Page 98 - Geothermal Energy Systems Exploration, Development, and Utilization

74 2 Exploration Methods
enough to be of use in P-wave delay surveys occur only rarely, and a long period of
recording time and relatively huge investments are necessary.
The first target of passive seismic methods is to determine hypocenters, whose
location is directly linked to those of faults – including those created by stimulation
and hydrofracturing – and to the tectonic signature of the area. In addition,
information about the geology and tectonics can be obtained from fault plane
solutions and first motion studies of these earthquakes, which are valuable in
determining whether the earthquake activity in a prospect area is anomalous or
typical for the region. If there are enough microseismic events and if they are
homogeneously distributed with respect to the recording stations and potential
targets, a 3D distribution of seismic velocities can be constructed. As the different
seismic velocities v p , v s ,and theratio v p /v s depend on various physical parameters
in a geothermal environment including fluid content of a rock, mapping of v p /v s
can be a powerful tool both in the exploration as well as for monitoring during
exploitation of a geothermal reservoir. Changes in v p , v s ,and theratio v p /v s
are expected when the steam volume increases, since it causes a strong P-wave
attenuation and an even sharper drop of v p /v s . Extensive fracturing of a liquid-filled
rock causes a minor reduction in the P-wave velocity and a significant reduction
in theS-wavevelocity, so that v p /v s is higher than normal. In addition, as v s is
more sensitive than v p to anisotropies of the rocks, v p /v s canvarywithazimuth.
Such variations can contain important clues about preferred orientation of fluid
circulation.
One method that takes advantage of anisotropies is the analysis of shear wave
splitting (SWS), which is based on the separation of the shear wave into a fast wave
traveling parallel to the fracture direction and a slow one traveling perpendicular
to the fluid-filled fractures (Crampin, 1981; Hudson, 1981) (Figure 2.13). The time
delay is proportional to the number of cracks per unit volume along the path of
the wave. Provided polarization of the fast wave and time delay are observed, the
detailed analysis of the polarization of shear waves in the seismograms allows the
determination of fracture orientation and of fracture density (Rial, Elkibbi, and
Yang, 2005). Tomographic inversion and the differences in arrival times can be
used to map the 3D distribution of the fractures, crack geometry, and thus regions
of potentially productive reservoir rocks. The SWS method is therefore highly
useful for the detection and development of EGS reservoirs, particularly if one well
has already been drilled and is used for stimulation procedures. The seismicity
induced by these operations provides excellent sources of shear waves near the
area of interest. Thus the method can be used even where natural seismicity is
scarce.
The largest data sets on SWS connected to geothermal reservoirs have been
collected by the University of North Carolina at Chapel Hill (J. Rial and his group).
From their experience gathered so far, there is also a list of limitations for the
method. One major problem can be the scarcity of detectable seismic events, which
is often the case in sedimentary basins. This can be overcome by long-term surveys
or permanent arrays. An assumption and prerequisite for all successful SWS
analysis is the mechanical isotropy of the uncracked rock volume. Any preexisting