Page 95 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 71
the subsurface, if structures to be determined are of uniform geometry and if
the geology of the area is nearly 2D, as in some sedimentary environments.
For 3D structures, several closely spaced lines are necessary to provide adequate
coverage of lateral changes. Even though 2D seismic profiling is a standard
procedure in exploration, for which abundant off-the-shelf software packages exist
for processing and interpretation, each reflection survey needs to be designed
specifically to optimize the measurements for the required information. Geologists
and geophysicists need to communicate the problem to be addressed clearly to the
contractors doing the profiling.
Resolution with depth was shown to depend on the wavelength and thus the
frequency of the signal. As higher frequencies are lost with depth, resolution can
be improved with a higher energy signal, requiring a stronger shot, which is not
always feasible. Lateral resolution also depends on wavelength and thus decreases
with depth. However, a crucial point which can be controlled by the layout is
receiver spacing: it should be sufficiently narrow to allow reliable correlation of
reflections from the reflection interfaces.
To get a 3D image of the subsurface and of a potential reservoir, 3D seismic sur-
veys are highly desirable. When fractures are important a 3D approach is, usually,
also required. With receivers arranged on and shot points moved along a grid, pro-
cessing and interpretation of data is usually very time consuming and additionally
complex. In result such surveys are rather expensive such that large-scale 3D sur-
veys are rarely performed in geothermal prospecting. Perhaps, more importantly
they have been developed primarily for oil exploration in sedimentary environ-
ments that usually display less structural complexity laterally than, for example,
volcanic areas or other areas favorable for geothermal exploration (Figure 2.1). A
rare example of such a survey was conducted in the Italian geothermal area of
Travale in 2003 (Cappetti et al., 2005). Despite difficult terrain, the survey generated
sufficient data to significantly improve the deep geothermal reservoir of the area,
although severe reprocessing was required (Casini et al., 2010).
One of the limitations of seismic signals generated and detected at the surface is
their restriction to horizontal or gently dipping reflectors. To detect and image more
vertically situated structures, vertical seismic profiling (VSP) was developed, which
takes advantage of measurements within an existing well. An array of receivers
and the setup of one or more sources well adjusted to the problem not only allow
resolution of vertical reflectors such as faults but also provides a highly reliable
calibration tool for surface seismic measurements. VSP is also very useful when
dealing with seismic anisotropy.

2.4.2.2 Seismic Anisotropy and Fractures
Most commonly, the reflections of P-waves are used to image the presence and
orientation of fractures at depth. The underlying assumption in this approach is
that fractures cause P-wave reflection anisotropy, with the fast and high amplitude
direction parallel to the fractures and the slow and low amplitude direction
perpendicular to the fracture. Fractures are also assumed to be the cause for P-wave
attenuation. Stress can close cracks, water and/or steam can influence the crack