Page 96 - Geothermal Energy Systems Exploration, Development, and Utilization

72 2 Exploration Methods
properties as well. It is therefore important to measure as many azimuths as
possible (i.e., a dense 3D grid) to detect and distinguish the potential influence
of crack orientation and fluid-fill on the signals. With increasing variety of crack
orientations (greater heterogeneity), it becomes more difficult to derive solutions
for fracture orientation from observed signals, such that there is almost always
some ambiguity in the results and careful processing and interpretation are
crucial.
Outof the manypossible more or less advanced processing and reprocessing steps
and procedures, the amplitude variation with offset (AVO) and amplitude versus
azimuth (AVA) methods deserve special mentioning, as they are often applied
to address fracture anisotropy. Variations in the AVA are analyzed assuming
that fractures attenuate the P-waves as they travel across the fractures. Thus,
an analysis of the variation of amplitude measured from different angles can
yield information about the fracture anisotropy. Similarly, analysis of AVO uses
variations in amplitude as function of reflection angle to derive information about
anisotropy. Approaches such as AVO assume that there is a dominating set of
fractures with a certain orientation. The detection of this preferred trend would
help understand the potential anisotropy in permeability and thus be of great
importance for geothermal exploitation. However, AVO does not always work.
Especially, older terrains with a complex geological history tend to have multiple
fracture sets of various orientations, some of which may be open simultaneously
despite unfavorable orientation of the stress field. So, before AVO analysis is
carried out, it has to be determined if the rock physics and fluid characteristics of
the target reservoir are likely to give a usable response. Such a step will include
seismic forward modeling including realistic geological and petrophysical boundary
conditions of the area. AVO quality is also dependent on depth, as signal-to-noise
ratio gets worse and higher frequencies are more attenuated, geology often gets
more complicated, making AVO less applicable with increasing depth. A detailed
coverage of AVO, its strengths and pitfalls, is given by Avseth, Mukerji, and Mavko
(2005).
Generally, current technology can often locate fracture trends, but it usually does
not provide the accuracy in locating high permeability zones to site wells. Seismic
attributes such as P-wave anisotropy, AVO, or AVA are helpful in defining overall
fracture properties and the detection of fracture zones. But these approaches have
not been able to define the specific fracture sets that control permeability. Theo-
retically, the resolution with depth would allow precise localization of productive
zones. But, even with the highest theoretical data quality of today, determining the
significance of the underground images obtained remains the greatest challenge;
there appears to be an agreement among seismic experts that a large part of the seis-
mogram is not yet understood and contains valuable information that may one day
be retrievable. The challenge is to define seismic properties that might image flow
properties in the reservoir and permeability, rather than simply geologic features.
While it is theoretically possible to reach this goal with adequate conditions such
as enough measurement points of sufficient quality, enough computing power,