Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 60

Our method is based on the evaluation of event location accuracy, which provides important parameters and is useful for optimal design of microseismic monitoring (e.g., Eisner et al., 2010; Verdon et al., 2012; Oye et al., 2013; Takagishi et al., 2017; Wuestefeld et al., 2018). We use the following procedures (Fig. 4.1) for our optimal design of seismic network for cost‐effective microseismic monitoring.

Figure 4.1 The procedure for optimal design of seismic network for microseismic monitoring.

1 Build a velocity model for a field site based on site characterization. The velocity model should be based on previous geology and geophysics studies in this region. When an accurate S‐wave velocity model is not available, the common practice is to assume a constant ratio of V P/V S where V P is the P‐wave velocity and V S is the S‐wave velocity.

2 Define target monitoring regions and generate synthetic microseismic events within target monitoring regions. The target monitoring regions where microseismic events would occur depend on the injection locations, the reservoir depth, and the surrounding geology. The fault that is the closest to the injection site is often of great interest for monitoring. Synthetic true locations of microseismic events may be within reservoirs and/or on faults.

3 Define the distribution region of seismic stations. The range of the area where seismic stations may be distributed depends on the locations and magnitudes of potential microseismic events.

4 Determine the event locations for a given seismic network. First, we calculate the synthetic travel times for P‐waves and S‐waves for all seismic stations using the true locations of microseismic events and the P‐wave and S‐wave velocity models. We add random noise to the calculated travel times, and then use these travel times to solve for the event locations assuming we know the velocity models.

5 Repeat Step 4 for all possible seismic networks. Networks with different seismic stations can be created by varying the spacing of seismic stations.

6 Analyze the relationship between the event location accuracy and the number of seismic stations. The optimal seismic network corresponds to the distribution with the best trade‐off between the event location and the number of seismic stations (or cost).

We demonstrate how to employ these procedures to design an optimal microseismic monitoring network using a synthetic model for the Kimberlina site in California.