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2.4 Geophysics 75

S (fast)
2

S (slow)
1
Aligned fluid-filled
fractures

After Rial et al. (2005) Incident S-wave

Figure 2.13 Shear wave splitting. The incident S-wave with
arbitrary orientation is split into a fast S-wave oscillating par-
allel to the direction of the fractures, while the slow S-wave
oscillates perpendicular to the fracture orientation.

lithologic anisotropy or strong heterogeneity can severely limit the usefulness of
the SWS method. In addition, the volume of aligned cracks needs to be sufficiently
large to produce a measurable effect at the surface. A layer of limited thickness at
great depth maybe below the resolution limits of the technique, even if the fractures
would present a good target for EGS operations.
Success in the application of the SWS analysis is critically dependent on the
data acquisition. Fracture parameters such as density, strike, dip, fluid-fill content,
and/or aspect ratio can be determined only if SWS data are collected from many
different azimuths and incident angles. A dense network of stations can usually
overcome this limitation. To determine where along the ray path the cracked areas
responsible for the SWS are located, an even denser spacing may be required,
which is of course a cost factor. Fracture orientation can most easily be determined
among the desired parameters, particularly for parallel vertical cracks. If cracks
are shallow dipping or more than one crack system with varying orientations
exist, the analysis requires the very dense seismic arrays mentioned above. The
determination of fracture dip is also more strongly dependent on ray path coverage
quality.
In addition to the dense seismic arrays, a high sampling rate is necessary to
not only measure fast shear wave polarization orientations but also track ray
path–dependent variations in observed time delays (Rial, Elkibbi, and Yang, 2005).
The high sampling rates are particularly important for the accurate determination
of fracture density, as variations in time delay are subtle compared to that of