Seismology and Enhanced Geothermal Systems

Seismology and Enhanced Geothermal Systems
Seismology and Enhanced Geothermal Systems
Bruce R. Julian
Foulger Consulting, Palo Alto, CA 94306
Gillian R. Foulger
Dept. of Earth Sciences, Durham University, Durham, DH1 3LE, U.K.
Recent technical advances increase the potential utility of seismological methods for
monitoring physical processes in geothermal systems, and in Enhanced Geothermal
Systems (EGS) experiments in particular, although many challenges remain.
Differential methods for locating microearthquake hypocenters relative to one another
can delineate the shapes of failure zones with a resolution of a few tens of meters, and the
resulting images of failure zones can greatly increase the uniqueness of interpreting
source mechanisms in terms of physical processes. This kind of combined interpretation
has, for example, demonstrated the occurrence of tensile failure in both natural and
exploited geothermal systems. Differential methods are poor for determining absolute
locations, however. Approaches that combine absolute and differential arrival times can
improve this situation, but using calibration data from timed explosions provides the
greatest absolute accuracy.
The extension of microearthquake source mechanism studies beyond “fault-plane
solutions” to the more general moment-tensor representation has produced evidence of
volumetric seismic processes such as tensile failure. Expected processes such as unsteady
fluid flow require a further extension to include net forces in source mechanisms, and we
have recently extended source-mechanism inversion methods to include forces.
Three-dimensional models of seismic wave speeds provided by tomographic methods can
significantly improve the accuracy achievable by all the above methods. In addition,
tomography has direct applications, because geothermal exploitation, and probably
natural processes, can cause the wave speeds to vary with time. Recent extensions of
tomographic methods increase the objectiveness with which temporal changes can be
detected. A shortcoming of local-earthquake tomographic methods is their restriction to
the shallow, relatively cool region above the maximum earthquake depth. Rays from
regional earthquakes that pass through the deeper portions of geothermal reservoirs and
their heat sources may provide data that can extend resolution to greater depths.
We will illustrate the application of seismological methods using data from natural and
exploited geothermal systems, and from EGS experiments currently in progress.
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