Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

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Inverting shear-wave splitting
measurements for fracture properties
Images / split the truth / in fractions.
Denise Levertov
3.1 Introduction
Seismic anisotropy refers to the situation where the velocity of a seismic wave is dependent on its
direction of propagation and/or polarisation. Seismic anisotropy in sedimentary rocks can have many
causes, which act at many length-scales. These mechanisms include mineral alignment (e.g., Valcke
et al., 2006), alignment of grain-scale fabrics (e.g., Hall et al., 2008), which can be distorted by nonhydrostatic
stresses (e.g., Zatsepin and Crampin, 1997; Verdon et al., 2008), larger scale sedimentary
layering (e.g., Backus, 1962) and the presence of aligned fracture sets (e.g., Hudson, 1981). In hydrocarbon
settings, the most common anisotropic mechanisms are horizontally aligned sedimentary
layers, and horizontally aligned mineral and grain-scale fabrics. Such an anisotropic system will have
a vertical axis of symmetry, and is referred to as Vertical Transverse Isotropy (VTI). A second source
of anisotropy is often introduced with vertically aligned fracture sets. Such an anisotropic system will
have a horizontal axis of symmetry, and is referred to as Horizontal Transverse Isotropy (HTI). The
combination of such VTI and HTI mechanisms leads to anisotropic systems with orthorhombic or
lower symmetry systems. The presence of fractures has a significant impact on permeability and fracture
alignment leads to anisotropic permeability. The detection of seismic anisotropy has the potential
to image aligned fracture sets, and so can be a useful tool to help guide CCS injection strategies.
Shear wave splitting (SWS) is probably the least ambiguous indicator of seismic anisotropy. As a
shear wave enters an anisotropic region it is split into two orthogonally polarised waves, one of which
will travel faster than the other. The polarisation of the fast wave (ψ), and the time-lag (δt) between
the arrival of the fast and slow waves, characterises the splitting along a raypath. The splitting along
many raypaths characterises the overall anisotropy symmetry system. Usually, δt is normalised by
the path length to give the percentage difference in S-wave velocities, δV S .
SWS is used as a matter of course in global seismological studies (e.g., Kendall et al., 2006) to
identify such features as fractures (e.g., Crampin, 1991; Boness and Zoback, 2006), melt inclusion
alignment (e.g., Blackman and Kendall, 1997; Kendall et al., 2005), alignment of crystals caused by
mantle flow (e.g., Blackman et al., 1993; Rümpker et al., 1999; Barruol and Hoffmann, 1999), and the
nature of the Earth’s solid inner core (Wookey and Helffrich, 2008). SWS has even been suggested as
a tool for predicting the occurrence of earthquakes (Crampin et al., 2008). Despite these successes,
SWS is rarely used to detect seismic anisotropy in reservoir settings.
In hydrocarbon settings, the shear waves used to measure SWS can come from two very different
sources: the first being controlled source multicomponent reflection seismics, the second being mi-
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