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Bidirectional Displacement Waveforms of Hhz Induced Microearthquakes - Evidence for Volumetric Shear-Slip Distributions in Ambient Crust Hydraulic Stimulation

Description: 

We examine HHZ displacement waveforms of microearthquake (MEQ) emissions induced by fluid injection into crystalline rock at 6 km depth and recorded at 2kHz rates by seismic sensors at 2.5km depth above the stimulation volume. The HHZ displacement waveforms for magnitude -1 < m < 1 MEQ are bidirectional, inconsistent with the standard unidirectional fault-zone-like planar slip distributions. While the HHZ waveforms are dominated by scattering for up to four-second codas, there is little intrinsic amplitude attenuation (Q_absorb ~ 3000). We find that the unscattered leading 10-20ms P-wave displacement wavefields is commensurate with dislocations on ambient fractures activated by fluid injection. Our HHZ event data set includes examples that show multiplets with several or more bi-directional dislocations.

The observed HHZ bidirectional quasi-sinusoid P-wave first motion displacements can be modeled by using Haskell’s elastodynamic integral over radially directed source-slip emissions including over-pressured volumetric permeability structures. Far-field MEQ HHZ bidirectional displacements waveforms arise from summing over radially expanding distributions of slip velocity. Arrivals emitted by radial slip towards sensors at slip sites nearer the sensor are followed by later-arriving reversed-sign displacements from radial slip away from sensors, as emitted by sites further from the sensors. Hypothetically, EGS stimulation of the ambient crust proceeds via high-pressure fluid breakouts causing slip dislocations that increase the pore connectivity (permeability). Our poster will discuss this connection, which is expressed by the poro-perm relation κ(x,y,z) ~ exp(αφ(x,y,z), and how this relates to the relative size distribution of the observed HHZ events.

Session: De-risking Deep Geothermal Projects: Geophysical Monitoring and Forecast Modeling Advances [Poster]

Type: Poster

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Rami Hofstetter

Student Presenter: No

Invited Presentation: 

Authors