Experimental study on the seismic and aseismic deformation during the failure of granitic rock
Creators: name: Niu, Zihua
Publisher: name: ETH Zurich
Year: 2021
Subjects: fiber optics
acoustic emission
rock mechanics
induced seismicity

description: During the exploration of geothermal energy, fluids are often injected into the high
temperature host rocks in the upper lithosphere for efficient heat or electricity production.
It is known that large-scale injection activity will perturb the subsurface stress field,
resulting in nucleation and propagation of fractures or reactivation of the existing faults.
These mechanisms damage the rock and are accompanied by both static (aseismic) and
dynamic (seismic) deformation of the subsurface materials. The related dynamic processes
are associated with induce seismicity that are continuous to raise the public safety concerns
for the geothermal projects. Mitigation of the injection-related seismic hazards has become
a major challenge to geoscientists and geothermal engineers. Solutions to the problem rely
on the in-depth understanding of how aseismic and seismic deformation are correlated
during the progressive failure of rocks. While the injection is conducted at the scale of
meters to decameters, nucleation of the dynamic fault propagation might initiate from
failure at millimeter-scale or even lower. Laboratory studies have been instrumental in
developing a fundamental understanding of rock deformation and failure processes and
continues to expand our insight.

This study performed three tests on intact specimens of Rotondo granite in a triaxial
configuration which aimed to fill the gap between lab-scale and field-scale observations of
the physical processes during the progressive failure of rocks. A toolbox for events
localization, moment tensor inversion, stress inversion and seismic statistics analysis is
developed for the seismic data collected from the broadband PCT-LBQ acoustic emission
(AE) sensors. Seismic analysis is combined with the cutting-edge distributed strain field
measurement with fiber optics (FO) cables. Results and interpretations of the data in the
experiments can be summarized as follows: (1) Aseismic deformation is the predominant
process during the failure of rock in the confined compression test. Seismic deformation
only accounts for [0.07 to 4]×10^{-2}   percent of the total anelastic deformation. (2) Fractal
dimension ( D_q   ) and b-value in Gutenberg-Richter relation decrease as the sample
approaches failure in general sense. The correlation between    and b-value is rather
complex and can be correlated to the details on how the faults nucleate and propagate. The
angles between normal and slip vectors of moment tensors drop from ~108 degrees to ~87
degrees, indicating the change from compressive to expansive seismic events. (3) At the
early stage of failure prior to the formation of large-scale fractures, the perturbation on the
strain field is found to be nearly inversely proportional to   r^{-n}, where r is the distance to
seismic sources and n is found to be around 4, which follows the theoretical estimation of
the strain field perturbation from a point source dislocation. (4) Preliminary efforts into
unstructured 3D stress inversion, derived from the seismicity clouds, has been validated by
the distributed strain field determined using fiber optics technologies. The combination of
AE and FO proves promising in revealing how seismicity is related to the progressive failure
of the rock.