Better study of geological layers through structural geology modeling

Structural geology as a partition of geological science aiming at describing the structures–joints, faults, folds at various scales. The anisotropy and inhomogeneity that can occur in natural rock masses. Open fault breccias facilitate the movement.

Рубрика Геология, гидрология и геодезия
Вид статья
Язык английский
Дата добавления 21.04.2016
Размер файла 4,5 M

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Structural geology (a partition of geology aiming at describing the structures-joints, faults, folds, etc. - at various scales) can be used in the field of rock mechanics and rock engineering, and particularly in underground engineering works (tunneling and rock caverns) together more reliable data for empirical stability analyses and deterministic calculation models. Methods of structural geology are introduced, and their applications in rock mechanics/rock engineering are considered a priority in particular through the observation of faults and joints arrest. Structural geology permits a better understanding of the origin, the chronology and the mechanical conduct of discontinuities, and furthermore a more accurate rock mass characterization and rock mass classification, as well as a validation of the actual stress regime. Illustrations chose from different places of using structural models are also given with emphasize on the obligatoriness and the way to build a model at each stage of an underground draft, from sector selection to detection and building, to ensure the quality and reasonableness of rock mechanical data and hypothesis. Besides the geological models could make forward modelled and restored, permitting to visualise the education and development of geological structures and connect this to the final model geometry and map pattern. In particular, all the features of rock can now be incorporated into such analyses, i.e. discontinuities, in homogeneity, anisotropy and non-elastic behaviour. However, in order for the simulations to validly represent the in situ rock mass, it is essential that the geological nature and structure of the rock mass are understood and incorporated in the modelling. We discuss this structural geology investment to rock mechanics in the context of geological structures, fractures, scale,water flow and modelling, plus rock engineering design.

Geological structures and the rock mechanics modelling problem.

In Figures 1 & 2, we begin by illustrating the anisotropy and inhomogeneity that can occur in natural rock masses. The angular box folds that are visible in both examples reflect the high mechanical anisotropy of the two systems. Immediately, it is evident that representing the rock mass as a homogeneous and isotropic continuum may not be valid, even as a first approximation. Both the inhomogeneity and anisotropy can be formed as a result of primary processes such as sedimentation which results in the formation of bedding, either within a single bed, such as a shale, or multiple beds, such as a cyclothemic sequence of repeated sandstone, limestone and mudstone. In addition to such an intrinsic anisotropy, rocks often develop induced anisotropies during geological deformation events, for example, the formation of rock cleavage and schistosity in response to increases in the differential stress and temperature linked to metamorphism (see Fig. 2). Depending on the requirements of the computer modeling for rock engineering design, it could well be essential to accommodate such inhomogeneity and anisotropy within the numerical program being used--which a detailed study of the geology can reveal.

Figure 1. The regular succession of sandstones

Figure 2. Folded slaty cleavage, an anisotropy and shales that characterize the folded turbidites induced in the rock by metamorphism of Northcote Mouth, Bude, S.W. England represent an important intrinsic mechanical anisotropy

Moreover, rock masses contain fractures, and a knowledge of the fracture type and filling can be crucial for adequatem fracture representation in the modelling, as illustrated in Figures 3 & 4. Additionally, fracture networks are generated by the gradual superposition of individual fracture sets over geological time, each linked to a different stress regime. The properties of the resulting networks are complex and have a major impact onthe subsequent movement of fluids through the rock mass. Because of the interaction of early fractures with later fractures (the early fractures act a barriers to the propagation of later fractures which characteristically end, i.e. abut, against them) the final geometry of the network is sensitive to the order in which the component fracture sets are superimposed.

Figure 3. Open fault breccias facilitate the movement

Figure 4. Fine grained fault gouges generally inhibit of fluids along faults. luid movement

Modelling.

The key to rock engineering design is the ability to predict the future, i.e., to be able to predict what will happen when an excavation with a given geometry is made at a certain depth and at a certain orientation in a given rock mass. This means that some form of model is required. This modelling support for rock engineering is illustrated in Figure 5 which summarises the eight main methods supporting rock engineering design.

Figure 5

geological rock structural

The first new effective module is Fault Analysis Modelling. This module allows rapid evaluation of throw distribution, across-fault juxtaposition and fault sealing capacity in 3D. Combined with statistical analysis of fault displacement and scaling relationships, the tool provides powerful validation of geological interpretations and insights into the economic significance of faults. Uniquely, the module can be integrated with restoration workflows using Move's 3D Kinematic Modelling and Stress Analysis modules to provide a complete temporal fault displacement and seal investigation. This workflow delivers key information on potential baffles or conduits to flow at the time of hydrocarbon generation and migration. The sealing potential of faults and joints encountered in a wide range of mineral and ore systems can also be investigated using this approach.

The second new module for is Fault Response Modelling.The Fault Response Modelling module is a highly versatile tool that can be used to validate your interpretation, identify highly fractured zones and realistically model stress perturbations around faults and other discontinuities. The module considers mechanical properties to reproduce fault-related deformation and provides a quantitative assessment of the surrounding fracture system. Faulting is simulated using a boundary element method with triangular elastic dislocations. This approach allows complex faulting scenarios to be quickly tested and evaluated.

Advantages and disadvantages.

Like any engineering tool, Fault Analysis Modelling and Fault Response Modelling has advantages and disadvantages. For example it provides a logical means to model and analyse system failure modes even for large systems. It is oriented to identifying faults that have a bearing on the undesired event. In addition as a modelling technique for assessing the reliability of systems it is well developed and accepted and it is an efficient tool when it comes to modeling the potentially large number of events and event combinations that can lead to failure. Disadvantages of these models-is that they may not follow a system flow diagram and, as a result, it may not be easy to relate the system flow to the logic that leads to failure in the model. Also FAM and FRM are not unique, because they can be constructed in different ways and, therefore, have different appearances.

"Application".

Methods of structural geology are used in rock mechanics with a view exploring the rock faults and joints arrest. Also Structural geology allows a better understanding of the origin, the chronology and rock mass classification, as well as a validation of the actual stress regime. The Fault Response Modelling and Fault Analysis Modelling, combined with the structural modelling and restoration functions of Move has many potential applications in earthquake-induced displacement and stress modelling, hydrocarbon exploration and the mining industry. Furthermore they using in order to plot 3D colour maps and create 2D strike projection for multiple faults and to define lithologies and rock properties database. For example they used to predict the orientation of fractures in a pop-up block within the La Concepcion oil field in the north-western Maracaibo Basin of venezuela and used to model displacements and stress changes that followed the 2008 Nura earthquake in Kyrgyzstan.

"Conclusion".

Geological structures and the rock mechanics modeling are usually have various problems, such as mechanical anisotropy of rocks during geological deformation events. These include formation of rock cleavage and schistosity in response to increases in the differential stress and temperature linked to metamorphism. To study these problems and avoid them we use modelling. To sum up, in this chapter we use new Fault Analysing Modelling and Fault Response Modelling, that can us help better provide a logical means to model and analyse system failure modes even for large systems. In addition, using structural geological modelling, we take a step in the further development of qualitative studies of rocks, and obtain high-quality products from the rock, while not making a lot of complicated efforts.

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