Multiscale characterisation of chimneys/pipes: Fluid escape structures within sedimentary basins

Evaluation of seismic reflection data has identified the presence of fluid escape structures cross-cutting overburden stratigraphy within sedimentary basins globally. Seismically-imaged chimneys/pipes are considered to be possible pathways for fluid flow, which may hydraulically connect deeper strata to the seabed. The properties of fluid migration pathways through the overburden must be constrained to enable secure, long-term subsurface carbon dioxide (CO2) storage. We have investigated a site of natural active fluid escape in the North Sea, the Scanner pockmark complex, to determine the physical characteristics of focused fluid conduits, and how they control fluid flow. Here we show that a multi-scale, multi-disciplinary experimental approach is required for complete characterisation of fluid escape structures. Geophysical techniques are necessary to resolve fracture geometry and subsurface structure (e.g., multi-frequency seismics) and physical parameters of sediments (e.g., controlled source electromagnetics) across a wide range of length scales (m to km). At smaller (mm to cm) scales, sediment cores were sampled directly and their physical and chemical properties assessed using laboratory-based methods. Numerical modelling approaches bridge the resolution gap, though their validity is dependent on calibration and constraint from field and laboratory experimental data. Further, time-lapse seismic and acoustic methods capable of resolving temporal changes are key for determining fluid flux. Future optimisation of experiment resource use may be facilitated by the installation of permanent seabed infrastructure, and replacement of manual data processing with automated workflows. This study can be used to inform measurement, monitoring and verification workflows that will assist policymaking, regulation, and best practice for CO2 subsurface storage operations

Full waveform inversion procedures with irregular topography

Full waveform inversion (FWI) is a form of seismic inversion that uses data residual, found as the misfit, between the whole waveform of field acquired and synthesized seismic data, to iteratively update a model estimate until such misfit is sufficiently reduced, indicating synthetic data is generated from a relatively accurate model. The aim of the thesis is to review FWI and provide a simplified explanation of the techniques involved to those who are not familiar with FWI. In FWI the local minima problem causes the misfit to decrease to its nearest minimum and not the global minimum, meaning the model cannot be accurately updated. Numerous objective functions were proposed to tackle different sources of local minima. The ‘joint deconvoluted envelope and phase residual’ misfit function proposed in this thesis aims to combine features of these objective functions for a comprehensive inversion. The adjoint state method is used to generate an updated gradient for the search direction and is followed by a step-length estimation to produce a scalar value that could be applied to the search direction to reduce the misfit more efficiently. Synthetic data are derived from forward modelling involving simulating and recording propagating waves influenced by the mediums’ properties. The ‘generalised viscoelastic wave equation in porous media’ was proposed by the author in sub-chapter 3.2.5 to consider these properties. Boundary layers and conditions are employed to mitigate artificial reflections arising from computational simulations. Linear algebra solvers are an efficient tool that produces wavefield vectors for frequency domain synthetic data. Regions with topography require a grid generation scheme to adjust a mesh of nodes to fit into its non-quadrilateral shaped body. Computational co-ordinate terms are implemented within wave equations throughout topographic models where a single point in the model in physical domain are represented by cartesian nodes in the computational domains. This helps to generate an accurate and appropriate synthetic data, without complex modelling computations. Advanced FWI takes a different approach to conventional FWI, where they relax upon the use of misfit function, however none of their proponents claims the former can supplant the latter but suggest that they can be implemented together to recover the true model.Open Acces

Paleokarst reservoir modelling - A concept-driven approach

A significant proportion of the world's hydrocarbon production comes from paleokarst reservoirs. Although these reservoirs boast some of the most productive wells in oil history, the recovery factor is relatively low (RFmean: 32%) compared to other carbonate reservoirs (RFmean: 37 - 51%). The low recovery could relate to current reservoir modelling approaches potentially yielding inaccurate resource estimates or early water-breakthrough. Conventional industry-standard reservoir modelling software suites do not have dedicated workflows or add-ins for handling the complex morphologies commonly associated with paleokarst. Current modelling approaches are often datadriven (conditioned on available seismic and well data) and employ adapted or modified versions of stochastic reservoir modelling workflows used for siliciclastic and carbonate reservoirs. However, many paleokarst features are below seismic resolution, and the representativity of individual well data is often challenging to assess. Consequently, data-driven models often fail to render the connectivity, geometry, and volume of karst features. Karst is the predecessor to paleokarst, and therefore a genetic approach employing existent information from recent karst systems may be a good starting point for generating analogues to paleokarst reservoirs. A concept-driven approach, in combination with current data-driven modelling approaches, may enable model rendering that more closely echoes actual paleokarst reservoir architectures. However, only a few conceptual modelling methods are publicly available and described in the literature. The drawbacks with the available methods are that they under-/overestimate the cave volumes, fail to provide realistic cave morphologies, and forecast clastic sediment infill, and do not differentiate between preserved and collapsed caverns. Consequently, post-collapse reservoir morphologies, volumes and facies distributions may be rendered inaccurately. This thesis aims to address the shortcomings of currently available conceptual methods and present a novel concept-driven workflow for paleokarst reservoir modelling. A novel methodology for geocellular rendering of karst systems is presented in this thesis. The method utilizes modern cave-survey data to generate dense, equally spaced point-clouds (infilling the cave periphery). These point clouds can be used to discretize the karst systems in a geocellular framework by geometrical modelling. The volumetric and geometric rendering of the method is compared with two pre-established methods and benchmarked against the cave survey. The results show that the new method offers improved volumetric and geometric geocellular rendering compared to the preestablished methods and are comparable to that of the cave survey. A pilot study using a well-known and pre-established geophysical method, electrical resistivity tomography (ERT), was carried out in the Maaras cave system in northern Greece to evaluate the large-scale volumetric significance and spatial distribution of clastic sediments infilling karst cavities. ERT proved to be a practical and useful method for differentiating mesoscale (>2.5 m2) stratigraphic heterogeneity. Resistivity contrasts allowed the identification of sedimentary thickness variations, interbedded breccias, and cave floor. Results showed that the siliciclastic sediment thickness varied from 25 m to >45 m, occupying a minimum of 69-95 % of the available accommodation space. Finally, a novel interactive tool for evaluating cavern stability and forward model collapse and infill processes was developed. The tool employs conventional cave survey data, field measurements and geomechanical data of the host rock to simulate potential post-collapse morphologies and generate spatial output data suitable for geocellular modelling. Collapse propagation, and eventually the volume affected by the collapse, is controlled by user-defined paleokarst facies proportions and associated average porosities following a “mass-balance-principle” (i.e., porosity is final and only redistributed over a larger volume). Three different collapse scenarios were modelled using the Agios Georgios cave system in northern Greece as an analogue. The results show that it is feasible to use cave surveys to simulate collapse and infill processes and estimate the final paleokarst reservoir architecture. The morphology, volume and relative facies-proportions rendered in the reservoir models are comparable to those calculated in the forward collapse modelling tool, indicating that the geocellular model echoes the simulation. The results also show that the vertical continuity and target volume of a reservoir increases significantly with increasing bedding dip. This suggests that improved forecasting of the final reservoir architecture may optimise well positioning, production planning and eventually improve recovery prediction.Doktorgradsavhandlin

Surveying and Three-Dimensional Modeling for Preservation and Structural Analysis of Cultural Heritage

Dense point clouds can be used for three important steps in structural analysis, in the field of cultural heritage, regardless of which instrument it was used for acquisition data. Firstly, they allow deriving the geometric part of a finite element (FE) model automatically or semi-automatically. User input is mainly required to complement invisible parts and boundaries of the structure, and to assign meaningful approximate physical parameters. Secondly, FE model obtained from point clouds can be used to estimate better and more precise parameters of the structural analysis, i.e., to train the FE model. Finally, the definition of a correct Level of Detail about the three-dimensional model, deriving from the initial point cloud, can be used to define the limit beyond which the structural analysis is compromised, or anyway less precise. In this work of research, this will be demonstrated using three different case studies of buildings, consisting mainly of masonry, measured through terrestrial laser scanning and photogrammetric acquisitions. This approach is not a typical study for geomatics analysis, but its challenges allow studying benefits and limitations. The results and the proposed approaches could represent a step towards a multidisciplinary approach where Geomatics can play a critical role in the monitoring and civil engineering field. Furthermore, through a geometrical reconstruction, different analyses and comparisons are possible, in order to evaluate how the numerical model is accurate. In fact, the discrepancies between the different results allow to evaluate how, from a geometric and simplified modeling, important details can be lost. This causes, for example, modifications in terms of mass and volume of the structure

3D printing porous proxies as a new tool for laboratory and numerical analyses of sedimentary rocks

The study of geological processes at the pore-scale has significant implications to understanding many real-world phenomena related to flow in porous media (e.g., hydrogeology, petroleum geology and engineering, CO2 sequestration). While numerical and experimental analyses of sedimentary-rock pore systems have advanced to the characterization of nanometer-scale features, correlation of data across multiple scales of investigation (e.g., between seismic data, core samples, thin-section images, and SEM images) is still challenging. The differences arise in petrophysical properties (e.g., permeability) calculated on the same pore network under varying experimental conditions (e.g., pressure, temperature). 3D printing is a rapidly evolving technology that enables the manufacture of intricate 3D pore-network models (defined in this research as proxies) that can be investigated experimentally and compared to numerical simulations repeatedly. The main objective of my Ph.D. research has been to improve our understanding of the accuracy of 3D-printed pore networks in comparison to natural rocks. In addition, the researched aimed at: 1) the improvement of building and post-processing workflows for accurate geometric replication of pore networks by each 3D printing technique; 2) the establishment and enhancement of validation workflows to test transport properties of rock proxies (e.g., porosity and permeability); and 3) the characterization of artifacts related to 3D printing, post-processing, and validation methods for several common 3D printing methods. While all 3D printers build models layer-by-layer, the physical and chemical properties of build materials, the build process itself, and post-processing methods vary widely. My research results provide the extent to which major 3D printing techniques (binder jet, polyjet, stereolithography, and fused depositional modelling) and associated materials (powders, polymers, resins, and plastics) can generate useful proxies of common porous sandstones (Idaho gray, Berea, and Fontainebleau) that can be tested in the laboratory as natural porous rocks. The accuracy and resolution of each technique was evaluated by testing the 3D printers with simple pore proxies (built from simple numerical models) and natural rock proxies (built from computed tomography data of natural porous rocks). With future advances in 3D printer resolution and materials, the fidelity with which we can reproduce natural rock pore systems should improve

Seismic Waves

The importance of seismic wave research lies not only in our ability to understand and predict earthquakes and tsunamis, it also reveals information on the Earth's composition and features in much the same way as it led to the discovery of Mohorovicic's discontinuity. As our theoretical understanding of the physics behind seismic waves has grown, physical and numerical modeling have greatly advanced and now augment applied seismology for better prediction and engineering practices. This has led to some novel applications such as using artificially-induced shocks for exploration of the Earth's subsurface and seismic stimulation for increasing the productivity of oil wells. This book demonstrates the latest techniques and advances in seismic wave analysis from theoretical approach, data acquisition and interpretation, to analyses and numerical simulations, as well as research applications. A review process was conducted in cooperation with sincere support by Drs. Hiroshi Takenaka, Yoshio Murai, Jun Matsushima, and Genti Toyokuni

Strategies for visco-acoustic waveform inversion in the Laplace-Fourier domain, with application to the Nankai subduction zone

Waveform inversion is a non-linear and ill-posed inverse problem, with the objective of utilizing the full information content of recorded seismic waveforms. A Laplace-Fourier domain implementation allows a natural `multiscale\u27 approach that mitigates the non-linearity and ill-posedness by inverting low-frequency, early arrival data in the initial stages of inversion. High-frequency components, and late arrivals are incorporated at a later stage. This allows the development of robust inversion strategies capable of handling large wide-angle crustal surveys, leading to reliable, high-resolution velocity and attenuation models of crustal structures. I apply waveform inversion to extract a P-wave velocity model of the active megasplay fault system in the seismogenic Nankai subduction zone offshore Japan, using controlled-source Ocean Bottom Seismograph data. The resulting velocity model includes detailed thrust structures, and low velocity zones not previously identified. The connection of large low-velocity zones in the inner and outer wedge suggests a significant distribution of overpressured regions in the vicinity of the megasplay fault, with the potential to strongly influence coseismic rupture propagation. I identify six-fold key strategies for successful waveform inversion; i) the availability of low-frequency and long offset data, ii) a highly accurate starting model, iii) a hierarchical approach in which phase spectra are inverted first, and amplitude information is only incorporated in the final stages, iv) a Laplace-Fourier approach, v) careful preconditioning of the gradient, vi) strategies for source estimation. Chequerboard tests and point-scatter tests demonstrate the resolution and the limitations of the acoustic implementation. I also compare four misfit functionals for optimization, and demonstrate that velocity information may be reliably extracted from phase alone, and that amplitude information is secondary in updating the velocity model. Finally I develop inversion strategies for retrieving both velocity and attenuation models. Cross-talk between these two classes of parameter estimates arises from the lack of parameter scaling in the gradient of the objective function, and primarily affects the attenuation model. I show the cross-talk can be suppressed by the combination of an appropriate attenuation damping parameter, and by the use of smoothing constraints. Initial velocity-only inversions also help in reducing the effects of cross-talk in subsequent velocity-attenuation inversion

Image resolution analysis: a new, robust approach to seismic survey design

Seismic survey design methods often rely on qualitative measures to provide an optimal image of their objective target. Fold, ray tracing techniques counting ray hits on binned interfaces, and even advanced 3-D survey design methods that try to optimize o?set and azimuth coverage are prone to fail (especially in complex geological or structural settings) in their imaging predictions. The reason for the potential failure of these commonly used approaches derives from the fact that they do not take into account the ray geometry at the target points. Inverse theory results can provide quantitative and objective constraints on acquisition design. Beylkin??s contribution to this ?eld is an elegant and simple equation describing a reconstructed point scatterer given the source/receiver distribution used in the imaging experiment. Quantitative measures of spatial image resolution were developed to assess the e?cacy of competing acquisition geometries. Apart from the source/receiver con?guration, parameters such as the structure and seismic velocity also in?uence image resolution. Understanding their e?ect on image quality, allows us to better interpret the resolution results for the surveys under examination. A salt model was used to simulate imaging of target points located underneath and near the ?anks of the diapir. Three di?erent survey designs were examined. Results from these simulations show that contrary to simple models, near-o?sets do not always produce better resolved images than far-o?sets. However, consideration of decreasing signal-to-noise ratio revealed that images obtained from the far-o?set experiment are degrading faster than the near-o?set ones. The image analysis was performed on VSP ?eld data as well as synthetics generated by ?nite di?erence forward modeling. The predicted image resolution results were compared to measured resolution from the migrated sections of both the ?eld data and the synthetics. This comparison con?rms that image resolution analysis provides as good a resolution prediction as the prestack Kirchho? depth migrated section of the synthetic gathers. Even in the case of the migrated ?eld data, despite the presence of error introducing factors (di?erent signal-to-noise ratios, shape and frequency content of source wavelets, etc.), image resolution performed well exhibiting the same trends of resolution changes at di?erent test points

Fault architecture and related distribution of physical properties in granitic massifs: geological and geophysical methodologies

El desarrollo de varios proyectos para la caracterización de macizos graníticos nos ha permitido evaluar la capacidad de resolución y eficacia de diversas metodologías de trabajo geológicas y geofísicas. La distribución espacial de la densidad de fracturación en macizos graníticos permite identificar el núcleo de las zonas de falla y las áreas dañadas de su entorno. Medidas del índice de fracturación procedentes de la superficie y de los sondeos, pueden ser usadas para generar modelos 3-D estocásticos que proporcionan una imagen de la arquitectura de las zonas de falla y la distribución de propiedades físicas en el cuerpo granítico. Los métodos geoestadísticos proporcionan la posibilidad de integrar los datos geológicos y geofísicos para visualizar la distribución de fracturas en el macizo rocoso. Experimentos de sísmica de reflexión de alta resolución, junto con perfiles sísmicos verticales adquiridos en sondeos con azimut variable y experimentos tomográficos 3-D adquiridos en superficie, han demostrado ser metodologías válidas para resolver la estructura interior de los granitos