Mapping Diffuse Seismicity for Geothermal Reservoir Management with Matched Field Processing: Year 2 Report
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Detecting and locating seismic events in managed underground geothermal reservoirs can become increasingly difficult during certain reservoir management operations. For example, during fluid injection procedures, large numbers of microearthquakes frequently occur during a relatively short amount of time. On seismic records, these events can overlap or have poor signal-to-noise ratios (SNR). This can obscure the onset of smaller signals and complicate the picking of seismic phases needed for earthquake location algorithms. To accurately illuminate the active areas of the underground reservoir through time however, a complete earthquake catalog down to very small magnitudes is necessary. To aid in the seismic characterization of reservoir fracture networks, we propose to complement traditional earthquake detection and location techniques with the empirical matched field processing (MFP) method. The principle of the empirical MFP method is to calculate propagation from source to receivers for pre-existing events, also called master templates. Then MFP matches the spatial structure of incoming seismicity observed by a network of sensors to master templates keyed to potential event locations. We demonstrate that empirical MFP can complement existing catalogs and techniques by using archived seismic information to increase earthquake catalog completeness. We identify and construct representative master templates using the Southern California Earthquake Data Center (SCEDC) earthquake catalog and archived waveform data of events originating within the Salton Sea Geothermal Field (SSGF). We identify 231 master events that have good quality seismic records at four or more seismic stations. We apply the empirical MFP method to continuous seismic data collected at the SSGF between November 2009 and December 2010. The MFP method successfully identified 5357 local events, while the original catalog only reported 1536 events. This increase in the number of events allows us to better determine the active fractures of the geothermal reservoir. We model the Coulomb stress change due to events between M2 – M3.5 that occurred during the January 2010 seismic swarm. Our results show a good correlation between regions of increased Coulomb stress change due to double-couple shear events on faults and the locations of subsequent microseismicity. In addition, we investigate the lower event detection magnitude threshold of our surface network using the MFP method. It clearly shows the event detection threshold is extended to include many smaller earthquakes with magnitudes down to M0.0. Therefore, we believe that the empirical MFP method, when combined with conventional methods, significantly improves network detection capabilities.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); USGS; National Science Foundation (NSF)
- DOE Contract Number:
- W-7405-ENG-48; AC52-07NA27344
- OSTI ID:
- 1077185
- Report Number(s):
- LLNL--TR-635216
- Country of Publication:
- United States
- Language:
- English
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