When do fractured media become seismically anisotropic? Some implications on quantifying fracture properties

Abstract

Fractures are pervasive features within the Earth's crust and have a significant influence on the multi-physical response of the subsurface. The presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems. In this study, we confirm the general scale-dependence of seismic anisotropy and provide new results specific to shear-wave splitting (SWS). We find that SWS develops under conditions when the ratio of wavelength to fracture size (λ S /d) is greater than 3, where Rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model (EMM) theory is qualitatively valid. When 1 < λ S /d < 3 there is a transition from Rayleigh to Mie scattering, where no effective anisotropy develops and hence the SWS measurements are unstable. When λ S /d < 1 we observe geometric scattering and begin to see behaviour similar to transverse isotropy. We find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio. More importantly, we observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 to 2 wavelengths depending on the fracture density and compliance ratio. The existence of a transition zone means that inversion of seismic anisotropy parameters based on EMM will be fundamentally biased. More importantly, we observe that linear slip EMM commonly used in inverting fracture properties is inconsistent with our results and leads to errors of approximately 400% in fracture spacing (equivalent to fracture density) and 60% in fracture compliance. Although EMM representations can yield reliable estimates of fracture orientation and spatial location, our results show that EMM representations will systematically fail in providing quantitatively accurate estimates of other physical fracture properties, such as fracture density and compliance. Thus more robust and accurate quantitative estimates of in situ fracture properties will require improvements to effective medium models as well as the incorporation of full-waveform inversion techniques.

Anisotropic P-wave travel-time tomography implementing Thomsen's weak approximation in TOMO3D

“…Some authors argue that it is not possible to distinguish TTI from VTI in real experimental cases without a priori information (Bakulin et al, 2009). Assuming the most general anisotropic media has also become rather usual, in particular with the improvement of computational resources, allowing for a more detailed and complex reconstruction of the subsurface physical properties (e.g., Zhou and Greenhalgh, 2005), although successful field data applications are yet to be achieved to Figure 3. For both selected meridians, 0 rad and π/4 rad azimuths, relative travel-time errors in percentage with respect to the analytic value for each of the four simulations used in the sensitivity analysis.…”

“…Anisotropy is a multiscale phenomenon, and its causes are diverse. In the crust, it can be produced by the preferred orientation of mineral grains or their crystal axes (Schulte-Pelkum and Mahan, 2014; Almqvist and Mainprice, 2017), the alignment of cracks and fracture networks and the presence of fluids (Crampin, 1981;Maultzsch et al, 2003;Yousef and Angus, 2016), or the bedding of layers much thinner than the wavelength used to explore them (Backus, 1962;Johnston and Christensen, 1995;Sayers, 2005). In the mantle, anisotropy is related to the alignment of olivine crystals due to mantle flow (Nicolas and Christensen, 1987;Montagner et al, 2007), aligned melt inclusions (Holtzman et al, 2003;Kendall et al, 2005), large-scale deformation (Vinnik et al, 1992;Vauchez et al, 2000) and preexisting lithospheric fabric (Kendall et al, 2006), among others.…”

Anisotropic P-wave traveltime tomography implementing Thomsen's weak approximation in TOMO3D

“…Anisotropy is a multiscale phenomenon, and its causes are diverse. In the crust, it can be produced by the preferred orientation of mineral grains or their crystal axes (Schulte-Pelkum and Mahan, 2014;Almqvist and Mainprice, 2017), the alignment of cracks and fracture networks and the presence of fluids (Crampin, 1981;Maultzsch et al, 2003;Yousef and Angus, 2016), or the bedding of layers much thinner than the wavelength used to explore them (Backus, 1962;Johnston and Christensen, 1995;Sayers, 2005). In the mantle, anisotropy is related to the alignment of olivine crystals due to mantle flow (Nicolas and Christensen, 1987;Montagner et al, 2007), aligned melt inclusions (Holtzman et al, 2003;Kendall et al, 2005), large-scale deformation (Vinnik et al, 1992;Vauchez et al, 2000), and preexisting lithospheric fabric (Kendall et al, 2006), among others.…”

Acoustic Reflectivity From Variously Oriented Orthorhombic Media: Analogies to Seismic Responses From a Fractured Anisotropic Crust

“…Many, if not most, rocks and rock masses are elastically anisotropic with respect to the propagation direction of seismic waves. Observations of seismic anisotropy inform us about in situ metamorphic strain textures (e.g., Almqvist et al, 2010;Godfrey et al, 2000;Ji et al, 2013;Kern & Wenk, 1990;Mainprice & Nicolas, 1989), layering (e.g., Backus, 1962;Brown et al, 1991;Grech et al, 2002;Kebaili & Schmitt, 1997), and properties of the natural and induced fracture's network (e.g., Grechka & Tsvankin, 2004;Nakagawa et al, 2003;Sayers & Kachanov, 1995;Yousef & Angus, 2016). This information is a key to understanding in situ geological structures and processes, but properly obtaining it requires knowledge of the seismic wavefield behavior in anisotropic media that are addressed here both experimentally and numerically.…”