Fuggi, Antonio (2021) Genetic full waveform inversion to characterise fractures. PhD thesis, University of Leeds.
Abstract
Active seismic methodologies provide a non-invasive tool to remotely characterise the physical properties of fractures at a wide range of scales, and have a positive impact in helping to solve rock engineering problems in a variety of geo-industrial applications. With current advances in seismic processing tools, such as full waveform inversion (FWI), and accurate models of seismic wave interaction with fractures, seismic characterisation of fractures can be tackled by utilising the entire seismic wavefield recorded at the receiver locations. A two-step strategy, using the genetic algorithm (GA) for global optimisation and the Neighbourhood Algorithm (NA) for evaluating uncertainties, was developed to simultaneously estimate the fracture properties (both fracture specific stiffness and equivalent fracture stiffness) and the background material properties directly from seismic waveforms. The optimisation involves minimising the difference between the observed (measured) and forward-modelled full waveforms through the finite difference code WAVE3D. The development, named Genetic Algorithm Full-Waveform Fracture Inversion (GAFWFI), looks beyond conventional seismic methods which focus on characterising fracture-induced anisotropy, by reducing the need to manually condition the data (e.g. manual picking of seismic phases), and by providing a robust means to explore multiple solutions. The development also allows the gap between different representations of fracturing to be bridged within a comprehensive method which can employ both discrete fracture and effective fracture models. GA-FWFI is tested initially on synthetic ultrasonic experiments with parallel fractures. Results confirm that the method can effectively invert for physical properties such as fracture stiffness, location, background material properties, while the posterior probability density (PPD) show that inversions are very well constrained. GA-FWFI is then applied to waveforms from a laboratory experiment investigating fracture slip and again results show high degree of accuracy. GA-FWFI is then utilised to unveil the coupling between discrete fracture networks (DFNs) and their equivalent fracture zone properties. The results reveal that the transition from a medium with open cracks to one with welded interfaces leads to the equivalent media having the equivalent medium stiffness non-linearly related to the crack specific stiffness. An attribute χ is proposed which helps guide the interpretation of a cracked medium by giving a range of likely values for crack size and crack stiffness. This work paves the way for novel strategies to seismically characterise fractures.
Metadata
Supervisors: | Hildyard, Mark W and Clark, Roger A and Hooper, Andy and Brittan, John |
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Keywords: | Waveform inversion, fractures, seismic inversion, fracture characterization |
Awarding institution: | University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Institute of Geophysics and Tectonics (Leeds) The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Institute of Geological Sciences (Leeds) |
Identification Number/EthosID: | uk.bl.ethos.865254 |
Depositing User: | Mr Antonio Fuggi |
Date Deposited: | 14 Nov 2022 13:30 |
Last Modified: | 11 Dec 2022 10:53 |
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