Seismic modeling is a technique for simulating wave propagation through the subsurface. For a given geological model, seismic modeling allows us to generate snapshots of wave propagation and synthetic data. In my dissertation, for real seismic events I have chosen to implement the finite-difference modeling technique. When adequate discretization in space and time is possible, the finite-difference technique is by far one of the most accurate tools for simulating elastic-wave propagation through complex geological models. In recent years, a significant amount of work has been done in our group using 2D finite-difference modeling. For complex salt structures which exploration and pro- duction industries meet today, 2D finite-difference modeling is not sufficient to study subsalt imaging or the demultiple of subsalt models. That is why I have developed a 3D finite-difference modeling code. One of the key challenges that I have met in developing the 3D finite-difference code is to adapt the absorbing boundary conditions. Absorbing boundary conditions are needed to describe the infinite geological models by limited computing domain. I have validated the 3D finite-difference code by comparing its results with analytic solutions. I have used 3D finite-difference program to generate data corresponding to 3D complex model which describes salt and subsalt structures of Gulf of Mexico. The resulting data include reflections, diffractions and other scattering phenomena. I have also used finite-difference program in anisotropic context to show that we can effectively predict shear-wave splitting and triplication in the data. There are new sets of events that are not directly recorded in seismic data, they have been called virtual events. These events are turning to be as important as real events in modern data processing. Therefore we also have to learn how to model them. Unfortunately, they cannot yet be modeled directly from finite-difference. Here I will describe how to model these events by using cross correlation type representation theorem. As illustration of how important of virtual events for seismic data process- ing, I also described an internal multiple attenuation technique which utilized virtual events.

<p dir="ltr">The Alpine Fault, located in the South Island, New Zealand, is a subject of intense geological study due to its potential to trigger large earthquakes. It encompasses a complex system with the interplay of mechanics, thermodynamics, and fluid. Gaining insights into these systems not only enhances our understanding of the fault but also holds the potential to guide risk mitigation efforts.</p><p dir="ltr">The damage extent and fracture networks within the metamorphic rock mass adjacent to the fault can be effectively characterized by seismic anisotropy, an elastic property of rock, where seismic waves travel at different speeds with variation directions. This thesis presents a comprehensive exploration of seismic anisotropy in the hanging wall immediately adjacent to the principal slip zone of the Alpine Fault in New Zealand. Leveraging the borehole seismic data from a unique scientific drilling project and advanced numerical modeling techniques, the ultimate goal is to invert and parameterize the bulk seismic anisotropy.</p><p dir="ltr">Motivated by these challenges, the thesis undertakes several key initiatives: The first effort focuses on gaining a comprehensive understanding of an innovative method for seismic measurement: Distributed Acoustic Sensing (DAS) – examining its operational principles, factors influencing observed wavelets, and how it contrasts with traditional point sensors for accurate interpretation. Subsequently, the research introduces the implementation of an open-source seismic wave solver designed for modeling elastic wave propagation in complicated anisotropic media. This solver is further optimized for computational efficiency with its performance rigorously benchmarked.</p><p dir="ltr">With this preparedness, the inversion is further facilitated by high-performance computing (HPC) and a deep-learning algorithm specifically designed for automatically picking transit times. The inverted bulk elastic constants, compared to the intact rock, reveal 28% to 35% reductions in qP-wave velocity, characterizing the damage due to mesoscale fracture. Further analysis sheds light on the existence of orthogonal fracture sets and an intricate geometrical arrangement that agree with the previous borehole image log. This represents an advancement in our ability to characterize and understand the geologic processes with seismic anisotropy.</p>

The oil and gas industry defines ultra-deep-water regions as areas in which water depths are greater than 1500 m. It is now well established that there are hydrocarbons in these regions. The reservoirs in these areas are generally located below basalt rocks or below salts. The focus of this thesis is to understand reflections, refractions, diffractions and scattering for acoustic lenses located below basalt rocks. The results of this study can potentially be used to understand the effect of tube waves on borehole seismic data in ultra-deep waters. Finite-difference modeling technique was used for this study. Finite-difference modeling allowed us to model refractions, reflections, diffractions and scattering; actually all events in surface seismic data, as well as borehole seismic data can be modeled. However, because of limited computational resources, this study will be based on a 2D finite difference instead of a 3D finite difference. This limitation implies that laterally infinite lenses were used to describe cylindrical boreholes. The four main characteristics of the geological constructs used here in simulating the ultra-deep-water regions were the size of the water column, the topography of the sea floor, the interfaces of basalt layers with the surroundings rocks, and the structure of heterogeneities inside the basalt layers. The average wavelength of wave propagation below the basalt layer is 125 m, which is very large compared to the size of a typical borehole (0.1 m). A lens with a thickness of 2.5 m, which corresponds to a dimension 50 times smaller than the average wavelength, sub-basalt was constructed. Also included were some lateral extensions in the construction of the lens to simulate wash-out zones. This study investigates the wave propagation below the basalt rocks and the effect of tube waves on borehole seismic data below the basalt layer by using these lenses instead of a cylindrical borehole. As the borehole geometry is different from that of the lens, the results are considered preliminary. Results suggest that tube waves are negligible in ultra-deep waters below basalt rocks because the wavelength of the seismic waves is large in comparison to the wash-out zone (192 times larger).

La ferrite résiduelle δ est présente dans les microstructures de coulée des aciers inoxydables austénitiques. Elle résulte de la transformation incomplète δ→γ ayant lieu l'étape de solidification. Sa présence peut nuire à la forgeabilité à chaud des aciers inoxydables et peut conduire à la formation de criques de rives et de pailles en J lors du laminage à chaud des brames. Ce travail de thèse a pour but de comprendre les mécanismes de la transformation δ→γ à haute température dans les aciers inoxydables austénitiques via une modélisation expérimentale et numérique. La transformation a été étudié dans un alliage ternaire Fe-Cr-Ni coulé par lingot et de composition proche de celle des alliages industriels. Trois morphologies de ferrite ont été mises en évidence à l'état brut de solidification: lattes au bord du lingot, vermiculaire et lattes au centre. Leur cinétique de dissolution est étudiée à des températures allant de 1140°C à 1340°C et caractérisée en termes de fraction de ferrite et profils de composition du Cr et du Ni. La dissolution de la ferrite vermiculaire comprend trois étapes : une croissance initiale transitoire suivie par deux régimes de dissolution à haute puis à faible taux de transformation. D'un autre côté, il a été possible d'étudier la dissolution de la ferrite dans des microstructures multicouches élaborées par l'empilement de plaques de ferrite et d'austénite du système Fe-Cr-Ni et soudées à l'état solide par Compression Isostatique à Chaud puis réduits en épaisseurs par laminages successifs. L'étude et la caractérisation de la cinétique de dissolution de la ferrite est plus facile dans ces microstructures étant donnée la planéité initiale des interfaces δ/γ. L'analyse des résultats expérimentaux a été menée via le développement d'un modèle numérique, à interface mobile, de la transformation de phases δ→γ pilotée par la diffusion. La diffusion peut être traitée dans les géométries plane, cylindrique et sphérique. En guise de validation, le modèle a été utilisé pour analyser la dissolution de la ferrite dans les microstructures multicouches. Par la suite il a été appliqué au cas de la ferrite vermiculaire en usant d'une approche novatrice où la morphologie des dendrites est approximée par une combinaison de cylindres et de sphères. Malgré la simplicité des hypothèse sous-jacentes, le modèle a permis d'expliquer les mécanismes de croissance initiale et de changement de régime de dissolution. D'autre part, via une étude paramétrique, l'effet des données d'entrée a été étudié et les plus pertinentes d'entre eux en termes de prédiction quantitative ont été mises en avant, en particulier la description thermodynamique du digramme Fe-Cr-Ni, le gradient initial et la distribution des rayons des particules de ferrite. / Residual δ-ferrite is widely encountered in the as-cast microstructure of austenitic stainless steels. It stems from the incomplete high temperature solid-state δ→γ transformation occurring upon the solidification stage. Its presence has a detrimental effect the hot workability of stainless steels, leading to the formation of edge cracks and sliver defects during slabs hot rolling. This PhD work aims at bringing more understanding of the kinetics of high temperature δ→γ transformation in austenitic stainless steels via experimental and numerical modeling. The transformation was studied in a ternary Fe-Cr-Ni ingot-cast alloy with composition close to the industrial alloys. Three ferrite morphologies were identified: lathy at the edge of the ingot, vermicular and lathy at the center. Their dissolution kinetics were established at temperatures ranging from 1140°C to 1340°C and characterized in terms of ferrite fraction and Cr and Ni diffusion. The vermicular ferrite undergoes a transient growth followed by a high then a low rate dissolution regimes. On the other hand, ferrite dissolution was also studied in the multilayered microstructures. such microstructures were elaborated by alternating ferrite and austenite sheets of the Fe-Cr-Ni system, diffusion-bonded by Hot isostatic Pressing and reduced in thickness by successive rollings. Dissolution is easier to handle in such microstructures thanks to the initial planar δ/γ interfaces. Analysis of the experimental results were carried out with a numerical moving-boundary model of diffusion-controlled δ→γ transformation. Diffusion can be treated in the planar, cylindrical and spherical geometries. As a preliminary validation, the model was used to analyze kinetics of ferrite dissolution in the multilayered microstructures. It was then applied to the cast alloy using an original descriptive approach combining spheres and cylinders as equivalent morphology of dendritic ferrite. Although based on simplifying assumptions, the model was able to reproduce experimental results with satisfactory agreement. Mechanisms underlying the initial growth of vermicular ferrite and the transition in dissolution regimes were outlined. The effect of a wide range of input parameters has been considered and relevant parameters for quantitative calculations were brought to light, such as thermodynamical descriptions of the Fe-Cr-Ni system, composition gradients and distribution of ferrite's radii.

Energy recovery ventilation (ERV) systems are designed to decrease the energy consumed by building HVAC systems. ERV’s scavenge sensible and latent energy from the exhaust air leaving a building or space and recycle this energy content to pre-condition the entering outdoor air. A few studies found in the open literature are dedicated to developing detailed numerical models to predict or simulate the performance of energy recovery wheels and desiccant wheels. However, the models are often computationally intensive, requiring a lot of time to perform parametric studies. For example, if the physical characteristics of a study target change (e.g., wheel diameter or depth) or if the system runs at different operating conditions (e.g., wheel rotation speed or airflow rate), the model parameters need to be recalculated. Hence, developing a mapping method with better computational efficiency, which will enable the opportunity to conduct extensive parametric or optimal design studies for different wheels is the goal of this research. In this work, finite difference method (FDM) numerical models of energy recovery wheels and desiccant wheels are established and validated with laboratory test results. The FDM models are then used to provide data for the development of performance mapping methods for an energy wheel or a desiccant wheel. After validating these new mapping approaches, they are employed using independent data sets from different laboratories and other sources available in the literature to identify their universality. One significant characteristic of the proposed mapping methods that makes the contribution unique is that once the models are trained, they can be used to predict performance for other wheels with different physical geometries or different operating conditions if the desiccant material is identical. The methods provide a computationally efficient performance prediction tool; therefore, they are ideal to integrate with transient building energy simulation software to conduct performance evaluations or optimizations of energy recovery/ desiccant wheel integrated HVAC systems.

<p>Composite Plate Shear Walls - Concrete Filled (C-PSW/CF), also known as SpeedCore walls, are increasingly used in commercial buildings. C-PSW/CF offer the advantages of modularization and expedited construction time. The performance of C-PSW/CF under wind and seismic loading has been extensively studied. As such, building codes permit the use of these walls in non-seismic and seismic regions. In addition to these lateral loads, C-PSW/CF may be exposed to fire loading during their service life. Elevated temperatures resulting from the fire loading subject structural components to a set of forces and deformations. These elevated temperatures result in the significant degradation of the material properties. Thus, fire loading may lead to the failure of structural components during fire incidents within the buildings.</p> <p>This dissertation describes (i) experimental, numerical, and analytical studies conducted to evaluate the performance of C-PSW/CF and (ii) the development of design guidelines for C-PSW/CF subjected to fire and gravity loading. The results from prior experimental investigations were compiled, and five additional fire tests were conducted to address gaps in the experimental data. The fire tests were conducted on laboratory-scale specimens subjected to axial compressive loading and simulated standard fire loading (heating). The parameters considered in the tests were axial compressive loading (21% – 30% of section compressive strength, <em>Ag f’c</em>), steel plate slenderness (24 – 48, tie spacing-to-steel plate thickness ratio), and uniformity of heating (all-sided versus three-sided heating).</p> <p>Numerical and analytical studies were conducted using two independent methods namely Finite Element (FE) and Finite Difference (FD) methods. The developed models were benchmarked to test data, and the benchmarked models were used to conduct parametric studies to expand the database. The thermal and structural material properties recommended by Eurocode standards were applied in these models. The parameters considered were the wall thickness (200 mm – 600 mm), wall slenderness (story height-to-concrete thickness ratio, <em>H/tc</em>= 5 – 25), axial load ratio (<em>Pu</em> ≤ 30% section concrete strength, <em>Ac f’c</em>), heating uniformity (uniform versus non-uniform heating), boundary conditions (pinned versus fixed), cross-sectional steel plate reinforcement ratio (<em>As/Ag</em> =1.3% – 5.3%), steel plate slenderness ratio (<em>stie/tp</em> = 20 – 75), tie bar spacing-to-wall concrete thickness ratio (<em>stie/tc</em> = 0.5 – 1.0), and concrete compressive strength (<em>f’c</em> = 40 MPa – 55 MPa).</p> <p>Symmetric nonlinear thermal gradients were developed through wall thickness for the walls exposed to uniform fire loading. Due to the low thermal conductivity of concrete, the temperature decreased nonlinearly through the wall thickness towards the mid-thickness of the walls. For the non-uniform fire exposure, temperatures through the wall thickness decreased nonlinearly towards the unexposed surface of the walls. A consistent trend was observed in the axial displacements of C-PSW/CF under combined fire and gravity loading. The observed trend consisted of several steps including (i) thermal expansion, (ii) gradual axial shortening, (iii) fast axial shortening, and (iv) failure.</p> <p>Local buckling of steel plates between tie bars was observed in all walls. However, this phenomenon did not cause any significant degradation in structural performance or failure of the walls. The results from parametric studies indicated that wall slenderness ratio (story height-to-wall thickness ratio), wall thickness, applied axial load ratio, and end boundary conditions have a significant influence on the fire resistance of C-PSW/CF. Higher wall slenderness ratios and load ratios had a detrimental effect on the fire resistance of walls. Global buckling was the dominant failure mode for the walls with high slenderness ratios (e.g., <em>H</em>/<em>tc </em>³ 15). In thicker walls, the lower temperatures in the middle regions of the concrete helped to maintain the axial compressive capacity of walls under fire loading. Limiting the steel plate slenderness ratio could slightly improve the fire resistance of unprotected walls by arresting the extent of local buckling between tie bars.</p> <p>The results from the parametric studies have been used to develop an approach for designing C-PSW/CF subjected to combined fire and gravity loading. The total (linear) length of the wall was discretized into unit width columns, where each unit width column corresponded to a length of wall equal to the tie bar spacing (<em>stie</em>). Thus, each unit is like a column with steel plates on two opposite surfaces, concrete infill, and tie bars distributed uniformly along the height. The axial load capacity of C-PSW/CF can be estimated as the axial load capacity of the unit width column, calculated using the developed approach, multiplied by the linear length of the wall divided by the unit width (tie bar spacing). For this approach, the wall slenderness ratio (<em>H/tw</em>), has a limiting value of 20. Walls with wall slenderness ratios greater than 20 should be fire protected. The expansion of the material on the exposed surface of walls generated moments through the wall cross-section in non-uniform fire scenarios. This phenomenon caused the early failure of walls (~40 minutes) with wall slenderness ratios greater than 20. An approach was developed to conservatively estimate the fire-resistance rating (in hours) of unprotected C-PSW/CF exposed to the standard fire time-temperature curve. The fire-resistance rating of C-PSW/CF depends directly on the applied axial load ratio, wall slenderness ratio, and wall thickness.</p> <p>The temperature profile through the wall thickness can be calculated by discretizing the section into fibers (or elements). Since the temperature of the elements is uniform along the height and length of walls, 1D thermal analysis (through wall thickness) can be performed using heat transfer equations or the fiber-based program developed in the study.</p> <p>Vent holes are recommended to relieve the buildup steam pressure as the moisture content of concrete evaporates at temperatures exceeding the boiling point of water. A rational method was developed to design the vent holes as a function of the maximum temperature and thermal gradient through the wall thickness, heating duration, moisture content, and the acceptable level of pressure buildup on the steel plates. However, in typical cases, unprotected C-PSW/CF walls can be provided with 25 mm diameter vent holes spaced at a distance equal to story height or 3.6 m (maximum) in the horizontal and vertical directions to relieve the buildup of steam or water vapor pressure.</p> <p>This research study also led to the development and validation of a computer program that can be used instead of the design equations to more accurately model and calculate the thermal and structural performance of composite C-PSW/CF. This program is based on a fiber-based section and member analysis method that can be used to evaluate the performance and axial (gravity) load capacity of unprotected and protected C-PSW/CF subjected to uniform or non-uniform heating. The analysis can be conducted by implementing standard (ISO 834 or ASTM E119), Eurocode parametric, or user input gas (or surface) time-temperature curves.</p> <p>The proposed equations and the recommendations in this study can be used to develop design guidelines and specifications for fire resistance design of C-PSW/CF under combined fire and gravity loading. A code change proposal will be proposed to AISC <em>Specification</em> - Appendix 4 (Structural Design for Fire Condition).</p>