Page 97 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 73
and sufficient frequency content, several practical obstacles prevent its successful
attainment.
In a study for the oil and gas industry, Majer (2003b) lists a number of
problems and potential ways to address them. The limitation of image resolution
by the amplitude and frequency content of the seismic waves and by the level of
complexity of the ambient and signal-generated noise fields is hard to overcome
with current techniques and surface sources. A major cause for this problem is
the heterogeneity and thickness of the weathered surface layer which can attenuate
the high frequency content and the coherence of the signal severely. This problem
is partly solved by VSP, as receivers and sources are placed beneath the surface
layer in a vertical array within the well. An approach to reduce imaging limitations
is the incorporation of S-wave properties and the converted waves (P to S and S
to P) generated by the multiple reflections in the earth. Majer (2003b) also points
out that including amplitude and converted waves in the analysis could even make
surface methods more useful, particularly where P-, S- and converted waves can
be examined directly. S-waves have already become more and more common parts
of the analyzed wave spectrum in recent years, which is of specific use for the
definition of anisotropy and fracture orientation of a rock. Generally, to make use
of the full potential seismic methods have to offer, three-component data including
P- and S-wave reflection as well as VSP is required.

2.4.2.3 Passive Seismic Methods
The passive seismic method takes advantage of naturally occurring seismicity. The
energy of seismic events is high enough to be detected by standard seismometers,
even if it is not felt by the population. Such low magnitude earthquakes occur
quite frequently in tectonically active regions, where most geothermal reservoirs
are located. Moreover, microseismicity is often associated with hydrothermal
convection, thus responding directly to the resource to be detected. Thus, passive
seismic studies have been found to have a promising potential in pinpointing active
faults or fracture systems that are not always found on the surface, as well as their
elevation and inclination.
Seismic surveys of microseismicity require a sufficiently dense network of
recording stations placed around the potential reservoir and an extended period of
recording time, usually several months. Several well-located events are necessary to
reliably characterize an active fault. If these active faults are located, sophisticated
use of recording and the recorded data can help to construct a three-dimensional
image of fluid flow in the reservoir, as fluid circulation occurs in open faults and
fracture systems, which are often responsible for the observed microseismicity.
The frequencies associated with fluid circulation in open fractures are usually at
the lower limit of the recording spectrum. This problem can be solved by the use
of broadband stations that record a much broader spectrum of frequencies than
standard seismometers. Since an increase in temperature results in the reduction
of P-wave velocity over a large volume in the crust, the measurement of delay times
from teleseismic events (distant earthquakes) have been used to locate large hot
bodies that act as the source of geothermal systems. However, teleseismics far