Genetic full waveform inversion to characterise fractures

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.