George Dresen: Fault structure, damage and induced microseismicity - what do we learn from the lab?

Oct 22, 2018 - 12:00 PM EDT

Speaker: 

Prof. George Dresen (U. Potsdam)

Dr. George Dresen, Profesor at U. Postdam and the German Resources Center for Geosciences, presents "Fault structure, damage and induced microseismicity -  what do we learn from the lab?" at the MIT Earth Resources Laboratory.

"Seismic events induced at about reservoir depth and large enough to be felt at the surface and thus raising public concern are commonly found to result from reactivation of preexisting faults. Faults typically display characteristic structural patterns that evolve in space and time resulting in complex anastomosing networks of slip zones showing aspects of band-limited self-similarity across a broad range of spatial scales. Individual slip zones show surface varying roughness and are embedded in zones of distributed damage. Field studies and laboratory experiments suggest that structural heterogeneity and fault zone roughness may affect seismic characteristics such as earthquake magnitudes, recurrence intervals, b-values, radiated energy, stress drops and source type distributions. In an effort to investigate the evolution of faulting-related damage and related microseismic activity we performed an extensive series of experiments on granite, quartzite and sandstone rock samples with 40-50 mm diameter and 100-125 mm length. The tests were performed in a servo-controlled MTS loading frame in triaxial compression at confining pressures between 20-150 MPa. The experiments include fracture tests of intact samples and stick-slip tests on saw-cut and pre-fractured samples with varying roughness. In addition, hollow cylinder hydrofracturing tests were performed were performed.  

Acoustic emissions (AE) were continuously recorded and ultrasonic velocities were monitored in 20-40 s intervals using up to 16 P-wave sensors attached directly to the sample surface. Full waveforms of AEs were stored in a 16-channel transient recording system (Proekel, Germany) with a bandwidth of 16 bit and 10 MHz sampling rate. Event location is based on automatic P-wave picking and updated time-varying velocity models. We calculated full moment tensors (FMT) using P-wave amplitudes corrected for incidence angle and coupling. Spatio-temporal changes in AE event densities, fault mechanisms, local damage and stress distribution, FMT components, and magnitude-frequency distributions (b-values) from fracture tests and multiple stick-slip events were analyzed and compared to post-mortem fault structures using optical microscopy and Xray tomography.

In general, we observe a correlation of fault roughness with AE hypocenter patterns indicating fault asperities. Spatio-temporal changes in AE activity and b-value distributions correlate with changing AE source mechanisms, in particular varying contributions from non-double components (NDC). Major stick-slip events associated with pronounced stress drops are spatially correlated with large AE events possibly indicating shearing of asperities along combined R- and P-type shears. Post-slip increase in b-values and increasing NDC contributions possibly indicate gouge compaction by grain crushing and sliding. Our studies suggest that the observed seismic characteristics are controlled by boundary conditions (confining pressure), sample material (porosity), and spatio-temporal changes in fault zone structure and stress heterogeneities."