ABSTRACT

We have developed a method for tracing the rupture propagation of microseismic events. We referred to it as microseismic rupture propagation imaging (MRPI), which is an adaptation of the back projection technique from global seismology. Hence, we shifted back recorded waveforms to a grid of possible source locations and obtained a coherent phase stack that migrated according to the migration of the rupture front. Using synthetic ruptures and the corresponding waveforms obtained by finite-difference modeling, we tested the viability of the approach for a reservoir model with the properties and geometry of the monitoring system of the Basel-1 geothermal reservoir. First, we found that an estimation of the rupture location, orientation, direction, and length was feasible in this environment. The method was then applied to the four largest events (ML=3.13.4) recorded at the Basel-1 reservoir. We found that the obtained rupture lengths and orientations were reasonably consistent with independent estimates from seismic moments, stress drops, and fault-plane solutions. MRPI allowed us to solve the ambiguity between the actual fault plane and the auxiliary plane. The derived fault planes and rupture directions for the three best-determined events indicated that the failure process was directed preferentially from the periphery toward the injection source. This agreed with the observation that hypocenters of large-magnitude-induced events tend to occur on the edges of the stimulated volume. The results also corroborated the recently proposed idea that induced events were more probable to occur on preexisting faults if the potential rupture surface lay within the stimulated volume.

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