Abstract
A key question for the development of geothermal plants is the seismic
detection and monitoring of fluid injections at several kilometers
depth. The detection and monitoring limits are controlled by several
parameters, for example, the strength of seismic sources, number
of receivers, vertical stacking, and noise conditions. For a known
reference reflector at 2.66 km depth at a geothermal site in northern
Germany the results of a simple surface seismic experiment were therefore
combined with numerical forward modeling for different injection
scenarios at 3.8 km depth. The underlying idea is that changes of
reflectivity from the injection at 3.8 km must be larger than the
variance of the measurements to be observable. Assuming that the
injection at 3.8 km depth would produce a subhorizontal disklike
target with a fracture porosity of 2\% or 5\% (the critical porosity)
the water injection volume has to be at least 443 and 115 m3, respectively,
to be detectable from the surface. If the injection on the other
hand does not create subhorizontal but subvertical pathways or only
reduces the seismic velocities via the increased pore pressure in
the immediate vicinity of the bore hole, the injection is undetectable
from the surface. The most promising approach is therefore to move
sources and/or receivers closer to the target, that is, the use of
borehole instrumentation. 10.1785/0120040124
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