Recent Milestones in Unraveling the Full-Field Structure of Dynamic Shear Cracks and Fault Ruptures in Real-Time: From Photoelasticity to Ultrahigh-Speed Digital Image Correlation

The last few decades have seen great achievements in dynamic fracture mechanics. Yet, it was not possible to experimentally quantify the full-field behavior of dynamic fractures, until very recently. Here, we review our recent work on the full-field quantification of the temporal evolution of dynamic shear ruptures. Our newly developed approach based on digital image correlation combined with ultrahigh-speed photography has revolutionized the capabilities of measuring highly transient phenomena and enabled addressing key ques- tions of rupture dynamics. Recent milestones include the visualization of the complete displacement, particle velocity, strain, stress and strain rate fields near growing ruptures, capturing the evolution of dynamic friction during individual rupture growth, and the detailed study of rupture speed limits. For example, dynamic friction has been the big- gest unknown controlling how frictional ruptures develop but it has been impossible, until now, to measure dynamic friction during spontaneous rupture propagation and to understand its dependence on other quantities. Our recent measurements allow, by simul- taneously tracking tractions and sliding speeds on the rupturing interface, to disentangle its complex dependence on the slip, slip velocity, and on their history. In another application, we have uncovered new phenomena that could not be detected with previous methods, such as the formation of pressure shock fronts associated with “supersonic” propagation of shear ruptures in viscoelastic materials where the wave speeds are shown to depend strongly on the strain rate

Multi-Site Application of the Geomechanical Approach for Natural Fracture Exploration

In order to predict the nature and distribution of natural fracturing, Advanced Resources Inc. (ARI) incorporated concepts of rock mechanics, geologic history, and local geology into a geomechanical approach for natural fracture prediction within mildly deformed, tight (low-permeability) gas reservoirs. Under the auspices of this project, ARI utilized and refined this approach in tight gas reservoir characterization and exploratory activities in three basins: the Piceance, Wind River and the Anadarko. The primary focus of this report is the knowledge gained on natural fractural prediction along with practical applications for enhancing gas recovery and commerciality. Of importance to tight formation gas production are two broad categories of natural fractures: (1) shear related natural fractures and (2) extensional (opening mode) natural fractures. While arising from different origins this natural fracture type differentiation based on morphology is sometimes inter related. Predicting fracture distribution successfully is largely a function of collecting and understanding the available relevant data in conjunction with a methodology appropriate to the fracture origin. Initially ARI envisioned the geomechanical approach to natural fracture prediction as the use of elastic rock mechanics methods to project the nature and distribution of natural fracturing within mildly deformed, tight (low permeability) gas reservoirs. Technical issues and inconsistencies during the project prompted re-evaluation of these initial assumptions. ARI's philosophy for the geomechanical tools was one of heuristic development through field site testing and iterative enhancements to make it a better tool. The technology and underlying concepts were refined considerably during the course of the project. As with any new tool, there was a substantial learning curve. Through a heuristic approach, addressing these discoveries with additional software and concepts resulted in a stronger set of geomechanical tools. Thus, the outcome of this project is a set of predictive tools with broad applicability across low permeability gas basins where natural fractures play an important role in reservoir permeability. Potential uses for these learnings and tools range from rank exploration to field-development portfolio management. Early incorporation of the permeability development concepts presented here can improve basin assessment and direct focus to the high potential areas within basins. Insight into production variability inherent in tight naturally fractured reservoirs leads to improved wellbore evaluation and reduces the incidence of premature exits from high potential plays. A significant conclusion of this project is that natural fractures, while often an important, overlooked aspect of reservoir geology, represent only one aspect of the overall reservoir fabric. A balanced perspective encompassing all aspects of reservoir geology will have the greatest impact on exploration and development in the low permeability gas setting

3D-modelling of fault-induced small-scale secondary fracturing in crystalline rocks

The objective of this Thesis was to develop new methods to model the microstructures within the bedrock, as these models improve the understanding of the properties of the micro-scale fracture networks and could be further applied to improve the interpretation of kinematics in the deformation zones and micro-scale hydrological properties of the different rock types. The second aim of the study was to compare datasets generated from the same samples by two alternative methods: X-ray Ct-scanning and the new 3D- grinding method. The study area is located in the municipality of Geta, in the northern parts of the Åland Islands, southern Finland. The Geta fault is a sub-vertical NE-SW trending dextral strike-slip fault. The 3D character of the fault and its well-developed damage zone allows studying the fault and its deformation zone in various scales and methods. Setting for the 3D-samples was defined by field observations, 2D fracture and fault mapping from orthophotographs and 3D-photogrammetry models, which allow correlation of the 3D-fracture network characteristics in variable scales and with regional 2D-datasets from recent and ongoing investigations (Orrengrund and other MIRA-3D project targets). 3D-samples were drilled into 50*50/60mm sized drill cores for the 3D-grinder. Two of the 3D-samples were first Ct-scanned and grinded afterwards with 3D-grinder for making possible the micro-scale topology analyses in different sample depths and 3D-modelling on microstructures. Results show that grinding tomography images are accurate and many different details can be viewed from them. The grinding tomography method allows generating data based on which microstructures can be modeled and observed with micrometer accuracy. The results of the 3D-modelling indicate that the orientations and dips of the micro-scale secondary fracturing corresponds to the macro-scale fracturing within the damage zones. However, fracture intersection affects the fracture geometries in micro-scale but not in larger scale. Micro-scale topology analyses show very little variations compared to macro-scale analyses

Lateral spreading and associated slope processes in fractured rock slabs

Landslides of the lateral spreading type, involving brittle geological units overlying ductile terrains, are a common occurrence in the sandstone and limestone plateaux of the northern Apennines of Italy. These instability phenomena can become particularly risky, when historical towns and cultural heritage sites built on the top of them are endangered. Neverthless, the mechanisms controlling the developing of related instabilities, i.e. toppling and rock falls, at the edges of rock plateaux are not fully understood yet. In addition, the groundwater flow path developing at the contact between the more permeable units, i.e. the jointed rock slab, and the relatively impermeable clay-rich units have not been already studied in details, even if they may play a role in this kind of instability processes, acting as eventual predisposing and/or triggering factors. Field survey, Terrestrial Laser Scanner and Close Range Photogrammetry techniques, laboratory tests on the involved materials, hydrogeological monitoring and modelling, displacements evaluation and stability analysis through continuum and discontinuum numerical codes have been performed on the San Leo case study, with the aim to bring further insights for the understanding and the assessment of the slope processes taking place in this geological context. The current research permitted to relate the aquifer behaviour of the rocky slab to slope instability processes. The aquifer hosted in the fractured slab leads to the development of perennial and ephemeral springs at the contact between the two units. The related piping erosion phenomena, together with slope processes in the clay-shales led to the progressive undermining of the slab. The cliff becomes progressively unstable due to undermining and undergoes large-scale landslides due to fall or topple

Distict element numerical modelling of volcanic debris avalanche emplacement geomechanics.

Catastrophic collapse of volcanic edifices is a relatively common phenomenon in the geological record, representing the largest subaeriallandsliding events on Earth. Subsequent volcanic debris avalanche (VDA) runout lengths often exceed 50 km and inundated areas may be greater than 1,000 km2. The geomechanical processes that occur during emplacement, however, remain poorly understood as emplacement processes must generally be inferred from deposit analysis. Summarizing the literature, this thesis first introduces the general factors that control edifice collapse, mechanisms thought to control avalanche mobility and commonly observed deposit features. The mechanisms which have led to the formation of characteristic deposit features specifically are then reviewed; commonly discussed themes are then used to develop a general emplacement model which summarizes the geomechanical evolution of VDAs. This model is then tested by analyzing orthophotographic images of VDA deposits; common deposit morphologies are observed in each case, suggesting a common deformation sequence may occur during emplacement. To better understand emplacement processes, a distinct element numerical model is then created. Initial unbonded particulate avalanche simulations allow spatial/temporal variations in avalanche body stress, energy and deformation to be considered in relation to the development of characteristic deposit features. A more sophisticated bonded particle model is then utilized to allow the consideration of emerging brittle behaviour. Resulting simulations display the development of characteristic VDA deposit features from initial block sliding and horst and graben development. Evolution to a fully-flowing granular avalanche occurs through the initiation and propagation of faults generated due to stresses in the avalanche body, reflective of the proposed common deformation sequence. Features commonly observed in VDA deposits, such as toreva blocks and surface hummocks, are created in the bonded avalanche simulations. Use of this innovative numerical model therefore allows for new insight into the geomechanical evolution of rock and debris avalanches to be developed

Mechanical Modeling of Natural and Anthropogenic Fluid-Rock Interactions: Volcano Deformation and Induced Seismicity

abstract: The dynamic Earth involves feedbacks between the solid crust and both natural and anthropogenic fluid flows. Fluid-rock interactions drive many Earth phenomena, including volcanic unrest, seismic activities, and hydrological responses. Mitigating the hazards associated with these activities requires fundamental understanding of the underlying physical processes. Therefore, geophysical monitoring in combination with modeling provides valuable tools, suitable for hazard mitigation and risk management efforts. Magmatic activities and induced seismicity linked to fluid injection are two natural and anthropogenic processes discussed in this dissertation. Successful forecasting of the timing, style, and intensity of a volcanic eruption is made possible by improved understanding of the volcano life cycle as well as building quantitative models incorporating the processes that govern rock melting, melt ascending, magma storage, eruption initiation, and interaction between magma and surrounding host rocks at different spatial extent and time scale. One key part of such models is the shallow magma chamber, which is generally directly linked to volcano’s eruptive behaviors. However, its actual shape, size, and temporal evolution are often not entirely known. To address this issue, I use space-based geodetic data with high spatiotemporal resolution to measure surface deformation at Kilauea volcano. The obtained maps of InSAR (Interferometric Synthetic Aperture Radar) deformation time series are exploited with two novel modeling schemes to investigate Kilauea’s shallow magmatic system. Both models can explain the same observation, leading to a new compartment model of magma chamber. Such models significantly advance the understanding of the physical processes associated with Kilauea’s summit plumbing system with potential applications for volcanoes around the world. The unprecedented increase in the number of earthquakes in the Central and Eastern United States since 2008 is attributed to massive deep subsurface injection of saltwater. The elevated chance of moderate-large damaging earthquakes stemming from increased seismicity rate causes broad societal concerns among industry, regulators, and the public. Thus, quantifying the time-dependent seismic hazard associated with the fluid injection is of great importance. To this end, I investigate the large-scale seismic, hydrogeologic, and injection data in northern Texas for period of 2007-2015 and in northern-central Oklahoma for period of 1995-2017. An effective induced earthquake forecasting model is developed, considering a complex relationship between injection operations and consequent seismicity. I find that the timing and magnitude of regional induced earthquakes are fully controlled by the process of fluid diffusion in a poroelastic medium and thus can be successfully forecasted. The obtained time-dependent seismic hazard model is spatiotemporally heterogeneous and decreasing injection rates does not immediately reduce the probability of an earthquake. The presented framework can be used for operational induced earthquake forecasting. Information about the associated fundamental processes, inducing conditions, and probabilistic seismic hazards has broad benefits to the society.Dissertation/ThesisDoctoral Dissertation Geological Sciences 201

Longwall mining-induced fracture characterisation based on seismic monitoring

Despite several technological advancements, mining-induced fractures are still critical for the safety of underground coal mines. Rocking fracturing as a natural response to mining activities can pose a potential hazard to mine operators, equipment, and infrastructures. The fractures occur not only around the working face that can be visually measured but also above and in front of the working face and where geological structures are affected by mining activities. Therefore, it is of importance to detect and investigate the properties of mining-induced fractures. Mining-induced seismicity has been generated due to rock fracturing during progressive mining activities and can provide critical fracture information. Currently, the application of using seismic monitoring to characterise fractures has remained relatively challenged in mining because mining-induced fractures are initiated by stress change and strata movement after mineral extraction. Compared to seismic monitoring in the oil and gas industry, the fractures and seismic responses may show different characteristics. Therefore, seismic monitoring in mines lacks a comprehensive investigation of received seismic signals to the properties of induced fractures and the effect on mine workings by these fractures. Additionally, constraints such as the quality of seismic signals and the deficiency of correlation analysis of seismic events in underground mining pose great challenges in using seismic data for hazard prediction. This thesis aims to address these challenges in using seismic monitoring to understand and characterise mining-induced fractures by (1) calculating fracture properties related to seismic source location, magnitude and mechanism based on uniaxial seismic data, (2) spatial and temporal correlation analysis of seismic events, and (3) inspecting fracture distributions and simulation of the fractured zone in longwall coal mines. Firstly, since cheap and easily removable uniaxial geophones close to production areas are preferable in coal mines, a novel method to use uniaxial signal and moment tensor inversion to generate synthetic triaxial waves is designed for a comprehensive description of the fracture properties, including location, radius, aperture and orientation. Secondly, to apply seismic data for advanced analysis, such as rockburst prediction and caving assessment, the correlation of seismic events is proved to be quantitatively assessable, and their correlations may vary throughout the mineral extraction process. The spatial and temporal correlation of seismic event energy is quantitatively analysed using various statistical methods, including autocorrelation function (ACF), semivariogram and Moran's I analysis. In addition, based on the integrated spatial-temporal (ST) correlation assessment, seismic events are further classified into seven clusters to assess the correlations within individual clusters. Finally, several source parameters such as seismic moment (M0), seismic source radius (R), fracture aperture (τ), failure type and fracture orientation were used to characterise fractures induced by longwall mining. This thesis also presents the fracture patterns induced caused progressive longwall mining for the first time. Besides, a discrete element method (DEM) model with seismic-derived fractures is generated and proves the impact of mining-induced fractures on altering stress conditions during mineral extraction. In addition, with the analysis of the seismic source mechanism and a synthetic triaxial method, a discrete fracture network (DFN) is generated from monitored seismic events to restore complete induced fractures. Overall, the outcomes of this study lead to a comprehensive assessment of mining-induced fracture properties based on real-time seismic monitoring, demonstrating its significant potential for hazard prediction and improving the safety of resource recovery