Geophysical Monitoring for Geologic Carbon Storage. Группа авторов

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Geophysical Monitoring for Geologic Carbon Storage - Группа авторов


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Valley is the southern extension of the elongate Great Valley. Site characterization was performed for this region (Wagoner, 2009; Coblentz et al., 2014). The area has operational oil fields, depleted oil and gas reservoirs, and saline aquifers. The target reservoir is a saline sandstone formation, the Vedder formation, located at a depth of approximately 2,100 m. The Vedder formation is a dipping formation with a thickness of approximately 400 m at the proposed injection location. The overlying low‐permeable Temblor‐Freeman shale provides the caprock. The three major faults at the Kimberlina site are the Greeley Fault, Pond‐Poso Creek Fault, and New Hope Fault. The proposed injection well is located between the Greeley Fault and Pond‐Poso Creek Fault.

Schematic illustration of the location of the Kimberlina CCUS (carbon capture, utilization, and storage) pilot site.

      4.3.2. Synthetic Event Locations With Different Surface Seismic Networks

Schematic illustration of p-wave velocity model for the Kimberlina site. Schematic illustration of the true locations of microseismic events (black dots) used in the synthetic study: (a) map view and (b) depth view.

      For each network distribution, we compute synthetic P‐wave and S‐wave arrival times for the true locations of microseismic events (Fig. 4.4) through ray tracing using the velocity model in Figure 4.3. We add a random Gaussian error with standard deviations of 0.02 s or 0.05 s to the computed travel times (e.g., Kijko, 1977b). We then use these travel times to solve for the locations of the microseismic events assuming that the velocity models are known. We obtain the linearized least‐squares solutions of event locations and origin times using an iterative inversion scheme (Paige & Saunders, 1982). The initial locations (blue dots in Fig. 4.5) are generated by adding a random Gaussian error with a mean of approximately 500 m to the true locations, and can be considered as an initial guess or some preliminary locations. Because microseismic events are clustered, we also use differential travel times between pairs of events in addition to absolute travel times to better locate the events (Waldhauser & Ellsworth, 2000; Zhang & Thurber, 2003). Improved relative locations using differential travel times are useful for studying the geometry of fractures and faults.

      The location results obtained using both P‐wave and S‐wave arrival times with a Gaussian time error of 0.02 s are shown as red dots in Figure 4.5. By increasing the total number of seismic stations, the location results improve with varying degrees. For the horizontal direction, the located events all seem to follow well with true locations (the left column (a), (c), (e), (g) of Fig. 4.5). For the vertical direction, the events are poorly constrained by the 4‐station network because of the lack of a station with small epicentral distances (Fig. 4.5b). The addition of one station at the center greatly improves the depth location accuracy for the events beneath the station (Fig. 4.5d). Clear improvement in the event depth is also observed when increasing from 5‐station to 9‐station, and then to 16‐station networks. The event location results with the 25‐station, 36‐station, and 49‐station networks do not seem to show significant difference (Fig. 4.5 f and h).

      4.3.3. Location Accuracy for Different Surface Seismic Networks


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