Geoscience Reference
In-Depth Information
4.
Temporary local seismic arrays can be installed to find faults, determine source
mechanisms, decrease error in location of seismic events, and increase resolution
of future events.
g
ap
Simple geometric considerations to help visualize subsurface problems and identify
cases that deserve further attention are in most cases absent. Developing these kinds of
simple analyses could, for example, be applied to understand the length scale affected by
a single well or by multiple wells relative to depth or proximity to major faults and to the
surface.
p
roposed
a
ction
In locales where a causal relationship may exist between subsurface energy activities
and seismicity (even for small earthquakes of
M
between 3 and 4), a local seismic array
should be installed for seismic monitoring. An appropriate body to determine whether such
an array is necessary may be the permitting agency for the well(s) thought to be involved
in the seismicity. Installation of such an array may require significant resources (including
instrumentation and analysis). Existing groups, such as the U.S. Geological Survey, national
laboratories, state geological surveys, universities, and private companies have the expertise
necessary to install arrays and conduct the necessary analyses. Full disclosure of the data
and results of such monitoring is required.
Geothermal Energy
F
indings
1.
The induced seismic responses to injection differ in cause and magnitude with
each of the three different forms of geothermal resources. At the vapor-dominated
Geysers field hundreds of earthquakes of
M
2 or greater are produced annually
with one or two of
M
4, all apparently caused principally by cooling and contrac-
tion of the reservoir rocks. The liquid-dominated field developments generally
cause little if any induced seismicity because the water injection typically replaces
similar quantities of fluid extracted at similar pressures and temperatures. The high-
pressure hydraulic fracturing into generally impermeable rock associated with the
stimulation operations at enhanced geothermal systems (EGS) projects can cause
hundreds of small microseismic events and an occasional earthquake of up to
M
3
due mainly to the imposed increased fluid pressures.
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