receiver patches of the Brunswick No. 6 3D seismic survey generated artefacts after dip moveout processing that reduced the overall quality of the seismic volumes. By using a filtering approach based on the application of a weighted Laplacian-Gaussian filter in the Kx–Ky domain, we reduced the noise and improved the continuity of reflections. We also imaged the short and flat reflections observed previously only in the shallow part of prestack time migrated data. These short reflections appear as diffractions on the filtered stacked section with dip moveout corrections, indicating that they originate from small geological bodies or discontinuities in the subsurface.

Modelling of rock slope failures due to geological discontinuties to minimize risk from road cut failures

Rock slope failures are common issues on road cuts. Geological discontinuities are one of main causal factors for the occurrence of rock slope failures and most rocks exposed in road cuts show slope failures as rock falls. Most of the roads constructed across hilly areas have risk from such slope failures resulting in adverse effects on road uses. The aim of the present study is to model rock slope failures which can be used at the design stage to reduce the risk from structural geological features especially planer structures (foliation and joint planes). Two road traces which run across Sri Lankan mountain terrains (Avissawella – Hatton and Balangoda – Haputale roads) were used for the study. Field data on orientation of structural geological features and the type of rock involved in rock falls were collected from 44 locations along both road stretches. Dip Anlysist 2.0 was used for modelling the slope failures. Wedge failure, Toppling and Plane failure modes were considered. Wedge failure is the most common rock slope failure observed. When compared with field observations, the model shows 64% precision from 44 locations studied. The model revealed that the risk from road cut failures can be minimized by optimising existing road orientation and slope angle of the cut. The slope failures can be reduced by using this type of model analysis during the design stage of new roads.

Evaluation of the Effect of Fault on the Stability of Underground Workings of Coal Mine through DEM and Statistical Analysis

The rock mass around an excavation is generally traversed by different geological discontinuities such as faults, folds, slips, joints, etc. Fault is one of the major geological discontinuities which creates lot of difficulties during underground winning of coal. Entire stress regime and ground conditions in the formation are altered in and around the faults. Faults also impose detrimental effects by introducing impurities, including clay and various forms of mineral matter into the coal seams; opening of pathways for the influx of water and gas into the underground workings; displacing the coal seams upward/downwards making the coal seams difficult or sometimes impractical to mine. Appropriate evaluation of the effect of the fault on the stability of the underground workings is a requisite for safe design of the underground mining structures. In this paper, a study has been carried out to assess the effect of the fault on the stability of underground coal mines by numerical simulation with distinct element method (DEM). On the calibrated DEM model, parametric study has been performed by varying the selected parameters, the dip and the friction angles of the fault. The analysis of variance (ANOVA) shows that both the factors have statistically significant effect on the strength of the coal pillar. Similarly, the displacement of the immediate roof and the height of the disturbed strata are evaluated by the DEM modelling and statistical analysis when the fault passes through the middle of the gallery. The results of ANOVA for both cases indicate that the both factors have significant effect on the displacement of the immediate roof and the height of the disturbed strata. It is obtained from the study that the low angle fault causes high instability in the immediate roof. The paper has been supplemented with the field observations where instability in underground roadways of a coal mine in India is caused by the fault. It was observed in VK-7 incline mine of Singareni Collieries Company Limited, India that there was sudden failure of immediate roof of a roadway where a low angle fault crosses the middle of the roadway. The findings of the paper help to understand the behaviour of the coal pillar and the surrounding rock mass in the presence of the fault. The study would also help to take appropriate decisions about the unstable regions of the working safeguarding safety in underground coal mines.

Probabilistic estimate of rock mass static and dynamic demands for underground excavation stabilisation

Excavation damage under high in situ stress depends largely upon the potential block size associated with any violent ejection. The size and shape of the dynamic instability are largely controlled by the location, orientation and extent of the pre-existing geological discontinuities. A new methodology is presented in which the rock mass demand can be expressed in terms of the mass in tonnes of unstable rock that is ejected per unit area of the excavation surface where failure occurs. A probabilistic approach has been implemented to estimate the potential rock mass instabilities and their associated static and dynamic demands. The new methodology considers that the strain energy released by the rock mass during violent stress-driven failure is largely converted into kinetic energy of ejection for blocks. The estimated dynamic demand has been favourably compared with observations of rock mass damage in a number of underground excavations.

Numerical and Experimental Investigations of the Interactions between Hydraulic and Natural Fractures in Shale Formations

Natural fractures (NFs) have been recognized as the dominant factors that increase hydraulic fracture complexity and reservoir productivity. However, the interactions between hydraulic and natural fractures are far from being fully understood. In this study, a two-dimensional numerical model based on the displacement discontinuity method (DDM) has been developed and used to investigate the interaction between hydraulic and pre-existing natural fractures. The inelastic deformation, e.g., stick, slip and separation, of the geologic discontinuities is captured by a special friction joint element called Mohr-Coulomb joint element. The dynamic stress transfer mechanisms between the two fracture systems and the possible location of secondary tensile fracture that reinitiates along the opposite sides of the NF are discussed. Furthermore, the model results are validated by a series of large tri-axial hydraulic fracture (HF) tests. Both experimental and numerical results showed that the displacements and stresses along the NFs are all in highly dynamic changes. When the HF is approaching the NF, the HF tip can exert remote compressional and shear stresses on the NF interface, which results in the debonding of the NF. The location and value of the evoked stress is a function of the far-field horizontal differential stress, inclination angle of the NF, and the net pressure used in fracturing. For a small approaching angle, the stress peak is located farther away from the intersection point, so an offset fracture is more likely to be generated. The cemented strength of the NF also has an important influence on the interaction mechanism. Weakly bonded NF surfaces increase the occurrence of a shear slippage, but for a moderate strength NF, the hybrid failure model with both tensile and shear failures, and conversion may appear.

Computing the damage and fracture energy in a coal mass based on joint density

Coal joints and cleats are geological discontinuities that are the most important factors that affect the mechanical responses of a coal mass under stress. The joint and coal mass interaction and the mode of failure dominate the mechanical behaviour of jointed coal masses, and therefore the stability of coal excavations. The shear or mixed shear/tensile failure changes to tensile failure by increasing the confining pressure, discontinuity length and angle. This paper extends a thermodynamic approach to constitutive modelling of the coal mass by developing local and non-local damage models based on the joint and cleat density and the dip angle. A consistent and rigorous statistical framework is constructed, which incorporates both local and non-local features into the constitutive modelling. This is an important consideration in developing damage constitutive models based on the trajectory of the failure surfaces in a coal mass. An equation is derived to calculate the fracture energy which is a function of the joint density either in a single direction or crossed conditions.

Accurate diffraction imaging for detecting small-scale geologic discontinuities

Seismic diffractions contain valuable information regarding small-scale inhomogeneities or discontinuities, and therefore they can be used for seismic interpretation in the exploitation of hydrocarbon reservoirs. Velocity analysis is a necessary step for accurate imaging of these diffractions. A new method for diffraction velocity analysis and imaging is proposed that uses an improved adaptive minimum variance beamforming technique. This method incorporates the minimum variance, coherence factor, and correlation properties to improve the signal-to-noise ratio and enhance correlations. Our method can make seismic diffractions become better focused in semblance panels, allowing for the optimal migration velocity for diffractions to be accurately picked. Synthetic and field examples demonstrate that the migration velocity for the diffractions can differ from that for the reflections. The results suggest that the diffraction velocity analysis and imaging method is feasible for accurately locating and identifying small-scale discontinuities, which leads to the possibility of using this approach for practical application and seismic interpretation.

The Arsenic Fault-Pathfinder: A Complementary Tool to Improve Structural Models in Mining

In a mining operation, the structural model is considered as a first-order data required for planning. During the start-up and in-depth expansion of an operation, whether the case is open-pit or underground, the structural model must be systematically updated because most common failure mechanisms of a rock mass are generally controlled by geological discontinuities. This update represents one of the main responsibilities for structural geologists and mine engineers. For that purpose, our study presents a geochemically-developed tool based on the tridimensional (3-D) distribution of arsenic concentrations, which have been quantified with a very high-density of blast-holes sampling points throughout an open pit operation. Our results show that the arsenic spatial distribution clearly denotes alignments that match with faults that were previously recognized by classical direct mapping techniques. Consequently, the 3-D arsenic distribution can be used to endorse the existence and even more the real persistence of structures as well as the cross-cutting relationships between faults. In conclusion, by linking the arsenic fault-pathfinder tool to direct on field fault mapping, it is possible to improve structural models at mine scale, focusing on geotechnical design and management, with a direct impact in the generation of safety mining activities.

Attribute amalgamation-aiding interpretation of faults from seismic data: An example from Waitara 3D prospect in Taranaki basin off New Zealand

A suitable combination of seismic attributes amalgamated with interpreter's acquaintances results into a meta-attribute that augments interpretation of geological discontinuities from seismic data. Application to time migrated 3D seismic data, acquired over the Waitara prospect covering an area of ~122 km2 in the Taranaki basin off New Zealand, clearly demonstrates this fact. We condition the seismic data to make the geologic structures free from noise, and then use it to define a set of attributes grouped into three different cases e.g., Case I, II and III. Finally, the attribute sets are trained over example locations selected from the data volume through a fully connected multilayer perceptron (MLP) based on artificial neural network (ANN) to generate a meta-attribute called as fault cube (FC) for each case. It is observed that the FCs obtained from these cases efficiently illuminate geological discontinuities. However, the FC, obtained from Case III attribute amalgamation, brings out the thinned and sharpened fault images from seismic data. This demonstrates the efficacy of amalgamation of suitable attributes for an efficient interpretation of geologic structures from seismic data.

Estimation of petrophysical parameters using seismic inversion and neural network modeling in Upper Assam basin, India

Estimation of petrophysical parameters is an important issue of any reservoirs. Porosity, volume of shale and water saturation has been evaluated for reservoirs of Upper Assam basin, located in northeastern India from well log and seismic data. Absolute acoustic impedance (AAI) and relative acoustic impedance (RAI) are generated from model based inversion of 2-D post-stack seismic data. The top of geological formation, sand reservoirs, shale layers and discontinuities at faults are detected in RAI section under the study area. Tipam Sandstone (TS) and Barail Arenaceous Sandstone (BAS) are the main reservoirs, delineated from the logs of available wells and RAI section. Porosity section is obtained using porosity wavelet and porosity reflectivity from post-stack seismic data. Two multilayered feed forward neural network (MLFN) models are created with inputs: AAI, porosity, density and shear impedance and outputs: volume of shale and water saturation with single hidden layer. The estimated average porosity in TS and BAS reservoir varies from 30% to 36% and 18% to 30% respectively. The volume of shale and water saturation ranges from 10% to 30% and 20% to 60% in TS reservoir and 28% to 30% and 23% to 55% in BAS reservoir respectively.

Studying of Environmental Effects of a Disposal Area Leachates with Geophysical and the Water Analyses Methods

Yapılan çalışmada Sivas deponi alanından çıkan sızıntı sularının yarattığı kirliliğin yayılımı jeofizik ve su kimyası yöntemleri ile incelenmiştir. Sızıntı suyunun akış yönü bölgede egemen olan jeolojik süreksizlikler ve jeomorfolojik yapılar tarafından denetlenmektedir. Buna göre, yüzeydeki sızıntı suyu Haçin deresinden, yakınındaki Kızılırmak nehrine doğru akmaktadır. Bu çerçevede, Haçin deresinin iki tarafında yayılan sızıntı suyu kirliliğinin zemindeki yayılımı irdelenmiştir. Kirliliğin varlığı ve yayılımı, elektromanyetik iletkenlik (EMC-Electromagnetic Conductivity) yöntemi ve alınan su örneklerinin su kimyası analizleri ile belirlenmiştir. EMC ölçümlerinden elde edilen görünür iletkenlik grafikleri, sızıntı suyunun yataydaki yayılım sınırlarını göstermiştir. Su kimyası analizleri ise sudaki kirliliğin sızıntı suyundan kaynaklandığını göstermiştir. Sonuç olarak yöredeki Kuvaterner yaştaki toprak örtünün yanı sıra yüzey ve yeraltı su kaynaklarının sızıntı suyundan ileri gelen kirliliğin tehdidi altında olduğu belirlenmiştir. Oluşan kirliliğin ise katı atık deponi alanının daha uzun süre kullanılacağı gözetilerek, acilen ÇED (Çevresel Etki Değerlendirme) süreci çerçevesinde denetim altına alınması gerektiği ortaya konulmuştur. 

Caracterização hidroquímica e isotópica dos aquíferos fissurais da região de Itabuna/BA

Os aquíferos cristalinos representam importantes reservas de águas em termos mundiais e são caracterizados pela complexidade que governam o movimento e o armazenamento da água em suas descontinuidades geológicas. Igualmente complexos são os mecanismos atuantes na interação rocha/água. O entendimento dos processos físicos e geoquímicos atuantes nestes aquíferos é crucial para sua gestão. Este trabalho apresenta uma caracterização hidroquímica preliminar das águas do aquífero fissural na região de Itabuna/BA, a partir da identificação dos tipos hidroquímicos dominantes. As análises químicas indicam elevada variabilidade da composição química da água, que pode estar associado à ocorrência de vários processos distintos, incluindo a interação com a água marinha e diferentes graus de interação com as rochas que hospedam a água em suas descontinuidades geológicas. As amostras de águas subterrâneas foram reunidas em dois grupos distintos: o primeiro grupo é representado por água do tipo cloretado sódico e menores valores de condutividade elétrica, associados às amostras coletadas nas porções mais rasas de sub-superfície; o segundo grupo é representado por tipos hidroquímicos bicarbonato sódico, cálcico ou cálcio-magnesianos, provavelmente associadas às águas com elevado grau de interação com as rochas do aquífero, proveniente de recarga pretérita. Os resultados de análise isotópica da água subterrânea e meteórica sugerem a existência de um leve excesso de deutério, decorrente de condições climatológicas locais.

Assessment of landslide occurrences in Serra do Mar mountain range using kinematic analyses

Due to its meteorological, geological and geomorphological settings, Brazilian’s Atlantic coast have suffered severe landslide phenomena, particularly along the cost of São Paulo, Rio de Janeiro, and Santa Catarina states. Studies have shown that these processes are complex and triggered by several factors. However, within the study area, many landslide locations are placed on gneiss migmatitic terrain with pronounced geological discontinuities. The objective of this paper is to assess landslide occurrences in Serra do Mar (Caraguatatuba, SP) and investigate, by means of kinematic analysis, how they correlate with foliation and fractures present on the rocks. Kinematic analysis is used to investigate which discontinuities are more favorable to landslides occurrence and their relationship with slope directions. Structural information and kinematic data models are used to evaluate cause–effect relationship of landslide processes. Results show that dip slopes, with NE–SW direction, have greatest potential for the occurrence of planar landslides. On the other hand, anti-dip and cross-dip slopes have a greater potential to develop wedge landslides. Rock weathering and fluid flux along foliation planes can intensify mass movement. Such effect is also seen in residual soil, since they are able to preserve relict structures of the rock. This study contributes to a better understanding of how geological structures, preserved in tropical residual soils, influence shallow landslides.

Influence of seismic diffraction for high-resolution imaging: applications in offshore Malaysia

Small-scale geological discontinuities are not easy to detect and image in seismic data, as these features represent themselves as diffracted rather than reflected waves. However, the combined reflected and diffracted image contains full wave information and is of great value to an interpreter, for instance enabling the identification of faults, fractures, and surfaces in built-up carbonate. Although diffraction imaging has a resolution below the typical seismic wavelength, if the wavelength is much smaller than the width of the discontinuity then interference effects can be ignored, as they would not play a role in generating the seismic diffractions. In this paper, by means of synthetic examples and real data, the potential of diffraction separation for high-resolution seismic imaging is revealed and choosing the best method for preserving diffraction are discussed. We illustrate the accuracy of separating diffractions using the plane-wave destruction (PWD) and dip frequency filtering (DFF) techniques on data from the Sarawak Basin, a carbonate field. PWD is able to preserve the diffraction more intelligently than DFF, which is proven in the results by the model and real data. The final results illustrate the effectiveness of diffraction separation and possible imaging for high-resolution seismic data of small but significant geological features.

Relationship of the discontinuities and the rock blasting results

The Geological discontinuities such as joint are the most common discontinuities present in the rock mass. A model scale study was carried out to evaluate the effect of the joints on rock blasting. Single hole tests at three selected burdens (optimum, less than optimum and more than optimum) were done on six different joint orientations. The joint orientation angles were 0°, 30°, 60°, 90°, 120° and 150° rotating in anticlockwise direction from the floor of the bench in a plane perpendicular to the free face. Bench models of dimensions 515x335x215 mm with a bench height of 50 mm were prepared by binding sandstone slabs of 25 mm thickness with an adhesive. The models were blasted by n°6 electric detonators. The dynamic and static properties of sandstone are given. The bench crater formed and the fragmentation produced were predominantly influenced by the position of charge with respect to the joint orientation. Severe toes were noticed in models with vertical joints and with joints dipping away from the face. Over breaks were observed in horizontally bedded models and in models with joints dipping towards the free face. Over breaks were observed in horizontally bedded models and in models with joints dipping towards the free face. The size of the broken fragments at 20 mm burden was found to be finer than the fragments obtained at 30 mm and 40 mm burdens for all joint orientations except vertical.

Assessing the tensile strength of rocks and geological discontinuities via pull-off tests

Assessing the tensile strength of rocks and geological discontinuities via pull-off tests

Analysis of the State for a Coal Massif Enclosing in-Seam Working and a Geological Discontinuity

A model of the geomechanical state for a rock massif with strength anisotropy has been developed. Themassif encloses an in-seamworking near the geological discontinuity. The developed models implemented on the basis of pseudo-load method and displacement discontinuity method. Based on the results obtained, stress diagrams and discontinuity zones of the massif in the vicinity of the mine working are constructed. The regularities of changing the state of the massif when its parameters are varied and the relative positions of the mine working and geological discontinuity are revealed.