Seismic imaging is a technique that uses seismic echoes to map and detect underground geological structures. The conventional seismic image has the resolution limit of λ/2, where λ is the wavelength associated with the seismic waves propagating in the subsurface. To exceed this resolution limit, this thesis develops a new imaging method using resonant multiples, which produces superresolution images with twice or even more the spatial resolution compared to the conventional primary reflection image. A resonant multiple is defined as a seismic reflection that revisits the same subsurface location along coincident reflection raypath. This reverberated raypath is the reason for superresolution imaging because it increases the differences in reflection times associated with subtle changes in the spatial location of the reflector. For the practical implementation of superresolution imaging, I develop a post-stack migration technique that first enhances the signal-to-noise ratios (SNRs) of resonant multiples by a moveout-correction stacking method, and then migrates the post-stacked resonant multiples with the associated Kirchhoff or wave-equation migration formula. I show with synthetic and field data examples that the first-order resonant multiple image has about twice the spatial resolution compared to the primary reflection image. Besides resolution, the correct estimate of the subsurface velocity is crucial for determining the correct depth of reflectors. Towards this goal, wave-equation migration velocity analysis (WEMVA) is an image-domain method which inverts for the velocity model that maximizes the similarity of common image gathers (CIGs). Conventional WEMVA based on subsurface-offset, angle domain or time-lag CIGs requires significant computational and memory resources because it computes higher dimensional migration images in the extended image domain. To mitigate this problem, I present a new WEMVA method using plane-wave CIGs. Plane-wave CIGs reduce the computational cost and memory storage because they are directly calculated from prestack plane-wave migration, and the number of plane waves is often much smaller than the number of shots. In the case of an inaccurate migration velocity, the moveout of plane-wave CIGs is automatically picked by a semblance analysis method, which is then linked to the migration velocity update by a connective function. Numerical tests on synthetic and field datasets validate the efficiency and effectiveness of this method.

L'imagerie sismique de la Terre permet de retrouver la structure de sous-sol. Cette opération est classiquement décomposée en deux étapes : une première phase a pour objectif de déterminer la cinématique de propagation des ondes (modèle de vitesse de référence) ; la seconde phase vise à retrouver la position des interfaces dans le sous-sol. Si cette seconde phase est maintenant classique, la détermination du modèle de vitesse de référence reste un sujet d’actualité en imagerie sismique. Une technique pour estimer le modèle de référence sépare les données en sous-ensembles, par exemple en points de tir. Une image partielle du sous-sol est obtenue pour chaque point de tir. Si ces images sont cohérentes les unes avec les autres, alors le modèle de vitesse qui a servi à obtenir ces images est dit correct. Cette technique s'est avérée très utile, en particulier pour l'imagerie des zones complexes, car elle ne requière pas a priori d'identifier des événements. Cependant, la technique est intrinsèquement liée aux données réfléchies. Elle ne prend pas en compte les arrivées transmises, les réflexions multiples, les ondes de surface, … L'objectif de la thèse de doctorat est de prendre en compte à la fois les arrivées réfléchies et les arrivées transmises (ondes directes et plongeantes) pour l'analyse de vitesse. Pour cela, le formalisme de l'analyse de vitesse doit être revu pour prendre en compte des développements récents (modèle « étendu » défini en 2008). Si une telle approche s’avère possible, alors il ne serait plus nécessaire d'extraire les ondes réfléchies des données. Par ailleurs, les ondes réfléchies ont des trajets essentiellement verticaux. Les ondes transmises ont des trajets plutôt horizontaux. La prise en compte des deux types d'ondes permettrait donc de mieux contraindre le modèle de vitesse et alors de déterminer des paramètres d'anisotropie. Au cours de la thèse, il s'agira de modifier le formalisme actuel d'analyse de vitesse pour prendre en compte les ondes réfléchies et transmises. Des applications seront développées sur les données synthétiques et réelles pour démontrer l'apport de l'approche combinée. Enfin, une analyse fine devra être menée pour voir si les ondes réfractées (en opposition aux ondes plongeantes) peuvent également être prises en compte. / Seismic imaging aims at retrieving the Earth's structures. It is classically split into two steps: firstly, the objective is to retrieve the background velocity model containing the large-scale structures of the velocity model. Secondly, one needs to determine the reflectivity part with the positions of the interfaces. The first part still remains a difficult challenge. Migration Velocity Analysis consists of migrating subsets of the total input data set (e.g. single shots). If all associated images are consistent, then the model used for imaging is said to be correct. This is a very useful method, in particular for the imaging of complex structures, as it does not a prioiri require any picking nor event identification. However, it is intrinsically restricted to reflected data: it does not take into account transmitted waves, multiple reflections, surface waves, ... The objective of the Ph.D work is to simultaneously consider reflected and transmitted (direct and diving) waves in the context of Migration Velocity Analysis. For that, one needs to reconsider the basis for velocity analysis. Recent developments around the « extended model » should be considered. If feasible, then it would not be anymore needed to extract reflected energy from a shot gather. More importantly, the combined inversion would better constrain the model: reflected waves are mainly associated to vertical propagation, whereas transmitted waves recorded at large offsets are more associated to horizontal propagation. This could offer the possibility to better estimate anisotropy parameters.

Les expériences sismiques actives sont largement utilisées pour caractériser la structure de la subsurface. Les méthodes dites d’analyse de vitesse par migration ont pour but la détermination d’un macro-modèle de vitesse, lisse, et contrôlant la cinématique de propagation des ondes. Le modèle est estimé par des critères de cohérence d’image ou de focalisation d’image. Les images de réflectivité obtenues par les techniques de migration classiques sont cependant contaminées par des artefacts, altérant la qualité de la remise à jour du macro-modèle. Des résultats récents proposent de coupler l’inversion asymptotique, qui donne des images beaucoup plus propres en pratique, avec l’analyse de vitesse pour la version offset en profondeur. Cette approche cependant demande des capacités de calcul et de mémoire importantes et ne peut actuellement être étendue en 3D.Dans ce travail, je propose de développer le couplage entre l’analyse de vitesse et la migration plus conventionnelle par point de tir. La nouvelle approche permet de prendre en compte des modèles de vitesse complexes, comme par exemple en présence d’anomalies de vitesses plus lentes ou de réflectivités discontinues. C’est une alternative avantageuse en termes d’implémentation et de coût numérique par rapport à la version profondeur. Je propose aussi d’étendre l’analyse de vitesse par inversion au domaine des données pour les cas par point de tir. J’établis un lien entre les méthodes formulées dans les domaines données et images. Les méthodologies sont développées et analysées sur des données synthétiques 2D. / Active seismic experiments are widely used to characterize the structure of the subsurface. Migration Velocity Analysis techniques aim at recovering the background velocity model controlling the kinematics of wave propagation. The first step consists of obtaining the reflectivity images by migrating observed data in a given macro velocity model. The estimated model is then updated, assessing the quality of the background velocity model through the image coherency or focusing criteria. Classical migration techniques, however, do not provide a sufficiently accurate reflectivity image, leading to incorrect velocity updates. Recent investigations propose to couple the asymptotic inversion, which can remove migration artifacts in practice, to velocity analysis in the subsurface-offset domain for better robustness. This approach requires large memory and cannot be currently extended to 3D. In this thesis, I propose to transpose the strategy to the more conventional common-shot migration based velocity analysis. I analyze how the approach can deal with complex models, in particular with the presence of low velocity anomaly zones or discontinuous reflectivities. Additionally, it requires less memory than its counterpart in the subsurface-offset domain. I also propose to extend Inversion Velocity Analysis to the data-domain, leading to a more linearized inverse problem than classic waveform inversion. I establish formal links between data-fitting principle and image coherency criteria by comparing the new approach to other reflection-based waveform inversion techniques. The methodologies are developed and analyzed on 2D synthetic data sets.

New emission legislation regarding air pollution control, as instructed by the Department of Environmental Affairs (DEA) to Eskom Generation Power Stations, implies a particulate emission limit of 100 mg/Nm3 for all existing power stations by 2015 and 50 mg/Nm3 for all new and existing power stations by the year 2020. Some of Eskom’s power stations which are equipped with Electrostatic Precipitators (ESP’s) were not designed for this stringent legislation. It is also experienced that ESP’s and coal quality in Eskom have deteriorated over time, resulting in the performance of the ESP’s not meeting the legislative requirements. Eskom is in the process of introducing various ESP enhancement projects to improve performance and aligning the operating philosophy to comply with the more stringent particulate emission legislation. An ESP efficiency test was conducted at Lethabo Power Station to determine the current state of the plant and performance. The results of the test were compared with the original design base specifications to determine the relevant deficiencies which contribute to high emissions and poor ESP performance. It was aimed to develop an ESP simulation model and validate the outputs with the test data. This study endeavours to demonstrate the greater impact on ESP performance when the ESP is operated outside the design specification. It is further aimed to demonstrate that a solution to the problem of high emissions is not only contributed by the variables within the ESP itself. This study is a coal to stack evaluation considering the ESP variables and the upstream conditions of the ESP that form part of the entire process. The intention of this study is to demonstrate the importance of operating an ESP at the designed parameters and highlight the significance of proper maintenance. It was learned that before any ESP enhancement technology can be implemented, the ESP and upstream conditions must be in accordance with design specifications. The implementation of an ESP enhancement technology will have no merit or justification on a unit that is being operated outside of its design specifications. The results obtained from the ESP simulation model correlated well with the ESP efficiency test data. The expectation of the model to assist operators and engineers to operate ESP’s according to the designer’s specifications was conceded. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

New emission legislation regarding air pollution control, as instructed by the Department of Environmental Affairs (DEA) to Eskom Generation Power Stations, implies a particulate emission limit of 100 mg/Nm3 for all existing power stations by 2015 and 50 mg/Nm3 for all new and existing power stations by the year 2020. Some of Eskom’s power stations which are equipped with Electrostatic Precipitators (ESP’s) were not designed for this stringent legislation. It is also experienced that ESP’s and coal quality in Eskom have deteriorated over time, resulting in the performance of the ESP’s not meeting the legislative requirements. Eskom is in the process of introducing various ESP enhancement projects to improve performance and aligning the operating philosophy to comply with the more stringent particulate emission legislation. An ESP efficiency test was conducted at Lethabo Power Station to determine the current state of the plant and performance. The results of the test were compared with the original design base specifications to determine the relevant deficiencies which contribute to high emissions and poor ESP performance. It was aimed to develop an ESP simulation model and validate the outputs with the test data. This study endeavours to demonstrate the greater impact on ESP performance when the ESP is operated outside the design specification. It is further aimed to demonstrate that a solution to the problem of high emissions is not only contributed by the variables within the ESP itself. This study is a coal to stack evaluation considering the ESP variables and the upstream conditions of the ESP that form part of the entire process. The intention of this study is to demonstrate the importance of operating an ESP at the designed parameters and highlight the significance of proper maintenance. It was learned that before any ESP enhancement technology can be implemented, the ESP and upstream conditions must be in accordance with design specifications. The implementation of an ESP enhancement technology will have no merit or justification on a unit that is being operated outside of its design specifications. The results obtained from the ESP simulation model correlated well with the ESP efficiency test data. The expectation of the model to assist operators and engineers to operate ESP’s according to the designer’s specifications was conceded. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014