In the southern Sichuan Basin, China, the recent increase in the seismic activity has been suspected to be related to hydraulic fracturing stimulation for producing the shale gas. In this study, we used the monitoring data from a local seismic network within the shale gas blocks to study the earthquakes near the shale gas production wells that have detailed injection data. Comparison of the timing of earthquakes and stimulation schedule of the studied well pads indicates an apparent correlation between the seismic activity and hydraulic fracturing. The results of seismic velocity tomography reveal that the reactivation of preexisting faults due to fluid diffusion is the primary cause of the observed earthquakes. Focal mechanism analysis combined with geomechanical modeling indicates that the increased pore pressures resulted from hydraulic fracturing are sufficient to trigger seismic slip on the faults.

We have developed a new seismic tomography method, back azimuth constrained double-difference (DD) seismic tomography, which is suitable for downhole microseismic monitoring of hydraulic fracturing. The new method simultaneously locates microseismic events and determines three-dimensional (3D) Vp and Vs models for the fracturing zone using

Focal-time analysis is a straightforward data-driven method to obtain robust stratigraphic depth control for microseismicity or induced seismic events. The method eliminates the necessity to build an explicit, calibrated velocity model for hypocenter depth estimation, although it requires multicomponent 3D seismic data that are colocated with surface or near-surface microseismic observations. Event focal depths are initially expressed in terms of zero-offset focal time (two-way P-P reflection time) to facilitate registration and visualization with 3D seismic data. Application of the focal-time method requires (1) high-quality P- and S-wave time picks, which are extrapolated to zero offset and (2) registration of correlative P-P and P-S reflections to provide [Formula: see text] and [Formula: see text] time-depth control. We determine the utility of this method by applying it to a microseismic and induced-seismicity data set recorded with a shallow-borehole monitoring array in Alberta, Canada, combined with high-quality multicomponent surface seismic data. The calculated depth distribution of events is in good agreement with hypocenter locations obtained independently using a nonlinear global-search method. Our results reveal that individual event clusters have distinct depth distributions that can provide important clues about the mechanisms of fault activation.

Seismic imaging with active seismic sources can be used to image subsurface structures such as faults, fractures, and layer interfaces with higher resolutions than tomographic methods. However, for characterizing large‐scale three‐dimensional structures, oftentimes it is too expensive and thus impractical to conduct active seismic survey. Seismic imaging using passive sources has been used to image structures through the “pseudoactive” source imaging by assuming source information is known. However, in practice there exist some uncertainties on source information including location, origin time, source time function, and radiation pattern, which can inevitably affect the final imaging result. In this study, we propose a new source‐independent passive seismic imaging method based on reverse‐time imaging with converted waves and a new imaging condition. This method uses the interference between P and S waves at conversion points for structure imaging by back‐propagating seismograms recorded by all receivers. Since no forward modeling is needed in this method, source information is not needed. A new grouping imaging condition is proposed to improve the image resolution, in which seismic receivers are divided into several groups and the separated P and S wavefields of different groups are multiplied and then stacked over time to get an image. We test the new method on both synthetic and real data sets based on a downhole microseismic system that is used for monitoring a shale gas hydraulic fracturing treatment. The results show that the proposed passive seismic imaging method does not depend on knowing source information and can achieve higher imaging resolution.

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